Actual PRV size selection

When prelimitwy PRV sizes are calculated for various relief cases, select the largest one as the required PRV orifice area. From Manufacturer s catalog or API 26, we will be able to select which relief valve to use, and what arc its inlet and outlet nozzle sizes. In general, PRV is designated with a letter 1o indicate its size. Commonly used PRV and its effective orifice area arc as follows  

Note 1. All coefficients mentioned in this PRD sizing section are for preliminary sizing. For more accurate and final sizing, coefficients recommended by manufacturer should be used. 

ASME code stamp relief rate is based on actual PRV orifice area and rated discharge coefficient (=0.9 elective discharge coefficicnc). 

Most equipment is hydrotested at 150% or 130% design pressure or MAWP. 130% hydrotest pressure is new code requirement, but some companies still use 150% hydrotest pressure for conservative reason. These hydrotest pressures show that ASME code required system ovopressure (10%, 16%, or 21%) is on the conservative side. 

ASME/ANSI B31.3, Process Piping Guide, states that piping pressure can exceed 20% design pressure or allowable stress for 50 houi at any one time and for less than 500 hours per year or it can exceed 33% design pressure or allowable stress for 10 hours at any one time and for less dian 100 hours per year. 

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