NTPC Training Report
NTPC Training Report
NTPC Training Report
ANKIT CHAWLA
2K11-MRCE-ECE-007
ECE (A) VIII Semester
To
ACKNOWLEDGEMENT
I express my deep sense of gratitude to my project supervisor Mr.V.K.Bhatia and
Mr.Umesh, NTPC LIMITED, for their immense support and excellent guidance. They have
helped me to explore this vast topic in an organized manner and provided me with all the ideas on
how to work towards the research-oriented venture.
I acknowledge with gratitude the help extended by all Mr.Puran Singh and technical staff
of NTPC FARIDABAD for their co-operation and guidance that helped me a lot during the course
of training. I have learnt a lot working under them and I will always be indebted of them for this
value addition in me. Last but not the least, I thank my family and friends for their constant
encouragement and wholehearted support during the entire project work.
TABLE OF CONTENTS
1. ABOUT THE COMPANY
1.1 Corporate Vision
1.2 Core Values
1.3 Installed Capacity of NTPC
2. FARIDABAD GAS POWER PLANT
2.1 Introduction
2.2 Salient Features
2.3 CONVERSION FROM GAS TO ELECTRICITY
3. FUEL
3.1 Natural Gas
3.2 Naphtha
3.3 Operations and fuel handling department
4. CONTROL & INSTRUMENTATION DEPARTMENT
4.1 AUTOMATION& CONTROL
4.1.1 The benefits
4.1.2 Process Structure
4.1.3 Control system Structure
4.1.4 System Overview
4.2 CONTROL & MONITORING MECHANISMS
4.3 POWER STATION INSTRUMENTATION
4.3.1 Presentation of information
4.3.2 Selection of instruments
4.3.3 Concept of instrumentation in thermal power station
5. INTERPRETATION of INSTRUMENT READINGS
6. PRESSURE MEASUREMENT AND MEASURING INSTRUMENTS
6.1 Pressure measurement-introduction
6.2 Pressure Measuring Devices
6.2.1 Manometer elements
6.2.2 Diaphragm, Capsule and bellows
6.2.3 Bourdon Tube Gauges
6.2.4 Transmitters for pressure measurement and differential pressure
measurement
7. LEVEL MEASUREMENT & MEASURING INSTRUMENTS
7.1 level measurement- introduction
7.2 methods
7.2.1 float and liquid displacement
7.2.2 Head pressure measurement system
7.2.3 Electrical/electronic methods
7.2.4 Ultrasonic methods
CHAPTER 1
B-Business Ethics
E-Environmentally and economically sustainable
C-Customer Focus
NTPC's core business is engineering, construction and operation of power generating plants and
providing consultancy to power utilities in India and abroad.
In October 2004, NTPC launched its Initial Public Offering (IPO) consisting of 5.25% as fresh
issue and 5.25% as offer for sale by the Government of India
NTPC thus became a listed company in November 2004 with the Government holding 89.5% of the
equity share capital.
In February 2010, the Shareholding of Government of India was reduced from 89.5% to 84.5%
through a further public offer. Government of India has further divested 9.5% shares through OFS
route in February 2013. With this, GOI's holding in NTPC has reduced from 84.5% to 75%. The rest
is held by Institutional Investors, banks and Public.
NTPC is not only the foremost power generator; it is also among the great places to work. The company is
guided by the People before Plant Load Factor mantra which is the template for
all its human resource related policies. NTPC has been ranked as 6th Best Company to work
for in India among the Public Sector Undertakings and Large Enterprises for the year 2014, by
the Great Places to Work Institute, India Chapter in collaboration with The Economic Times.
CAPACITY(MW)
COAL
17
33,675
GAS/LIQUID FUEL
4,017
HYDRO
600
RENEWABLE ENERGY
110
33
38,402
NTPC OWNED
PRODUCTS
TOTAL
OWNED BY JVs
COAL AND GAS
6,196
TOTAL
40
44,598
STATE
BY NTPC)
COMMISSIONED
CAPACITY(MW)
Singrauli
Uttar Pradesh
2,000
Korba
Chhattisgarh
2,600
Ramagundam
Telangana
2,600
Farakka
West Bengal
2,100
Vindhyachal
Madhya Pradesh
4,260
Rihand
Uttar Pradesh
3,000
Kahalgaon
Bihar
2,340
Dadri
Uttar Pradesh
1,820
Talcher kaniha
Orissa
3,000
10
Uttar Pradesh
1,050
11
Talcher Thermal
Orissa
460
12
Simhadri
Andhra Pradesh
2,000
13
Tanda
Uttar Pradesh
440
14
Badarpur
Delhi
705
15
Sipat
Chhattisgarh
2,980
16
Mauda
Maharashtra
1,000
17
Barh
Bihar
1,320
Total
Coal Based Joint Ventures:
33,675
Table 3
S.NO
STATE
COMMISSIONED
CAPACITY(MW)
Durgapur
West Bengal
120
Rourkela
Orissa
120
Bhilai
Chhattisgarh
574
Kanti
Bihar
415
IGSTPP,Jhajjar
Haryana
1500
6
Total
Vallur
Tamil Nadu
1500
4,229
GAS BASED(OWNED BY
STATE
NTPC)
COMMISSIONED
CAPACITY(MW)
Anta
Rajasthan
419.33
Auraiya
Uttar Pradesh
663.36
Kawas
Gujarat
656.20
Dadri
Uttar Pradesh
829.78
Jhanor-Gandhar
Gujarat
657.39
Kerala
359.58
Haryana
431.59
Kayamkulam
7
Faridabad
Total
S.NO
4,017.23
GAS BASED(OWNED BY JVs)
STATE
COMMISSION
CAPACITY(MW)
RGPPL
Maharashtra
1967.08
HYDRO BASED
Koldam(HEPP)
STATE
APPROVED
COMMISSION
CAPACITY(M
CAPACITY(M
W)
W)
800
600
Uttarakhand
520
Uttar pradesh
Himachal
Pradesh
Tapovan Vishnugad
(HEPP)
Singrauli CW
Discharge(Small
Hydro)
Lata Tapovan
Uttarakhand
171
Rammam
West Bengal
120
Total
1,519
600
Project
State/UT
Capacity (MW)
1.
Dadri Solar PV
Uttar Pradesh
2.
3.
Telangana
10
4.
Odisha
10
5.
Faridabad Solar PV
Haryana
6.
Unchahar Solar PV
Uttar Pradesh
10
7.
Rajgarh Solar PV
Madhya Pradesh
50
8.
Singrauli Solar PV
Uttar Pradesh
15
Total
110
Wind Energy:
Projects under Tendering (80 MW)
Hydro Energy:
Projects under Execution (8 MW)
Geothermal Energy:
Distributed Generation
Indias growth story is not possible unless the entire country marches in tandem. This means that
the growth trajectory of rural India where two-thirds of our population lives ramps up
considerably. Economic development with the supply of reliable energy is a must for this goal to
be realized. Subsequently efficient energy management is crucial for rural development. Some of
the villages are located in remote & inaccessible areas where it would be either impossible or
extremely expensive to extend the existing power transmission network. To this end, distributed
generation is the solution to the challenge in providing power to off-the-beaten track village
clusters. Currently 16 decentralized distributed generation power projects with a combined
capacity of 340 KW have been commissioned for benefitting 2280 households with a population
of 12500 in four states.
Awards:
IEEMA Power award-2009 in the category of Excellence in Distributed Generation.
NTPC Distributed Generation film Energizing villages has been awarded in category
development venture by Public Society of India, Hyderabad.
CHAPTER 2
2.1 INTRODUCTION
NTPC-Faridabad was approved on 25th July 1997. The total project cost was 1163 cr. INR.
The plant was fully functional in the year 2000 with an installed capacity of 432 MW. The
plant under an agreement with the Haryana government supplies the entire power
generated to the state of Haryana only.
Faridabad gas power plant is a combined cycle power plant having a net capacity of
432MW. It consists of two gas turbines with capacity of 138MW each and a steam
turbine with the capacity of 156MW. The main fuel used here is natural gas. In case of
the unavailability or shortage of natural gas the alternative fuel used is naphtha. Since
the production cost with naphtha differs from that with natural gas by Rs. 6-7 per unit,
naphtha is not used as a main fuel.
The power plant is called the combined cycle power plant because the heat energy
liberated during the combustion of natural gas is not dissipated into the environment but is
utilized for the generation of steam which rotates the steam turbine.
1. Project
2. Location
3. Plant Capacity
4. Plant Configuration
Haryana
432 MW
Gas Turbine 1 - 138 MW
Gas Turbine 2 - 138 MW
Steam Turbine - 156 MW
5. Mode of Operation
6. Fuel
7. Alternate Fuel
8. Average Gas requirement
Base Load
Natural Gas
Naphtha / HSD
2 million cubic meters per day
9. Fuel Source
10. Cooling water
11. Naphtha Storage
The exciter of the turbine as well as other machinery is coupled to both the BSDG as well as main
power supply. In case of power failure, the power source is automatically switched to BSDG
supply.
Efficiencies are very wide ranging depending on the lay-out and size of the installation.
Developments needed for this type of energy conversion is only for the gas turbine. Both waste
heat boilers and steam turbines are in common use and well-developed, without specific needs for
further improvement.
Figure 2.1: Flow diagram depicting process of conversion of Natural Gas to Electricity
The above diagram depicts the process of conversion of natural gas to electricity. It is a combined
cycle which utilizes the heat of the fuel gas to heat the water and operate the steam turbine.
The step by step description of the process and the machinery is as follows:
2. Air filters
The air obtained from the environment contains numerous pollutants and unwanted compounds
which may harm the machinery and reduce the efficiency of the system. These unwanted
compounds may also react with the surface of the machinery and cause scaling which would
subsequently reduce the lifetime of the machinery. To overcome this problem, the air is passed
through the filter section. This section consists of an array of 576 filters to eliminate all the
unwanted particles and compounds present in the air.
3. Air compressor
The filtered air is then passed through the compressor section. The compression of air takes place
in 16 stages. The compression reduces the temperature of air. To compensate the heat loss and
prevent the temperature shock in the next stage, heat addition is done in the next stage of
combustion.
4. Combustion chamber
After compression, the air is sent to the combustion chamber where 11 parts of air is burnt with
one part of natural gas. This leads to the expansion of air which is used to rotate the turbine.
Each turbine section is preceded by two combustors. Each combustor consists of eight burners.
Fuel enters the front of the burner as an atomized spray or in a pre-vaporized form. Air flows in
around the fuel nozzle and through the first row of combustion air holes in the liner. Air near the
burner nozzle stays close to the front liner wall for cooling and cleaning purposes. Air entering the
opposing liner holes mixes rapidly with the fuel to form a combustible mixture. Air entering the
forward section of the liner re-circulates and moves upstream against the fuel spray. During
combustion, this action permits rapid mixing and prevents flame blowout by forming a lowvelocity stabilization zone. This zone acts as a continuous pilot for the rest of the burner. Air
entering the downstream part of the liner provides the correct mixture for combustion. This air
also creates the intense turbulence necessary for mixing the fuel and air and for transferring
energy from the burned to the un-burnt gases. Since an engine usually has two ignition plugs,
cross ignition tubes are necessary in the can and can-annular types of burners. These tubes allow
burning to start in the other cans or inner liners.
5. Gas turbine
It is a single shaft (with line compressive unit). It is a 50 Hz; 135MW machine which runs on
natural gas could also be operated on the liquid Naphtha. The gas turbine is very heavy, industrial
type, within line compressor multistage flow type. The combustion chamber is of annular type.
According to the flow of the air ; compressor is placed first, combustion chamber is next to it and
turbine at the end of gas turbine. Two bearings are placed to support the shaft of the machine,
these turbines are provided at the compressor starting end, and other are placed at the turbine end.
The shaft of the unit is provided with the blades in the turbine region.
i) Basic parts of the Gas Turbine:
1. Compressor: Is a fuel stage axial type. It is provided with a variable inlet guide system to
enable efficient operation. Filters are provided at the top of the compressor to filter any unwanted
material from entering the turbine. In the compressor region, there are 16 stages of blade, one set
of blade, one set of blade on shaft and other set of fixed blade comes alternatively.
2. Combustion chamber: There are two chambers in the gas turbine, one on each side of the
shaft, connected vertically and parallel to each other. The combustion chambers are cylindrical in
shape and attached to the unit in between the compressor and turbine.
3. Turbine: It is provided at the end of the gas turbine unit. It consists of four stages of blades it
also has the gearing to support the shaft at its end.
Exhaust of the turbine is connected to the bypass stake which is further connected to WHRSG.
The bypass is take is provided with two gates namely diverter damper and gelatin gates.
SIEMENS(Germany); model-
capacity
compressor
turbine
burner
combustors
Air intake filters
V 94.2
137.76 MW
16 stage
4 stages
Hybrid dual fuel
SILO type
Pulse cleaning(576 in
By pass take
Ambient temperature
Ambient pressure
numbers)
Vertical 70 m in height
27 deg c
1013 Mbar
v) Gas turbine
generator
specifications:
Relative humidity
Voltage rating
60%
10.5 KV+/-5%
Current
Power factor
KVA( Apparent
9354A
0.85 lagging
170.12 MVA
power)
Excitation current
Excitation voltage
Insulation type
Connection type
833 Amp
410 V
Class F micalastic
AA
Stator Winding
Speed
YY
3000 rpm
6. Generator
The rotation of gas turbine leads to the rotation of the rotor part of the generator which is
connected to the same shaft as that of the turbine.
Generator
7. Step up transformer
The electricity is generated at 10.5KV. But this voltage is very less for the purpose of transmission
over a long distance and hence the step-up transformer is used to step up the voltage from 10.5KV
to 220KV.
iii). Specifications of HP
Turbine:
TYPE
No. of stages
Total H.P main steam
Single flow
25 reaction stages
76.4 bar
pressure
HP main steam temp.
HPT exhaust pressure
HPT exhaust temp.
