Back-Pressure, Correction

All relief valves are affected by reaching critical flow, which corre-spond.s to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pre.ssures above 50%. API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow. 

T = flowing temperature, "R K(, = back-pressure correction factor... 

Kj = valve coefficient of discharge = 0.92 Pi = flowing pressure, psia MW = molecular weight of gas = 17,4 Z = compres-sibility factor = 0.9561 C = gas constant based on ratio of specific heats Cp/C T = flowing temperature, R Kb = back-pressure correction factor... 

Figure A8.6 BACK PRESSURE CORRECTION. FACTOR (FOR NON-CHOKED FLOW)...

The existence of biases between labs is a clear indication that the operating principles that are used to describe a measurement process are not being realized in practice. In some cases the operator s technique fails to account for a factor that influences the measurement. In other cases the model that is used to describe the measurement process contains approximations that are violated in using the method. Examples include the back pressure correction, which can be important in situations where the gas is highly soluble in the polymer, the correction for the volume increase in manometric measurements using mercury columns 5,... 

For preliminary estimates, the coefficient Kj can be taken as 0.975 for a relief valve and 0.62 for a bursting disk. The back-pressure correction factor, Ky, can initially be assumed to be 1.0 for critical flow. The combination correction factor, K, is used when a rupture disk is used upstream of the relief valve (see next section), in which case it is 0.9. If no rupture disk is used, then is 1.0. For vessels designed in accordance with AS ME BPV Code Sec. VIII, Pj = 1.1 times the maximum allowable working pressure. 

The values of k for some gases are given in Chapter Three. is the back pressure correction factor due to back pressure and is dependent on the type of relief used. Figure 5-5 gives Kj, for conventional spring reliefs and Figure 5-6 for balanced bellows reliefs, repectively. 

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). 

Kf, = back-pressure correction factor, obtained from Figure 4.14 = 1 for conventional and pilot-operated PRV = 1 if back pressure is less than 30% of the set pressure... 

Figure 4.14 Back-pressure correction factor, K, for balanced-bellows pressure relief valves (for vapors and gases). (Courtesy American Petroleum Institute, 1220 L Street, NW, Washington, D.C.)...

Balanced-bellows PRVs for subcritical flow are designed using Equation 4.44. The back-pressure correction factor is obtained from tiie manufacturer. 

For balanced bellows, if the back pressure is more than 50% of the set pressure, the program asks for manual input of the back-pressure correction factor (which by default is 0.69). 

Back pressure correction factor Combindtion correction factor Required discharge area Type of orifice required... 

When the calculate button is pressed, an input form appears as shown in Figure 4.28. This is because the back-pressure ratio exceeds 50%, and the program will require the value of the back-pressure correction factor. A value of 0.69 is entered. 

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