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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