Group 2:

Pressure api

PRESENTED BY:
ASAD TANVEER MCQS (28-56) SLIDES (1-27)
AAMIR SAJJAD MCQS (57-83) SLIDES (28-48)
SIKANDAR KHAN MCQS (84-112) SLIDES (49-68)
ALI RAZA MCQS (113-143) SLIDES (69-94)
UMAIR AKRAM MCQS (144-174) SLIDES (95-112)
 Introduction
Pressure (P ) expresses the magnitude of normal force (F-N)
per unit area (a-m2) applied on a surface(crowe et al. 2005)
F F
P  or P 
A 2 A
UNITS OF PRESSURE: Pressure may be measured in british
(fps) or in metric (SI)
Pa(= N/m2), psi(=lbf/in2), bar (=105 pa=100 kpa), mbar (=100
pa=1 hpa), atm (=101.3 kpa), mmhg (or torr), inhg, etc.

Pabs  Patm  Pgage

WHERE PABS : Absolute pressure
Patm : atmospheric pressure
(standard is: 101.3 kpa =14.696 psi=760 mmhg=29.92 inhg)
Pgage : gage pressure

Umair Akram 08-04-2015

 Different Types of
Pressure
• Gauge Pressure
• Absolute pressure
• Vacuum or differential pressure
• Static pressure and velocity pressure

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Gauge Pressure:
A gauge that indicated zero at atmospheric pressure measures the difference between
actual and atmospheric pressure. This difference is called “gauge pressure”. It is
abbreviated as psig.
Absolute pressure:
Absolute pressure is actual total pressure (indicating atmospheric pressure) acting on a
surface. It is abbreviated as psia.

Vacuum or differential pressure:

Gauges that indicate gauge pressure may be designed to indicate pressures below zero.
Such a gauge is called a “vacuum gauge”. Gauges that indicate absolute pressure cannot
indicate pressures below zero, because zero is perfect vacuum.
In a differential pressure measurement, the gauge pressure is the difference between the
absolute pressure of the fluid and the atmospheric pressure.

Static pressure and velocity pressure:

When the fluid is in equilibrium, the pressure at a particular point is identical in all
directions and independent of orientation. This is called “static pressure”.
Velocity pressure is the difference between the total pressure and static pressure;

Velocity pressure = total pressure – static pressure

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Figure: Illustration of (a) gauge pressure versus absolute pressure and (b) delta or
differential pressure.

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 These instruments may be grouped into four categories:
1. Gravity-based: barometer, manometer, deadweight piston.
2. Elastic deformation: bourdon tube (metal and quartz), diaphragm,
bellows,strain-gage, optical beam displacement.
3. Gas behavior: gas compression (McLeod gage), thermal conductance (Pirani
gage), molecular impact (Knudsen gage), ionization, thermal conductivity, air
piston.
4. Electric output: resistance (Bridgman wire gage), diffused strain
gage,capacitative, piezoelectric, potentiometric, magnetic inductance, magnetic
reluctance, linear variable differential transformer (LVDT), resonant frequency.

Umair Akram 6 08-04-2015

 Pressure Instrument

Is a device which is used for the measurement of the

pressure exerted by fluid.
Pressure instrument measure a difference between
Two Pressures.
Mostly one of hem is Atmospheric pressure.
 Pressure Methods

Manometer method
Elastic Pressure Transducers
Pressure measurement by
Measuring Vacuum
Pressure measurement by
Balancing Force produced on a
known area by a measured force.
Electrical Pressure Transducers
 Types of Measurement

Mechanical
Manometers (U-tube manometer, well type manometer, Inclined
manometer,
micro manometer)
 Elastic Pressure Transducers:

Bourdon tube (C-type Bourdon tube ,Spiral and Helix Bourdon

tube
 Diaphragm pressure transducer ( Metallic diaphragm gauge
,Slacke diaphragm gauge and Bellows)
Electrical
 Strain Gauge, Capacitive sensor, Potentiometric, Resonant Wire,
Piezoelectric, Magnetic, Optical
 Barometer

A barometer is an instrument for measuring

astmospheric pressure.

