Why & How these in Boilers??

1-How do you decide Right hand side & Left hand side of a Boiler?

Boiler front is decided based on boiler outlet duct.

If you stand by facing towards boiler out let duct, then the most front part of you is deemed as Boiler front , next to that is boiler rear & LHS & RHS side of you is deemed as Boiler left hand side & right hand side.

2-Why in Some HP boilers drums are aligned at an angle 2 to 4⁰ towards right or left?

Drum slanting side depends on the connection of CBD line. If CBD line is connected at LHS side of the drum then the drum is slanted towards LHS side that is it is made down by an angle 2 to 4^0. This is because to avoid the blow down of excess water throughout the drum length & for such drums CBD line is extended for only for short distance only .And also it is been ensured that all the sludge will collect at slant position only.

Why do the Boilers Explode???

3-How it is been decided that RHS or LHS safety valve of steam drum is set at higher pressure?

It is decided based on the slanting angle of steam, drums & CBD line connection.

If drum is aligned horizontally then you can set any side of the Safety valve at higher pressure as there is uniform spreading of sludge in drum.

Whereas if drum is slanted (made down)towards right or left from where CBD line is connected then it is needed to set that side safety valve at higher pressure to avoid carryover of sludge into the safety valve if t blows first. Such sludge will deposit on safety valves disc & seat which again leads into leakages & wrong operation related issues. So safety valve of such location is always set at higher pressure.

4-Why thickness of steam drum dish end is somewhat lesser than other area

Because dish end has spherical shape, so there develop hoop or circumferential stresses & on the other part of the drum longitudinal stress.

 For Hoop stresses

σc =  Pd/2tη

Thickness t = Pd/(2 ησ)

& For longitudinal stresses σl = Pd/4tη

Thickness t = Pd/(4 ησ)

Where P = Pressure acting & d is internal diameter of the drum

Based on above relations thickness for spherical part of the drum that is dish end, the thickness is lesser than other part of the drum.

Boilers troubleshooting guide

5-Why the start up vent is used in Boilers?

Start up vent is used for

• To provide minimum steam flow from the boiler during start up, shutdown & sudden load cut off
• It is used to manual relieve of excess pressure
• Used to give excess flow for temperature rising during start up or partial loads
• Used to take excess load on boilers during peak load test

6-Why the super heater safety valve is set at lower pressure than drum safety valve

If drum safety valves set at lower pressure, then there will be very less or no steam flow to super heaters.

In order to save super heater coils from starvation due to no flow of steam during steam blow from drum safety valves, the super heater safety valves are always set at lower pressure than drum safety.

7-Why LHS/RHS water wall panels expand more (towards down) than front & rear water wall panels?

Side water panels are usually straight hence expansion readings show more value  where as front & rear water wall panel will have bends.

8-Why pressure gauges fitted at boiler firing floors show more pressure than actual (that of fitted at steam drum EL.level)

Pressure gauges show 2 to 3 kg/cm2 higher pressure due to addition of hydraulic head in PG impulse line laid from actual location to firing floor

9-Why there is a pressure difference between main steam line pressure & drum pressure?

Main steam line pressure shows lower pressure than drum pressure due to pressure loss in super heater coils. And this pressure difference increases as the number of super heater coils & steam flow increase.

10-Tips to reduce LOIs in Boilers

10-Why do high pressure Boilers have higher efficiency & lower fuel consumption?

Because:

1-High pressure boilers have higher  saturation temperature

2-High pressure boilers have higher feed water temperature at economiser inlet

3-High pressure boiler have lower enthalpy of evaporation (latent heat)will be less

H = Hf + Hfg + Cps x (Tsup-Tsat) -

Where,

Hf = Enthalpy of liquid at operating pressure

Hfg = Latent heat

Tsup = Suepr heated steam temperature

Tsat = Saturated temperature of steam

Example:What amount of heat would be required to produce 5000 kg of steam at a pressure of 65 kg/cm2 and temperature 485 °C from water at temperature 175 °C?