528 deg c
5.1 bar
175 deg
double flow
No. of stages
7 reaction stages
46 T/hr
4.38 bar
v) Steam turbine
specification:
manufacturer
type
capacity
Maximum terminal outputs
Main steam pressure
Condenser vacuum
speed
Steam turbine generator
Stator current
coolant
insulation
Power factor
Excitation system
Rotor voltage
Rotor current
The high pressure turbine receives HP steam i.e. 85 Kg/cm2, while the low pressure turbine
receives the low pressure steam i.e. 4-5 Kg/ cm2.
12. Condenser
The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be
pumped. If the condenser is made cooler, the pressure of the exhaust steam is reduced and
efficiency of the cycle increases.
The surface condenser is a shell and tube heat exchanger in which cooling water is circulated
through the tubes. The exhaust steam from the low pressure turbine enters the shell where it is
cooled and converted to condensate (water) by flowing over the tubes.
a) L.P. Drum: these drum store the L.P steam produced during the flow of water in the L.P.
evaporator. It is small in the size than the H.P drum and has a blow of cork at its top to avoid
blasting at high steam pressure.
b) H.P. Drum: this drum store the H.P steam produced during the flow of water through H.P. super
heater. It also has a blow cork for safety purposes.
Working of the WHRSG:
The boiler feed pumps feed the water to the HP & LP economizers, where the temperature of the
water rises close to the saturation temperature after flowing to the economizers the water is passed
to the steam drums through feed control system, then water is taken to the bottom header of the
evaporator through the downpipes, here water gets converted into a mixture of steam and water.
The mixture is carried to the tubes through rigor pipes. In the drum mixture is passed through
centrifugal separators, where water is passed for recirculation through the down pipes. In the
super heater steam gets superheated, to control the temperature of steam it is passed through spray
type de-superheated is provided between HP economizer 1&2. This steam at the outlet of the
super heater is carried to the steam turbine through feed pipes.
S.No.
1.
2.
3.
4.
5.
6.
Parameter
Design
Pressure (bar)
working
pressure (bar)
Steam
temperature
(deg. C)
Steam flow
(T/hr)
Total heating
surface,
Superheater
(m2)
Total heating
surface, water
tubes (m2)
HP System
83
LP System
9
63
5.5
488
207
162.67
39.1
8980
584
55910
23859
15. De-aerator
The water from the condenser is led here by the condensate extraction pumps. The de-aerating
boiler feed water system eliminates the need of expensive oxygen scavenger chemicals and also
offers the following advantages:
Raises the boiler feed water temperature, eliminating thermal shock in boilers.
Feed water pumps are sized for each individual application - assuring total compatibility
and optimum operation.
PRINCIPLE: Paging scheme-high temperature breaks down the gases and expels the air.
HOW DOES IT WORK? Aerated water is fed into the de-aerator through the inlet water
connection. This water passes through the steam-filled heating and venting section. The water
temperature is raised and many of the un-dissolved gases are released. As the water passes
through the assembly, it flows to a scrubber section where final de-aeration is accomplished by
scrubbing the water with oxygen free steam. This steam is induced through a stainless steel spray
valve assembly which causes the high velocity steam to break the water down to a fine mist
through a violent scrubbing action. The de-aerated water spills over to the tanks storage
compartment for use by the boiler, and the gases are vented to the atmosphere.
De-aerator
Figure 2.15 De-aerator
The de-aerated water is then stored into the feed storage tank and is pumped out when required.
17. Boiler feed pumps
They are used to pump the water from the feed storage tank to the respective boiler drums. They
are classified as high and low pressure boiler feed pumps based on the boiler drum to which they
pump the water.
18. Cooling towers
A cooling tower is equipment used to reduce the temperature of a water stream by extracting heat
from water and emitting it to the atmosphere. Cooling towers make use of evaporation whereby
some of the water is evaporated into a moving air stream and subsequently discharged into the
atmosphere.
The tower vary in size from small roof-top units to very large hyperboloid structures that can be
up to 200 meters tall and 100 meters in diameter, or rectangular structure that can be over 40
meters tall and 80 meters long. Materials are chosen to enhance corrosion resistance, reduce
maintenance, and promote reliability and long service life. Galvanized steel, various grades of
stainless steel, glass fibre, and concrete are widely used in tower construction
CHAPTER 3
FUEL
Gas turbines are capable of burning a range of fuels including Naphtha, crude oil and natural gas.
Selection of fuel depend upon several factors including availability of fuel, fuel cost, cleanliness
of the fuel. Natural gas is an ideal fuel because it provides efficiency and reliability with low
operation and maintenance cost. Liquid fuels particularly heavy oils; usually contain
contaminants, which cause corrosion and fouling in gas turbine. Contaminants which cannot be
removed from fuel may leave deposits in gas turbine, which reduces the performance and adds to
the maintenance cost.
Duel fuel system is commonly used, enabling the gas turbine to burn up the fuels when primary
fuel sources are not available. Duel fuel system can be designed to fire both fuels simultaneously.
FGPP works on natural gas but if there is shortage of natural gas then plant is run on Naphtha.
Naphtha as compared to natural gas has less calorific value but there is no alternate fuel other than
Naphtha. It is cheaper than any other fuel and the amount of flue gases that comes out of the
Naphtha can also be sent out to boiler to boil the water for manufacturing of steam for running the
steam turbine.
Naphtha is highly inflammable and highly explosive fuel. When it makes a mixture with air, it
forms a very highly dangerous explosive mixture. When the supply of natural gas cuts off, the
pipelines are filled with air. So while using Naphtha, it is necessary to remove that air because it
can make explosive mixture with Naphtha. So for flushing this air a high-speed diesel (HSD) is
sent to the pipeline, which removes the air present. In this way HSD enters the combustion
chamber and working continues. This is the procedure for working of gas turbine when it has to
feed on Naphtha without stopping the while plant, which was previously feed on natural gas.
Naphtha before entering the pipelines undergoes filtration various times so that there should not
be any impurity in that when it enters the combustion chamber. The main difference between
Naphtha and Natural gas is that, natural gas enters the combustion chamber in the form of gas but
Naphtha enters in form of liquid spray. Then it is compressed in it and due to high compression it
burns and leaves very highly pressurized flue gases, which in turn is used to rotate the gas turbine.
This entire process of using the Naphtha as a fuel is known as Naphtha firing.
This department handles the operations of the plant by managing the fuel and all the machinery
related to the monitoring and controlling the various parameters of the fuel.
The natural gas is supplied to the plant through the HBJ pipeline based on the requirement. It is
then used as per requirement.
For emergency purposes, the plant stores 16000Kl of naphtha at any point of time.
During the generation of electricity using naphtha, the naphtha is dosed with HSD (High Speed
Diesel) to form a favorable and efficient combustible fuel.
Storage of naphtha: Naphtha is piped through the Asavati pipeline as per the order placed and is
pumped into the reservoirs using two unloading pumps. Its flow into the reservoirs is then
monitored using the proper measuring instruments.
Naphtha is stored in two 8000Kl reservoirs made of cement and RCC walls. The reservoirs are
inverted bowl floating roof type to prevent any air inside the chamber which may cause a harmful
reaction with naphtha. A pressure of 28Kg/cm2 is maintained inside the reservoirs. Also, the
chamber is insulated completely in order to control the temperature and prevent combustion of
naphtha owing to high temperature.
Forwarding of the fuel: Naphtha is sent to the reservoirs using the unloading pumps. When
required, it is pumped out and sent to the dosing chamber using three forwarding pumps.
The HSD is also pumped from the reservoirs to the dosing chamber by two forwarding pumps.
and process plants need to control different process variable from a remote distance control room,
the further measuring, transmitting indicating, recording, abnormality alarm system and
innovated. The process of innovation is marching ahead in fast rate. In the near future, we are
certainly to enter in towards more and more sophistication n C&I stream.
The Control & Instrumentation Department(C&I) is responsible for the operation of all the
electronic sensors, actuators and controllers besides maintaining the pressures, temperatures, level
and the flow in the various tanks, pipes and also in the various heat exchangers present in the
plant. It is thus the brain of the plant.
The main functions of the C&I dept. at NTPC Faridabad are:
1. Measurement and display of various parameters.
2. To control the various parameters by Automatic feedback controlling which involves the taking
of decision based on inputs from measurements by the processor.
3. Protection of various equipments (pumps, generators etc.) and workers from hazards by
automatically tripping a cycle when hazardous conditions are reached.
4. Alarm generation in case of a mechanical or an electrical failure.
The Faridabad plant has outsourced its automaton to various companies on a Package Based
Deal.
The C&I dept. besides also undertakes the modifications or up gradation of its systems.
This division basically calibrates various instruments and takes care of any faults occur in any of
the auxiliaries in the plant. It deals with metallurgical problems. In any process the philosophy of
instrumentation should provide a comprehensive intelligence feedback on the important
parameters viz. Temperature, Pressure, Level and Flow. This department is the brain of the plant
because from the relays to transmitters followed by the electronic computation chipsets and
recorders and lastly the controlling circuitry, all fall under this.
Faster sequence of control actions compared to manual ones. Even a well- trained
operator crew would probably not be able to bring the plant to full load in the same
time without considerable risks.
2) Ensure and maintain plant operation, even in case of disturbances in the control system, via:
Coordinated ON / OFF and modulating control switchover capability from a sub process
to a redundant one.
Prevent sub-process and process tripping chain reaction following a process component
trip.
3) Reduce plant / process shutdown time for repair and maintenance as well as repair costs,
via:
Bringing processes in a safe stage of operation, where process components are protected
against overstress.
Speed
Temperature
Current
Voltage
Pressure
Eccentricity
Flow of Gases
Vacuum Pressure
Valves
Level
Sensors
1. Sensors are instruments used for measurement purposes.
2. They measure various parameters and convert them to electrical output which is supplied to
controllers.
3. This data is then displayed which aid the engineers make the judicious decisions.
These sensors measure the following parameters:
1. Level of fluids in pipes and tanks e.g. LVDT can be used for this measurement.
2. Flow of fluids such as fuel steam etc. done by sensors such as Venturimeters or Rotameters.
3. Pressure in tanks and pipes also can be measured
Its done by sensors such as Gauges, Pressure Switches, Capacitive Transducer etc.
4. Temperatures can be measured anywhere in the system.
Instruments such as Thermocouples and RTDs are used
Controllers
These are devices which receive data from the sensors, process it and give instruction to the
actuators based on the processed data. They are analogous to the human brain.
Various types of control systems used at NTPC Faridabad are:
1. Single control system
This consists of a single processor which controls the entire process. This system is not quite
efficient as it draws large amount of power and also if the processor fails then the entire unit has to
stop.
2. Distributed control system
This consists of various processors which are responsible for various parts of the process and have
a channel of communication between them enabling them to work efficiently. Such a system is
more efficient and is favored.
DCS is extensively used within the plant to control various processes of the plant.
Actuators
Actuators are analogous to motor organs like hands or legs of the body. Actuators are the
instruments which are responsible for carrying out control commands from the controller like
closing of a valve etc.
Actuators are of manly three types:
1. Pneumatic
These actuators carry out mechanical tasks using compressed air. These are very accurate but are
not very strong and get damaged easily. The pressure in it is in the range of 3-15 psi.
2. Hydraulic Actuators
These use the pressure of compressed oil to perform their tasks. These actuators are quite strong
but lack a high degree of accuracy.
3. Electronic Actuators
These make use of electronic motors to perform their tasks.
Vital information which is required by operators at all times for the safe operation of the
plant. This information is presented through single point indicator/recorder, placed on the
front panels. Main steam pressure, temperature, condenser level, vacuum, drum level,
furnace pressure etc. are some such parameters.
The second group of information is generally not vital under the normal operation of the
plant. But they become vital whenever some sections of the plant start malfunctioning .
Such needs are met through multipoint indicators/recorders placed in the front panels.
Temperature and draft across the flue gas path bearing temperature of the motors of fans
etc are some such examples
.The last group of information is not required by the operators but for the efficiency
engineers. This information are given by recorders mounted on back panels or local
Panels. D.M. make up quantity, fuel oil flow quantity etc. are some examples.
4.3.2 SELECTION OF INSTRUMENTS
Instruments engineers are required to work in close association with the system design as well as
the equipment design engineers in selecting instruments and sensing system. After deciding the
capacity of Thermal Power Station the designs of Boiler turbine and auxiliary equipments such as
mills, pumps, fans, de-aerator, feed heaters etc. are taken up.
Based on the design of the main and the auxiliary equipments, the parameter values for efficient
and economic operation determined load are specified. The instrument and system design
engineers decide the location for the measurement of various parameters such as level, pressure,
flow, differential pressure, temperature and other parameters based on the system design and
layout conditions.
Then the instrument engineers select the appropriate instruments influenced by following factors:
i) Required accuracy of measurement
ii) Range of Measurement
iii) The form of final data display required
iv) Process media
v) Cost
vi) Calibration and repair facilities required/ available
vii) Layout restriction
viii)
changes with the change in temperature. Resistance thermometers are generally used up to
300 0
C.
Above 3000 C, thermocouples are used as primary sensor. The common types of thermocouples
used in thermal power station are chromel-alumel or chromel-copel depending upon the
temperature. Iron constantan is another thermocouple in use. The secondary instruments for
thermocouple sensor are pyrometriv millivolt meters or electronic potentiometers. Null balance
method is used for the very accurate measurement of millivolts generated by thermocouples
sensing the process temperatures.
The electronic bridges and potentiometers can be either indicators, or indicator cum recorders
with alarm/protection contracts and with remote transmission facilities.
PRESSURE MEASURING INSTRUMENTS
The pressure measurement in thermal power station ranges from 1 Kg/cm 2 (nearly) at condenser
to hydraulic test pressure of boiler. Here again many medias exist such as steam/water,
lubricating oil, fuel oil, air, fuel gases, hydrogen etc.
For local indication of pressure and differential pressure, bourdon tube, type and diaphragm type
gauges or liquid monometers either electronic or pneumatic coupled with a secondary instrument
indicator/recorder. Many varieties of transmitters are in use. In these transmitters the mechanical
movement of sensing elements such as bourdon, bellows, diaphragm etc. due to the pressure
causes an electrical property change such as current, voltage, resistance, capacitance, reluctance,
inductance etc. which is utilized as a measure of pressure in the secondary instruments. The
secondary instruments are either indicators or recorders which may incorporate signaling
contacts.