It consists of long glass tube

filled with mercurry. Tube
is turned over into disk.
Some of the mercurry runs
out of tube and into dish.
 Principle of barometer:

If one end of liquid is at zero absolute pressure, then

difference in height of liquid from zero reference
indicates the absolute pressure.
The pressure in evacuated
portion of barometer is not
really absolute zero but
rather the vapor pressure of
filling fluid,mercurry, at
ambient temperature.
It is seen that level of mercury column falls , leaving an empty space
called a vacuum.
Atmospheric pressure acts on the surface of mercury in trough as
shown in fig.
This pressure is equal to pressure at base of column of mercury in
inverted tube i.e. pressure of atmosphere is supporting the column
of mercury.
If atmospheric temperature
falls the barometer height
decreases and vice versa.
So atmospheric pressure
measured in terms of height
of mercury.
 Types of Barometer:

Fortin barometer
Aneriod barometer
 Fortin
Barometer:
Fortin barometer is an example
of mercury barometer that
enables barometric heights to be
measured to a high degree of
accuracy.

A fortin barometer shown.

Mercury is contained in a
leather bag at the base of
mercury reservoir and height H
of mercurry in reservoir can be
adjusted using the screw at base
of barometer to depress or
release the leather bag.

To measure atmospheric
pressure screw is adjusted untill
pointer at H is just touching the
surface of mercury and height
read by vernier and main scale.
 Aneriod
barometer
Aneroid barometer consists
of a circular , hollow sealed
vessel and usually made
from thin flexible metal.

Air pressure in vessel is

reduced to zero before
sealing so that change in
atmospheric pressure will
cause the shape of vessel to
expand or contract.

These small changes can be

measured with a lever and
be made that to move a
pointer over calibrated
scale.
 Manometer:

A simple manometer consists of a glass tube having

one of its ends connected to a point where pressure
is to be measured and other end remains open to
atmosphere.
Manometer is simplest
measuring instrument used
for gauge pressure measurement
by balancing the pressure
against the weight of a coloumn
of liquid.
The action of all manometers
depend on the effect of pressure
exerted by a fluid at depth.
 Types of Manometer:
U-Tube manometer
Well type manometer
Inclined manometer
Micro manometer
U TUBE
MANOMETER:
Simplest form of
manometer used
for experimental
work in
laborateries.
By suitable
choice of liquids
it can be used for
recording wide
range of
pressures.
U-tube
manometer
construction:
It consists of transparent
(glass) tube constructed in
the form of an elongated U
and is partially filled with a
liquid, most commonly
water or mercury.

Water and mercury are used

because their specific weight
for various temperatures are
known exactly and they
don’t stick to tube.

One end of tube is

connected to one pressure
tap and other end connected
to other pressure tap, or it
may be left open to
atmosphere.
 U-tube
manometer
working:

 When there is a pressure

difference between two ends
of tube , the liquid level does
down on one side of tube up
on other side.

The difference in liquid levels

from one side to another
indicates the difference in
pressure.

From fig diffrential pressure

is obtained by
- =(ρ-)()g

If to measure low pressure

then use water as liquid in
manometer, mercury for high
pressure.
Well type
manometer:
It consists of a very
large vessel (well)
connected on one
side to a very small-
sized tube.thus zero
level moves very little
when pressure is
applied.
Even this small error
is compensated by
suitably distorting
the length of scale.
 Well type
manometer
However such an
arrangement is sensitive
to non-uniformity of the
tube cross sectional area
and is thus considered
somewhat less accurate.
In a single leg-
instrument ,high
accuracy achieved by
setting the zero level of
the well at zero level of
the scale before each
reading is taken.
 Inclined
manometer:
 Inclined manometer is
an enlarged leg
manometer with its
measuring leg inclined
to the vertical axis by
some angle.
The angle of
inclination is of order
of .The inclination is
done to expand the
sclae and thereby to
increase sensitivity.
 Inclined
manometer:
The inclined
manometer is used
to measure very
small pressure
differences (in
hundreds of inch of
an inch of water).
The manometer is
tipped so that liquid
moves a longer
distance through
tube as it rises.
 Micro
Manometer:

