How to calculate the Boiler safety valve discharge capacity?

 

The discharge capacities of the safety valves depend on their type and fluid they are handling.

As per IBR regulation-292, following are the three Major types of safety valves.

1-Ordinary lift safety valve-In this safety valve, head lifts automatically a distance of at least D/24 with an over pressure not exceeding 10% of the set pressure. Here ‘D’ is the minimum diameter of the Body seat.

2-High lift Safety Valve- In this safety valve, head lifts automatically a distance of at least D/12 with an over pressure not exceeding 10% of the set pressure. Here ‘D’ is the minimum diameter of the Body seat.

3-Full lift safety valve- In this safety valve, head lifts automatically a distance such that the area of discharge which limits the flow through the valve is between 100% and 80% of the minimum area at any section at or below the body seat. This lift is achieved by a rapid opening within an over pressure not exceeding 5% of the set pressure.

Factors considered for safety valve discharge capacity calculation:

1-Steam condition/quality: Safety valves discharge capacity or flow depends on steam quality, it is different for saturated steam and super-heated steam.

2-Set pressure: Safety valves discharge capacity or flow depends on steam operating pressure.

3-Safety valve seat bore:

4-Valve type

Which formula do you use for discharge capacity calculation???

For saturated steam, the rated discharge capacity of safety valve shall be calculated by using following equation.

E= C X A X P

Where, E = Rated discharge capacity of saturated steam in kg/hr.

P= Highest pressure of any safety valve mounted on the Boiler in absolute bar

A= Area of seat bore in mm²

And C = Constant, it depends on type of valve.

C value for various valves;

1-Ordinary lift safety valve: 0.05

2-High lift Safety Valve: 0.1

3-Full lift safety valve: 0.24

For ordinary & high lift safety valves, Area A is the minimum bore diameter of the body seat. However, for full lift safety valve area A is the area of discharge to be obtained from OEMs i.e safety valve manufacturers.

For super-heated steam, the rated discharge capacity of safety valve shall be calculated by using following equation.

Es = E / √(1+2.7XTs/1000)

Where, Es = Rated discharge capacity of super-heated steam in kg/hr.

Ts= Degree of super heat in Deg C

Note: Steam discharging shall have direct access to the safety valve from Boiler drum or line without flowing through the internal pipes/accessories.

Safety valves discharge pipes should be as short & straight as possible and be fitted with an open drain to avoid accumulation of condensate water in line and extended to safest location.

Calculations for safety valves discharge capacity.

1-A 100 TPH (100000 kg/hr) Boiler drum safety valves is of full lift type. The other details of valves are as below, calculate its discharge capacity

Operating pressure: 110 bar

Safety valve set pressure = 127 bar

Size of seat bore: 2 inches i.e 50 mm

Solution:

Given data,

Set pressure, P = 127+1 = 128 BarA

Diameter of sear Bore, D = 50 mm

Area of seat bore, A = π X D² / 4

A = 3.142 X 50²/4

A =1963.75 mm2

Constant C for full lift safety valves is 0.24

Therefore,

Safety valve discharge capacity, E = C X A X P

E = 0.24 X 1963.75 X 128 =60326.4 kg/hr = 60.33 TPH (Tons per Hour)

 

2-A 90 TPH (90000 kg/hr) Boiler drum & super heater safety valves are of full lift type. The other details of valves are as below, calculate its discharge capacity

Operating pressure: 110 bar

Super heater Safety valve set pressure = 117 bar

Drum Safety valve set pressure = 127 bar

Size of seat bore: 2 inches i.e 50 mm

Operating temperature (Main steam) : 540 deg C

Solution:

Given data,

Set pressure of drum safety valve, P = 127+1 = 128 BarA

Set pressure of Super heater safety valve, P = 117+1 = 118 BarA

Diameter of sear Bore, D = 50 mm

Area of seat bore, A = π X D² / 4

A = 3.142 X 50²/4

A =1963.75 mm2

Constant C for full lift safety valves is 0.24

Degree of super heat Ts = Main steam temperature-Saturation temperature of steam

Saturation temperature of steam at this operating pressure is 323 deg C

Ts = 540-323 =217 Deg C

Therefore,

Drum Safety valve discharge capacity, E = C X A X P

E = 0.24 X 1963.75 X 128 =60326.4 kg/hr = 60.33 TPH (Tons per Hour)