Steam pressure P = 65 kg/cm2

Steam temperature Tsup = 485 °C

At above parameters, saturated temperature Ts = 282.7 °C

hf = 298.82 kcal/kg, hfg = 364.47 kcal/kg

Now, enthalpy of 1 kg of superheated steam

Hsup= hf + hfg + Cps (Tsup - Ts)

hsup = 298.82 + 364.47 + 0.5 X (485 - 282.7)

hsup = 764.44 kcal/kg

Amount of heat already associated with 1 kg of water = 1 X 1 X (175 – 0) X 175 kcal/kg

Therefore net heat to be supplied per kg is 764.44 – 175 = 589.44 kcal/kg

Procedure for Boiler gauge glass line up

11-Why there is more CO in flue gas?

More CO in flue gas is due to improper combustion, that is due to

• Less excess air
• In adequate turbulent
• Lower furnace/bed temperature
• Higher FC in fuel

IBR acts, regulations & IBR forms

12-Why Does boiler furnace pressure fluctuate?

• Interrupted fuel flow
• Leakage in boiler pressure parts
• Furnace combustion controller not working properly
• Malfunctioning of fans pneumatic dampers
• Higher moisture in fuel

13-Why there is more NOX in flue gas?

• Higher NOx is due to
• Higher bed temperature or furnace temperature
• Higher excess air
• More N2 in fuel

14-Why there is no Temperature gauge (TG) is fitted on steam drum of any Boiler?
In steam dream the phase of water is at saturated state, so no any necessary of providing TG. However temperature gauges are provided at drum inlet feed water line & drum outlet saturated line.

In some drums whose thickness is > 100 mm, there you may find thermo couples for measuring skit temperature. So in order to avoid  weakening of steam drums due to making number of drill holes for unnecessary instruments, the TG is not generally provided for drums.

For example if  drum PG showing  pressure 110 kg/cm2, then its temperature will be around 320 deg c. (Refer steam table for saturation temperature). And generally not used in any calculation or performance analysis, if required one can refer steam tables for saturated water

The temperature gauge or thermo couple provided at the drum outlet lines is used during plant start up.

It's all about HP heaters

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

Why does Boilers main steam temperature increases more than design?

Why thermal expansion is necessary in Boilers

Understanding the term expansion & contraction

                                

When a body is heated it will expand & when it cooled it contracts. So body expands & deforms when heated & cooled. Change in temperature of a free body causes body to expand & contract without inducing stress. When the deformation of the body is restricted by means of any external force, there will be huge chances of stress induction. Such induced stresses are called temperature stresses. These may be tensile or compressive in nature.

Boiler calculations for Boiler operation engineer (BOE) exam

Viva Questions & answers for preparation of BOE exam & interview

Here L = Original length of the steel bar

∆t = Change in metal temperature deg C

ɑ = Coefficient of thermal expansion

Temperature strain e = Free deformation / Original length = ∆L / L

E = L ɑ ∆t / L = ɑ ∆t

Temperature stress σ= Young’s modulus X Strain = E X e

Temperature stress σ = E ɑ ∆t

Stress induced in a rigid or constrained body

P = Force exerted by a rigid support of constraint

We have σ = P / A

P = E ɑ ∆t A

Expansion in Boilers

Boiler is made up of plates, tubes, pipes and simple steel of various grades depending upon the duty conditions. Depending on the service such as cold air/hot air/cold flue gas/hot flue gas/cold water/hot water/saturated steam/super heated steam, thermal expansion movement of steel materials takes place to different extent in Boiler. Ignorance of thermal expansion movement of boiler components in design/installation may lead to failure of boiler components. The damage to boiler components can be costly affecting human life in some cases.

There are two types of expansions in Boilers

Factors considered for Boilers Engineering & Design

Absolute Expansion: Boiler whole mass expands.