A micro
manometer is
used for
accurate
measurement
of extremely
small pressure
differences.
 Micro
Manometer
construction:
A micro manometer
consists of a well
connected to a flexible
tube whose one end is
inclined
A magnifier is attached
to the inclined portion
of tube for observation
of the fluid level.
A micrometer is
connected to the well for
observation of reading.
 Micro
manometer
working:
 The micro manometer is
initially adjusted so that
when pressure in well
and inclined portion
become equal i.e , the
meniscus in the inclined
tube is located at a
reference point given by
a fixed hairline viewed
through a magnifier.
The reading of
micrometer used to
adjust the well height is
now noted.
 Micro
manometer
working:
Now application of an
unknown pressure
difference causes the
meniscus to move off the
hairline which can be
restored to its origional
position by raising or
lowering the well with
micrometer. The
difference between
initial and final
micrometer readings
gives change in height
and thus the pressure.
 Manometer:

ADVANTAGES DISADVANTAGES

They are simple and They are large and

time proven. bulky
They have high accuracy They need leveling
and sensitivity. They are not portable.
Wide range of filling
No over range
fluids available.
protection in it.
Cost is reasonable.
Condensation leads to
Suitable for low pressure
problems.
C Type Bourdon Tube Pressure Gauge

Principle
 The Bourdon tube works on a simple principle that
a bent tube will change its shape.

 As pressure is applied internally, the tube

straightens and returns to its original form when the
pressure is released.

 The tip of the tube moves with the internal pressure

change and is easily converted with a pointer onto a
scale.
31 06/05/2020
Construction and working

32 06/05/2020
Advantages
 Inexpensive
 Wide operating range
 Fast response
 Good sensitivity
 Direct pressure measurement

Disadvantages
 Primarily intended for indication only
 Hysteresis on cycling
 Sensitive to temperature variations
 Limited life when subject to shock and vibration

33 06/05/2020
Bourdon tube is probably the
most common of the expandable
element gages
 The gage consists of a hollow,
curved tube made from flexible
metal
 One end of tube is connected through a valve to the
source of pressure

34 06/05/2020
 When the valve between
the process and the
bourdon tube is closed,
pressure in tube is low and
the tube remain curved but
when the valve is open,
pressure increases and tube
tends to straight out

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 By connecting the tube to the
pointer through a gear train,
pressure is indicated on dial
 The gear turns to the right, the
pointer moves toward the right
hand side of dial
 The dial shows an increase in
pressure as pointer moves from
left to right
 It also called C tube

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 Spiral and Helix Bourdon
tubes

 Bourdon tube can be made

sensitive by changing the
shapes

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Compare C type and Spiral Bourdon tubes
Spiral is more sensitive
38 06/05/2020
 Pressure by Elastic Transducers

Bourdon
Bellows
Diaphragm
 C-type Bourdon tube

It is the most frequently used pressure gauge because

of simplicity and rugged construction. Its range is from
0-15 psig o 0-100,000 psig.
 Construction and working:

I consist of a long thin-walled cylinder of non-circuler

cross-section, sealed a one end of bonze, steel and
beriluim copper, attached b a light line work o he
mechanism which operates the pointers.
Other end is open for the application of the pressure
which is to be measured as the fluid under pressure the
oval shape of tube change the circular.
The resulting moment of the free of the tube cause the
pointer to move over the scale
The tip of bourden tube is connected to a segment leaver
through an adjustable length link.
The segmental leaver is suitably pivoted and the spindle
hold the pointer
A hair spring is sometime is used to fastened the spindle
to the frame of instrument to provide tension for proper
mashing of the gear teath
Thereby freeing the system from black lash
Any error due to friction in the spindle baring is known
as loss motion
There are two types
 Helical Type
 Spiral Type
 adjustment

Here are two type of adjustment of burden ube

a) Multiplication adjustment
Because of stress develop in a bourdon tube actual
travel is non-linear in nature. However, for a small
ravel of he op his can be considered o be linear and
parallel o he axis of he link. He small linear op
movement is matched with rotational pointer
movement. His is known as multiplication and can be
adjusted b adjusting he length of lever. A shorter lever
gives large rotation for the same amount of tip travel.
 Cont….

b) Angularity:
When the approximately linear motion of the tip is
converted to a circular motion with the link lever and
pinion attachment, a one to one correspondence
between them may not occur and a distortion results.
This is known as “angularity”.
 Advantages:

Following are the advantages of bourdun tubes:

Their cost is low.
They ha e simple construction.
They have been time-tested in application.
These tubes are available in a wide variety of ranges,
including very high ranges.
They are adaptable to transducer designs for
electronic instruments.
 Disadvantages:

Following are the disadvantages of bourdun tube.

