How do you calculate Boiler HP dosing system capacity ???

Phosphate treatment is preferred in drum type boilers. In this Phosphate especially Tri sodium phosphate is added at the boiler drum which is operating at high pressure (so called HP dosing system) through HP dosing system.

If it is dosed at feed pipe, it may react with impurities and sludge may be deposited at the feed line.

Tri sodium Phosphate (TSP) is generally used for HP dosing. This is directly dosed into steam drum to remove carbonate & bicarbonate scale sludge & to increase pH. Phosphate reacts with calcium and magnesium forms less sticky, loose and non-adherent sludge instead of hard scale. This sludge remains in the boiler water in suspended condition and then, removed from the boiler through blow down Other than TSP, disodium phosphate (Na2HPO4) and monosodium phosphate (NaH2PO4) are used in boiler for dosing.

Questions & Answers on AFBC Boilers

CONSIDERATIONS & ASSUMPTIONS FOR SYSTEM DESIGN:

1. Boiler capacity, operating pressure & temperatures be 100 TPH, 110 kg/cm2 & 530 deg C respectively

2-Residual Phosphate in drum water be 8ppm (Acceptable range 5 to 10ppm)

3-Residual Hydrazine in feed water collected at BFP outlet be 0.02ppm

4-Make up water requirement is 5%

5-Boiler maximum possible TDS be 50ppm

7-Feed water TDS be 5ppm

Let us calculate phosphate consumption

Percentage of blow down =Make up % X Feed TDS / (Boiler TDS-Feed TDS)

                                                = (5/100) X 5 X 100 / (50-5) =0.55%

Water loss in blow down = 100 X 0.55/100 =0.55 TPH =550 kg/hr

Loss of phosphate in blow down water = (Residual phosphate X Blow down) / 1000

                                                                       =8 X 550 / 1000 =4.4 Gm/hr

Consumption of TSP in Gm/hr = Residual phosphate X Phosphate loss = 8 X 4.4 =35.2 gm/hr

Consumption of TSP in a day in kg = 35.2 X 24 /1000 =0.85 kg/day

That is we need to dose 0.85 kg of TSP in a day to maintain phosphate level & pH in drum water

So phosphate dosing in mg/liter or ppm = 0.85 X 10⁶ / (100*1000 X 24) =0.35

So per hour we have to dose (0.35 X 100 X 1000) / 1000 =35 gram of TSP into steam drum.

Pump selection

Considerations:

1-Pump type is positive displacement plunger type

2-Discharge pressure = Drum operating pressure X 110% =115 X 110% = 126.5 kg/cm2, select 130 kg/cm2

3-Pump will be operated at 50% (Can operate up to 75%) stroke to avoid frequent maintenance issues.

4-TSP solution is prepared twice in a day (0.88 / 2 =0.44 kg / half day)

Now we have to choose a pump for dosing TSP

There for Total solution of TSP to be dosed in Boiler water at 15 LPH @ 50% stroke (Standard dosing required is 12-15 LPH for 100 TPH boiler)

So pump rated capacity = 15 X 2 = 30 LPH (At 100% stroke)

Note:

Selection of pump stroke depends on pump capacity & maintenance reliability. So better to operate the pump at 50% stroke

Tank selection:

Select a cylindrical tank

Capacity of the tank = Maximum operating stroke X 24 =30 X 24 =720 liters

Optimum size of the tank = Operating stroke X 24 hours X 115% =15 X 24 X 120% = 432 liters

Select tank capacity = 450 litres

Note:

Pump can be operated at higher stroke by reducing the concentration of TSP in tank

Pump can be operated even at less stroke (<50%) if TSP concentration in tank is increased

But standard practice is to dose chemical at higher flow rate with less chemical concentration to get good result.

10-Tips to reduce LOI in Boilers

Boiler Feed Pumps Design factors & Pump Capacity calculation

22-Most likely Question Answer Guide for Boilers troubleshooting

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1.What are the emergencies that are expected to happen in boiler operation?