Differential Expansion: Individual parts expansion. There are places where there is a relative expansion movement, which can cause stress in those parts.

In boilers expansion pointers are attached to all the pressure parts header to under stand the direction & value of expansion.

Expansion pointers are used for verifying the expansion movement of the boiler. These are attached to the drum ends/bottom or top header ends. When the boiler is under commissioning stage the expansion must be monitored. Depending on the anchor points in X-axis and Y-axis, the expansion is predicted by designers. The same is counter checked at site. Deviations in the form of non-uniform expansion should be checked.

Modern boilers expand towards bottom, during start ups & shutdown it is very important to observe the Boiler expansion.

Expansion of the metals depends on;

• Change in metal temperature

• Length & area of the materials

• Coefficient of expansion of the materials

It's all about HP heaters

Provisions for thermal expansions in boilers:

• Boilers all headers bottom space should be free from obstacles

• For air & flue gas ducts expansion bellows may be of metal or fabric should be provided

• Expansion loops for all steam lines should be provided

• Spring supports & hangers for steam line & hot water lines

• Rocker washers for steam drum & super heater headers supports

• Provision of stay bolts

IBR acts, regulations & forms used

 Precaution to be taken for free expansion:

• After shutdown, ensure all foreign material from Boilers & ductings have been removed completely

• Ensure all temporary supports & platforms have been removed from boilers & ducting

• Ensure there is no welding between any pressure part & hot ducts with platform or other or beams which can restrict the expansion

• Ensure enough space is available for all bottom headers for free expansion

• Ensure steam line drains line are free to expand with steam lines

• Ensure there are no any uneven expansions

Procedure for Boiler Gauge glass line up

Examples:

A square rod of size 20mm X 20 mm in cross section & 2000 mm in length is allowed to expand by fixing both of its ends. Determine the force developed if the rod is heated from 25 degree c to 150 deg C.

L = 2000 mm

A =20 X 20 = 400 mm2

E = 2 X 10⁵ Mpa

ɑ  = 12 X 10^{-6 0}C

∆t = 150-25 = 125 ⁰C

We have             P = σA

P = E ɑ ∆t A

P =2 X 10⁵ X 12 X 10^-6 0C X 125 X 400 = 30000 N
Force developed at the end P = 30 KN

What is the expansion of Boiler side water wall panel , whose total length is 18 meters & metal temperature is 295 deg c. Consider atmosphere temperature 30 deg c &  ɑ  = 12 X 10^{-6 0}C

Expansion of side water wall panel ∆L =L ɑ ∆t = 18 X = 12 X 10^-6  X (295-30) =0.068mm = 68mm

10-Tips to reduce LOI in Boilers

Restricted expansion in Boilers or steam lines will lead to..

• Damages to the restricted part

• Damage to the refractory & sealing

• Leakages

• Secondary failure

• Tube/pipe puncture

• Explosions

Why do the Boilers explode???

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

Available @ Flipcart/Amazon/Notion press

Boiler Feed Pumps Design factors & Pump Capacity calculation

Power plant Safety QnA

Why does Boiler back fire?

Why  does boiler back fire?

Following potential situations may cause back fire in biomass or bagasse fired boilers

Protections & Interlocks in power plants

1-Sudden trip of ID fans or closing of fans suction or discharge dampers:

If there is no interlock on ID fans trip to FD fans, a very huge back fire occurs in boiler which has potential to cause damages to the Boiler & operators.

Sudden closure of ID fans suction or discharge dampers will cause same back fire as above.

2-Malfunctioning of pneumatic dampers positioner may cause closure of ID fans dampers resulting into huge back fire in Boiler

3-APH tubes jam : Area required  for the flue gas flow is equals to Boiler capacity/9. Due to accumulation of ash in tubes, the flue gas flow area gets reduce. This situation causes backfire

This situation occurs if Boiler is run at lower load for long time without bypassing APH for FD/SA air, as at lower load flue gas temperature will be less, which get condense in APH tubes.

