Calculate API 520 steam pressure relief valve (PRV) and rupture disk size.
The flow through the relief valve nozzle is analysed using the Napier equation. For temperatures above 1200 F (922 K), the gas PRV calculation should be used. If the back pressure is greater than the critical (sonic) pressure the flow is sub sonic (M < 1).
Friction losses are accounted for using the discharge coefficient Kd. For initial sizing of PRV's the effective nozzle diameter should be used with the discharge coefficient Kd = 0.975. The actual nozzle diameter and rated coefficient of discharge should be used to verify that the selected PRV is suitable for the required flow rate. The PRV effective diameter is taken from API 526 (letter designation D to T). Changes in phase are not accounted for.
The calculation can also be used for rupture disks. The rupture disk diameter should be substituted for the nozzle diameter, with a discharge coefficient Kd = 0.62. Rupture disks can also be analysed as part of a relief vent system using the flow resistance method.
Note : The ideal gas calculators use the ideal gas compressible flow equations. The API 520 gas and steam calculations use an approximation of the ideal gas compressible flow equations. Use the ideal gas calculators for a comparison with the API 520 calculators.
Reference : API 520 Sizing, Selection And Installation Of Pressure Relieving Devices (2014)
Change Module :
Related Modules :