They have low spring gradient.
They are susceptible to shock and vibration.
They are susceptible to hysteresis.
 Diaphragm pressure transducers:

Diaphragms are widely used for pressure and draft

measurements. Particularly in a very low ranges.
They can detect a pressure differential ever=n in the
range of 0-4mm.
 Types of diaphragm pressure transducers

There are two types of

diaphragms are generally
used:
a)Metallic diaphragm gauge
b)Slack diaphragm gauge
 Mettalic diaphram gauge:

It consists of thin flexible diaphram made up of brass

and bronze
A pointer is attached with diaphram.
The force of pressure causes deflection in diaphram
The diaphram is attached with the mettalic pointer
which shows the reading.
This is capable for low pressure measurement.
 Slack diaphragm gauge

It is more difficult to measure pressure below atmospheric

pressure because changes are small.
The full range from atmospheric pressure to a perfect vaccum is
only 14.7 psi.
The pressure in this range can be measured with a slight
modification of the diaphragm.
A diaphragm with the large area produces large change in force
from a small change in pressure.
Similarly making the diaphram slack rather than tight allows it
to move a large distance in response of small pressure change
Slake diaphram made up of rubber or flexible material\
Range is 0.01-0.40 mmHg.
 Diaphram Pressure Transducers:

advantages disadavantages

Their cost is moderate. They lack good

Hiigh over range vibration and shock
characteristics resistence.
Good linearity
They are difficult to
Small in size
Adjustable to slurry services
repair.
They are limited to
They are adoptable to
absolute and differential relatively low
pressure measurement mpressures.
 Topics

Pressure (types, units)

Calibration and maintenance of pressure measuring
instruments
Care and troubleshooting of instruments(all from S.K SINGH)
The bellows gages
Operating problems with pressure gages
How the gages are sealed?
Interpreting the pressure reading(all from pressure api)


 Calibration of pressure measuring instruments

It is the process of adjusting the output of instrument to

match the known range of pressure. Basically it includes
zero error span linearity adjustment. Calibration is
carried b applying air or liquid pressure whose value is
accurately known
Calibration of low pressure gages
For very low pressure( 3inch) inclined water gauge may be
used.
For pressure (144inch)mercury manometer in used.
Dead weight piston must be used for for much higher
pressure.
 Calibration of pressure transmitter
(25-125psig pressure range)

Turn on the transmitter and allow the internal

components to reach normal temperature.
Use a dead weight test to apply 25 psig .
Adjust the zero to set exactly 4mA output
Supply 125 psig to instrument and adjust the span to get
the reading of 20mA.
Apply 75psig and adjust the linearity to bring the output
signal within specified tolerance of 12mA.
In most cases adjusting the span may be throw off zero
adjustment therefore it is necessary to repeat entire
calibration procedure several times
 Maintenance of pressure measuring
instruments
Maintenance is necessary for proper working of instrument
and accurate reading.
Generally the instructions for periodic maintenance are given
in the manual supplied by the manufacturer which explains
the working of instrument. It is called the periodic
maintenance. It includes 3 things
1.Visual inspection (it tells about damage in piping and
wiring )
2.Blowdown and venting (it prevents the clogging by
removing dust and foreign materials.
3.Cleaning and lubrication (controls the leakage and
misalignment by checking leakage regularly
 Care and troubleshooting for pressure measuring devices

Liquid filled manomater must be leveled. All the

gauges must be mounted correctly. The zero error
and calibration of the instrument must be checked.
If the instrument has mercury its level must be
checked.
Troubleshooting helps in finding the failure in
instruments. It observes the it in operation and write
down its symptoms. Troubleshooting charts should
be used for guiding.
 THE BELLOWS GAGES

They work o the principle similar to diaphragm

gages which are used for various pressure
measurements.
When the pressure is applied on one side of gage the
segments of bellows expand.
This expansion is transferred throughout linkage
and the pointer is indicated on dial.
This gage has larger area to be acted upon by
pressure on it.
Bellow and diaphragm gages.
 Contd.