4-Higher moisture in the fuel: If the fuel has higher moisture that required, then there will be more requirement of combustion air, while maintaining this air fuel mixture back fire may occur.

5-Frequent variation in fuel moisture : This is experienced in sugar plants due to variation in bagasse moisture at mill outlet due to variation in imbibition water added to extract juice from cane.

Upon continuous variation of fuel moisture it become very difficult for operator to maintain balanced draught in furnace.

6-Improper spreading of fuel : Due to this furnace may experience bed heap combustion of fuel, due to this there will not be uniform distribution of bottom combustion air causing boiler back fire.

IBR acts, regulations & forms used

7-Sudden drop of boiler load: Due to this boiler air fuel ratio & draught system gets disturbed causing boiler back fire.

8-Fouling in Super heater coils : Higher moisture in fuel cause accumulation of bagasse & unburnt on super heater coils, which blocks the flue gas path causing boiler back fire

9-Accumulation & deposition of ash in ESP fields: This situation causes more resistance to flow of flue gas.

10-Improper air fuel mixture during start up & shutdowns may cause boiler back fire

11-Air leakage into & out of the system : Due to the air ingress & flue gas leakages, it becomes very  difficult for operator to maintain balanced draught.

12.Leakage or puncture of water wall tubes: Leakage or puncture of water wall tubes will result into high furnace pressure

Why do the Boilers explode???

16-Perfect reasons for increasing the fuel consumption of Boilers

Questions & Answers on AFBC Boilers

Basic calculations on fuels & combustion

Fuel:
It is a substance which releases heat energy on combustion. The principal combustible elements of each fuel are carbon and hydrogen.

Fuels classification:

Primary Fuels: These fuels directly available in nature, Ex: Wood, Peat, Lignite coal, Petroleum and Natural gas.
Secondary Fuel: These are prepared fuels, Ex: Coke, Charcoal, Briquettes, Kerosene, fuel oil, petroleum gas, producer gas etc.

Fuels are also classified as Solid fuels, liquid fuel and Gaseous fuel.

Different types of coals: Peat, Lignite, Bituminous coal, Anthracite coal and Coke.

Gaseous fuels:Natural gas, Coal gas, Coke oven gas, Blast furnace gas, producer gas, Water gas and Sewer gas.

Energy producing elements in fuel: Carbon, oxygen, hydrogen and Sulphur.

Formation of Charcoal & Coke:

Charcoal:It is obtained by destructive distillation of wood.

Coke is formed by destructive distillation of certain types of coal.

Calorific value of fuel: Calorific value (GCV) is the amount of heat released on complete combustion of unit quantity of fuel.It is measured in kcal/kg.

GCV is also known as Higher Calorific Value (HCV),it is given by following formula

GCV or HCV = (8084 X C% + 28922 X (H2%–O2%/8) + 2224 X S%)/100…kcal/kg

Where C, H2, O2 and S are percentage of Carbon, hydrogen, oxygen and Sulphur respectively in fuel.

Net Calorific Value (NCV) or Lower Calorific Value (LCV):

The total heat released by fuel during combustion is not completely utilized. Some heat is taken out by water vapour which is produced during combustion of hydrogen. Such heat value taken by considering heat taken away by water vapour is called NCV or LCV.

LCV = HCV – (9 X H2% X 586), Where H2 = Hydrogen% in fuel and 586 is latent heat of steam

in kcal/kg.

Useful heat Value (UHV) of coal:

NCV = GCV - 10.02 X Percentage of total moisture.

Ultimate analysis:Ultimate analysis indicates the various elemental chemical constituents such as Carbon, Hydrogen, Oxygen, Sulphur etc.

Proximate analysis of coal:Proximate analysis determines fixed carbon, Volatile matter, moisture and percentages of ash.

Volatile Matter (VM) & is its significance:

VM is generally a composition of methane, hydrocarbons, hydrogen and carbon monoxide and other incombustible gases like CO2 and Nitrogen. It is the indication of presence of gaseous fuel in the fuel.

