Vapor pressure relief device sizing

Pc is critical pressure at PRD outlet, in psia PI is PRD inlet pressure, in psia k is heat capacity ratio or isentropic expansion coefficient for ideal gas Cp is ideal gas (not the real gas per API 520 section 5.6.2.4) vapor beat capacity at relief temperature, btu/lb moie- F. 

Pc/Pl is the critical pressure ratio for sonic flow, If P2/P1 is below the critical pressure ratio, flowthrough PRD is sonic flow. Otherwise, the flow through PRD is subsonic flow. 

For preliminary calculation, PI can be assumed to be equipment relieving pressure at 10% ovetpressure for non-fire case or 21% overpressure at fire case. After PRD inlet and outlet piping are designed, an updated PI and P2 based on hydraulic calculation should be used to recheck the vapor relief is sonic or not 

A is required effective PRV oriflce area or rupture disk area, in inch W is vapor relief load, in Ib/hr T is relief temperature, in R Z is vapor compressibility factor at relief conditions. Let Z=1.0 for ideal gas. (see note 5 at end of this section) M is relief vapor molecular weight C is a coefficient calculated by q. (7b) or let it be 315 Kd is effective discharge coefficient Kb is backpressure correction factor Kc is a correction factor for install a rupture disk at PRV inlet, set kc to 1.0 for rupture disk sizing only Pi is PRD inlet pressure, in psia, 

Kd equals 0.975 for PRV sizing, if manufacturer s Kd is not available. It equals 0.62 for rupture disk sizing. 

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VILl.l Vapor pressure relief device sizing at sonic flow Calculate required PRV oriflce area or rupture disk area by follow equation ... 

The method used for the safe installation of pressure relief devices is illustrated in Figure 8-1. The first step in the procedure is to specify where relief devices must be installed. Definitive guidelines are available. Second, the appropriate relief device type must be selected. The type depends mostly on the nature of the material relieved and the relief characteristics required. Third, scenarios are developed that describe the various ways in which a relief can occur. The motivation is to determine the material mass flow rate through the relief and the physical state of the material (liquid, vapor, or two phases). Next, data are collected on the relief process, including physical properties of the ejected material, and the relief is sized. Finally, the worst-case scenario is selected and the final relief design is achieved. 

Figure 7-7A. Pressure level relationship conditions for pressure relief valve installed on a pressure vessel (vapor phase). Single valves (or more) used for process or supplemental valves for external fire (see labeling on chart). Reprinted by permission, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1 Sizing and Selection, API RP-520, 5th Ed., July 1990, American Petroleum Institute.

Often, the RD is mounted under the PSV so that it is sealed tight and protects the relief valve from being contacted by corrosive, plugging, hazardous, freezing, or regulated processes. This way the best characteristics of both devices are utilized. The RD can also be installed after the PSV. This installation can be used when the valve discharges into a vent header that might contain corrosive vapors. Table 3.153 provides data on RD materials, sizes, and minimum rupture pressures. 

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