Please give the formula for pressure setting of steam safety valve of boiler?

At least two safety valves have to be fitted to the boiler. They may be both mounted on a common manifold with a single connection to the boiler. The safety valve size must not be less than 38mm in diameter and the area of the valve can be calculated from the following formula 

C x A x P = 9.81 x H x E 

where 

H= Total heating surface in m3 

E = Evaporation rate in Kg steam per m2 of heating surface per hour 

P = Working pressure of safety valves in MN/m2 absolute 

A = Aggregate area through the seating of the valves in mm2 

C = the discharge coefficient whose value depends upon the type of valve. 

C=4.8 for ordinary spring loaded valves 

C=7.2 for high lift spring loaded valves 

C= 9.6 for improved high lift spring loaded valves 

C= 19.2 for full lift safety valves 

C= 30 for full bore relay operated safety valves

There are two components linked with

safety valve. 1. Tonnage 2. Pressure.


The safety valves of boiler shall be reset at a pressure at least 2.5% below, but not more than 5% below the safety valve set pressure.