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)

 20- metallurgical differences among Carbon steel, Alloy steel and Austenitic (Stain less) steel tubes in Boilers.

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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