Boiler Preservation Methods Dry and Wet

July 19, 2018 Boiler articles, Boiler Commissioning, Boiler Safety Articles Leave a comment 9,461 Views

Boiler preservation is required when boiler is out of service or in shutdown condition for
a long period to protect boilers internal surface from corrosion and atmospheric oxygen.
There are two types of boiler preservation methods each of them is used in specific
conditions.
1. Wet preservation of boiler
2. Dry preservation of boiler
Boiler preservation through nitrogen method is also used in power plant boilers.

These methods are designed to limit corrosion caused by the normal range of boiler and
atmospheric contaminants.
Wet Preservation of boiler
If Boilers are required in stand by condition for sudden and quick demands of steam are
preserved by the wet method. This method is also used for large boiler units which are
difficult to drain.
Total Boiler pressure parts economiser, steam drum, water walls, super heater tube are
filled with feed water having high concentration of Hydrazine (200 PPM). Such high
concentration of hydrazine ensures, there is no dissolved oxygen available in feed
water. Boiler is kept under pressure, so that atmospheric air can’t enter in to Boiler
pressure parts.
Procedure of wet Preservation
The boiler should be filled with feedwater to the normal water level then
90% water level of gauge glass. Maintain ph up to 9.5-11.
Hydrazine 200 PPM is to be dosesed with the water. Ensure uniform concentration.
The results of boiler water sample, in case of N2H4 level is low from 100ppm then to
inject N2H4 and in case of N2H4 level is above 200ppm then to perform dilution process
(by feeding water into boiler and by taking blow down frequency.
Sodium sulphite (Na2SO3) 300-400 PPM can also be used if Haydazine is not available
in case.
The boiler should be steamed to circulate chemicals to uniform concentration.
Do not use wet preservation method in very cold environment minus zero temperature
or freezing water condition.
Dry Preservation of boiler
This method is used when boiler is required to be shutdown for a long period. Dry layup
requires the removal of all water and the moisture of air to maintain a relative humidity
less than 50%, and preferably less than 40%. This prevents corrosion by hygroscopic
salts. Dry-air (dehumidified) storage is highly effective, and its continued effectiveness is
easy to monitor. Dry layup allows easy and safe access for maintenance, with no
potential for suffocation and no exposure to toxic chemicals.
Procedure for Dry preservation of boiler
The water is to be drained out of the boiler. The drum man hole, super heater,
blowdown, air cocks, gauge glass and waterwall box drains should be opened.
Completely dryout the internal surface of boiler with hot air.
After drying silica gel on activated alumina (1.3 kg/m3 or 0.25 kg/m2 of heating surface)
should be placed inside the drum. Unslaked lime (5.1 kg/m3 of heating surface can also
be used as dessicant.
Closed airtight all the pressure parts so that no moisture and air leaks in.
Boiler inspection should be carried out once a month.
Replace the lime trays if necessary.
Nitrogen preservation
In the Steam generators even where water is present, corrosion can be prevented by
eliminating oxygen from the environment. Oxygen can be eliminated by sealing and
pressurizing the entire boiler, or the space above water level, with nitrogen to prevent air
in leakage. In the absence of acids and other oxidants, eliminating air stops corrosion.
Nitrogen blanketing is a highly effective method for preventing corrosion. It is easy to
monitor its pressure and alarm, so effective preservation can be assured. However, the
boiler must be well sealed to prevent excessive leakage.

Boiler Feed Water Pump Working Principle and Operation

Today we will discuss about (BFWP) boiler feed water pump working principle and
operation of a boiler feed water pump. The boiler feed pump function is to supply
pressurized feed water to the boilers maintain the water inventory at various operating
loads. Boiler feed water pumps are commonly centrifugal pumps. To ensure a high
enough head and pressure, Boiler feed Pumps have been arranged in series (multistage
pumps). A feedwater pump is most often driven by an electric motor for energy
efficiency. Often two steam turbine-driven (turbo pump) and two electric feedwater
pumps are used.

