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Proper Use of Conventional PRV Discharge Coefficients

By Jung Seob Kim, Sunlake Co., Heather Jean Dunsheath, Covestro, Hyun Ji Woo and Nayeon Kim, SK E&C |

In order to correctly size pressure relief valves (PRVs), a robust understanding of discharge coefficients for vapor, liquid and two-phase flow is crucial A pressure relief valve (PRV) is used to prevent the pressure in a process vessel from exceeding the allowed pressure rating of the vessel. The required PRV orifice area is determined by dividing an ideal nozzle orifice area by a valve discharge coefficient. Therefore, proper use of PRV discharge coefficients is very important for sizing PRVs and preventing potential overpressure of process vessels. PRV manufacturers provide certified discharge coefficients that were determined experimentally for liquids and vapors. Certified PRV discharge coefficient values are reported in the Pressure Relief Device Certifications, NB-18 [ 1] from the National Board of Boiler and Pressure Vessel Inspectors. The certifications are based on isentropic ideal nozzle-flow equations for incompressible flow of water and compressible flow of air and steam. However, there is no certified discharge coefficient for two-phase flow. As many others have noted in the literature, the authors have found that the discharge coefficient for gases is significantly greater than the discharge coefficient for liquids. The…
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