Significance:

• Helps in easy ignition of coal by increasing the flame length
•   Sets minimum limit on the furnace height and volume.

Properties of Coal:

·         Caking Index: Indicates binding property of coal.

·         Swelling Index: Indicates caking capacity of coal.

·         Slacking Index: Indicates the stability of coal when exposed to open atmosphere.

·         Grinding Index: It gives the idea of ease of grinding of coal.

·         Abrasive Index: Indicates the hardness of coal.

Combustion:

Combustion is the rapid oxidation of fuel accompanied by the production of heat

Oxygen is the major element on earth, making up to 21% (by volume) of our air. Carbon, hydrogen and Sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and Sulphur dioxide releasing tremendous heat.

Oxygen is the major element on earth, making up to 21% (by volume) of our air. Carbon, hydrogen and Sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and Sulphur dioxide releasing tremendous heat.

Basic requirements of combustion:Fuel, Oxygen and 3T’s

C + O2 = CO2+ Heat 8084 kcal/kg of Carbon.

2C + O2 = 2CO + Heat 2430 kcal/kg of Carbon.

2H2 + O2 = 2H2O 1 Heat 28922 kcal/kg of Hydrogen.

S + O2 = SO2 + Heat 2224 kcal/kg of Sulphur.

Products of Combustion: CO2, CO, O2, SO2 and ash.

Spontaneous combustion: is a phenomenon in which coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal.

Major contents of ash:

• Silica (SiO2)
• Aluminum oxide (AlO3)
• Iron Oxide (Fe2O3)
• Sodium Oxide (Na2O)
• Potassium Oxide (K2O)
• Calcium Oxide (CaO)
• Magnesium Oxide or Magnesia (MgO)

Fly ash & Bottom ash :

Fly ash 70–80%, Bottom ash 20–30%.

Fly ash at Economiser : 7–8%, APH: 10–12% and ESP: 80–82% of total fly ash.

Oxygen & Nitrogen present in the air :By weight Oxygen 23% and Nitrogen 77% and by volume Oxygen 21% and Nitrogen 79%.

Stoichiometric air fuel ratio: A mixture of air and fuel, which contains sufficient amount of oxygen for complete combustion.

Rich mixture: Mixture with deficiency of air

Lean mixture: Mixture with excess air

Power generation phenomenon in steam Turbo generators

Theoretical air of combustion:

Minimum amount of air that supplies the sufficient amount of oxygen for the complete combustion of all carbon, hydrogen and any other elements in the fuel that may oxidize is called theoretical air.

Theoretical air required for combustion of carbon:

We know that Carbon on oxidation with Oxygen forms Carbon dioxide.

C + O2 = CO2

12 + 32 = 44 (Molecular weights of Carbon and Oxygen are 12 and 16 respectively)

1 + 2.67 = 3.67

So, 1 kg of Carbon requires 2.67 Kg of Oxygen for complete combustion into 3.67 kg of carbon dioxide.

Similarly

Hydrogen on oxidation forms Water

2H2 + O2 = 2H2O

4 + 32 = 36 (Molecular weights of Hydrogen and Oxygen are 1 and 16 respectively)

1 + 8 = 9

So, 1 Kg of Hydrogen requires 8 Kg of Oxygen for its combustion & forms into water.

Excess air: Amount of extra air given to ensure complete combustion is called excess air.

3T’s of combustion:

• Temperature: High enough to maintain the ignition of the fuel.

• Turbulence: Is the mixing of fuel and air.

• Time: Sufficient enough for combustion.

Specific heat Cp and Cv:

Specific heat is the amount of heat in kcal needed to raise the temperature of 1 kg of substance by 1 °C. Cp and Cv are the specific heat at constant pressure and constant volume of gas.

