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Reg. 293

Minimum Aggregate Area

 
a) Discharged capacity of Safety Valves:- (a) Saturated steam:- The rated discharge capacity of safety valve which discharges saturated steam shall be calculated using the following equation:

E = CAP Eqn.(78)

where,

E = rated discharge capacity of saturated steam (kg/hr):
F = highest pressure of any safety valve mounted on the boiler (bar absolute);
A = area (mm2)

For the ordinary lift and high lift safety valves, A is the area of the minimum bore of the body seat. For full lift safety valves, A is the area of discharge described in clause (c) of regulation 292 and its value can only be obtained from the safety valve manufacturers.

C is a constant taken from the following table, appropriate to the type of the valve or as established by tests carried out in accordance with Appendix L:

Type of valve Value of the constant ‘C’
Ordinary lift 0.05
High lift 0.10
Full lift 0.24
b) Superheated steam:

The rated discharge capacity of a safety valve which discharge super heated steam shall be calculated using the following equation:-

Es = E
Eqn(79)
1 + 2.7Ts
                  1000

where,

Es is the rated discharge capacity of super heated steam (kg/hr);
E is the rated discharge capacity of saturated steam calculated using equation 78 (kg/hr).
Ts is the degree of superheat (oC)

Equation 79 may be used to determine the rated discharge capacity of safety valves at super-critical steam pressure. In these cases the saturation temperature of the steam shall be taken as 375oC.
c) The total peak load evaporation of a boiler shall be calculated on the basis of evaporation not less than 30 kg/hr./m2 of heating surface (exclusive of superheater and non-steaming economiser). In the case of Waste Heat Boilers however, when the evaporation per square metre of the heating surface is certified by the manufacturers to be less than 30 kg., the minimum number of safety valves required may be calculated on the basis of the actual maximum evaporation of the boiler.
 
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