ASME Section VIII and Inlet Pressure Drop
ASME Section VIII and Inlet Pressure Drop
(OP)
Dear Colleagues,
Per ASME Section VIII, M-6, the inlet pressure losses will be based on the valve nameplate capacity corrected for the characteristics of the flowing fluid. Are there any exceptions to this requirement spelled out in ASME VIII? In other words, does ASME Section VIII also define specific casess/scenarios where owner engineer can take exception and use the actual relief requirement to calculate the relief device inlet pipe pressure drop?
Thank you in advance for your help.
Per ASME Section VIII, M-6, the inlet pressure losses will be based on the valve nameplate capacity corrected for the characteristics of the flowing fluid. Are there any exceptions to this requirement spelled out in ASME VIII? In other words, does ASME Section VIII also define specific casess/scenarios where owner engineer can take exception and use the actual relief requirement to calculate the relief device inlet pipe pressure drop?
Thank you in advance for your help.





RE: ASME Section VIII and Inlet Pressure Drop
So while almost everyone follows App M and API 520/521, you could probably deviate from it if a client insists on it.
RE: ASME Section VIII and Inlet Pressure Drop
1) Fire - the contingency flow was used and not actual valve capacity. The reason - Fire is typically slow pressure generation and the valve is not likely to fully open
2)Liquids - the contingency flow was used and not actual valve capacity. I don't remember the rational
3)Allowable inlet loss limit was 5% instead of 3%. This was at a 60+ year old refinery. Revalidation of safety valves were finding a large fraction of deficiency over 3%. So they made a facility exemption to save money.
RE: ASME Section VIII and Inlet Pressure Drop
Good luck,
Latexman
RE: ASME Section VIII and Inlet Pressure Drop
I was 10 yards away from an oversized PSV that went off like a very big jack hammer. Everything shook. A guy standing next to the valve stepped backwards in shock onto an unguarded platform edge. It was only his flapping arms that kept him on the platform. I came within an ace of needing a new pair of trousers. Neither of us was very impressed with the Process Engineer who spec'd the valve. He thought is was funny and that over sizing was not his problem.
RE: ASME Section VIII and Inlet Pressure Drop
I concur with Latexman. What I am trying to figure out is whether or not deviating from this requirement would result in Code violation. If not, it will be much easier to get this approved by the site safety committee provided that a reason for deviation is based on sound engineering judgement/experiences.
RE: ASME Section VIII and Inlet Pressure Drop
ASME Section VIII UG-135 (b) says the flow characteristics of all pipe and fittings between a pressure vessel and its pressure relieving device shall be such that the pressure drop will not reduce the relieving capacity below that required, or adversely affect the proper operation of the pressure relieving device (i.e. chatter).
This aligns with what is in M-6, "inlet pressure losses will be based on the valve nameplate capacity corrected for the characteristics of the flowing fluid". The "valve nameplate capacity (i.e. valve wide open) corrected for the characteristics of the flowing fluid" is the relieving capacity of the installation.
In both cases the Code is referring to the flow that occurs with a wide open safety relief valve and it's pipe and fittings from the pressure vessel to the safety relief valve. There is no mention of using a sizing or required flow rate for the basis. There is mention that the relieving capacity (i.e. valve wide open) cannot fall below that required (i.e. sizing scenario). That means the upstream system must be designed with the safety relief valve wide open as the basis. I see no other way.
Chatter is the rapid opening and closing of the valve that will result in an unpredictable reduction in relief capacity, damages to the seating surfaces, and possible destruction of the safety valve. To avoid chatter, one must design the upstream system with the safety relief valve wide open, because that is what is really going to happen, and meet certain criteria.
IMO, substituting a flow rate from a sizing scenario is just folly, which may lead to future legal issues and liabilities.
Of course, the above is about compressible flow only; not liquids.
Good luck,
Latexman
RE: ASME Section VIII and Inlet Pressure Drop
Also, SRVs do not, as a rule, go to 100% lift (wide open) on initial pop. They typically go about 60% to 70% on initial pop unless it is a low set pressure where the spring is a little weaker. The full lift is only achieved at 107% to 110% overpressure. This is why One RIng Design, Process Safety-Relief Valves are not certified for ASME Sec. I Boiler Service. A typical SRV cannot meet 100% of relieving capacity by 103% overpressure as required by ASME Sec. I.
ASME Sec. VIII, Div. 1, Para. UG-134(f) is a recent revision which requires more attention to UG-135.
JAC
RE: ASME Section VIII and Inlet Pressure Drop
Good luck,
Latexman
RE: ASME Section VIII and Inlet Pressure Drop
RE: ASME Section VIII and Inlet Pressure Drop
JAC