Emergencies that can happen in boilers operation are:

• Boilers tubes leakage
• Failure of feed water control station
• Unbalanced draft in furnace
• Furnace explosion
• Blow down valve failure
• Feed water pumps failure
• Secondary combustion in super heaters and ESP

Also read 16-Perfect reasons for increasing the fuel consumption of Boilers

2.List out the potential reasons for boiler tubes failure.

Reasons for boiler tubes failure:

• Failure due to overheating
• Failure due to internal scales
• Failure due to aging
• Failure due to uneven expansion
• Failure due to ash and flue gas erosion
• Failure due to material defects
• Failure due to internal and external tube corrosion
• Improper or effected circulation due to opening/passing of low point drains

3.List down the sequential action taken after boiler tube leakage to restore the boiler.

Following activities are done to restore the boiler:

• After tube leakage, Shutdown the boiler as soon as possible
• Allow to cool the boiler continuously by circulating feed water
• If necessary start fast cooling by keeping ‘ON’ ID and FD fans
• After cooling the boiler, identify the leakage tube/coil
• Drain the water up to the leakage point
• Find out the root cause of leakage
• If leakage is minor like pin hole, crack then weld the tube (follow welding procedure)
• If leakage is major like fish mouth damage, then replace the damaged part
• Allow to cool the joint for two hours
• Fill up the boiler and carryout hydraulic test at 1.25% of maximum operating pressure
• After ensuring the joint is free from leakage, drain the boiler up to normal drum level
• Start the boiler as per start up procedure

It's all about HP heaters

4.What are the reasons for boiler drum level fluctuation?

Possible reasons for drum level fluctuations are:

• Sudden loading and unloadingof Boiler
• Fluctuation in steam pressure
• Safety valve sudden pop up
• Priming and foaming
• Failure of drum level controller
• Leakage or passing of blow down valves
• Uncontrolled firing rate
• Leakages in tubes or coils

IBR acts, regulations & forms used

5.How do you handle a boiler, if steam drum safety valve lifts before lifting of super heater line safety valve?

Drum safety valve lifting may cause failure of super heater coils due to less steam flow to coils. In such cases maintain sufficient steam flow to super heater coils by maintaining drum level normal by allowing more feed water flow into drum. And reduce the firing rate and drum pressure for re-seating drum safety valves quickly.

Procedure for Boiler Gauge glass line up

6.What are the possible reasons for furnace draft fluctuations?

Following are the some reasons for furnace draught fluctuation:

• Unbalanced draft
• Fuel moisture variation
• Sudden load change
• Failure of ID, FD fan
• Flue gas path choke up generally in APH
• Air and flue gas leakages
• Loose or improperly fitted fans guide vanes.

7.What action you will take if boiler blow down valve fails?

Following immediate actions shall be taken during boiler blow down valve failure:

• Allow more feed water flow and maintain drum level normal.
• Reduce the boiler load and check the quantity of water flowing through the blow down line.
• If the water loss is less and acceptable run the boiler till upcoming planned shutdown, If it is more than acceptable, shutdown the boiler and get done the repair work of valve.

Why do the Boilers explode???

8.What are the main causes for boilers explosion?

Major causes of boilers explosion:

• Unbalanced draft i.e. high positive pressure
• Secondary combustion
• Bypassing the boiler protections and interlocks
• Overheat
• Scale and corrosion of pressure parts

9.What are the causes of higher flue gas outlet temperature of boiler? And how do you troubleshoot?

• Improper Heat Transfer in Economizer, APH: Clean Economiser coils and APH tubes regularly, maintain flue gas temperature as per manufacturer guide.
• Accumulation of Soot on Super Heater Coils, Economizer: Operate soot blowers regularly.
• Burning of Higher Moisture Content Fuel: Limit the moisture content of fuel.
• Higher Economizer Feed Water Inlet Temperature: Reduce steam flow to HP/LP heaters.
• Excess Heating Surface of Boiler: Modify the boiler pressure parts design in consultation with manufacturers and designers.