Commissioning of a boiler feed water pump

To ensure adequate feedwater flow a boiler must have at all times a slightly higher
capacity for feedwater than for steam generation capacity of boiler. This pump is fitted
with mechanical seal or labyrinth glands to prevent leakage along the shaft and a
balance piston to achieve hydraulic balance and balancing line from first stage to last
stage to accumulate the thrust in feed water pump. The pump body consists of a double
casing with a radially split inner casing held together with long bolts. Labyrinth glands or
mechanical seals are fitted to prevent the leakage of hot feed water outwards along the
shaft sleeves. Condensate from the condensate pump discharge is piped into the
control portion of the gland where it is directed inward along the shaft to the balance
chamber and then out again and back to the condenser. The pump utilizes fully
shrouded impellers and volute casings The materials of the impellers, shaft and volute
passageways are stainless steel to reduce corrosion.

Feed water booster pump takes suction from deaerator. The primary function of the
booster pumps are to deliver feedwater to the main feed pump at the required NPSH.
The booster pumps are relatively low speed, low head, and have lower required NPSH.
There is also a recirculation line originating the minimum flow valve of the pump
discharge check valve and discharging into the deaerator storage tank.
Start-up of BFWP boiler feed water pump
Conditions required to start up the boiler feed pump unit:
(1) Feed water tank filled (to above minimum water level)
(2) The pump and pipings are fully primed and vented.
(3) Suction valve fully open
(4) Discharge valve closed
(5) Minimum flow shut-off valve open
(6) Cooling water system running
(7) Oil pump in operation.
(8) Differential temperature between top and bottom on barrel within 20°C
Pumps Start and Check:
(1) Oil supply pressure
(2) Pump suction pressure
(3) Pump discharge pressure
(4) The passing of the minimum flow
(5) Bearing temperature
(6) Shaft seal The seals should only show very small sings of leakage. Check its
temperature.
(7)Check the oil flow rates at the lubricating points of the pump.
Shutdown of the pump
(1) Close discharge valve
(2) Switch off the driver and observe the pump run down smoothly to a standstill. The
pump rotor should not stop with a sudden jerk.
(3) Switch off the oil pump.
(4) The suction valve should remain open unless the pump is being taken out of service
for prolonged period and is being drained
Boiler inspection Procedure requirement and
Recommendations
May 30, 2018 Boiler articles, Boiler Safety Articles Leave a comment 4,690 Views

Pressure vessels and Boiler Inspection are done to determine the amount of
maintenance required to have equipment operate properly until the next turnaround. A
comprehensive equipment history must be maintained that includes conditions found
during inspection and testing, maintenance work done, changes made, and date of
each. ASME Code Standards require boiler inspection to be done at least every year.
There is no mandatory inspection requirements for heaters. Inspection intervals are
based on service and experience.
Boiler Inspection is not an exact science and requires the use of judgement and
experience.
Boiler Steam Drum Inspection