LEVEL MEASUREMENT
Level measurement is generally carried out as differential pressure measurements. In power
stations, level measurement in open tanks such as DM storage Tank and Fuel Oil and Lub Oil
tanks and in closed tanks such as de-aerator, condenser hot well, boiler drum and L.P. & H.P.
heaters are to be made. Gauge glasses and floats are used for local indication of levels and the
transmitters used for measuring the differential pressure are used along with the secondary
instruments for remote level measurements.
The measurement of the boiler drum poses many problems because of varying pressure and
temperature and many computations and corrections are to be made in order to get correct levels.
A recent development in this area is the Hydra step. Though it is very costly it improves the
accuracy and the reliability of this measurement.
Other problem area is the solid level measurement where the coal bunker levels and dust
collector hopper level are required. In both these cases continuous level measurement is not
possible. However fairly reliable and accurate provisions are available to indicate the extreme
level on either directions (low or high). The nucleonic level gauges or the capacitance and
resistance type sensors serve in these areas very well.
FLOW MEASUREMENT
Flow measurement of solids, liquids and gases are required in Thermal Power Stations. Though
the liquid flow measurements are made very accurately, the gas flow measurement cannot be so,
water flow measurements are done fairly easily and accurately whereas steam flow measurement
requires density correction under varying pressures. The air and flue gas flow measurements
suffer accuracy and reliability due to variation in pressure, temperature, duct leakage, dust
accumulation etc. The solid flow measurement is very difficult and only on a rough area is
arrived at about the P.F. flow through inferential means. In Power stations flow measurement are
based on inferential principles. Differential pressures are created by placing suitable throttling
devices in the flow path of the fluids in the pipes/ducts. The throttling devices are suitably
selected depending upon the media, flow quantity etc. from among office, venture, flow nozzle
dall tube etc. the differential pressure developed across such sensing devices is proportional to
the square of the flow quantity. The differential pressure is measured by the devices discussed in
9 with additional square root extraction facilities.
ANALYTICAL INSTRUMENTS
Apart from the above there are few quality measurements necessary in thermal power generation
plants of high capacities. These include feed water quality measuring instruments such as
conductivity, pH, dissolved oxygen, and sodium instruments, steam quality measuring
instruments, such as conductivity, silica and pH analyzers. The combustion quality is accessed by
the measurements of the percentage of oxygen, carbon monoxide or carbon dioxide in the fuel
gases. The purity of the oxygen inside the generator housing is measured by utilizing the thermal
conducting capacity of the hydrogen gas.
The water and steam purity is measured as the electrolytic conductivity by electronic bridge
method in which one arm form the electrodes of conductivity cell dipped into the medium.
The volume percentage of oxygen in combustion gases is made utilizing the paramagnetic
properties of oxygen. The carbon monoxide percentage is measured by the Absorption of
Electromagnetic radiation principle.
Both these gas analyzers require elaborate sampling and sample conditioning system resulting in
poor reliability and availability of these measurements. Recent developments in these fields have
brought out on line in-situ instruments for these two parameters where the problem of sampling
is dispensed with.
The Analytical Instruments as the above instruments had been the neglected lot so far in the
power stations. But now the authorities seem to think their importance for the process.
TURBOVIBORY INSTRUMENTS
The turbovibory instruments have become very important in modern day turbines where the
materials have been stressed nearer to the yield points and the internal clearance have become
the minimum .Shaft eccentricity, vibration (both shaft and bearing pedestal) differential
expansion of shaft and cylinders, overall some of the turbovibory measurements. These all
measurements are interrelated and interdependent.
LIST OF INSTRUMENTS
All these measurements discussed above and their correct interpretation enables the operators to
check and watch the behavior of the process and the equipments and take necessary corrective
actions in time.
A typical list of important measurements carried out in Thermal Power Stations is given below:
Temperature
a) Steam temperature at boiler outlet, super heater stages, steam legs before ESVS, CVS
after ESVS, IVS and at turbine Curtis wheel-indicators/indicator-cum recorders with
alarm and protection facilities in control room.
b) Steam temperature at turbine HP cylinder outlet, hot reheat and exhaust hood
temperatures.
c) Metal temperature of turbine casing and metal temperature of super heaters and reheatersindicators, indicator cum recorder in U.C.B. with multipoint selection.
Other temperature measurement in various zones of boiler indicator
a) Flue gas temperature measurement in various zones of boiler indicator and indicator cum
recorder in control room.
b) Air temperature at inlet and outlet of air pre-heater.
c) Turbine bearing oil drain temperature-indicator cum recorder in U.C.B.
d) Generator winding and core temperature-indicator cum recorders in control room.
e) Temperature of auxiliary equipments bearing such as mill ID, FD and P.A. fans etc
indicator cum recorder in U.C.B.
Pressure
a) Condensate pressure after condensate pumps and before the ejectors-indicator in U.C.B.
b) De-aerator pressure-indicator cum recorder in U.C.B with electrical contacts for
interlocking facilities.
c) Feed water pressure after feed pumps-individual indicators for each pump.
d) Feed water pressure before and after feed regulating stations-indicators in U.C.B.
e) Drum pressure indicator cum recorders in U.C.B. with alarm signaling facilities.
f) Super heater steam pressure at boiler outlet 2 Nos. indicators one for each side in U.C.B.
and at local with alarm protection facilities. Measurement is done at the outlet of superheater and before boiler stop valves.
g) Steam pressure 1 No. indicator cum recorder, one of the lines before turbine stop valves
in U.C.B.
h) Steam pressure at emergency stop valves and TVS.
i) Steam pressure after control valves indicators in local panel for Pressure of each valve.
j) Steam pressure at Curtis wheel indicator cum recorder in U.C.B. with alarm contacts.
k) Steam pressure in H.P. turbine exhaust indicator in U.C.B. for cold reheat steam.
l) Vacuum in condenser indicator cum recorder in U.C.B. with alarm facilities and separate
vacuum relay for protection.
m) Hot reheat pressure indicator in U.C.B. with signaling contacts.
n) Steam pressure at the exhaust of I.P. cylinders-indicators in local panel.
Pressure: Fuel and Lubricating Oil
a) Heavy oil pressure indicators in U.C.B. with signaling contacts. Measurement is made
before and after pressure regulating valves.
b) Light warm up oil pressure indicators in U.C.B. with signaling contacts. Measurement is
made before and after the flow control valves.
c) Ignition oil pressure indicator in U.C.B.
d) Governing oil pressure-indicator in U.C.B. with signaling contacts.
e) Lubricating oil pressure-indicator in U.C.B. Measurement is made after oil coolers.
Pressure: Air Flue Gas
a) Air pressure indicators in U.C.B. before and after air heater for secondary air.
b) Indicators in U.C.B. before and after air heater for primary air.
c) Wind box pressure indicators in U.C.B.
d) Furnace draft-indicators and recorders in U.C.B. Measurement is made averaging left and
right side drafts.
e) Flue gas draft before and after economizer-indicators in U.C.B.
f) Draft after air heaters two indicators in U.C.B. one for each air heater.
g) ID fan suction 2 Nos. indicators in U.C.B. one for each fan.
LEVEL MAESUREMENT
a) Drum level indicators and indicators cum recorders (total 3 Nos from different tapping) in
U.C.B. with alarm and protection facilities. Normally 3 types of measurement are
adopted:
i) Local gauge glass
ii) Remote gauge glass and
iii) Remote indirect measurement
b) Drip level in H.P. and L.P heaters-indicators in U.C.B. with alarm and
protection facilities.
c) Condensate level-indicator in condenser-indicator in U.C.B. with alarm and protection
facilities.
d) De-aerator level-indicator in U.C.B. with signaling contacts for alarm.
e) The various storage tank level such as D.M. water, fuel oil, lubricating oil etc. are
measured by the local direct gauge glasses.
Flow
a) Condensate flow to de-aerator-indicator/recorder in U.C.B. with integrator unit for
totalizing in two locations (i) between air ejectors and L.P. heater No. 1 and (ii) between
the final L.P. heater and de-aerator.
b) Feed water flow indicator/recorder in U.C.B. with integrator unit. Measurement is made
between final H.P. heater and feed regulating valves.
c) Super heated steam flow 2 Nos. indicators cum recorders one for each pipe with
integrator unit in U.C.B.
d) Re-heater steam flow 2 Nos. indicators cum recorders one for each side of the boiler.
Measurement is made at the inlet to re-heater.
e)
Air flow-2 Nos. indicators cum recorders one for each FD fan in U.C.B. and
measurement is made at the discharge of th FD fans.
f) Fuel Flow
The fuel oil flow to the unit is given by two indicators cum recorders in U.C.B., one
measuring the oil in the incoming line and the other in the return line. Normally the coal
flow is measured for the whole station by the belt conveyor weighers.
AUTOMATIC CONTROL
The importance of maintaining a balance in the process was discussed under section 1 whenever
the process gets disturbed due to the deviation of process elements behavior; they are to be
brought back to the balance condition. Since a lot of process elements are involved and
disturbances are very frequent, the correction can be carried out efficiently and quickly only by
the introduction of automatic control system eliminating any possible human error. The
following are the important automatic control loops in the thermal power station.
Automatic Boiler Control
i) Steam pressure always called as Boiler Master
ii) Combustion control
iii) Furnace draft control
iv) Boiler feed regulation or drum level control
v) Super heater/ re-heater steam temperature control
Analogue from scanned in sequence one at a time converted into digital form and transmitted to
the central information system for display or control purposes.
BURNER MANAGEMENT
For higher capacity boiler, fuel firing rate is also higher. Explosion occurs within 1 to 2 secs of
fuel accumulation. Therefore leaving the management of fuel firing to the operators will lead to
explosion because human reflexes will be little slower. A complete automatic burner
management system called furnace safeguard supervisory system FSSS in short has been
introduced to manage the present day boilers.
This system takes care that every increment of fuel input corresponds to the available ignition
energy inside the furnace.
The following functions are entrusted to such an automatic burner management system:
i) Furnace purge supervision
ii) Igniter control
iii) Warm up oil control
iv) Pulverize control
v) Secondary air damper control
vi) Flame scanner intelligence
vii) Boiler trip protection
The above discussion gives some synopsis of the instrumentation in thermal power station.
5.1 INTRODUCTION
To give the operation engineers a correct picture of the happenings inside the
plants enormous numbers of instruments are required to be installed. Also those
instruments are to be mounted in panels in centralized locations to avoid many
personals watching the readings and interpreting on their own way. Also these
instruments are to be arranged in such a way that they give plant behaviors in a
systematic way and with minimum complexity. Here comes the aesthetic and
ergonomics view of the installation.
TELEMETERING
For the centralized instrumentation remote indication facility is required and
consequently telemetering was introduced. The method of placing the
instruments at a distance from the measuring point is called telemetering. This
type of metering is very common in power stations as nearly all the instruments
for measuring and controlling the power flow are centrally mounted on a panel.
Electrical instruments are now widely used for this purpose because they are
convenient to install, reliable and reasonably accurate. Also it is cheaper to
transmit an electrical signal by a cable than pipe lines in case of pneumatic.
Transmission lag is very negligible. However if the telemetering is required for a
short distance, pneumatic system is used.
INTERLOCKING SYSTEM
A power station is a combination of many individual equipments and systems
and for better performance relies upon the performance of these individual
equipments. The equipments are interdependent and interrelated with each other,
and therefore they are to operate in coordination with each other. Electrical
interlock systems connect these individual equipments and operate them with a
required sequence. For example boiler is a system comprising milling
plant, ID fans, FD fans, PA fans etc. These equipments are interlocked in such a way that they are
started / shut down in specific sequences in order to avoid damage to equipments and men. For
example in a milling system the coal feeder is interlock such a way that it will not start unless its
succeeding system to crush and discharge the coal into the furnace such as exhauster and mill are
in operation. These schemes may very little with different manufacturers but generally all P.F
and oil fired boilers have common sequences.
Also equipment is so interlocked that in case the failure of the running equipment to deliver the
good, automatically the reserve one is put in to service. For example in case a feeds pump which
is running at time, fails to meet the demand of the boiler, the interlock system will put an
idle/reserve pump into service to meet the demand.
ANNUNCIATORS
The operation Engineers attention should be drawn towards a parameter which deviates much
from the desired value. This is done by the annunciators installed in the control panels in front of
him by audio, visual or both means. Whenever the system deviation occurs, a relay gets
energized by the signal received from the deviated system which sets up a flashing light, an
audible alarm. These are to be received by the operator till then they continue to be on. It
becomes necessary for him to take necessary remedial action to correct the deviation.
REMOTE OPERATION OF EQUIPMENTS
As discussed earlier that power station comprises many types of equipment, it become necessary
to operate them from a centralized control room. Moreover as the capacity of plant increases its
operating electrical supply potential also increases which is very dangerous on safety point of
view. As a result the indirect way of remote operations came into practice. A very low voltage
level such as 110 V or 240 V AC/DC is used to close a breaker of the electrical motor of 3.3 or
6.6 KV voltage level. The low voltage switches are usually provided on the operating desks in
the control room. Where D.C is used for station batteries are provided as standby. Circuit
breakers are provided with protection relays.
INTERRELATION OF INSTRUMENTS
In order to have stable generating conditions the heat energy supplied through the fuel must
balance the electrical energy output of generator plus the normal losses. But often this balance is
disturbed due to fluctuations in temperature, pressure, steam flow or electrical output. A large
number and variety of instruments are required to measure and indicate the cause and amount of
disturbance so that steps can be taken to keep the energy flow in balance. Each instrument has its
own function to perform but the value of its measurement often depends on the accuracy of the
other instruments associated with it. The interdependent and interrelations of these instrument
readings play very significance roll in the stability and efficiency of the heat energy balance.
Furnace Draught
The balance between the induced and forced draught fans is produced by measuring and
controlling the furnace suction. Balance draught usually occurs when there is slight suction
inside the combustion chamber. This is achieved by properly adjusting the speed or the dampers
of the fans. Disturbances in the draught can cause unstable combustion and this in turn will affect
the readings on many of the other instruments associated with the boiler.
CO2, CO, and O2 Measurement
These instruments are valuable guides to know:1) The quantity of air supplied.