Solved examples:

 Example-1:A Biomass (Bagasse) contains 23% of carbon, 22% of oxygen, 3.5% of Hydrogen and 0.05% of Sulphur, then calculate the theoretical air required for its combustion.

We have, Theoretical air requirement = (11.6 X %C + 34.8 3 (%H2 - %O2/8) + 4.35 X %S)/100… 

Kg/kg of fuel.

Therefore, Th air requirement will be = (11.6 X 23 + 34.8 X (3.5 – 22/8) + 4.35 X 0.05))/100

                                                             = 2.9 kg of air/kg of fuel.

Example-2:What is the quantity of excess air, if O2 measured in the Boiler outlet flue gas is 6%?

We have, Excess air = (O2%/( 21 - O2%)) X 100

                                 = (6/(21 – 6)) X 100 = 40%

Example-3:A complete combustion requires 2.9 kg of theoretical and 20% of excess air, then calculate the total air consumed for complete combustion per kg of fuel burnt.

Actual quantity of air supplied = (1 + Excess air %/100) X theoretical air

                                                   = (1 + 20/100) X 2.9 = 3.48 kg of air/kg of fuel.

Example-4: A flue gas has 10% of CO2 and theoretical calculated CO2 is 12%, then calculate percentage of the excess air.

                                         % of Excess air = ((Theoretical CO2%/Actual CO2%) - 1)) X 100

                                                                   = (12/10) - 1) X 100 = 20%

Example-5: A Coal sample contains 10% of ash, coal required is 300 MT/day, assuming 100% combustion calculate the mass of ash generated in a day.

                                                   Mass of ash generated = (300 X10)/100

                                                                                        = 30 MT

Example-5:A Indonesian coal contains 58% of Carbon, 4.2% of Hydrogen, 11.8% of Oxygen and 0.5% of Sulphur, also needs 20% of excess air for its complete combustion. Calculate the Total air required for complete combustion and O2% in flue gas.

We know that,

Theoretical air required for combustion is = (11.6 X 58 + 34.8 3 (4.2 - 11.8/8) + 4.35 X 0.5)/100

                                                                    = 5 7.7 Kg/Kg of Coal

                                                      Total air = (1 + EA/100) X Theoretical air

                                                                    = (1 + 20/100) X 7.7

                                                                    =  9.24 Kg/Kg of Coal

                               Also we know that EA = (O2 %/(21 - O2%)) X 100

                                                               20 = (O2/(21 - O2)) X 100

                                                               O2= 3.5%

Safety valves and its basic concepts

Pressure Relief Valve:This device is generally fitted on liquid lines like water, oil line. In this, valve the opening is proportional to increase in the line or vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually. In relief valve valves may not open 100%, as the line pressure reduces valves closes gradually. Pressure relief valves have higher flow capacities

Pressure Safety Valve: It is fitted on compressible fluid or gas lines. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.

Pressure safety valve & relief valves are used for system, equipment & man power protection.

Pressure reducing valve: These may be of hydraulic or pneumatic type used for water lines. This valve reduces the pressure of the water that goes through it, and is used to obtaining a regulated and constant value at its outlet.

Pressure control valves: These may be of hydraulic or pneumatic type used for steam lines

Safety valve:

A safety valve must always be sized and able to vent any source of steam so that the pressure within the protected apparatus cannot exceed the maximum allowable accumulated pressure.Here the valves sizing, manufacturing, installation, positioning & setting are more important.

Factors to be considered for selection/design of a pressure safety valve:

• Connection size and type

• Operating pressure 

• Operating Temperature

• Back pressure

• Service

• Required capacity

• Thermodisc

• Thermal compensation

• Blow down & Operating gap

Terminology used in safety valves:

Set pressure: It is the pressure at which safety valve lifts or pops up.It is usually 106-107% of operating pressure.

Reseat pressure: It is the pressure at which Safety valve seats.

Blowdown: It is the Blowdown is the difference between set pressure and reseating pressure of a safety valve expressed as a percentage of set pressure.