Firebox
Tube and Drum External Inspection
Tubes are inspected externally for excessive fire scale (fireside scale), deformation
(bulging) or sagging that may indicate overheating. Sagging roof tubes are especially
susceptible to overheating. Drums and shells are inspected externally for excessive fire
scale, cracking of joints and any indication of leakage (deposits) at the tube to drum
connections.
The tube and drum thickness are measured ultrasonically to determine a corrosion rate.
Tube samples may be taken for inspection of the metal deterioration and measurement
of deposits.
Tubes and drums are inspected at support points to see if there is any binding or wear.
Supports are inspected for corrosion and damage and to make sure they are functioning
properly.
Firebox Refractory/Insulation
Refractory and insulation are inspected to determine if there is any deterioration.
Refractory is inspected for breakage, slagging, crumbling, spalling, and open joints.
Leakage of hot boiler gases through joints when the edges have crumbled or when the
tile or insulating concrete has fallen out may expose the supporting steel to high metal
temperatures, rapid oxidation, and corrosion.
Fly-ash corrosion may occur when fly-ash and refractory are in contact. Fluxing occurs
and produces a slag that may be fluid at boiler operating conditions. Metal oxides found
in fuel oil are fluxing agents. The general effect of slagging is to decrease the thickness
of the refractory and thereby reduce its insulation properties. Slagging may cause rapid
deterioration of boiler hardware such as tube hangers and spacers.
Severe spalling will require repair. Spalling of refractory usually results from a startup
heatup being done too rapidly. Spalling can also result from overheating by direct flame
impingement. Sagging would indicate problems with the refractory/insulation support.
Support problems can be caused by overheating or corrosion of the supports.
External deposits may indicate the need for external water washing. The water washing
procedure includes sealing the refractory with a bitumen sealer to prevent water
damage of the refractory and the use of 0.5% soda ash solution to minimize stress
cracking of austenetic steels such as stainless steels. Under no circumstances is salt or
raw water to be used for water washing. The bitumen sealer is burned off in normal
operation.
External firebox insulation is inspected for rust spots, cracking and bulging that may
indicate corrosion beneath the insulation.
Fire Scale
Tubes are externally inspected for excessive fire scale. Fire scale may indicate the
metal has experienced high temperatures and has oxidized. Areas with heavy fire scale
should be carefully inspected and monitored for corrosion. Ultrasonic measurements
should be taken.
Boiler Convection Section Deposits
The convection section is inspected for deposits especially when oil has been fired.
There may be deposits that are not readily removed by water washing. Ash from fuel oil
firing can form a glass like deposit which may be very difficult to remove even with
chemical cleaning. Sometimes the tubes must be replaced. Any damage to the
extended surfaces should be noted for evaluation of required maintenance.
Boiler Steam Mud Drum Deposits
Boiler steam and mud drums and the tube ends are inspected for deposits. The boiler
inspection may indicate the need for chemical cleaning to remove deposits. Severe
deposits indicate the need to review boiler water treatment and blowdown controls.
Thickness Measurement
Thickness measurements are made to detect corrosion as shown in picture.
Ultrasound – Ultrasound measurements
Ultrasound measurements of boiler tube and drum metal thickness are taken to
establish an average corrosion rate of tubes and drums. Normal ultrasound
measurements indicate an average metal thickness but may not give any indication of
pitting or cracking.
Pitting and Cracking
Boiler Inspection for pitting and cracking includes radiographic examination, magnetic
particle examination (magnaflux), and liquid penetrant examination. Welds may be
radiographed (x-ray) to determine cracks and penetration of welds. Magnetic particle
inspection
and liquid dye penetrant inspections are used to locate small surface cracks especially
in autenistic steels. A pit gauge is used to measure the depth of pits.
Tube Deposits scale
Boiler tubes can be internally inspected by a fiberscope type of instrument that can give
a TV picture of the inside of the tubes. Sometimes sections or tubes may be cut out for
inspection and replacement. Internal boiler scale (deposits) may indicate the need for
chemical cleaning. The tube in Figure 3 is in definite need of chemical cleaning.
Boiler Hydrostatic Testing
Once maintenance has been completed the furnace or boiler is hydrotested at 1.5 times
the design or maximum allowable working pressure to make sure there are no leaks.
Read Boiler Hydrostatic Testing procedure.
Bench Test
The ASME code requires testing of boiler pressure relief valve and safety valves. All
boiler PZVs must be tested in place for the final set pressure and blowdown (reseating)
adjustments as required by the ASME code. The in-place test is performed as per a
written approved operating procedure. The in-place test has provisions to prove full lift
capacity and accuracy of adjustments.
Water Tube Boiler Parts and Functions
July 25, 2017 Boiler articles, Power plant Articles, Water tube Boilers 2 Comments 41,718 Views

What is a water tube boiler?

