safcon
Specifier/Regulator
- Jan 12, 2004
- 10
I posted the following in the Chem-E section but received no response. Might this forum help?
A question has come up in a process hazards analysis (PHA):
A relief valve (standard spring loaded) is fitted to a vessel. The MAWP of the vessel is 6,500 psig. The relief valve is set at 6,500 psig and bench tested at that pressure (with no back pressure on the relief valve). When in service, the relief valve must discharge into a sealed system running a constant 500 psig. I contend that the spring pressure and the back pressure are additive, and that the relief valve, when in service, won't lift until the vessel reaches at least 7,000 psig (the 6,500 psig spring load plus the 500 psig back pressure).
I further believe that the 7,000 psig lift pressure may actually be a conservative estimate. Because the relief valve plug is cone shaped (with the small end of the cone facing the vessel), more surface area of the plug is exposed to the 500 psig of back pressure. Assuming a 6" diameter flow port with a 3" taper to the plug diameter, the larger, discharge side of the plug will have a 9" diameter, and a surface area of 63.6 square inches. Opposing that force will be the 6" diameter of the process side of the plug with a surface area of 28.3 square inches. The difference in area ratios will mean that the force on the larger surface will be multiplied by 2.25. Since the larger (9" diameter) surface sees 500 psig, there will actually be an additional 1,125 psig (2.25 times 500 psig) required on the smaller (6" diameter) surface of the cone to reach stasis.
If this is correct, the 6,500 psig of spring pressure will be added to the equivalent 1,125 psig of back pressure giving an actual relief valve lift pressure of 7,625 psig. Is this correct?
A question has come up in a process hazards analysis (PHA):
A relief valve (standard spring loaded) is fitted to a vessel. The MAWP of the vessel is 6,500 psig. The relief valve is set at 6,500 psig and bench tested at that pressure (with no back pressure on the relief valve). When in service, the relief valve must discharge into a sealed system running a constant 500 psig. I contend that the spring pressure and the back pressure are additive, and that the relief valve, when in service, won't lift until the vessel reaches at least 7,000 psig (the 6,500 psig spring load plus the 500 psig back pressure).
I further believe that the 7,000 psig lift pressure may actually be a conservative estimate. Because the relief valve plug is cone shaped (with the small end of the cone facing the vessel), more surface area of the plug is exposed to the 500 psig of back pressure. Assuming a 6" diameter flow port with a 3" taper to the plug diameter, the larger, discharge side of the plug will have a 9" diameter, and a surface area of 63.6 square inches. Opposing that force will be the 6" diameter of the process side of the plug with a surface area of 28.3 square inches. The difference in area ratios will mean that the force on the larger surface will be multiplied by 2.25. Since the larger (9" diameter) surface sees 500 psig, there will actually be an additional 1,125 psig (2.25 times 500 psig) required on the smaller (6" diameter) surface of the cone to reach stasis.
If this is correct, the 6,500 psig of spring pressure will be added to the equivalent 1,125 psig of back pressure giving an actual relief valve lift pressure of 7,625 psig. Is this correct?