10.What are the problems you do come across during sudden load cutoff on boiler? How do you troubleshoot?

During sudden load cut off following abnormal operation may take place:

• Drum Level Low: Increase feed water flow proportionally.
• Safety Valve Lift: Drum level starts to rise, so control feed water flow. In case of drum safety valve lift there may be chances of failure of super heater coils due to less steam circulation (starvation of coils), so utmost care must be taken to maintain drum water level.
• Furnace Draft May Become Unbalance: Maintain furnace draft by controlling ID and FD fan speed manually.
• Higher Attemperation: Reduce firing rate and attemperator water flow.

Questions & Answers on AFBC Boilers

11.List down the causes and remedies of higher fuel consumption of boiler.

More fuel combustion is manly is due to:

• Incomplete Combustion: Achieve proper and complete combustion, limit excess air and fuel moisture. Maintain 3Ts of combustion.
• Low Economizer Inlet Feed Water Temperature: Increase bleed steam to HP and LP heaters.
• Formation of Soot on Heat Transfer Surfaces: Operate soot blowers regularly at rated pressure and temperature.

12.Why does the boiler backfire?

• Back firing of boiler is due to:
• Unbalanced draught
• Reduced speed of ID fan or more speed of FD fan
• Improper locking of fans damper (which may disturb in fan running due to air pressure)
• Sudden trip of ID fan or closing of damper
• High moisture fuel
• More excess air
• Jamming of APH

Reasons for Boiler back fire

13.List out the reasons for more unburnt in ash.

Excess unburnt is due to:

• Incomplete combustion
• Insufficient combustion air
• More moisture content fuel
• Not meeting 3T’s of combustion
• Not working of cinder reinjection system
• Lower GCV fuel
• High ash content in fuel
• Low over fire air

14.What are the trouble shoots during boiler start up?

• Boiler Flue Gas Ducts Explosion: Before light up purge the boiler properly.
• Trouble to Achieve Furnace Temperature: Maintain balanced draught (slightly positive).
• Fluctuation in Drum Level: Maintain drum level by taking feed water control valve in manual. Do not allow the drum level to go high (>90%) or less (<20%).
• Tripping of ID Fan or FD fans Due to Draught Fluctuation: Maintain furnace draft in manual mode until boiler load reaches more than 30% of MCR.
• High/Low Steam Temperature: Maintain attempteration in manual and keep open attemperator isolation valves.
• Flue gas condensation in APH: By pass the APH for FD air flow to avoid flue gas condensation.

15.How do you troubleshoot the formation of clinker?

Clinker formation can be avoided by following ways:

• Using low moisture coal
• Maintaining proper air fuel mixture
• Achieving proper combustion
• Using coal with no foreign materials
• Avoiding frequent startups and stops
• Use coal with high ash fusion temperature

17.How do you reduce the auxiliary power consumption of Boiler?

• Maintain proper air fuel mixture so that speed of ID, FD fans and fuel feeding system can be optimized.
• Avoid using high moisture coal as it requires more air for combustion.
• Use VFDs to fuel feeding system.
• If boiler is running on load less than <30% shutoff the stand by fans.
• Tune the feed water control valve for maximum opening to reduce the speed of Boiler feed pump.

Viva Questions & answers for preparation of BOE exam & interview

18.What are the possible reasons for higher main steam temperature?

• Failure of Attemperator control valve
• More boiler load
• More heat transfer in super heater zone due to more velocity of flue gas
• More firing rate
• Low Economiser inlet feed water temperature
• High moisture fuel
• More excess air
• Low drum level
• Leakage of super heater coil

Boiler calculations for Boiler operation engineer (BOE) exam

19.What are the effects of high and low drum levels?

High drum level:

• Results into carryover of water into super heater coils which may damage coils due to high thermal shock.
• Can damage safety valve seats.
• Disturbs circulation ratio.
• Carry over with sharp fall in super heater temperature.
• Water may enter turbine and serious damages to turbine may occur
• Super Heater deposit and fouling.
• Turbine Deposit and increase in specific steam consumption.