Now, calculate the discharge capacity of super heater safety valve Es

Es = E / √(1+2.7XTs)/1000

Es = 60326.4 / √(1+2.7XTs/1000)

Es= 78745.42 kg/hr (78.74 TPH or Tons per hour)

The safety valves shall be so designed that they attain rated discharge capacity with the over pressure not greater than 10% of rated pressure.

Safety valves shall be reset at a pressure at least 2.5% below the set pressure, but not more than 5% below the safety valve set pressure. In some cases, where valve seat bore diameter is < 32 mm, the limit 5% is increased up to 10%.Or safety valves whose set pressure is < 2 bar G can have reseat pressure 10% below the set pressure.

How do you calculate the blow down of safety valves?

The formula for calculation of safety valves blow down

Blow down % = (Set Pressure-Reset pressure) X 100/set pressure

For example, A boiler super heater safety valve pops up at 73 kg/cm2 and reseat at 69 kg/cm2, then its blow down % is;

Blow down% = (73-69) X 100/73

Blow down = 5.4%

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How to calculate the furnace temperature of a Boiler???

 

               How to calculate the furnace temperature of a Boiler???

The furnace temperature of a boiler depends on various factors such as fuel type, combustion efficiency, and heat transfer.

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 vapor is called NCV or LCV.

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

Boiler furnace efficiency is a measure of how effectively the combustion process converts fuel energy into useful heat. It is typically calculated using the combustion efficiency method or the direct and indirect methods.

Example-1

A coal fired Boiler with lower calorific value 4200 kcal/kg is burnt in a Boiler with air fuel ration 6:1. Neglect the ash generated, calculate the maximum temperature attained in the furnace of the Boiler.

Assume the total heat generated by combustion of coal is given to the production of combustion. Consider the average specific heat of flue gas 0.24 kcal/kg & Boiler atmospheric temperature 30 deg C.

Given data;

L.C.V of fuel: 4200 kcal/kg

Specific heat of fuel: 0.24 kcal/kg

Boiler area temperature: 30 deg C

Furnace temperature = Heat released by combustion

No losses have been considered

Mg X Cpg X (t2-t1) = 1 X LCV

Mg X Cpg X (t2-t1) = 1 X 4200

Where, Mg = Mass of flue gas in kg

Cpg = Specific heat of flue gas

T2 = Maximum furnace temperature

(7+1) X 0.24 X (t2-30) = 4200

3.2 X t2 -57.6 = 4200

Furnace temperature t2 = 1330.5 deg C

Example-2:

A Biomass fired Boiler with GCV  2200 kcal/kg is burnt in a Boiler with air fuel ratio 3.5:1, Neglect the ash generated, calculate the maximum temperature attained in the furnace of the Boiler.

Assume the 70% of total heat generated by combustion of fuel is given to the products of combustion & 30% is losses. Consider the average specific heat of flue gas 0.24 kcal/kg & Boiler atmospheric temperature 32 deg C.

The details of the ultimate analysis of the fuel is;

Carbon, C: 23%

Oxygen, O2: 22%

Sulphur, S = 0%

Hydrogen, H2: 3.2%

Moisture, M: 50%

 

Given data;

GCV of the fuel: 2200 kcal/kg

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

LCV = 2200-(9 X 3.2% X 586) = 2031.2 kcal/kg

Air to fuel ratio: 3.5:1

Combustion efficiency = 70%

Specific heat of flue gas, Cpg = 0.24 kcal/kg

Boiler area temperature t1 = 32 deg C

Therefore, we have;

Mg X Cpg X (t2-t1) = 1 X LCV

Where t2 = Furnace temperature to be attained

(3.5+1) X 0.24 X (t2-32) = 1 X 2031.2 X 70%

1.08 X t2 – 34.56 = 1421.84

Furnace temperature t2 = 1348.51 deg C

Example-3:

An oil-fired Boiler with a Lower calorific value (LCV) 10200 kcal/kg is burnt in a oil fired Boiler of capacity 100 TPH. The ratio of air fuel is 18:1, neglect the ash generation & heat loss in combustion, calculate the maximum temperature attained in the furnace of the Boiler.

