PSV set point adjustment for static head in liquid relief? 1

[hollerg]

It is not uncommon to see PSV's mounted 20 to 40 feet above vessels at grade.

1) When liquid overfilling and subsequent overpressure by a pump is a credible relief case, shouldn't the set pressure be lowered to account for the static head that will be in the pipe, so that the vessel never exceeds 110% of the MAWP?

2) This would seem to be a common mistake. Is there API or ASME comment on this issue?

Gary

 

[EGT01]

Gary,

In many cases, relief valves are elevated to allow the discharge piping to be free draining to some destination and yes, this elevation should be accounted for in the set pressure of the relief device.

I have seen this to be a common mistake especially when relief systems are designed during preliminary phases of a project. Ideally, they should be rechecked for final detail design.

I don't think I've seen any specific reference in API RP-520 or 521 but see ASME Sec VIII, Div 1, UG–134 PRESSURE SETTING OF PRESSURE RELIEF DEVICES
(c) The pressure relief device set pressure shall include the effects of static head and constant back pressure.

Regards,
Ellis