Low drum level may results less steam flow to super heater coils. 

20.How do you control boiler corrosion rate?

• Maintain pH of water in the desired range to control corrosion.
• O2 in feed water should be controlled within 0.007 ppm, primarily by good deaeration, supplemented by Oxygen scavenger dozing.
• Condenser leak should be immediately detected and acted upon.
• DM water storage tank should be properly sealed.
• Condensers and heaters should be air-tight.
• Proper wet lay-up of units during shut downs.

21.How do you identify the leakage of economizer tube? What is its effect?

 An economizer tube leakage can be detected by sound or increased make up water requirements. The leak should be investigated at the earliest possible time and the unit to be shut down in a normal manner. Water leaks in the economizer can cause considerable erosion damage to adjacent tubes. Water carried over from an economizer tube leak may lead to cold end corrosion also. 

22.What are the reasons for lifting of steam drum safety valve and how do you bring the boiler to normal operation?

Drum safety valve may lift due to following reasons:

Excessive fuel firing rate.

Sudden decrease in steam load.

Faulty setting of safety valve.

The following actions have to be taken to bring boiler into normal operation:

Reduce fuel-firing rate.

Open the start up vent valve.

Observe the steam flow of the super heater. If there is no flow, trip the boiler.

Maintain the steam drum level.

Questions & Answers on Boiler safety valves

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Questions answers & calculation on Boiler draught & Chimney

1-What do you mean by the term draught?

It is the pressure difference, which causes flow of gases to take places.

2-What is the function of draught in Boiler?

• To supply the combustion air required for proper combustion
• To evacuate the combustion products from the furnace or combustion chamber
• To evacuate the combustion products to the atmosphere
• So draught is the pressure difference between the inside the boiler and outside air. Inside the boiler pressure is due to combustion products (Flue gas) & outside pressure is due to fresh atmospheric air.

3-What are the different types of draughts used in power plant boilers?

• There are mainly two types of draughts.
• Natural draught & artificial draught
• Natural draught is generally obtained by Chimney
• Artificial draughts have main two types
• Steam jet: It is again classified into Induced & forced draught
• Mechanical draught: This is classified into Induced draught, forced draught & Natural draught

Protections & Interlocks in power plants

4-What do you mean by the Natural draught?

Natural draft is obtained by the use of Chimney. It produces the draft where the air and gas are forced through the fuel bed, furnace, and boiler passes.

5-How do you produce the mechanical induce draft?

It is produced by Induced draft fan

6-How do you produce the forced mechanical draft?

It is produced by Forced draught fan & secondary air fans.

7-How do you produce the balanced mechanical draft in Boilers?

Balanced draught is produced by ID & fans.

8-What are the advantages of mechanical draught?

Advantages:

• Reduced chimney height
• Easy control of combustion & evaporation
• Improvement in the efficiency
• Prevents smoke
• Can consume low grade fuel
• The fuel consumption per HP of mechanical draught boilers is 15% less than that of artificial boiler
• Grate required for fuel burning is less

Boiler IBR forms, acts & Regulations

9-What do you mean by static draught?

The difference of pressure causing the flow of gases is known as static draught. Its value is very small

10-What do you mean by Static pressure?

It is the potential energy put into the system by the fan. It is given up to the friction in the ducts and at the duct inlet as it is converted to velocity pressure.

11-What do you mean by velocity pressure?

Why do the Boilers explode???

It is the pressure measured along the line of the flow that results from the air flowing through the duct.

12-What do you mean by total pressure?

It is the sum of static pressure & velocity pressure

13-How do you calculate the air flow in the duct?

Volumetric air flow Q m3/sec = Duct area in M² X Velocity in m/sec

14-How much draft is maintained in the boiler furnace zone?

It is -3 to + 3 MMWC for proper combustion

15-What is the FD air draught loss in Air Pre heater?