This gage has ability to move farther for smaller

pressure changes,
This gage measures the difference process pressures
and atmosphere pressure.
This gage can also be used to measure the difference
between two processes.
It is shown above in diagram
DIFFERNTIAL PRESSURE OF TWO
PROCESSES
 Contd.

When pressure is greater on A pointer moves

towards right.
When there is no pressure difference pointer moves
towards zero point.
Another type of bellow gage is differential pressure
bellow gage.
Process pressure A pushes bellows inside and
process pressure B pushes it outside. When pressure
B is greater than A gage collapse.
Differential pressure gage
OPERATING PROBLEMS WITH PRESSURE GAGES

Pressure gages can be damaged by excessive

pressure. Damage can be caused by pressure greater
than 150% of the range of gage. If pressure is
doubled the gage can be permanently damaged.
If bourdon tube is used above pressure for which it is
designed it cab be ruptured
It is very important to know the pressure limit of
gage,it is also very important to know the range of
pressure you are measuring.
Differential pressure gage
 Contd.

Bourdon tube is usually housed in metal case which

protects it from dirt and weather.
If tube is over pressurized and rupture process fluid
is released in the metal case and the pressure in this
case will increase which can cause damaging of metal
case as well.
The case may be fitted with a blow out disc to protect
against over pressurizing. When pressure is greater
disc is forced out of the housing and by releasing this
pressure to atmosphere it protects the gage.
Contd.
 HOW THE GAGES ARE SEALED?

Some process materials damage the metal used in

the gage. In such cases the fluid enter in the pressure
elements.
To separate the gages from fluid gages in the steam
services are fitted with pigtails siphons.
Siphons traps the steam before it reaches the gage
and damage it.
Process fluid must apply pressure to gage
 Cont.

Process pressure must be transferred to gages.

Mercury manometer is scales with nonabrasive fluid.
Process pressure is transferred to mercury in the
gages through this sealing fluid. Process fluid does
not contact with the gage fluid.
A diaphragm transfer the pressure into sealind fluid
which is transferred to the gage.
Mercury manometer
 INTERPRETING PREESURE READING

Refinery operations are usually concerned with low

much less or more pressure than the atmospheric
pressure.
Pressure gages are calibrated to zero that indicates
the atmospheric pressure. This is gage pressure not
absolute.
Total pressure is sum of gage and atm.pressure.
Psia=psig+14.7
 71

PRESSURE MEASUREMENT
METHODS AND APPLICATIONS

Umair Akram 08-04-2015

 Pressure Measuring Devices
BOURDON GAGE:

72

http://www.efunda.com/DesignStandards/sensors/bourdon_tubes/images/Bourdon_tube_A.gif
http://www.cpigauges.com/images/gauges/WeldGageStlCsBM400psi.jpg http://www.hydraulicspneumatics.com/FPE/images/sensors1_1.jpg

Principles: change in curvature of the tube is proportional to difference of

pressure inside from that outside the tube
Applications: tire pressure, pressure at the top or along the walls of tanks or
vessels

Umair Akram 08-04-2015

Pressure Measuring
73
Devices
Helix and spiral tubes:
Helix and spiral tubes are fabricated from tubing into shapes as per their
naming.
With one end sealed, the pressure exerted on the tube causes the tube to
straighten out.
The amount of straightening or uncoiling is determined by the pressure applied.
The uncoiling part of the tube is mechanically linked to a pointer which
indicates the applied pressure on a scale.
This has the added advantage over the C-Bourdon tube as there are no
movement losses due to links and levers.

SPIRAL TUBE
HELICAL TUBE

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 Dead-weight pressure gauge:
9 3
7
1
2
5 8
2

4
6

 A cylindrical piston 1 is placed inside a stainless-steel cylinder 2.

 The measuring pressure is supplied through the vent 8 to the fluid 4.
 The gravitational force developed by calibrated weights 3 can balance this force
and the piston itself..
 The balance should be achieved for a certain position of the piston against a
pointer 9 of the stainless-steel cylinder.
 A manual piston pump 5 is used to achieve approximate force balance (to
increase pressure in the system), whereas a wheel-type piston pump 6 serves for
accurate balancing.
 A Bourdon-type pressure gauge 7 is used for visual reading of pressure.

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74