found a very helpful post
Summarized:
• The Safety-Relief Valve shall be set at or below MAWP (normally same as Design Pressure), Ref. ASME Sec. VIII, Div. 1, Para. UG-134(a).
Engineers try to be "safer" by having a relief device lift earlier than MAWP but it typically results in events that should have never happened.
• The allowable accumulation is 3 psi or 110% of MAWP, whichever is greater, Ref. ASME Sec. VIII, Div. 1, Para. UG-125(c).
(Accumulation pressure can be greater than MAWP. Just because a valve is set to pop at a given pressure doesn't mean that it is fully open at a set pressure. If a valve is set to open at 100 psig it is not fully open and flowing until it reaches over pressure, which in most cases is 10% over the set pressure the difference in over pressure and set pressure, is called accumulation pressure.
Snap acting pilot valves will go full open on initial pop for some high pressure, high performance service. These designs are more expensive. The majority of Sec. VIII, direct spring PRVs will not go fully open on initial pop. More likely, they go 50 to 70% open and then gradually reach full lift at or before 110% of Set Pressure (Accumulation Pressure). To set a valve at or below MAWP does not mean the system will not exceed MAWP when it is relieving.
For a conventional type PRV, the maximum operating pressure may needs to be approx. 10% lower than the set pressure to avoid unnecessary premature opening. Whilst for pilot operated PRV, it may be approx. 3 %.)
• The Set Pressure Tolerance for Safety-Relief Valves is +/- 2 psi up to 70 psi and 3% of Set Pressure above 70 psi, Ref. ASME Sec. VIII, Div. 1, Para. UG-126(d). (Manufacture/installation tolerance)
• When deciding on the PRV set pressure, at least two points you may have to look at.
first point where set pressure can same as or lower than MAWP.
Second point you have to looks at is premature opening of PRV by considering
- maximum operating pressure
- type of PRV
- superimpose back pressure
- blowdown
- net spring setting
- tolerance,
- etc
• If you have a string of equipment with the same MAWP (no rotating equipment or block valves in between them) and a single relief valve protecting all of them after the last piece of equipment (as is the case in some gas plants), you may want to set the relief valve at a pressure that takes into consideration the pressure drop through the system.
Say you have a pressure drop at normal operation of 200 psi through your system, and the MAWP for all equipment is 600 psig, your relief valve may be set at 400 psig to protect the equipment at the front end.
However I would still be very hesitant to place a new relief valve at the end of a chain of equipment like that. Pressure drop is not always a reliable value in gas plants due to the multiple variables that go into it and there are also lots of potential complications (methane hydrate formation, liquid slugs, etc) that could make reliving difficult. Almost without exception I would recommend placing the relief valve at the first equipment item. It makes more sense to place the relief valve at the front end, closer to the source of the overpressure in the example I gave.
3 years industry experience;
Oil & Gas industry in Canada.