It is 50 to 60 MMWC

16-What is FD air draught loss in SCAPH (Steam coil air pre heater)?

It is 20 to 30 MMWC

17-What is the draught loss in FD air in flow element?

It is around 10 to 20 MMWC

18-What is the draught loss of flue gas in Economizers?

Around 70-90 MMWC

19-What is the draught loss of flue gas in APH?

Around 50-60 MMWC

20-What is the draught loss of flue gas in ESP?

It is 20 to 30 MMWC

21-What is the draught measured at ID fan outlet?

Draught measured at 2 to 4 meter from ID fan discharge nozzle is up to +5 MMWC & as you go nearer to Chimney the draught goes into negative

22-How do you calculate the differential pressure of a ID fan?

Differential pressure of a ID fan =Pressure (draught) at suction side – Pressure at the discharge side

23-How do you calculate the differential pressure of a FD fan?

Differential pressure of a FD fan =Pressure (draught) at discharge side + Pressure at the suction side

24-What are the reasons for more positive draught in Boiler furnace?

It is due to:

• More FD air flow
• Sudden load drop on Boiler
• Improper spreading of the fuel
• Variation of fuel moisture

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25-What are the reasons for more negative draught in Boiler furnace?

It is due to:

• Higher ID fan speed & lower FD fan speed
• Low load on Boiler

26-Why the FD fan size and power rating is less than ID fan?

Because FD fan is used to handle cold air & ID fan for hot gases (FD air + Fuel). Hot gases have lower density than cold air, hence size & capacity of the to expel the gases is more.

27-What are the reasons for draught losses?

• Frictional resistance offered by the flues and gas passages to the flow of the flue gases
• Loss near the bends in the gas flow circuit
• Loss due to friction head in equipments like grate, economizer, super heater etc
• Loss due to imparting velocity to the flue gases

28-Why do prefer artificial draught over natural draught?

• To reduce chimney height
• To get more draught
• To reduce fuel consumption

29-What do you mean by the term Chimney?

Chimney is a vertical tubular structure built either of masonry, concrete or steel. Chimneys found application in power plants, textile plants, steel industries, Diesel plant etc. For smaller boilers up to 50 TPH steel chimneys are used and the boilers of capacity more than 50 TPH concrete chimneys are used

30-What are the functions of Chimney?

• To vent the flue gases produced in the Boiler
• To produce draught
• To reduce the load on Boiler ID fans
• To protect environment by venting the flue gas at suitable height

31-Explain the construction of concrete Chimney?

Chimney is normally cylindrical in construction. It is exposed to hot gases inside & cold air at outside. Inside of the chimney is lined with refractory bricks to protect concrete part from high temperature & erosion by flue gas. Chimney is specified based on its inside, outside diameters & height.

32-How does the draught produce in Chimney?

Draught in the chimney is due to density difference between the columns of the hot flue gases in chimney & cold air outside.

33-How do you specify the Chimney?

Chimney is specified based on its Outer diameter, inside diameter and height.

34-How do you calculate the draught produced in Chimney (Hw)?

Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)

H = Chimney height in meters

Ta = Atmospheric temperature in K

Tg = Flue gas temperature in K

Ma = Mass of air & Mass of flue gas = Ma+1

35-How do you calculate the velocity of the flue gas in the Chimney?

Velocity of flue gas V = √(2XgXH1)

H1 = Head in terms of gas column

Calculated as H1 = H ((Ma/(Ma+1)) X Tg/Ta)-1)

36-How do you calculate the mass of flue gas flowing through the Chimney?

Mg (kg/sec)= Density of gas (kg/m3) X Area of Chimney (m2) X Velocity of flue gas in Chimney (m/sec)

37-In which type of draught flue gas temperature leaving the Boiler is higher?

