Overpressure correction factor

Kp = Correction factor obtained from Figure 14 for overpressures less than... 

Figure 14. Capacity correction factors due to overpressure for relief and safety relief valves in liquid service. The curve shows that up to and including 25 percent overpressure, capacity if affected by the change in lift, the change in oriflce discharge coefficient and the change in overpressure. Above 25 percent, the valve is at fidl lift and capacity is affected only by overpressure.

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.

Kp = liquid capacity correction factor for overpressures lower than 25% for non-code liquids equations only, (see Figure 7-22). 

For steam service at 10% overpressure, we use the following formula based on the empirical Napier formula for steam flow. Correction factors are included to account for the effects of superheat, backpressure and subsonic flow. An additional correction factor, Kn, is required by ASME when the relieving pressure (Pj) is higher than 1500psia (10.340kPaA). 

A bellow style pressure relief valve is required to protect a vessel containing an organic liquid. The required relieving capacity is 310 gpm. Inlet temperature is 170°F. Set pressure is 100 psig. Allowable overpressure is 25%. Built-up back pressure is 25 psig. Specific gravity is 1.45 and viscosity is 3,200 cP, Determine the orifice size of the valve. The correction factors are ... 

In the case of a balanced-bellows relief valve, the impact of general back pressure depends on the percentage of overpressure and is known as the back-pressure correction factor. For 10% overpressure, the back-pressure correction factor is 1 up to 30% general back pressure. For 16% overpressure, the correction factor does not change (value = 1) up to 37% back pressure, and for 21% overpressure, fhe correction factor does not change (value = 1) up to 50% back pressure. 

Balanced-bellows PRVs can be used for higher back pressme up to 50% back pressure without reducing the back-pressure correction factor (for a 21% overpressure situation). 

---