2) The variation in the quality of the fuel being burnt.
3) The performance of the automatic control if in circuit.
The percentage of CO2 may not necessarily be an indication of efficient combustion. It may be
showing an optimum value yet the combustion must be incomplete due to the variation in noncarbon combustibles such as hydrogen, sulpher and chlorine.
Therefore the reading of the O2 may be a correct percentage according to the reading yet, it may
be found still in excess. Therefore in modern day practice the CO measurement is taken
as valuable information regarding the combustion. Air is adjusted till we get some traces of CO
in the flue gases.
Temperature
Accurate measurement of steam temperature is very important because of the high degree of
superheat used and boilers are operated with critical temperature margins. Steam temperature
measurement itself is comparatively easy but its control is more complicated due to the time log
and thermal inertia inherent in the system.
There will be wide variation in the moisture content of the coal flowing through the mill. Air
with varying temperature is to be sent to dry out that moisture. But the primary air temperature
variation affects the stability of the combustion, steam temperature, exit gas temperature etc.
Fuel Measurement
It is easy to measure the liquid fuel by the conventional instruments in volume quantities. In case
of solid fuel such as P.F. the measurement is not possible with conventional type of instruments.
Therefore their quantities are determined indirectly by measuring the quantities of primary air.
But a certain volume of fuel does not always have the same amount of heat units due to the
variation in calorific value moisture and ash contents. Variations in quality and quantity of fuel
affected the steam, air and gas flow as well as temperature pressure, CO 2, O2 and power output.
However the variations can be easily adjusted in the fuel flow system.
Measurement of Air And Gas Flow
Since boiler handles large volume of air/gas, it is difficult to measure the quantity correctly due
to the variation in pressure, Temperature, casing leakages, dust laden etc. The measurement of
the air/gas is used as a means of establishing the correct fuel/air ratio.
Normally measurement of air flow is very easy as compared to gas flow because of accumulation
dust, slag and varying temperature etc.
Turbovisory Measurements
The turbovisory readings such as differential expansion eccentricity, vibration, and temperature
differentials give a fairly clear picture of the behaviours and clearances and also the eccentricity.
Eccentricity in turn give a picture of vibrational level. All these parameters are interdependent
and interrelated.
AUTOMATIC CONTROL
Whenever the balance gets disturbed due to the deviation of process elements behaviour they are
to be brought back to the balance condition. Since lot of process elements are involved and
disturbances are very frequent, the correction can be carried out efficiently and quickly on by
introducing automatic system for elimination any possible human error. Thus automatic control
was established to maintain the system balance.
For example when an operator has to fire the boiler by regulating the fuel to the burners and at
the same time to adjust the position of dampers or the speed of the fans for the control of air
supply, haw well he does this depends of the type of fuel and his own ability. His mistakes in
assessing the things in a correct proportion will aggravate the disturbance. But an automatic
combustion control does this job, more quickly, efficiently and smoothly. Automatic control
system detects the changes signal and direct the regulator accordingly to correct the deviation.
Advantages Of Automatic Control System Are:
a) The values of the process elements such as steam pressure, temperature flow are kept
close to the desired value.
b) Combustion efficiency is improved resulting in:
i) Fuel Economics
ii) Reduction in boiler fouling
iii) Less atmospheric pollution
iv) Less carbon in ash and grit
c) Metal fatigue is reduced by maintaining stable metal temperatures.
Operator has more time to spend in regular operation and routine inspection.
Disadvantages
a) The equipment has to be much reliable.
b) The standard setting should have to be watched and adjusted to suit the varying
characteristics of fuel etc.
c) Sometimes control action goes on the reverse direction due to the time lag in the
measurement.
d) Control equipments are very expensive and require periodic maintenance.
COMPUTERS
With the increasing size of modern plant, the distances between items of plant run of four to five
hundred yards away from the centralized control rooms. Also large number of equipments
necessitate large number of instruments. It becomes very difficult to watch these many
instruments and supervise the operation of such a large number of equipments. A computer
relieves the operator from routine tasks leaving him free to concentrate on the overall inspection.
The operator cannot always watch every instrument and at the same time make the necessary
adjustments to suit the constantly varying conditions. However a computer can be programmed
to make all the necessary adjustment as and when required.
The starting up operations of a large unit involves somewhere around 1000 separate steps
including nearly 500 switching operations to bring on load. All those steps are to be carried out
in short time and in correct sequence. These all activities become cumbersome and any mistake
will lead to disastrous end. Computer fed with correct programme, performs these duties will.
It is very difficult by the operation engineer to keep a constant watch on these temperature
gradients. For a computer it is very easy job. In many cases computers are exclusively used to
run up a turbine. It allows the steam into the set at the appropriate temperature and accelerates
the set after monitoring the internal clearances temperature differentials and other mechanical
aspects, runs upto speed and synchronise the set. In the computers facilities are available to
check the efficiency of the plan then and there. The evolution of computer can be compared to
teaching animals to do tricks. Every trick has to be acquired by much study and experimentation
on a slow progressive basis.
DATA LOGGING
The conventional central control room is rather a cumbersome system. Large number of
instruments must be observed to know what is happening inside the plant. The data logging
simplifies this job by collecting all the measurements transmitted from the process, converting
them into digital form and printing them on the log sheets. All the important measurement at one
times are printed along a row. Data loggess thus reduce the use of graphical recorders.
Since data logging gives too many measurements at a time, it cannot be easily digested by the
control staff. Now data-reduction systems are finding their use where only the process quantity
deviated form normal value is shown.
SCANNING SYSTEMS
In a complex process extending over a considerable area, lot of messages are transmitted to and
from the process. Theses transmission channels are quite expensive and there may be danger of
loss of data owing to confusion of signals by extraneous electrical noise. In such case some
coded transmission, less sensitive to such noise is found useful. Hence all the process signals in
analogue form scanned in sequence one at a time converted into digital form and transmitted to
the central information system for display control purposes
CHAPTER 6
PRESSURE MEASUREMENT AND
MEASURING INSTRUMENTS
6.1 PRESSURE MEASURMENT
Pressure measurements are one of the most common measurements taken and recorded in the
Power Station ranging from very low, i.e. condenser vacuum to very high i.e. hydraulic pressures
in some actuator systems. Between these two limits of say 30-40 millibar absolute to 300 bar are
to be the measurements of different process media-steam, water, oil, air, gas etc. and each with
varying degree of accuracy and reliability.
If one end is sealed, then the manometer can be used for absolute pressure measurement. If the
area of one of the limb is made considerably greater then the other, then the measurement of the
differential pressure is represented by the height of the liquid column in the smaller tube with
negligible error. Such system is called the single limb manometer or cistern manometer since the
larger area pipe is in the form of a metal cistern. The manometer liquid normally used is water.
Sometime colored water is used to distinguish the column. The other liquids used are
i) Transformer oil having specific gravity 0.864
ii) Mercury having specific gravity 13.56
iii) Blended Paraffin liquid
Industrial type high pressure U tube manometers are available having metallic tubing. These
manometers employ a secondary system of linkages / leverages for indication purposes. Inclined
tube manometers are the special development to give increased length of column for less
differential pressure. The inclined tube carries the scale. Manometers are available with
adjustable inclination depending upon the range required. Fig. 6.1 to 6.2 shows a system of
manometers.
Figure 6.1 a)
Figure 6.1 b)
Figure 6.2 a)
Figure 6.2 b)
6.2.2 Diaphragm, Capsule and Bellows
The present days low pressure to medium pressure applications are met with diaphragms. Also
the introduction of these elements as greatly helped in remote measurement and control of
pressures even of very low range (0-4 mm wcL).
Material and Range of Measurement
The various types of diaphragm and below elements are made of steel of special composition,
phosphor bronze, nickel silver and beryllium copper etc.
Bellows and multistack are made from 80-20 brass, phosphor bronze, stainless steel and
beryllium copper.
For very low pressures, the diaphragms are required to be extremely flexible. For these
applications materials like colon leather, gold beater skin, nylon rubberized fabric etc. are used.
These groups of sensors are used for the measurement of very low pressure upto 20-25 kg/cm2
Ranges of Pressure
Burdon tube gauges are in use from the range 0-0.5 kg/cm2 to 6,000 kg/ cm2 and even higher
ranges occasionally. The practical range for each type of listed below.
Helical boundon
C Type boundon
Materials
Materials like phosphor bronze, steel, berrylium copper etc. are used depending upon the pressure
range and the medias corrosiveness. The chart given in Table I give more details of bounden tube
materials and their pressure ranges
Material
Joints
Composition
Percentage
Phosphor
Bronze
(Drawn)
Beryllium
Copper
(drawn)
Alloy
Steel
(machined)
Copper 95
Tin 5
Phosphorus
Trace
Beryllium 1.8
Cobalt 0.3
Pressure
Heat range
Treatment
Soft
Soldered
None
kg/cm2
1-70
Brazed
Precipitation
hardened
03-350
Carbon 0.26/
0.32, Chromoum
0.8/1.1,
Molybdenum
0.15/0.25
Screwed
Quenched
and
tempered
650-5500
K. Monel
(Machined)
Nickel 66
Copper 29
Aluminium 2.75
Iron 0.9
Screwed
Precipitation
hardened
70-1350
Stainless
Steel
(machined)
Stress
relieved
2-70
The simplicity and ruggedness of a Bourdon gauge makes it the most frequently used pressure
gauge. The reference pressure in a Bourdon gauge is atmospheric pressure. Hence, the dial
reading gives gauge pressure.
ERRORS IN BOURDON TUBE GAUGES
Errors that may occur in Bourdon gauge are zero error, range error, angularity error, and
hysterisis. For zero error the pointer is adjusted, for range errors the quadrant screw is adjusted,
for angularity error the linkage screw is adjusted. In case of error due to hysterisis the tube
should be replaced if the error goes beyond the specified value.
Hystersis is the difference in the indicated value of the gauge for an applied pressure during
the increasing cycle and during the decreasing cycle of pressure.
TESTING A BOURDON PRESSURE GAUGE
1. Gauge is tested at 5 points up and down before adjusting anything. Divisions
corresponding to about 10%, 30%, 50%, 70%, 90% are chosen.
2. About 1% pressure is applied then zero is set by removing and replacing the pointer to
read the pressure applied.
3. About 90% scale pressure is applied if necessary the range is adjusted by loosening
the shoulder screw and moving the linkage along the slot in the quadrant (towards the
pivot to increase the range, away from pivot to decrease the range).
4. Step 1 and 2 are repeated until gauge is correct at both points.
5. When zero and range are correct then angularity is adjusted if necessary. Half full
scale pressure is applied to the gauge, angularity adjustment screw is loosened and
adjustable linkage is slided until the angle formed by the quadrant and linkage is right
angle.
6. Approximately five points of the scale are checked with pressure increasing, the
readings are tabulated.
7. The same five points with pressure decreasing are checked and result tabulated.
8. Result from 6 and 7 are used to check for hysteresis.
9. Gauge is assembled. Pointer should not foul the glass over any part of its travel.
10. Result sheet is made the final condition of the gauge as a % of full scale. Gauge
should be within 1% of full scale.
Zero Error
A zero error can be observed easily by quickly testing at the cardinal points. A zero error will
have exactly the same amount of deviation at all points. in this type of error the pointer is
reset.
10
30
50
70
90
UP
29
49
69
89
DOWN
29
49
69
89
Gauge Reading
6.5
To remove the gauge, first valves A and B and closed, valve C is opened to equalized
pressure in both the part P1 and P2. The gauge is removed now. To check zero on plant the
same procedure is followed, but the gauge is now removed.
To restart the unit is installed with valves A, B and C losed. After installation value C and B
are opened. Then C is closed and A is opened.
SNUBBER
This is a protection device for pressure measuring instrument from violent pressure surges
and pulsation. Snubbers also known as deadners reduce the effect of pulsating pressure. They
result in the instrument indicating or recording an average pressure, instead of recording each
individual surge or pulse. Snubbers are used in pipe lines leading to the instrument.
In general, these snubbers reduce the velocity of fluid to the instrument and thus prevent
sudden extreme change in pressure from reaching the measuring element too rapidly. The
reduction in velocity can be achieved by several methods. The body consist of two parts, the
lower part and the upper part, lower part is connected to the pipe line. It contains a piston.
The pin piston assembly rises and falls with the pressure impulses and absorbs the effect of
shock and surge. Owning to the rise and fall of the piston the snubber is self cleaning. The
upper part of snubber is screwed to the lower part, on one side and to the pressure instrument
on the other. The upper part has a stop for piston. The stop has a hole in the centre for the
process fluid to pass to reach the instrument from the pipe line and vice versa.
The gauge has a pressure setting needle. This needle is set via a knob through the centre of
the glass of the gauge. The glass is generally of acrylic. The needle has a projection where
contact will be made. The needle is connected to a wire.
The pointer is also connected to a wire. It has a projection when it touches the projection on
the setting indicator a contact is made. When the pressure reaches the set value then the gauge
pointer touches the projection on the setting needle. The pointer and setting needle behave
like an open switch till the set limit is reached. The gauge can be connected to relay, hooter or
lamp for alarm or control. Sometimes a magnet is provided on the needle to enable quick
closing of contact Thereby avoiding chances of sparking.
In a similar manner alarm controls can be made at two points one for a low pressure limit and
the other for a higher pressure limit. If the working pressure reaches the low set limit or the
high set limit alarm contact is made.
DIGITAL PRESSURE GAUGES
Digital pressure gauges working with integral or remote pressure sensing transducers are now
becoming more easily available and these are usually of a very high accuracy. This means of
course that they can be used for calibration purposes or for efficiency monitoring. The
following pages describe two such devices and they have proved in practice to be very
valuable calibration standards for the Eggbrough commercial instrumentation In particular
the device is used for checking the transmitters which measure the condenser absolute
pressure.
The system for measuring condenser vacuum has had to be investigated at great length
initially, one tapping into the condenser steam space was used. Following test it was found
necessary to use one from each LP T/A exhaust in to the condenser and average the pressure
via a common manifold. This system now gives a representative average absolute pressure in
the condenser.