Blow down of safety valve = (Set pressure – Reseat pressure) X 100 / Set pressure

Blow down of safety valves is in the range of 2 to 5%.

Chattering: Excessive pressure loss at the inlet of the safety valve will cause extremely rapid opening and closing of the valve, this is called as chattering.

Chattering may result into lowered capacity as well as damage to the seating surface of the valve. Continuous chattering may result into damage to the other parts.

Following recommendation wil assists in eliminating the chattering

• The area of the inlet nozzle should be equal to the the inlet area of safety valve & that nozzle should be short as possible

• Inlet nozzle corners must be rounded to a radius of not less than ¼ of the diameter opening

Sonic Vibrations: Flashing, choked flow sudden flow or cut/off of steam in safet valves & related lines may result into  sonic vibration. This velocity is usually reached when the valve pressure drop rises to 50% of the upstream pressure. Vibration of long pipelines can also occur due to mechanical damage.

IBR acts, regulations & forms used

Precautions to avoid sonic vibrations:

• Safety valve should be installed at least 8D to 10D of pipe diameter down stream from any bend in steam line.

• Safety valves should not be installed closer than 8D to 10D to pipe diameters either upstream or down stream from the diverging or converging  “Y” fittings.

• The safety valve nozzle should never be installed in a steam line in a position directly opposite a branch line of equivalent size.

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.

25-Questions & answers on Boilers safety valves

Boiler safety valves overhauling steps

Consideration for installation of safety valve:

• Exhaust drain & cover plate vent piping must be installed so that they will not impose under pressure on the safety valve.

         Note: Do not plug the cover plate hole or do not reduce the hole piping size.

• Discharge pipe of the safety valve should not be supported on the valve body

• Clearance between the valve exhaust piping and the discharge stack should be sufficient to prevent contact when considering thermal expansion of the boiler valve

• Steam flowing vertically out of the discharge elbow produces a downward reaction on the elbow, in proportion on the quantity of steam flowing & its velocity.
  
  
• In no case should discharge piping smaller than  the outlet valve
  
  
• For optimum performance safety valves should be serviced regularly

• Valve assembly should be within 1⁰ vertical alignments.

• Gaskets fitted should be of correct size, should not close the valve inlet opening

10-Tips to reduce LOI in Boilers

Adjustment of set pressure:

Safety valves are set +/- 1% of set pressure.Set pressure should not be changed without the permission of manufacturing unit.

Before proceeding to check the popping (lift) pressure, ensure the pressure gauges used are calibrated. To adjust the popping pressure, remove the lifting gear, exposing the adjusting bolt lock nut. Loosen the lock nut if the opening pressure is low tighten (turn clockwise) the adjusting bolt, if it is high loosen (turn counter clockwise) the bolt. After each adjustment the lock nut should be securely tightened to prevent loosening of the bolt.

Adjustment of blowdown:

If the blow down is not as desired when the set pressure has been obtained, it is must to adjust the rings. The guide (adjusting) ring is the principal blow down control ring. To change its position, remove the guide set screw on the back of the valve body. Insert a screw driver or similar tool and engage one of the notches (these can be seen through set screw hole). The ring can then be turned to the right or left as desired. Turning the guide (upper) ring to the right raises it up and reduces the blow down. Turning the guide (upper) ring to the left lowers it and increases the blow down. After each adjustment always replace and tighten the set screw being careful that its point engages a notch and does not rest on the top of the tooth.

Note: Do not attempt to adjust blow down with lower ring

Power Transformers QnA

Factors which cause safety valve to damage or failure:

• Quantity & quality of the steam

• Discharge piping stress and back pressure

• Variation in ambient temperature

• Improper gagging

• Improper bolting of flanges

• Foreign material in the steam

• Improper method of assembly & disassembly

Guidelines for Boiler safety valve setting:

Preliminary checks

• Ensure calibrated pressure & temperature gauges are fitted.