Given data;

LCV of the fuel: 10200 kcal/kg

Air to fuel ratio: 18:1

Combustion efficiency :100

Specific heat of flue gas, Cpg = 0.24 kcal/kg

Boiler area temperature t1 = 30 deg C

Therefore, we have;

Mg X Cpg X (t2-t1) = 1 X LCV

Where t2 = Furnace temperature to be attained

(18+1) X 0.24 X (t2-30) = 1 X 10200

Furnace temperature, t2 =2266.84 deg C

 

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Top-14 reasons for Boiler Tubes failure

 

        What are the potential reasons for Boiler Tube failure??

Following are the direct causes for Boiler tubes or pressure parts failure.

1-Operation of the Boiler at higher pressure:

Boiler pressure parts are designed to operate at particular pressure for continuous operation. If they are being operated at higher pressure than recommended, leads to failure of pressure parts may be at welding joints or even at plain surfaces.

Boiler pressure parts lose their strength as they become older and older.

As per IBR-7, Boiler operating pressure should be reduced by 5% after 25 years of operation. That is A boiler operating at 109 kg/cm2 pressure should be operated at 103.55 kg/cm2 after 25 years.

Boiler operating pressure reduction chart

Age of the Boiler (From the date of first use in years	25	35	45	50	60	70	80	90	100
% of reduction in working pressure	5	10	15	20	30	40	50	60	70

 

2-Operation of the Boiler at higher temperature:

Operation of the Boilers at Higher temperatures can cause metallurgical weakening of the tubes, leading to deformation, cracking, or rupture. Continuous exposure to high temperatures causes creep, where the metal elongates and loses strength over time.

Rapid or excessive temperature variations cause thermal expansion and contraction, leading to cracks. Operation of the Boilers at higher temperature can also increase internal scaling.

3-Pressure parts or tubes internal scaling:

Poor quality of DM water can lead to internal scaling, this internal scaling of tubes results into poor heat transfer subsequently overheating and tube failure.

Calcium, Magnesium & silica present in water if not treated well can form hard scales on pressure parts internal surfaces.

Operation of the High Boiler at lower pH (< 8) for long time eventually leads filure of pressure parts.

In sugar-based cogeneration plants, there is the chance of entering sugar traces in process return condensate. Due to this, pH of feed water drops ( up to 6) quickly and result into tube failures.

Operation of the high-pressure Boiler at pH around 5 to 6 results into tubes failure in just 3 to 5 hours.

4-Over heating of tubes:

Over heating of pressure parts due to wrongly set burners, air and internal scaling, poor water circulation & soot build up leads into tubes failure.

5-Erosion of tubes:

Erosion of pressure parts is mainly due to high flue gas velocity, bed materials in case of AFBC & CFBC boilers and even due to high velocity of steam and water.

So, ensuring right flue gas velocity at various locations of the Boiler is Vital to take care of erosion of pressure parts.

In case of AFBC & CFBC Boilers, bed coils (Evaporator & Super heaters) and water wall tubes erode due to bubbling beds.

Wrongly designed economiser, evaporators tubes and super heaters coils may suffer from erosion due to high velocity of water and steam. Usually, steam/ water velocity increases due to under sized tubes/coil or operation of the Boiler at over load.

Velocity of the fluid increases as the size of the tubes decreases.

Velocity (m/sec) = Flow (m3/sec) / Area (m2)

Based on above relation, velocity of the fluid increases as the Area of the tubes/pressure parts decreases and Flow increases.