And all readings which originally disagreed because they tapped in to different points on the
condenser now agree.
Calibration over the full range 0-100 mbar absolute is achieved by use of the condensers
own vacuum when the T/A is on load and the use of a vacuum pump in series with the gauge
tapping.
DRAUGHT GAUGES
Draught gauges are used extensively throughout the Power Station to measure air and gas
pressures through the boiler and mills. The draught gauge is basically a diaphragm pressure
gauge with an elongated scale.
The readings are all transmitted as (0-10 mA) standard signals, if the process medium is not
allowed in the control room.
The foregoing comments on pressure gauge installation is appropriate to draught gauges also,
and since they are measuring relatively low pressures it is important that the pipe work is
installed very carefully with the added provision of a blow down facility to clear the lines of
dust.
For suction gauges it has been found that the drilling of a small hole in a draught gauge line
near the tapping point can give an automatic cleaning of the lines without loss of reading or
sensitivity.
Calibration of draught gauges is best achieved with a manometer. Manometers of reasonable
accuracy and the correct range can be obtained from various manufacturers. It is important
when calibrating a draught gauge in this way that the correct type of manometer is used
6.2.4. TRANSMITTERS FOR PRESSURE AND DIFFERENTIAL PRESSURE
MEASURMENT
A transmitter has a process signal such as pressure, flow, level or temperature as its input and
an electric or pneumatic signal as its output.
PR
FLOW
LEVEL
TEMP
TRANSMITTER
ELECTRIC/PNEUMATIC
OUTPUT SIGNAL
and produce an output current within a range. The output range is standardized to bring
uniformity in the construction of secondary instruments as well as to facilitate the test and
calibration work. The prevent output signal ranges are:
A. 4 20 MA DC
B. 0 - 20 MA DC
C. 10 - 50 MA DC
Transmitters are generally connected in a measurement loop according to one of the
following methods:
a. Four Wire Transmitters
In this method four conductors are led to the transmitter. One pair is used to carry the power
supply, which may be 220 VAC or 24 VDC. The other pair is used for signal transmission as
shown in Fig. No 6.7.
Figure 6.7
This is presently the most widely used method for transmitter connections. There are three
basic elements in this loop, namely a/c power supply, transmitter and the receiving
instrument. They are connected in series and the transmitter acts as a current regulator in the
series circuit. The current in the series circuit changes with respect to change in process
parameters as shown in Fig. No6.8.
Thus only two wires are needed for connecting one element to another. This simplifies
cabling and reduces erection and cable costs.
Being a series circuit, the input resistance of the receiving instrument plays an important
role, as higher input resistance will generally limit the loop current. For this purpose
transmitter manufacturers generally provide a load drive capacity curve for the transmitter.
Referring to Fig. 6.9 we find that, this curve gives the maximum value of input resistance that
can be connected at the operating power supply voltage, without affecting the output current
of the transmitter
Figure 6.9
The capacitance type and strain gauge type are definitely of superior design. They have
virtually no moving parts and hence are very accurate and have a good repeatability. They are
lightweight in construction and much smaller in size. Also al the adjustments such as zero,
span, damping are electronic therefore calibration becomes very easy
Figure 7.1
The float movement is limited to about 120o as a maximum, the motion being transmitted to
the pointer by a worm drive or similar arrangement. If the gauge is mounted below the liquid
level, there must be some seal between the gauge and the tank. Some gauges use a
magnetic method or pointer transmission.
Figure 7.2
LIQUID DISPLACER SYSTEMS
This gauge, embodying a displacer, relies on Archimedes principle for its operation.
According to this principle if an object is weighed in air and then in a liquid there is apparent
loss of weight which is equal to weight of the displaced liquid. The displacer is a long hollow
cylinder loaded to remain partially submerged, and is suspended in the liquid in the vessel or
in an adjacent small diameter chamber connected to the vessel. The apparent weight of the
displacer will decrease as the level of the liquid rises.
Figure 7.3
If the density of the liquid remains constant then the height of liquid above a datum (tapping)
point is directly proportional to the pressure measured at that datum point. Thus a pressure
measuring device can be used scaled in units of level.
Measurement of Liquid Level In Open Vessels
Since the static pressure at a chosen point of measurement (datum line) will vary directly
with the head of liquid above it, it can be seen this pressure can be measured and the gauge
calibrated directly in head of liquid. The tapping point is always taken above the sediment
level. The gauge will read directly the total depth of liquid in the tank.
The gauge can be pressure measuring device, for example, bourdon tube, bellows, U tube,
enlarged leg manometer etc.
Measurement of Liquid Level In Closed Vessels
With closed vessels in most cases the vessel is closed because the system is to be pressurized,
or to operate conditions other than atmospheric as per Fig. 7.4.
Figure 7.4
In these cases it is necessary to see that the same conditions exist on the reference side of the
indicator as inside the container, so the reference limb is fed back into the top of the vessel.
Closed Vessel With Condensable Vapour
With closed vessels a further condition that may produce errors is when the pressure in the
tank contains vapour and these vapours then to condense on top of metering fluid in reference
limb, again causing the pressure factor which must be taken into account.
To offset this condition condensing chambers are used, these are chambers with a
considerably greater area than the meter chambering areas, so that the level of liquid in it
does not change much when the metering liquid moves in the manometer. The whole line will
thus be filled with condensate, thus forming a pressure head of relatively constant value, any
additional condensation now overflowing back into the vessel.
Figure 7.5
Figure 7.6
Liquid Seals
When there is a danger that the liquid whose level is being measured will, due to its nature,
adversely affect, the manometer fluid or transmitter diaphragm material then liquid seals
should be used.
The sealing liquid must not mix with the vessel liquid, be attacked by it, absorb corrosive
elements from it. Of course it also must have no adverse effects on the manometer fluid or
diaphragm material.
Gas Purge System of Level Measurement
Basically this method consists of a tube which is inserted into a liquid whose depth is to be
measured. An air pressure is applied to the tube and the air pressure is built up until bubbles
just begin to escape from the bottom of the tube. Bubbles will only form only when the
pressure in the tube is negligibly higher than the pressure exerted on the bottom of the tube
by the height of the liquid above the bottom of the tube as per Fig.7.6.
When bubbles from the pressure in the pipe P = gh. When the density of the liquid is known
the pressure will be proportional to the height of the liquid above the bottom of the tube.
Therefore if the pressure in the tube is measured by the pressure gauge or U-tube the scale
can be calibrated in terms of depth of liquid on into any units required such as the volume or
weight of the liquid in the tank.
Air Trap System
In some cases where measurement of level is required, such as strong corrosives or at
working temperatures unsuitable for diaphragm, the air trap system can be used.
The box is covered by a plate with a small hole just large enough to allow liquid to enter. As
the level of liquid in the tank rises, the pressure on the air trap increases, liquid flows into the
trap and compresses the gas in the trap. When the air pressure plus the head of the liquid in
the trap is equal to the head of liquid above the trap no more liquid above the trap, no more
liquid enters the trap. The air pressure set up can be measured by a suitable indicator or
recorder which can be calibrated directly in terms of level.
Figure 7.8
Figure 7.9
If a constant voltage is applied across the terminals, then as level increase, resistance
decrease, hence the current flowing in the circuit will increase. Therefore current will be
proportional to the level. If an ammeter is placed in series with the circuit, then it will
indicate the current flowing in the circuit. Since the current is proportional to level the
ammeter can be calibrated directly in terms of level.
This method can be adapted for use in manometer level measurement system by locating the
electrodes in the mercury of one of the limbs.
In the sight gauge and head pressure manometer it has been assumed that the density of the
liquid remains constant throughout, but this is not necessarily true. If the temperature of the
liquid in the limbs varies then its density varies thus errors in level indication will occur.
The hydrastep vessel uses a side-arm method of attachment to the drum, and carries a number
of separate electrodes spaced vertically at intervals, usually of 25 50 mm (1-2 in), each of
which is associated with a separate channel of the electronic indicating system. The design of
the vessel, however, gives a very much reduced density error. The conventional visual gauge
body has a small cross sectional area and a small bore, with only a small flow of
condensate. A by pass tube is often fitted so that condensate from the stemp, pipework is
diverted from the gauge. Because of the small cross section, the heat flow in to the gauge
body occurs more or less equally from both the steam and the water, and because of the slow
flow, the temperature gradient of the water column is large.
The hydrastep vessel has a metallic cross-section some four times that the visual gauge, and a
bore cross-section of about 10 times. The reduced thermal resistance vertically permits a
substantial quantity of heat required by the lower half of the vessel to be supplied from the
steam space, which is of course maintained at saturation temperature. In addition to providing
a larger surface area for heat exchange purpose in the steam space, the large boar reduces the
turbulence of high condensate flow and encourages the formation of a significant boundary
layer on the inside of vessel well below the water/steam interface, and this layer acts as a
partial thermal insulator. Instead of the mean water column temperature for a half-full gauge
being about 90oC below saturation temperature, as in the visual gauge the hydrastep vessel
exhibits only about 8oC mean drop, which results in a density error of only one-sixth of the
visual gauge.
Figure 7.10
Other work shows that an adequate differential for Hydrastep exist between the resistivities of
the water and the stream in a side armgauge at boiler pressures up to about 216.5 bar (3140
lb. f / in2, Tsat 372.8oC).
The switching band for the Hydrastep electronic circuits also shows superimposed at
approximately midway between the water and steam resistivities. The anomaly shown at
140 180oC concerned the related water and steam readings taken during a severe steam
valve leak which resulted in steam entrainment in the vessel water column and water droplets
in the steam space.
To ensure absolute safety to personal the maximum voltage which appears at an electronic
terminal is 10v rms, and its maximum short circuit current is 10 uA, 50 Hz. Each electrode
circuit. therefore meets the requirements for intrinsically safe apparatus with a margin of
safety of five orders of magnitude in respect of current. At 10 uA, the maximum electrode
current is only one-fortieth of the 0.55% human perception current at 50 Hz. The mechanical
design of the electrode is such that its centre be ejected from the vessel in the event of the
failure of the ceramic insulation, and a guard is fitted to deflect any steam jet which may arise
from a faulty electrode or seal.
The potential on each electrode is applied to its own individual discriminator channel to
control an output electromagnetic relay carrying six sets of changeover contacts.
One set of contacts is used for display purpose in the control room of either water or steam,
as appropriate, for each channel or the electronics, each display module being arraigned in the
order corresponding to the disposition of the associated electrodes in the vessel. Two further
sets of contacts on each relay are used in a logic matric to raise an alarm should a fault. Occur
such that any channel is out-of-step, i.e. that it gives an indication which is physical
impossibility such as water above steam or steam below water.
The three remaining contact sets are available for high or low level alarm purposes, or for
additional logic configurations to provide validated control signals, alarms and / or
emergency tripping of the generator and its auxiliaries.
Further failoperative safe guards are provided by the connection of alternate channel of
electrodes on any one vessel to electrically separate power supplies, providing an interleaved
system. The loss of one power supply will still allow even a single Hydrastep to operate
within the terms of the Factory Inspectors Certificate of Approval, and the instrument can be
repaired with the generator still on load.
Failure characteristics
The appearance of the display under normal conditions is shown in the left hand column of
the drawing. A colour change principle is used for each display module to avoid the
ambiguity possible between a true fault and a burnt out lamp where a simple onoff
arrangement is used.
A power supply failure appears in either column 2 or 3, and the failure of a signal channel as
in the column 4 to 11. In no case of an electronic fault does the indication error exceed one
step. It is simple matter to include an automatic comparison between adjacent steps, on the
premise that water cannot exist about steam in the vessel in sufficient quantity to cause such
an indication (column 4 and 8). This comparison may be performed quit easily by means of
additional contacts on the relays controlling the display lamps. The usual station annunciator
operates when a connection is made between the alarm busbars. In the logic
matrix,wherever the water/steam interface may be, all channels above it should show steam
with their contacts in the upper position and all below should show water. If a fault should
indicate water more then one channel above the interface - for example, as shown doted on
channel 11 the bars are shorted through 8 and 7, causing an alarm. Other logic systems, and
techniques other than relay circuitry could be used, including station computer if spare
capacity is available.
Although electrically separate, the odd and even logic drive circuits are physically adjacent
for ease of inter-connection and are mounted with the power supplies close to the display
unit in the control room.
A design of colour change module using sub-mioiature long-life filament lamps has enabled
a small graphic display to be used, which could be mounted directly into the control console.
Two-Gauge Hydrastep
In the simplest multi-gauge instrument vessel A Carries the odd numbered electrodes and
vessel B the even, when the whole of the electronic system may be identical to that already
described for the single Hydrastep. Separate pipework for the two vessels is essential so that
pressure variations caused by a fault on one vessel will not affect the performance of the
other, and so that either may be shut off independently. Since the water steam interface within
the drum is not a plain surface under operating conditions, the inner ends of the waterside
pipes must terminate at substantially the same point in the drum so that the same head of
drum water is applied to both manometers. This prediction is not required for steam
connection.
It is worth noting that a leak or blockage on either the steam or the water side of a vessel or
its pipe-work will result in a fractional pressure drop in the vessel concerned, the manometer
will then rebalance the vessel showing a higher level than for the non-faulty one. This means
that the faulty half of a two-gauge Hydrastep arrangement may be identified and switched
out, so that the gauge still remains operative using the sound vessel. The only exception to
this condition occurs with a leak of such proportions that water cannot remain in the vessel.
Such a leak would normally have developed comparatively slowly from a minor leak which
should already have been recognised; but in any case, the operation of a low level alarm with
one half of the Hydrastep showing a level within normal tolerance will identify the fault.
Twin Hydrastep
The standard twin Hydarstep gives additional security by the provision of identical Hydrastep
units, A and B operating from both ends of the drum.
Not only does this extra redundancy permit the shut down of a complete end (e.g. to
exchange a faulty electrode or valve packaging), with the generator on load , but under
normal conditions the adjacent arrangements of the two electrically separate displays from
the drum ends gives the operator valuable information concerning endto-end level variations
either cyclic or static, which can occur under certain plant running conditions
Figure 7.12
Figure 7.13
The relay logic matrix is connected to give a three-out-offour system, that is three out of
four gauges have to indicate a low level before a trip is initiated.