• Gauges for each individual valves should be fitted

• Discharge piping has to be inspected for binding on the valves,supports and welds on piping.

• A rope appx. 6-7 meters with a hook one end should be attached to the valve lifting lever before starting the pressure rise. It will help in operating the lever to avoid chattering & over pressure

• Have the correct tooling available

• Establish the good communication system

Guidelines:

• If the unit has Electromatic safety valve, this valve should be in operation firts for more safety of the unit.

• Drum valves to should be tested first: Possibilities of valve part damage because of GIRL BLASTING are grater on superheated valves in contrast to the drum valves .If super heater valve is gagged after seat damage while testing other valves, the total valve damage will increase.
  
  
• Boiler temperature increases during the testing cycle of the Drum valves. Consequently higher temperature steam will be available for super heater steam valves and produce accurate results

• Keep water level low as possible, if drum level is high the safety valves may slugged with water causing long blow down & also may result damages to seat & disc.

• Maintain pressure rising in the range of 2-3 kg/cm2 per minute, slow pressure rising may result into simmering of the valve.

• If fuel feeding system fails at nearer set pressure, then reduce the boiler pressure at least 10% & raise again. Holding the boiler pressure nearer to set pressure for long time may result into simmering & valve lift erratically.

• If a valve has to be lifted several times, cooling off period is very must. Cooling period is around 20-30 minutes.

• If valves have not been tested with hydro test prior to the steam condition, it is recommended to hand lift before steam actuation.

Challenging situations & troubleshooting during boiler light up & start up

Safety valve floating procedure:

• Normally the highest set pressure valve is the valve floated first. While setting this valve other safety valves are gagged.

• Start the boiler as per cold start up procedure by modulating the firing.                
• When the drum pressure reaches about 60–70% of operating pressure gently tighten gage on other safety valve. 

• Raise pressure slowly by throttling start up vent valve. When 80% of popping up pressure is reached manually operate the safety valve under test. This will blow off any debris or dust left over in the valve internals.

• Raise the boiler pressure by modulating the firing

• When the pressure reaches nearer to the set pressure close the start up vent. While the safety valve pops (lift), open the start up vent valve and note down the lifting/set pressure value.When the valve sits back, note down the reset pressure

• Control of drum level is important to avoid possibility of water carry over from drum to the super heater.

• The set pressure is adjusted by either tightening or loosening the adjusting nut. Tightening the nut increases the set pressure and vice versa

•  Blow down is adjusted by upper rings adjustment.
• After setting the set pressure and blow down, bring down the boiler pressure to operating level.

Examples-1: A boiler steam drum safety valve lifts at 125 kg/cm2 and reseats at 120 kg/cm2, then calculate its blow down percentage?

                                   BD% = (125 – 120) X 100/125

                                                 =4.0%

Example-2: A boiler super heater safety valve has blow down 3% & has been set at 70 kg/cm2, calculate the reseat pressure.

3% = (70-P2) X 100/70

Reseat pressure P2 =67.90 kg/cm2

Precautions shall be taken during Super heater safety valve set at lower operating temperature than actual:

Safety valves blow down should be set more than required, as blow down percentage decreases as the steam temperature increases. An approximate rule is to add 0.5% of set pressure to the blow down for each 56.5 °C rise in SH steam temperature.

Example-3:
If a Super heater safety valve lifts at 189.5 kg/cm2 & reseats at 180 kg/cm2 at the temperature of 400 deg c, then calculate the blowdown calculation at 540 deg c

We have,

Lift pressure        = 189.5 kg/cm2

Reseat pressure =180 kg/cm2

Difference           =9.5 kg/cm2

Difference in temperature =540-400 = 140 Deg C

Asper above condition for every 55.6 deg c  rise in steam temperature blow down percentage increases by 0.5% of set pressure

140/55.6  X 0.5 X 189.5/100

=2.385 kg/cm2

Hence, blow down  at 540 deg c =9.5-2.385 =7.115 kg/cm2

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