safety valve back pressure

salar1976,

Not strictly correct, API 520 in 3.3.3.2.* etc gives correction factors in figures 30 and 31 that are applicable up to approximately 50% back pressure. However it goes on to say that for back pressures greater than 50% formulas in 3.6.2 can be used. It also states very clearly that for back pressure greater than 50% the valve manufacturer should be consulted for correction factors and for any other possible limitation for a specific valve.

So to answer the question again in more detail and assuming that the back pressure is variable and that you are taking into account the superimposed and the built up pressures;

• you must use a balanced valve design
• you can use balanced bellows, balanced piston or balanced pilot pilot-operated valve design
• you must consult the valve manufacturer for the correction factors for his specific valve
• you must ensure adequate safe venting to atmosphere of bellows or piston or pilot.