Hydrastep Display Unit
Instead of a sliding box as in the rest of equipment, the display unit uses a hinged door from
construction, with a removable rear cover. Sufficient clear space around the unit must be left
to allow access. The philosophy of continuous comparison between adjacent channels by the
logic matrix ensures that the indication presented to the operator has been fully verified. The
channels at either end of one vessel (i.e. channels 1 and 12 of both drum and indicators in the
case of the standard Twin Hydrastep) can each be verified on one side only, since channels 0
and 13 do not exist. If, for example, the case of steady falling water level is considered in
conjunction with a fault on channel 1 such that water is permanently indicated, it would
appear to the operator that some water still existed in the gauge although, in fact, the fall in
level had continued past this point. Had a channel 0 existed below channel 1, as the level
continued to fall, a water above steam condition would have appeared and the fault on
channel 1 would have been recognised. Without channel 0, therefore, channel 1 cannot be
fully verified and is not presented to the operator since it could be misleading; similarly
channel 12 could be suspect without a channel 13 for verification. To ensure that no
misinterpretation can occur, therefore, only channels 2 to 11 inclusive is displayed in normal
operation, channel 1 and 12 being covered by the hinged outer panel. This outer panel,
however, may be opened by the Instrument Engineer to gain access to the remote test
switches and telephone socket, when the indication from these channels can also be observed.
Because of the level differences between drum ends which can occur in normal operation, it
is impractical in a Twin Hydrastep to cross validate any channel on one vessel and the
corresponding channel on the other, and the restriction in the previous paragraph must apply
to both columns of the duel display. However, in the special case of the four gauge system,
whether or not the automatic tripping facility is connected, cross-validation may be
incorporated at all the available corresponding levels between the two vessels at the same end
of the drum and all indication including the extremes may be then presented to the operator.
Capacitance Methods
A capacitor is a device for storing electrical energy. In its simplest form it consists of two
plates of area, separated by a distance. The air between the plates is called the dielectric.
When a voltage is applied across the plates on electrical charge is stored proportional to the
applied voltage.
Figure 7.14
Capacitance level measurement involves the use of an electrode which extends the full length
of the tank and form a capacitance between itself and earth where earth may be the vessel, the
contents or a concentric cylinder around the electrode, depending on the type of electrode
involved.
A variation of capacitance will occur when the depth of the medium in the vessel alters
therefore the capacitance change will be proportional to level.
By this method the level of liquids, powders or granular solids may be measured.
Conducting Mediums
When the medium is a good conductor of electricity then the system works as a variable area
capacitance transducer. The electrode is one plate of the capacitor and is insulated with a
material that is compatible with the medium, the insulation forming the dielectric. The
medium in the vessel from the other plate of the capacitor. Thus, as the level changes, the
area of the capacitor plates varies. If level falls then area decreases and capacitance
decreases.
Non-Conducting Mediums
When the medium is non-conducting the electrode is not insulated and the system works as a
variable dielectric capacitance transducer. The dielectric is made up of, say, liquid in the
tank and the air or gas in the space above the liquid thus the electric constants will be
different (normally those of liquids are much greater then gases). As the liquid level varies
then the overall capacitance will change due to change in dielectric. A rise in level ill increase
capacitance and fall in level will decrease capacitance.
7.2.4. ULTRASONIC METHODS
Ultrasonic
Ultrasonic beams are a form of energy transmitted by means of mechanical vibrations and
carried through the transmitting converting one type of energy into mechanical vibrations
which are received by a device which detects the ultrasonic beams converting them into a
more readily usable form of energy. Above a certain frequency (20 kHz) it is known as ultra
sound or ultrasonic sound. For level switching a range between about 36 and 40 kHZ is used.
Ambient noises or their harmonics are ineffectual in this range.
Principle of Operation of the Sensors
When certain materials, mainly nickel, iron and cobalt, are placed within a magnetic field,
their lengths will very by an amount dependent on the strength of the magnetic field.
The fundamental generator is a nickel tube which carries the coil and bias magnet. The
current through the coil either weakens or strengthens the field, depending on the direction of
the current. Application of an alternating current causes the length of the tube to increase and
decrease at the supply frequency. owing to the mechanical properties of the tube it will tend
to oscillate longitudinally as a half-wave resonator.
Similarly with the receiver, a sound wave impinging on the diaphragm will cause a relatively
large amount of movement in the nickel tube, if within the band paths frequency, virtually
non if outside. Changing the length of the tube will cause a change in the magnetic strength
of the bias magnet, thereby generating an e.m.f within the coil. Hence the same cant be used
as either a transmitter or a receiver.
The system is unaffected by dirt, vapour, moisture etc. The sensors are temperature-sensitive;
the resonant frequency falls as the temperature rises but there is no effect if both sensors are
at the same temperature. Very little maintenance is required.
Impurities in the liquid are one of the problems of sight glasses systems as the glasses
becomes discolored and obscures the liquid meniscus.
Regular cleaning of the gauge glass is the common maintenance task. Other problems are
broken glass tubes or leaks and / or blockages at the connections.
The range of sight glasses largely depends on the nature of the liquid, the static pressure and
the temperature involved. Ranges of 0.2 to 2 metres are typical.
If the density of the liquid is constant then sight glasses are simple, accurate devices for level
measurement they can be calibrated by comparison with a dipstick or the addition of a known
volume of liquid.
WATER GAUGE WITH CLOSED CIRCUIT
Television (C.C.T.V.) Remote Display
One of the obvious problems with a simple sight glass system for, say, boiler drum level
measurement is that local indication only is provided. The use of a special type of side glass
(water gauge) with an associated c.c.t.v. system allows level display to be remotely located in
the Control Room as shown in Figure No. 25
Figure 7.15
The gauge is vertical tube of triangular wedge cross-section. Two faces of the three sides are
made up of glass and mica divided up into small compartments. Illumination is projected
through the gauge and the light is bent by the medium. The degree of bent depends on
whether the medium is water or steam. Because of the prismatic arrangement either the
water windows or steam windows are illuminated thus the level of water in the drum can
be determined.
A C.C.T.V. camera is mounted a few feet away from the gauge and is carefully aligned with
the light path through it. The camera and lens system being fully protected against fuel dust
and ash. The C.C.T.V. monitor is located in the Control Room.
This system is more difficult to operate with high steam pressures (120 bars) because the
refractive index of water closely approaches that of steam thus angular deflection of the light
paths is very small making level indication difficult. At pressures of 166 bars it is almost
impossible to accurately determine water level by direct level viewing means.
The problem can be overcome by gauge rear illuminators using quartz iodine lamps and by
using an optical magnifier to enlarge the small differential in the refractive index of the two
mediums.
The main problem with this system are that of faulty alignment, hostile environmental
conditions which affect the reliability of the camera and high degree of technical expertise
required for C.C.T.V. maintenance.
Ultrasonic flowmeters
the pipe flanges and is located by the flange bolts. The orifice is then concentric with the
internal bore of the pipe.
It will be convenient before describing particulars to see what occurs when an orifice plate is
inserted in a fluid stream in a pipe, and a liquid flow is considered. Fig. 8.1 illustrates the
action in a simplified manner.
pressure and from observing the different values we may trace the pattern of the pressure
changes as we proceed along the pipe. At position 1 and 2 there is no pressure change worth
specifying.
At 3 and 4, just before the orifice, we find a slight increase in pressure. The stream is then
constrained to flow through the smaller size of the orifice, from which it issues as a jet. At
position 5 and 6 there are lower pressures then at the up stream position due to the change in
the stream sectional area. Since this is similar the velocity has increased, and the pressure has
fallen. The stream or jet cross section decreases in area after leaving the orifice until it
reaches a point, indicated as 7 in the diagram, where it is a minimum and the velocity a
maximum. This is mainly due to the liquid being directed inwards as it approaches the orifice,
and, through inertia effects, persisting in this direction for a distance after it leaves the orifice.
The static pressure also reaches it minimum value at this position, which is known as the
vena contracts. The distance from the orifice varies with the ratio of orifice diameter to pipe
diameter but an average value be about one half the pipe diameter. From the vena contracts,
the steam station expends until it reaches the pipe diameter at 8. Two facts emerge from the
study of Fig.8.2. one is that the downstream static pressure never recovers its upstream value.
This would appear to be caused by the velocity changes being accompanied by considerable
turbulence with resulting dissipation of energy involving a pressure loss. Taking a typical
value of 0.6 for orifice to pipe diameter ratio, the percentage loss works out a 65 percent of
the differential pressure. Where pressure loss is important this factor should be borne in mind.
The second point which emerges is that there appears to be a variety of positions at which to
take pressure trappings or connections for obtaining the differential pressure.
The following are the main tapping positions (shown diagrammatically in fig.8.2 and 3
Figure8.2
Figure 8.3
One advantages of this type is that drillings etc. are carried out at the manufacturers works
and the errors due to site operations are eliminated.
Having established the possibilities of a definite constructive device, for fluid flow
measurement under ideal conditions, we must now examine what modifications are necessary
in practice.
Turbulent Flow
In practically all cases of the flow in pipes for industrial purposes the flow is turbulent, that is
the particles of the fluid do not follow paths parallel to the direction of flow. Some, if not all,
of the particles have a transverse motion as well as longitudinal one and form little eddies or
swirls giving rise to turbulence. Stream line or laminar flow formulae will not apply here
without modification and a new set of equations must be derived.
Discharged Coefficient
Due to friction and velocity distribution, the practical flow figures do not line up with
theoretical ones. Observe that the stream area contracts after leaving the orifice to the vena
contracts position. The cross-sectional area there may only be about 0.6 that of the orifice.
Orifice materials
Materials used for orifice plates include mild steel, stainless steel, monel, phosphor bronze,
gunmetal, depending on the application. A rough classification would be:
Water metering
Air metering
Steam metering
Stainless steel
Venture Tube
We have seen the effect of inserting an orifice plate in a fluid stream, causing a abrupt change
in stream area to produce a differential pressure. The operation can be accompanied by a
fairly high permanent pressure loss, and where pressure loss is important, it is necessary to
turn to other method of producing differential pressures. Let us consider devices with a
gradual change in area. The first of these is the venture tube
Figure 8.4
Constructional Features
To some extent, the construction of the Venturi tube depends on the application. For normal
uses, the section would be of gun-metal, cast iron, or Mechanite, and smoothly machined
liners of gunmetal or stainless steel inserted at the inlet and throat pressure tappings. The use
of gun metal or stainless steel reduces the risk of corrosion. To facilitated construction work a
victualic joint is sometimes inserted in the downstream cone. The extreme end of the cast
section are flanged to match with the pipe flanges, and with the adjacent section, and pressure
tappings are arranged for screw in or flanged connections depending upon the particular
installation condition.
For high pressure hot water flow as in boiler feed water in power station, the design is used,
and the gun metal lining is inserted. The lining is made in three sections: inlet cone, throat,
and outlet cone, profiled as for a standard Venture. This design is suitable for pressures up to
1400 lb/in2. Another pattern has a maximum working pressure of 2000 lb / in =2.
The Venturi tube possesses a big advantage over the orifice is that its section need not be
circular. Square or rectangular shapes have been used for measuring large volumes of fluid
flow. The non circular section lends itself to constructional materials other than metal, and
concrete has even been used for one or two very large flows. Note that the design renders the
tube useful for fluids containing suspended matter because of its gradual area changes.
Nozzles
The nozzles falls between the venturi tube and the orifice plate as a means of flow
measurement. Its approximates to a venturi tube with the curved form of approach, giving a
gradual change of sectional area and has the same order of discharge Coefficient. But the
absence of a downstream expansion core brings the pressure loss in to the same region as that
for an orifice plate. It is cheaper than a venturi tube, and at high velocity flow it is use in
place of an orifice plate may be necessary. See Figure 8.5
Figure 8.5
C)PITOT TUBE
Let us study the effect of placing a blunt object in a fluid stream as an obstruction to the flow
(Fig. 8.7). As the fluid approaches the object, the velocity will decrease until it reaches zero at
the point where it impinges on it. From the previous, a declaration should mean an increase in
pressure. This would follow from Bernouillis Theorem.
It is very convenient to be able to measure the static pressure in the close neighborhood of the
tube and standard Pitot tubes. Both designs consist basically of inner and outer tubes. The
inner one leads from the impact hole to one construction of the differential measuring
instrument. The outer tube, referred to, sometime, as the static tube, has a series of holes
bored into it so that its interior connects to the out side surface to be in contact with the static
pressure. This tube is joined to the second connecting of the measuring instrument.
The pitiot tube can only measure velocity at one position in the cross-section of a pipe. Now a
velocity of a fluid in a pipe, taken across the section, is not uniform, varying from zero at the
pipe surface to a maximum at some point (not necessarily the centre) along the diameter. To
find the mean velocity it is necessary to make a traverse of the pipe with the tube, taking the
differential pressure at certain specified positions. An ideal distribution curve is shown in fig.
8.7. For Reynolds numbers above 100 000, the ratio of average velocity to velocity at the
centre of the pipe is frequently specified as 0.82 or 0.83. Whereas this value would apply for
ideal cases for as curve of the type in Fig. 8.7, the actual curve may be different. The
desirability of carrying out a traverse, therefore, is obvious. Once having determined the ratio
value, the Pitot tube may be placed at the pipe centre and the instrument calibrated in terms of
average velocity.
Another theoretically possible means of determining the average velocity is to select a
position where the velocity corresponds to the average value. This has some practical
drawbacks. The location may be near the wall of the pipe a very approximate value being
0.25 of the radius in form the wall. It could be at a point where the velocity curve slope is
fairly steep and any misplacement could lead to significant errors in velocity determination.
At the centre of the pipe, by comparison, the curve is normally flatter and errors in location
are not so serious.
Figure 8.6
Figure 8.7
Two other types of Pitiot tube deserve mention. One is the double, tip pattern shown in
Fig.8.7. in which there are two holes, one facing upstream and the other downstream, the
former measuring the impact head and the latter the section head. The differential pressure
obtained is greater than with the standard types, but is not double the value. Actually, the
increase is between 35 per cent and 40 per cent depending on the position of the tube in the
pipe. The other type is the Pitot-Ventury. It is a combination of two concentric venturi tubes,
the out let cone of the inner one terminating in the throat of the outer. The throat pressure of
the inner tube and the impact pressure on an impact hole in the supporting tube give 710
times the differential produced with the normal types under the same conditions.
