Relief Temperature --- Rediculous
Relief Temperature --- Rediculous
(OP)
I have an unusual condition I am sizing a relief valve for:
low operating pressure but very high design pressure.
Here's my dilema: Pnormal = 50 psig (Pn=61.4 psia), barametric P = 11.4 psia; Pset pressure = 600 psig (for fire case, that's 737.4 psia); Tnormal = 127 F (586.7 R). Following equation 7b from page 17 of API RP 521, for the relieving pressure (constant pressure case):
Trelief = (737.4/61.4) X 586.7 R = 7,046 R (What?)
I seem to recall, and API hints at this, that the upper limit for carbon steel is 1,100 F (Of course, this is a 316SS tank).
Any consensus on what I should use for the real relief temperature? If 1,100 F, then does anyone have anything for stainless or other materials?
(Damn, I wish I hadn't stashed all my reference material in storage.)
Thanks in advance.
low operating pressure but very high design pressure.
Here's my dilema: Pnormal = 50 psig (Pn=61.4 psia), barametric P = 11.4 psia; Pset pressure = 600 psig (for fire case, that's 737.4 psia); Tnormal = 127 F (586.7 R). Following equation 7b from page 17 of API RP 521, for the relieving pressure (constant pressure case):
Trelief = (737.4/61.4) X 586.7 R = 7,046 R (What?)
I seem to recall, and API hints at this, that the upper limit for carbon steel is 1,100 F (Of course, this is a 316SS tank).
Any consensus on what I should use for the real relief temperature? If 1,100 F, then does anyone have anything for stainless or other materials?
(Damn, I wish I hadn't stashed all my reference material in storage.)
Thanks in advance.





RE: Relief Temperature --- Rediculous
The risk of failure under such conditions is so small, I would ignor this case.
RE: Relief Temperature --- Rediculous
RE: Relief Temperature --- Rediculous
However as i have understood it in general - the fire case is only to work as a last resort against catastrophic failure. The vessel will/may be ruined afterward. Normal deluge should prevent the temperature/pressure ever getting so high that the valve will actually vent.
Best regards
Morten
RE: Relief Temperature --- Rediculous
Nor, would the assumption of constant pressure be correct.The pressure should gradually build over several minutes as the liquid level drops to produce the vapor to fill the space above it. No, I think perhaps, API is wrong. The evaporating liquid should cool the contents of the vessel. If the vessel is well-insulated, the heat loss by conduction and convection should be slight. From my experience, as long as the temperature is below 200 F, the radiation should be low; insulation will keep the skin temperature of the outside of the insulation jacket well below 200 F. So, what I am thinking is that adiabatic might be the best assumption to use to predict the relief temperature.
Here's the comparison:
Constant Volume (API-RP-521):
P1 = 737.4 psia (600 psi + 11.4 barometric + 21% overpressure); Pn = 50 psi +11.4 = 61.4 psia; Tn = 127 + 459.67 = 586.67 R.
T1 = (P1/Pn)Tn = (737.4/61.4)586.67 = 7,046 R (?)
Adiabatic (Q=0):
T1 = (P2/Pn)^m, where m = k-1/k and k=Cp/Cv (Don't use ideal gas for Cv=Cp-1 because Pr<1 and Tr<1; Z = 0.707 by corresponding states for water) k = 1.291 (Cp, Cv from NIST site online).
T1 = (737.4/61.4)^0.225 X 586.67 = 1,026 R (566.7 F)
Note, I am concerned because this approaches the 1100 F limit for carbon steel, although the allowable stress for 316SS is 13,000 psi at 1000 F compared to CS(A106) with an allowable stress of only 2500 psi at the same temperature.
Let me know what you think.
Dirk Willard
RE: Relief Temperature --- Rediculous
I pointed out that API RP 520 is a recognized industry standards. Bring errors to their attention. Our contracts require compliance with API RP 520. We are not going to use another method of sizing and selecting relief valves based upon his clever technique. We would consider validating our relief valve calculations using recognized software. However the vendor's calculation would support the valves purchased. Also that the control systems engineers bought the relief valves. Most CSE's are electrical these days. Both chemical and mechanical engineers typically study more thermodynamics and fluidics than electrical engineers. If they want to assume the responsibility to specify and purchase the relief valves then they are welcome to take them over. The last that I knew PSV's were still handled by control systems engineers at that company - and my current home too.
RE: Relief Temperature --- Rediculous
FYI: this method of calculating the mass flow was proposed by A.K. Coker, "Size Relief Valves Sensibly," Chemical Engineering Progress, August 1992.)
W = Qload/DHlatent where Qload is calculated from one of the Q=F(C1)Aw^C2 equations. Be careful not to confuse storage tanks with pressure vessels. The equations for storage tanks from API 2000 should not be used for pressure vessels.