Certified liquid pressure relief device sizing

1 Certified liquid pressure relief device sizing ASME code requires that PRV used for liquid relief should have capacity certification, guaranty PRV full open at 10% overpressure. Following equations should be used for liquid PRD sizing  

Kc equals 1.0, if there is no rupture disk at PRV inlet. Kc equals 0.9, if there is a rupture disk at PRV inlet. (0.9 is uncertified Kc certified Kc is available from the National Board of Boiler and Pressure Vessel Inspectors). Kv is viscosity correction factor. At initial orifice area calculation (Eq. (9a)), let Kv equal to 1.0. Based on calculated orifice area, select a PRV orifice. Based on this orifice area, recalculate Re, kv, and A using Eq. (9c), Eq. (9b), and Eq. (9a). Repeat these calculations until selected PRV orifice doesn t change anymore. 

2 Non-cetlified liquid pressure relief device sizing This applies to liquid PRV before ASME code set capacity certification requirement. Interested reader can find its sizing requirement in API 520, section 5.9. [1] 

3 Two phase pressure relief valve sizing This sizing method is based on an equation of mass flux through a one dimensional isentropic nozzle,. see Eq. (10). It is derived based on the fluid is homogeneous (HEM model), which means no slip and no heat transfer between vapor and liquid, and its density is radially uniform. [4] 

G is mass flux of fluid, in lb/sec-fl2 x is weight fraction of vapor, vg is vapor specific volume, in fl3/lb vf is liquid specific volume, in fl3/lb Jis integration (from PI (PRV inlet pressure, in psia) to P2 (PRV outlet pressure, in psia)) dp is differential of pressure, in psi. 

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V11.2 Liquid pressure relief device sizing There are two different type of sizing depending on whether the PRD to be certified or not (by ASME code). 

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