In the water tube boiler, the water and steam flow inside the tubes and the hot gases
flow over the outside surfaces. Where as in fire tube boilers hot gases from combustion
travels through the tubes. flue gases produced from furnace where fuel is burnt.
Water tube boiler design and working
In a typical Water tube boiler design as shown in the figure, it consists of two drums the
steam drum and the water or mud drum. On this design straight tubes were rolled into
mud drum, and the front headers were connected to the steam drum. The rear header
was connected to a horizontal box header, which was also attached to the steam drum.
The tubes were inclined to promote water circulation. There was also a spring loaded
safety valve on top of the steam drum. In a water tube boiler fuel is fired into a water
cooled furnace. Sheet metal and refractory enclosing the boiler called casing. At the end
of the furnace, the flue gas turns into the convection section and travels towards the
stack.
All boilers have a radiant and a conviction sections, tubes around the furnace are called
generation or riser tubes about a half of steam is generated in this area rest of the steam
is produced in the convection section. Water inside the tubes become hot and due to
natural circulation steam is collected and separated in the steam drum. Pre heated
(economiser) Feed water is continually feed in the steam drum with feed water pump.
These boilers were common in the paper making, sugar and other Industries which had
big demands for electrical power and steam. Steam generated at high pressure and
superheated would be fed to a turbine driving an alternator to generate electricity. Low-
pressure steam leaving the turbine would then be used for the process.The smaller
water tube boilers can be built at the manufacturers and delivered to site as one
unitpackage water tube boiler. Most boilers, however, are built in sections and erected
on the site.
Water tube boiler parts and their functions
There are many pressure parts and non pressure parts in a water tube boiler we only
discuss main water tube boiler parts and function.
Steam drum
Steam drum is a collection vessel for steam & water. Here water & steam is separated.
It has steam separators. Steam goes from top side to superheater & water goes from
the bottom through down comer Mud drum, then to furnace bottom ring headers (bottom
of furnace water wall).

Steam drum of water tube boiler

It has two types of draining arrangement (a) CBD- continuous blow down used when
Si02 or TDS is on the higher side. (b) Emergency blow down is used when drum level is
high high. Boiler mountings and accessories like the safety valve, water level gauge and
pressure gauge, feed water inlet connection are installed on this drum. Drum internals
includes cyclone separators baffle plates rose pipe and the dry pipe. The main function
of the steam drum is to provide water storage and space to separate steam from water.
Mud or Water Drum
Mud or water drum the lower drum is directly attached to upper steam drum with large
no of straight tubes bundles called boiler bank tubes. Solids and mud can settle in this
mud drum for removal through periodic blowdown. Some times desuperheater coil also
installed in this drum to recover heat from superheated steam. Draining arrangement of
this drum is through one or two boiler blowdown connections to control tds or to fully
drain the boiler when out of service.
Water walls
Tubing arrangement around the furnace to extract heat from fuel to generate steam is
called water wall circuit. These water walls can be arranged in line arrangement or
stagger arrangement. Water walls get the heat from radiation and approximately absorb
the 50% of the total heat produced in the furnace.
Super heater
If the temperature of the steam is above its saturation temperature then it is called
superheated steam. The super heater (heat exchanger) is used to increase the
temperature of the steam. These are bundles of high strength tube which can bear
temperature 600C Depending upon the material of tubes Mostly SA-213 is used. In most
industrial water tube boilers the superheater is placed where flue gases make their turn
from the radiant to the convection section of the boiler. There are three types of super
heaters convection, radiant and conv-radient super heaters.

Economizer
Economizer (heat exchanger) is the boiler accessory used to recover the heat of flue
gas that leaving the boiler by heating feed water. The efficiency of a boiler can be
increased with an economizer. 60C rise in feed water temperature with the help of
economizer can save up to 1% of fuel. Typically economiser is used before the air
heater in flue gas path of athe boiler to increase the boiler efficiency.
Air Heater
Air supplied to a boiler for combustion is pre heated with the help of air heater by
recovering the heat of waste flue gas that leaves the economizer. 20C rise in
temperature of combustion air can save up to 1% of total fuel. Preheated air is also
required for the operation of pulverized coal furnaces. Primary air is needed for drying
coal in the pulverizer.
Air heaters are classified into two main types recuperative Air heaters and regenerative
Air heaters.
Boiler Fans
For combustion of fuel in the boiler furnace air is drawn from the atmosphere and
pushed through the ducts with forced draught fan to furnace where air reacts with fuel
and become flue gas, the flue gas is then extracted from the furnace with the help of
Induced draught fan. The fan used in large water tube boilers are FD fans, ID fans,
Primary air fans, Secondary air fans and Gas recirculation fans.
Other main water tube boiler parts are burning equipment burners and furnace and gas
cleaning devices like ESP Cyclone Separators and bag filters