D) DALL TUBE
The principle features of the Dall Tube are indicated in Fig. 8.8. It involves two truncated
cones separated by a narrow throat. The throat length is between 0.3d and 0.1d where d is the
throat diameter. The inlet cone has an included angle between 40 o and 50o, the out let cone
between 12o and 17o. The mouth diameter Dm, the inlet pipe diameter D and throat diameter
d are connected by the following relation
Dm4 d4 = K (D4 d4 )
Where K = 0.5 to 0.75
Observe the diameter of the inlet cone is less then that of the pipe, resulting in a sharp step.
This creates an impact pressure which is additional to the static pressure existing at the step.
The high or upstream connection is made just in front of the step. The other connection is
made at the throat where the relativity abrupt change in area results in a marked static
pressure depression. The original patent specification No. 689, 474 claims a pressure loss
expressed as 5% or 6% of the differential pressure. This compares with a loss of between 2
and 3 times this value with a normal venturi tube. In addition, the Dall tube has the advantage
of being considerably shorter than the normal venturi.
GLASS TYPE
The basic feature of this type of meter is the conical section glass tube. For accuracy, the
diameter of this must be maintained at very close limits. Clear borosilicate glass is used
which is highly resistant to thermal shock and chemical action, and the method of its
manufacture enables tolerances of I/10 000 of an inch to be observed. The use of glass
introduces the question of a safe working pressure for the fluid being measured. At present
this is about 500 lb/in2 and applies to the smaller diameter tubes. For larger sizes the safe
working pressure falls from this figure. (The normal diameters range from 2 mm to 60 mm
depending on the flow to be measured.) The tube is normally clamped in a metal frame, the
inlet and outlet being sealed into connections as required, e.g. flanged or screwed. Where
danger may occur from flying glass resulting from a fractured of the tube, Armour Plate
glass protection windows encase the instrument.
Figure 8.8
The standard float shape is indicated in Fig. 8.8, and is perfectly free. Viscosity immune
floats, however, may demand a guide, as the float disturbe the equilibrium of the liquid. In
one pattern, the guide is a central rod around which the float is made to rotate, so that visual
evidence that the float is moving freely is obtained.
The glass tube type measures from 2 cc/min up to 3000 liters/min of gas, and 0.5 cc to 225
liters/min of liquid.
The pressure drop will depend on the type of float being used and the nature of the fluid, but
varies between about 0.2 cm (0.078 in) w.g. for small gas flows and 3.5 cm (1.38 in) w.g. for
liquid flows.
METAL TUBE TYPES
For larger flows than the glass type tubes can accommodate, a conical metal tube pattern is
introduced. Here, the metal body is of gun metal, cast iron or stainless steel with a stainless
steel float. The latter is carried on a rod which moves between two guides, one at the lower
end and the other at the upper end of the tube. The guide rod passes through the upper part of
the tube in to a compartment with a glass scale, the end of the rod acting as an indicator (Fig.
8.9). This type of meter has typically ranges from 250 to I 20 000 liters/min of gas flow and
20 to 7000 liters/min of liquid. The maximum fluid working pressure is 500 lb/in 2. When
used with opaque liquids, a compressed air supply may be connected to the top of the scale
unit and the level of liquid depressed, so that a clear view of the indicator is obtained. Opaque
liquids may also be metered by the high pressure version.
Fig 8.9
The principle of the electro magnetic flow meter may be understood better if we first consider
a very thin disc of an electrically conducting liquid moving with a velocity V along a pipe of
internal diameter d. An external magnet system directs a magnetic field of strength H across
the section of the pipe so that it acts at right angles to the direction of motion of the disc.
Now, by Faradays Law of indication, when an electrical conductor of length L moves
through a magnetic field of strength H at a velocity V in a direction at right angles both to the
magnetic field and its length, an e.m.f. is generated of value.
E=K H L V
WHERE
(I)
K = a constant
Our disc liquid is a conductor obeying the general requirements of Faradays Law, and it can
be seen without much difficulty that L in equation (I) is replaced by d, the diameter of the
disc. If, now, there is an indefinite number of such moving disc continuous to one another, we
have the equivalent of a conducting liquid stream flowing continuously through the pipe
.(ii)
.(III)
Figure 8.10
ADVANTAGES OF THE ELETROMAGNETIC FLOWMETERS
1. Linear relation between flow rate and measuring signal as compared with the square
law relation of differential pressure devices. This results in a range ability of the order
of 100/1.
2. The measuring instrument can be arranged with a centre zero for measuring flow in
either direction. Alternatively the electrode leads may be changed over to measure a
reverse flow.
3. The only pressure loss is that due to the length of tube, forming the meter. But a
pressure loss would be present with the same length of ordinary pipe so that the
introduction of the meter cannot be said to involve significant additional pressure
losses.
4. There is no obstruction to flow which renders the meter suitable for liquids containing
suspended matter. Abrasion may be avoided by choosing a suitable lining material.
Wood pulp and paper mill stocks, cement slurries, sewage, food pulp are but a few
difficult fluids which may be metered.
5. The design lends itself to the metering of corrosive liquids since parts on contact with
the fluids may be made of corrosion-proof materials.
6.
It is not affected by velocity profiles, since the e.m.f. is at all points proportional to the
velocity of flow across the diameter.
-----------
(I)
(II)
(III)
C+V
with no flow,
to
=
d/c
The difference between t and to, t, is given by
Vd
t
..
C (C + V)
C for most fluids is of the order of 1500 meters/sec. whilst V for most industrial application
would be a few metres/ sec equation (III) then reduce to
Vd
t = . .. (IV) C2
Figure 8.11
........
(VI)
........
(VII
)
C V
........
(VIII
)
2
3. The measurement of t now involves some problems. It may be solved by pulse
C
techniques or a continuous wave beam may be used. In the latter case, the transmitting
transducers are driven from a common source and the phase difference between the
two received signals measured. The phase difference is given by
2 wvd
= ---------------C2
Where W = the angular frequency.
........
(IX)
4. Observe that it all the methods considered, C the velocity of sound is present. This
can
be eliminated if the methods of Fig. 35 is adopted. A short pulse is emitted from
transducer T1 and is received by T2. The arrival of the pulse from T 2 triggers another
one
from T1. The time between pulses is
d
t1 = --------------C+V
........
(X)
C+V
-------------
........
(XI)
d
A similar pulse is transmitted from T3 to T4 and calling the repetition frequency here
f2,
C-V
f2 = --------------
........
(XII)
.......
.(XIII)
f1 f2 = f = 2 V / d
Equation (XIII) is independent of C.
5. A further techniques used has been a differential arrangement across the pipe. It can
be shown that a beam of sound can be deflected in the downstream direction in the
traversing a pipe from one to the other. The deflection x is approximately
given by x = V d / C . . . . . . . . (XIV)
........
(XV)
Figure 9.1
Changes in volume of a liquid by the application of heat enclosed in a test body is utilised to
measure the quantity of temperature. The liquids normally used are, mercury and hydrocarbons such as ethylalcohol, for low temperature, metaxylene for medium range
temperature, tetrahydro naphthalene (tetralene) for higher temperature
Figure 9.3
The test bodies (bulb) are either glass or of steel material.
The liquid filled system, consists of an element sensitive to temperature change (i.e. bulb), an
element sensitive to volume change (bourden, bellow of diaphragm), means of connecting
these two and a device for measuring and indicating.
The liquids filled in a bulb from which a capillary is drawn which ends in a bourden or
bellow or a diaphragm. The entire system is filled completely with the liquid at 0 o C at high
pressure of the order of 1000 PSI. When the temperature rises, the volume of the liquid
increases thereby tending to enlarge the enclosure. As a result a mechanical motion is
achieved which is transmitted to the dial indication by lever arrangement or rack and pinion
arrangement. Instead of capillary connection, a short solid stem is also used. There is also a
possibility that the metal enclosed (bulb & capillary) also may increase in volume due to
thermal expension which will add to the error of the system. To remove this error, a
compensation means is provided.
COMPENSATING LINK
This method used two metals with different co-efficients of expansions. Instead in the
capillary as a link-chamber. The chamber contains a core of Invar having negligible co-
efficient of expansion. The wall of the chamber is made of the steel material. The space
between the core and the wall is filled with the system liquid. If the size of the chamber and
volume of the Invar material are carefully proportioned, then on any change in ambient
temperature, the volume of the angular space, due to the expansion of the outer wall is
sufficient to accommodate any variation in volume of the liquid in the capillary and so
prevent it exerting an effect on the bourdon tube.
DOUBLE CAPILLARY
The second method used a second capillary of the same diameter as the first one filled
with the same liquid under the same condition. This second capillary and is sealed off without
the bulb and run along the first capillary and connected to a second bourdon. This bourdon is
made to act on the instrument points in an opposite sense to that of the main bourdon.
Since both capillaries and bourdon tubes are subjected to the same conditions it can be seen
that the ambient temperature effect in the main system is counteracted by that of the second
system.
Bulb Design
All manufacturers keep the change of bourdon volume for all ranges in their production a
constant for commercial reasons. This leads to varying bulb sizes for various ranges.
Therefore for higher range smaller bulb volume is required.
EXPANSION OF GASES
Here the changes in pressure of the gases filled in test bodies (bulbs) of constant volume on
changes of temperature is utilised as means of measurement of the temperature. The gas used
normally is nitrogen. The system works on the gas law PV= RT. Therefore the pressure of the
gas is proportional to the temperature. The bulb is evacuated and filled by the gas at a
required pressure and then the system is sealed. Rest of the system is the same as the liquid
filled system and here the bourdon becomes sensitive to pressure changes.
EXPANSION OF VAPOUR
This works on the basic principle that all enclosed liquids at a given temperature will create a
definite vapour pressure if the liquid is only partially filled. This vapour pressure will
increase with temperature and this property is utilized in measurement.
THERMO ELECTRICITY
Thermocouples
Thermocouple consists of two wires of suitable materials which are joined together at the end
by twisting together and then joining the tipe by brazing or welding. The wires selected
should have the following characteristics.
i. They must physically withstand the temperature for which they are selected, rapid
changes in temperature and the effect of corrosive atmospheres.
ii. Their composition should not change at this temperature range.
iii. They should posses reasonably liner temperature e.m.f relation ships throughout the
range.
iv. They should develop an e.m.f per degree change of temperature that is detectable with
standard measuring equipment.
v. They should not change its characteristics by physical fatigue caused by some
materials.
Very few combination of wires are developed so far satisfying the above conditions.
They are:
a.
b.
Chromel Alumel
Type K range upto 1260 o C composition:
Chromel : Ni; 89% Cr. 9.8% Fe 1% Cobalt 0.2% Dull appearance, non-magnetic
Alumel : Ni 94.5% Al 2% s 1.0%., Mn 2.5%
Glossy surface and slightly magnetic.
c.
Copper Constantan
Type T range - 180 o C to +370 o C
d. Chromel Copel
Type E range 0-870 o C
e. Platanium R Hodium Platinum
Range 0-1480 o C with 10% Rhodium type S and with 13% Rhodium type R.
Extension Wire
Since it becomes extremely costly to take the thermocouple wires up to the measuring
instrument located at far of places some substitute wires are used to connect the
thermocouples to the instruments. These wires posses the same characteristics as that of
thermocouple wires but up to a lesser temperature. These wires are termed as compensating
leads.
Also ordinary copper wires are used as extension wires after compensating the e.f.m. at the
thermocouple terminals, the difference in temperature of the terminals and the cold junction
using a bridge circuit utilizing the law of intermediate temperatures.
Cold junction compensation circuit is a wheat stone bridge consisting of three arms with
constant resistance of mangan due to the temperature changes and the fourth arm with copper
wire wound resistance which is sensitive to temperature. This bridge is supplied with 4 volt
D.C and is balanced at the temperature of reference junction (say at 27 oC) when the
temperature change unbalance occurs due to change in resistance copper wire across diagonal
which adds or substracts the thermocouple e.m.f. accordingly. Other method of providing
cold junction compensation are by means of constant temperature oven or ice box in which
the reference junctions are kept inside a constant temperature oven.
Resistance Thermometers
The material selected for resistance thermometers should have the following properties:
i)
ii)
The specific resistance should be within the limit for easy construction.
iii)
Little change in the resistance due to non- temperature methods such as strains etc.
IV)
Three resistance thermometers are available having above properties. They are Nickle,
Copper and Platinum.
Figure 9.4
NICKEL RESISTANCE THERMOMETERS
Its characteristics are not linen throughout the range but is frequently used to its specific
resistance and less cost specific resistance 6.38 micro-cm., Temperature Co-efficient .0066
ohm/ohm (oC)
Copper Resistance Thermometers
It has got a linear characteristics; specific resistance of copper is very less of the order of 1.56
micro ohm-cm. Temperature co-efficient 53 ohm.
Platinum Resistance
Through costly platinum is more suitable than either copper or nickel. Its usage is restricted
to jobs that cannot be properly handled by the other two types of thermometers.
Specific resistance 9.38 microohm-cm
Temperature co-efficient 0.00385 ohm/ohm oC
Construction Details
The different types of wires are employed in different way depending upon the ranges and the
measuring media normally wire size 0.05 top 0.07 mm dia is used.
i. Platinum wire element wound on mica strip and protected by the mica strip.
ii. Coiled wires mounted on ceramic mendrals and casted in ceramic.
iii. Copper or Nickel element wound on an ebonite plate or on mental mendral.
iv. The coiled element wound on a mica cross.
The sensing elements wound as above are provided with porcelain beads, and inserted inside
a protecting sheath and the terminals brought to the porcelain blocks. The sheath is provided
with a head and cover where the block is kept.
Measuring Circuits
The change in resistance of the temperature sensitive resistance element can be measured by:
i. Cross coil indicators (CI) called ratiometer.
ii. Wheat stone bridge
a. Null balance method,
b. Deflection galvanometer type.