Flue gas temperature in Natural draught chimney is more as compared to artificial draught

38-Calculate the height of the Chimney required to produce a draught equivalent to 20 mmwc if the temperature of the FG  is 180 deg c and ambient temperature is 25 Deg C.And air required for complete combustion per kg of fuel is 7 kg

We have formula,

Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)

Ta = 273 + 25 = 298 K

Tg = 273+180 = 453 K

Ma = 7 kg

Ma+1 = 7 + 1 =8 Kg

Hw = 20 mmwc

20 = 353 X H (1/298 – 1/453 (8/7))

H = 43.17 meter

Factors considered for Boilers Engineering & Design

39-Calculate the mass of flue gas flowing through the Chimney when the draught produced is equal to 18 mmwc. Temperature of flue gas is 250 Deg C and ambient temperature is equal to 30 deg c. The flue gas formed per kg of fuel burnt are 15 kg.Take diameter of chimney 2 meter

Given

Hw = 18mmwc

Tg = 250 + 273 = 523 K

Ta = 30 + 273 = 303 K

Mass of flue gas Ma + 1 =15 kg

Chimney diameter D= 2 meters

Let us calculate Chimney height H

Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)

18 = 353 X H ((1/303 – 1/523 (15/14))

H = 34.28 m

We have H1 = H ((Ma/(Ma+1)) X Tg/Ta)-1)

H1 = 353 X 34.28 (((14/15) X (523/303))-1)

H1 = 20.94 meter

Velocity of flue gas inside the chimney V = = √(2XgXH1) = √(2x9.81x20.94) = 20.2 m/sec

Mass of dry flue gas Mg = A X V X Density of gas

Density of gas at temperature 250 deg c = 273 X1.293 / 523  = 0.67 kg/m3

Area of chimney A = 3.142 X 2²/4 = 3.142 M²

Mg = 3.142 X 20.2 X 0.67 = 42.52 kg/sec

39-Calculate the draught produced in Chimney of height 50 m, if density of the flue gas is 0.8 kg/m3 and atmospheric temperature is 25 deg c.

Density of atmospheric air = 273 X 1.293 / (273+25) = 1.18 kg/m3

Draught produced in Chimney is given by

Hw = H (Density of cold air-Density of flue gas) = 50 X (1.18-0.8) = 19 mmwc

40-How do you increase the draught produced in Chimney?

• By increasing the height of Chimney
• By increasing the flue gas temperature

41-How do you calculate the minimum height required for Chimney?

Minimum height of the chimney is calculated based on Sulphur dioxide emission.

H = 14 X Q^0.3

Q = Sulphur emission rate in kg/hr

42-How do you calculate the draught pressure for maximum discharge?

It is given by P = 176.5 X H / Ta

Hw = Chimney height in meters

Ta = Absolute atmospheric temperature in Kelvin

Hw = Draught in mmwc

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25 most frequently asked viva questions & answers on Boiler safety valves

Boiler safety valves question & answers:

1-Which type of safety valves are used in modern high pressure Boilers?

Open yoke, spring loaded, Semi Nozzle & full lift type

2-What is the over pressure range for safety valves?

It is around 6 to 7% of operating pressure of particular line/vessel

3-What is the recommended % of blow down for Boiler safety valves?

It is around 2 to 3%

4-How do you calculate safety valves blow down?

Blow down = SV (Set pressure – Reseat pressure) X 100 / Set pressure

5-What is the MOC of safety valve disc & spindle?

Disc: Stain less steel with 13% chromium & Spindle: SS 316

6-What happens if drum safety valve set at lower pressure than super heater safety valves?

Upon blowing of drum safety valve first, all the steam formed will blows out through the drum safety valves causing less or no steam flow to the super heater coils. This situation will lead to the failure of super heater coils.

7-When should be gags for safety valves applied & removed?

Safety valve gags should be applied & removed at 80-85% of the pressure of the low set valve, even at safety valve setting & during hydro. Test.

8-Why safety valve gags should not be applied in cold condition?

Spindle of the safety valves expands considerably with increase in temperature as pressure is raised. Due to constraint to the expansion may cause spindle bend & also cause damage to the seating surface.

9-How do you adjust the set pressure of a safety valve?