Read>>>Boiler interview questions & Answers

6-Fatigue Failure:

Repeated thermal and mechanical stress due to frequent start up and shutdown cycles, frequent load variation leads to thermal shocks & failure of pressure parts.

7-Chemical cleaning of The Boilers internals:

Excessive use of chemicals especially acids during Boiler chemicals cleaning leads to Pressure parts failure during operation.

8-Stress & corrosion cracking:

Tubes fail due to corrosion, corrosion is mainly due to low pH, dissolved oxygen and other impurities in water which lead to pitting and weakening of the tubes. If Boilers are being operated in such conditions pressure parts go into failure.

9-Poor welding joints:

If welding joints quality is poor i.e if No standard welding procedure was followed during installation stages the joints fails upon small variation load or during abnormal operations.

Joints fail in following cases;

1-Poor welding techniques followed

Read>>>>> What is the significance of post weld and Pre-weld heat treatments (PWHTs)?

2-Poor skill of welder

3-Selecting wrong electrodes etc

4-Wet atmospheric condition

10-Poor operations:

What are the poor operation methods which lead to failure of pressure parts>>>>>??

Boiler pressure parts failure largely depends on Boiler operation skills of the operators.

Pressure parts may fail in following poor operation techniques

1-Not following Boiler start up procedures

2- Not following Boiler shut down procedures

3-Quick start-up of the Boiler, not following the Boiler start up curve

4-Not operating the vents & drains as per requirement or SOP

5-Sudden load variation

6-Operating the Boiler by ‘Bypassing the Boiler protections & Interlocks’.

7-Not providing minimum circulation steam to Boiler during start-ups, shut downs or even in case of ab normal operations.

Not knowing the function of ‘start up vent’ is the main reason for super heater coils failure during Boiler shut downs or low load operations.

Start up vent is used to provide minimum required flow to the Boilers during start-ups, which helps in cooling the super heater coils during No load operations

8-Operating the Boiler on low drum levels or not taking the quick shut downs if there is no water flow into the Boiler.

9-Wrong operation of attemperator. Sudden opening and closing of attemperator valves lead to thermal shocks in super heaters.

10-Operating the Boiler at higher drum levels. Operating the boiler at higher drum levels leads to carryover of wet steam or water into super heaters, which leads to thermal shock & tubes failure.

11-Giving water wall bottom headers blow down at high steam rate: This leads the starvation of water wall tubes.

12-Wrongly set safety valves & higher blow down percentage of safety valves:

In a Boiler, if drum safety valves pop up frequently before popping up of super heater safety valves, then it leads to the starvation of super heater coils and even failure.

12-Hydrogen damage:

Reaction between steel and water at high temperatures produces atomic hydrogen, which diffuses into the metal & leads to embrittlement and cracking, resulting in a fish mouth rupture.

13-Un even thermal expansion of pressure parts:

Indian Boiler Regulations Book-

Un even of thermal expansion of Boiler tubes due to not following Boiler start up curves & start-up/Shut down procedures or restriction to thermal expansion lead to cracks or failure of joints of pressure parts.

 

Read>>>Why does the thermal expansion occur in Boilers??

14-Metallurgical defects:

Poor quality of metals used for manufacturing of Boiler pressure parts lead to failure of pressure parts during operation.

What are the main reasons for Boiler tubes ‘fish mouth’ opening??

Fish mouth openings are catastrophic failures in Boilers, It’s a type of rupture where the tube bursts open, resembling a fish's mouth. This failure is highly dangerous and usually indicates severe operational or maintenance issues. The main reasons for a fish mouth opening in boiler tubes are:

Following are the major reasons for Boiler tubes ‘fish mouth’ opening.

1-Short term over heating: Occurs due to localized overheating when tubes are exposed to direct flame or high temperatures without sufficient cooling

2-Internal scaling

3-Operation of the Boiler with low water level

4-Starvation of super heater coils & water wall tubes

5-Hydrogen damage

6-Erosion of pressure parts, mostly in case of AFBC & CFBC Boilers.

7-Operation of the Boiler at higher pressure & Temperature than recommended.

8-Over heating.

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