Ratiometer
The ratiometer consists of two crossed moving coiled placed in the field of permanency
magnet at an angle of 20oC approximately and connected detector of a wheat stone bridge
with the resistance thermometer constructing its own arm.
The current flowing through the crossed coils create deflecting and restoring moments. The
change in resistance of thermometer with temperature causes change in current through the
crossed coil producing deflecting moment to the pointers in such a way that the deflection is a
function of ratio of the two currents and measures the temperature on the scale.
To lead in and out the currents through the coils and to bring the pointer to zero in off
conditions
two hair springs are connected. See Fig.9.5.
Wheat Stonebridge
Figure 9.5
b.
C is much larger hence is more easily measured than the microscopic change of
Optical Pyrometer
The radiant energy is measured by photometric comparison of the relative brightness of the
object of unknown temperature with a source of standard brightness such as the tungsten
filament of an electric lamp.
Radiation Pyrometer
The radiation from the target a portion of the object whose temperature is being measured is
focussed by lens arrangement on thermopile (a number of small thermocouples connected in
series). This thermopile generates an e.m.f proportional to them amount of energy falling
upon. This e.m.f fed to a millivolmeter or poteniometer which indicates the temperature.
The radiation pyrometer is ideally suited for:
i) When very high temperature are involved, temperature beyond the practical beyond
the practical range for thermocouple measurement.
ii) Where furnace atmosphere is detrimental to thermocouples and cause erratic
measurement and short life.
iii) Where for other reasons, it is impractical to contact the material whose temperature is
to be measured.
CHANGE OF STATE OF TESTING BODIES
For pure chemical element or compounds change of state viz from solid to liquid to gaseous
etc. takes placed at a fixed temperature and this property thus gives a method to measure the
temperature.
Fusion Method
Fusion of different metals takes place at different temperature. Pyrometric comes are made
for different temperature and ar placed inside the furnace which will indicate the temperature
when the rated fusion temperature is attained.
Vapourisation Method
Vapourisation temperature of different volatile liquid are different. This property is utilised to
measure the temperature.
CHAPTER 10 PNEUMATIC INSTRUMENTS
10.1 INTRODUCTION
Pneumatic instrument systems were the main method of controlling and monitoring industrial
plant. Electrical instrument systems, with fast response times and ease of installation, have
already overtaken pneumatic systems and are now used for most applications previously
considered to be the duty of their pneumatic counterpart.
The slow response and costly installation problems of a pneumatic system are, however,
accepted when the prevailing conditions make electrical systems unacceptable.
Pneumatic instruments also find service in the smaller one off control system, where
transmission lags are small due to the size of the loop
FLAPPER/NOZZLE
Pneumatic instruments relay on the accurate conversion of mechanical movement to a
proportional pneumatic signal. In most cases this conversion is achieved with the use of a
transducer known as flapper/nozzle.
Air is supplied at a pressure of 1.5 bar. Due to the fact that the nozzle orifice is three times
larger than that the restrictor orifice air can, in fact, exhaust faster than it can pass through the
restrictor. This will result in gauge reading zero
Figure 10.1
If the flapper is now positioned so as to seal off the nozzle, the pressure will build up to the
supply pressure and be indicated on the gauge. In actual practice the flapper would be
connected through some form of linkage to the measuring element and it would be the
movement of the measuring element that moved the flapper. It follows that movement of the
measuring element changes the flapper relative to the nozzle and will, therefore, change the
air output pressure in a similar manner to that shown in the graph.
The flapper movement required to change the output from maximum to minimum is very
small, the actual movement/pressure change ratio will depend upon nozzle and restrictor sizes
but is usually about 0.02 mm.
Provided the flapper nozzle output is restricted to the straight line portion of the graph we can
say that output will be proportionately to measuring element movement.
While in principle the single flapper/nozzle is an effective transducer it does have some
serious drawbacks, for instance any change in supply pressure would affect the output
pressure and also since the amount of flapper movement is so small even the slightest amount
of wear on pivots or linkages would render the system useless. The difficulties may be
overcome by the use of negative feedback bellows. The feedback can be used to oppose the
measuring element force (force balance) or it can be used to change the position of the
flapper relative to the nozzle (position balance).
POSITION BALANCE PRINCIPLE
The flapper now flat and can be moved by the feedback bellows are well as the measuring
element. Assume the measuring element moves the flapper towards the nozzle, the output
pressure will increase and the feedback bellows will expand. The upper end of the flapper
will, therefore, be moved away from the nozzle and the effective movement of the flapper
about the nozzle is reduced. This increases the amount of measuring element movement
needed to give the complete range of output pressure and gives a proportional relationship
between measuring element movement and the corresponding output pressure. Small changes
in supply pressure will not effect the output. If the measuring element is in the position where
the out put pressure should be 0.6 bar for example, and the air supply suddenly increased, the
output pressure would tend to increase, but the increase in pressure would expands the
bellows, pushing the flapper away from the nozzle until 0.6 bar is again obtained. This
technique is used extensively in pneumatic proportional control. As shown Fig. 10.2
Figure 10.2
When the movement of the measuring element cause the flapper to move away from the
nozzle the drop in pressure cause the feedback bellows to reduce the force in opposition to
the measuring element force, until equilibrium is again established.
Since, with both the reverse and direct acting continuous bleed relay, output pressure is
maintained by venting excess air to atmosphere there is a continuous consumption of air.
Typically this will be about 0.5 cubic feet/minute and can be overcome by the use of a nonbleed type relay.
Non-Bleed Relay
The noozle pressure is applied to the exterior of the large outer bellows.
The control line pressure is exerted on the interior of the small bellows B and when the
forces due to the two are equal, a balanced condition exists. The relay flapper C the covers
both the exhaust nozzle D and the supply nozzle E.See Fig.10.6.
Figure 10.6
If the primary nozzle pressure increases the larger bellows are deflected downwards carrying
with it the smaller inner bellows. Exhaust nozzle D forces the flapper C away from month
of the supply port E, but remains closed itself. Air is admitted to the control line and the
interior of B. The force supplied to B increases and the bellows assembly is now moved
upwards until C is back to its original position and the support E is closed.
With a decrease of primary pressure the outer bellows move upward taking below B
upwards. The exhaust nozzle now comes into operation because of its mouth is uncovered in
the action of moving away from C relay flapper. Air bleeds away from the interior of
bellows B and also the control line via port F. Pressure is reduced and the bellow assembly
begins to move downwards until nozzle D meets the relay flapper C and the exhaust
passage is closed.
With this type of relay it should be noted that a dead spot may occur if a spring type of relay
flapper C is used to provide a positive closing force on the two ports D and E. Any
increase in primary nozzles pressure must overcome this spring pressure and so tend to
reduce the response time of the relay. In practice this is reduced to minimum by using an
outer bellows of an area as large as possible
ELECTRICAL/PNEUMATIC CONVERSION
Because of the modern trend towards electronic control and display equipment it is frequently
necessary to convert pneumatic signals to a proportional electrical signal or to convert an
electrical signal to a proportional pneumatic signal.
This is achieved by the use of a pneumatic/electrical converter as shown in fig. 10.7.
Figure 10.7
Electro pneumatic converts are also used where transmission signals cover great distances or
to improve response times of existing pneumatic equipment.
THE FIELDEN E/P CONVERTER
This Fielden E/P converter is a force balance device without feedback. Because of the lack of
feedback the setting up of the nozzle is critical. The device is supplied with air at 1.5 bar and
has restrictor and nozzle size ratios similar to be a conventional flapper nozzle system i.e 3:1
The beam is pivoted at one end whilst the other end is attached to a permanent magnet, the
plug of the primary valve is also connected to the bream. Zero adjustment is achieved by
varying the/ spring tension and positioning the primary valve plug relative to its seat (nozzle).
Current is applied to the coil and a magnetic field is set up, (the strength of which depends
upon the valve of current) the permanent magnet is forced down which brings the primary
valve closer to its seat, pressure builds up and forces the diaphragm down which seals off the
exhaust valve and opens the secondary valve, resulting in an increase in output pressure. If
the value of current falls the permanent magnet will rise relieving the pressure on top of the
diaphragm which closes the secondary valve in. Excess pressure is vented through the
exhaust, resulting in a drop in pressure. Oil damping is provided on the magnet to give
smooth operation. See fig.,10.8.
Figure 10.8
The output pressure of the regulator is fixed at say 3 bar. The air operated pressure regulator
will accept inputs between 0.2 and 1 bar, 1 bar input resulting in 3 bar output. The range of
output would be dependent upon piston specification and condition and would, therefore, be
set up in situ. It follows, therefore, that by varying the pressure on top of the piston we can
effectively position the piston rod anywhere within its length of travel.
A control valve must be capable of responding smoothly and rapidly to small changes in the
controller output signal. The quality of control will be impaired if any force; for example, that
due to friction of working parts, oppose the movement of the spindle and the valve plug. This
can be overcome by emplying mechanical feedback in the form of a positioner.
THE VALVE POSITIONER
The primary function of a valve positioner is to ensure that the control valve plug position is
always directly proportional to the value of the controller output pressure, regardless of glad
friction, actuator hysteresis, off-balance of forces on the valve plug etc. This is usually
achieved by incorporating a feed back lever that acts in opposition to the movement to the
input bellows.
The system can be either a position or force balance system but in practice force balance
systems are more common. Positioners can be incorporated into diaphragm, or cylinder type
actuators. See fig. 10.11
Figure 10.11
The controller output signal does not directly actuate the valve stem but is fed to a bellows
unit. Assume that the system is in equilibrium and then the controller output increases
slightly. The flapper is moved towards the nozzle and the relay output pressure beings to
increase.
This output pressure continuous to increase until the valve spindle moves, mechanical
feedback then restores the equilibrium. Thus the force applied to move the valve spindle is
sufficient to overcome the effect of all forces, no matter what the origin, which tend to
oppose the spindle movement. Without the positioner the slight change in controller output
signal may have been too small to initiate any corrective action/ The matching of input signal
range to valve travel range is achieved by changing the ratio of bellows/nozzle distance to
feedback arm/nozzle distance.
Positioners incorporate into pneumatic cylinders generally operated on a pilot valve principle
of which two are shown in Fig 10.12 and 10.13.
KENT MARK IV
The controller output acts upon the bellows, the pilot valve spool is attached to the bellows
via a connecting link. Assuming an increase in the controller output the bellows will expand,
unbalancing the spool of the pilot valve. Air is then admitted to the top of the piston and it
begins to move down. In doing so it takes the cam with it, as the cam moves down the bell
crank level turns about its pivot and through the spring opposes the movement of the bellows
and restores the spool of the pilot valve to its original position. The system is the back in
equilibrium. An equalizing valve is included to enable manual positioning of the piston
Figure 10.12
Figure 10.13
TYPES OF CYLINDER
The simplest type is the single acting cylinder (Figure 54).With this type air is used to make
the unit out stroke or extend (+). Once the pressure has been removed, the return or in stroke
(-) is achieved by mechanical means, in this case a spring. The cylinder can be air to extend
(application of a signal will push the piston out) or air to retract (application of a signal will
push the piston in).
Figure 10.14
In the double acting cylinder, if air is applied to P1 (with P2 open to exhaust) the piston will
outstroke (+); and if air is applied to P2 (with P1 open to exhaust) the piston will in-stroke (-).
The symbols + and are often used as a shorthand notation to indicate movement of the
cylinder, particularly when describing the sequence of operation of a circuit. For example,
there may be three cylinders A, B and C which operate in the sequence A+, B+, B-, A-, C+,
C-.
PNEUMATIC CYLINDER CUSHIONING
On high pressure systems, piston speeds can be in the order of 450 mm/sec. and impact forces
at the ends of the stroke can be great. In order that damage may not be cased by sudden
contract between the fast moving piston and the cylinder end housing, some form of buffer or
cushioning can be used. This does not limit the piston travel but allows gradual declaration in
the last 25 mm or so of travel, this is achieved as shown in fig 55
Figure 10.15
As the cylinder outstrokes under the action of applied pressure air is displaced from the other
side of the piston to atmosphere through the main part and needle valve. When the cushioning
boss enters the cushioning seal, the main port is blocked off, air can, therefore, only escape
through the needle valve at a much slower rate thereby causing the piston to slow down for
the premium period of travel. This results in the cushioning effecting shown in Fig.10.16
Figure 10.16
The basic three port valve consists of a two lobe spool running in a surface ground cylinder,
compressed air can be switched to the outlet by the application of a force to the spool.
The force can be removed and the outlet will remain connected to the air supply.
The application of a second force will return the spool to its original condition, main air will
then be isolated and the device connected to the outlet will exhaust from part 3.
FIVE PORT RELAYS
The basic construction of the five port relay is the same as the three port relay, the only
difference being the use of a three lobe spool. Compressed air can now be routed through the
valve whilst the same time a signal can be exhausted through it. The direction of force will
determine the routing of supply and exhaust
LIST OF TABLES
Table 1: Installed Capacity of NTPC
Table 2: Coal Based Power Stations owned by NTPC
Table 3: Coal Based Power Stations owned by Joint Ventures
Table 4: Gas Based Power Stations
Table 5: Hydro Based Power Projects
Table 6: Solar Energy Projects
LIST OF FIGURES
Figure 2.1: Flow Diagram depicting Process of Conversion of Natural Gas to Electricity
Figure 2.2: View of Air Intake System
Figure 2.3: Sectional View of Air Compressor
Figure 2.4: Cross sectional View of a combustor used in combustion chamber
Figure 2.5: Gas Turbine
Figure 2.6: Turbine Blades
Figure 2.7: Turbine and Generator mounted on a single shaft
Figure 2.8: step up Transformer
Figure 2.9: An auxiliary Transformer
Figure 2.10: A steam Turbine
Figure 2.11: Condenser
Figure 2.12: Constructional view of HRSG
Figure 2.13: View of steam Turbine Generator
Figure 2.14: Step up Transformer
Figure 2.15: De-aerator
Figure 3.1: Pipelines of gas source
Figure 3.2: Naphtha Specifications
BIBLIOGRAPHY
www.ntpc.co.in
Wikipedia
NTPC guide manual
Sample report
Power plant engineering-A.K.Raja, Manish dwivedi