Safety valve is set by adjusting the compression of spring

10-How do you adjust the blow down of a safety valve?

By turning up/down of upper ring.

To reduce the blow down: Upper ring is taken down & to increase the blow down upper wring is turned up.

11-How do you reduce the simmering sound of safety valve?

By adjusting the lower ring

12-What will happen if a safety valve has higher blow down than required?

Higher blow down of safety valve causes

1-Fluctuation steam demand & drum level

2-Increase in super heater steam temperature

3-Sudden & fast dropping of boiler pressure

4-Insufficient steam flow to super heater coils (In case of drum safety valve)

5-Loss of high temperature steam eventually leads loss of fuel

Basic concepts of Boiler safety valves

13-What will happen if a safety valve has lower blow down than required?

Lower blow down setting of safety valve leads to the pressurizing of boiler eventually damages to the pressure parts.

14-What are the potential reasons for failure/damage of safety valves?

Potential reasons for safety valve failure are:

• Crossing the quantity of steam to be discharged
• Quality of the steam is not up to the mark
• Higher back pressure
• Stress in discharge piping
• Improper gagging & bolting
• Foreign materials in steam
• Damages during disassembly & assembly in field

15-Why drum safety valves are set first instead of super heater safety valve?

Possibilities of valve part damage because of “girl blasting” are greater on the super heated valves in contrast to the drum valves. If a super heater valve is gagged after seat damage while testing of other valves continues, the total valve damage will most likely be increased.

And also steam temperature is more while testing the drum safety valves; hence more temperature for setting super heater valve will be available producing accurate results.

16-Why the drum level should be maintained lower in the range of 30-40% during SV setting?

Higher drum level will cause stugging of valves with water causing long blow down & damages to the disc & seat.

17-What is the pressure rising rate during safety valve setting?

Usually 2 to 3 kg/cm2 per minute. Slow rising may cause simmering & valve to lift very erratically

18-What precautions shall be taken for lifting the safety valves several times?

A cooling off period around 25-30 minutes should be given between every lift to get accurate results & to avoid damages to the valve

19-What are the reasons for simmering in safety valves?

Know about valves

• Improperly set lower ring
• Holding the boiler pressure at or nearer to the set pressure of valve
• Maintaining higher drum level during lifting of SV

Boiler Safety Valves overhauling steps

20-Why it is necessary to drop the pressure of boiler after setting a safety valve before removing gags?

Boiler pressure should be dropped 20% of valve set pressure to avoid valve lifting prematurely

21-Why it is been recommended that SH safety valve to be set at higher blow down rate if the steam temperature available is less?

Yes, as the steam temperature increase SH safety valve’s blow down decreases, hence at lower temperature it is always set at higher blow down.

22-What are the criteria for selecting the safety valves?

• Super heater safety valve should have blowing (relieving capacity) 30-35% Boiler MCR

• Drum safety valves (Including 1 &2) should have relieving capacity 90 to 95% of Boiler MCR

• All safety valves (SH & Drum) should have relieving capacity 125 to 130% of Boiler MCR

• Solenoid operated electromatic safety valves are generally deigned to relieve steam 12-15% of Boiler MCR.

23-On which side of steam drum higher pressure set safety valve is to be fitted?

It depends on the inclination of the drum & CBD line layout. Actually HP boiler drums are made inclined at an angle 2-3 for enabling collection of internal treated sludge. So in order to not to lift that side SV , that SV is set at higher pressure

24-What is meant by back pressure in safety valves? & how does it affect SV performance?

Back pressure is the static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system.

If pressure in the valve outlet varies while the valve is closed, the valve set pressure will change. If back pressure varies while the valve is open and flowing, valve lift and flow rate through the valve can be affected.

25-What is meant by accumulating test pressure?

The accumulation test is done on boilers to limit the excessive pressure rising while the safety valve is in open. The test is carried on new boilers or new safety valves with full firing condition with MSSV and feed water valves closed. It is conducted as long as water in drum permits generally 7 minutes for water tube boilers.

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