Sizing, safety relief Balanced valves

Sizing, safety relief, 436, 437-441 API liquid valve, 444 Balanced valves, 441 Conventional valves, 438 Critical back pressure, 440 Effects of two-phase flow, 437 Hydraulic expansion, 441 Rupture disks, 434 Sub-critical flow, 449 Slurry flow, process pipe, 142-147 Regimes, 143... 

VARIABLE OR CONSTANT BACKPRESSURE SIZING FACTOR. K,. FOR BALANCED BELLOWS SAFETY RELIEF VALVES (VAPORS AND GASES)... 

Figure 12. Variable or constant back pressure sizing factor Kb for balanced bellows safety relief valves (vapors and gases).

Figure 17. Variable or constant back pressure sizing factor, Kw for 25% overpressure on balanced bellows safety relief valves (liquids only). The curve represents conqiromise of the valves reconunended by a number of relief valve manufacturers. This curve may be used wiien the make of dw valve is not known. When the make is known, the manufacturer should be consulted for the correction factor.

For conventional valves, pressure drop or variations in back pressure should not exceed 10% of set pressure. Because most process safety valves are sized for critical pressure conditions, the piping must accommodate the capacity required for valve relief and not have the pressure at the end of vent or manifold exceed the critical pressure. Designing for pressure 30% to 40% of critical w ith balanced valves, yields smaller pipes yet allows proper functioning of the valve. The discharge line size must not be smaller than the valve discharge. Check the manufacturer for valve performance under particular conditions, especially with balanced valves w hich can handle up to 70% to 80% of set pressure as back pressure. 

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