Recomended overdesign value for Safety Valves Selection

[washuchem]

After calculating required capacity for a safety valve service, the final selection for the rated capacity and/or selected orifice area is normally X times the required capacity or required orifice area. What are recommended values for this "X" value? What might be the impact of selecting a PSV 10 or 20 times the required capacity/orifice area on the PSV performance / calibration needs? Thanks.

 

[MortenA]

you should never deliberately over design your PSV "just in case". C.f. API 520 you must check lead and tail pipe pressure drop at the design capacity (not just your design case) and this could quickly lead to a serious over design of your inlet/outlet piping.

If you have a planned modification some years down the road that require a higher design capacity that what the initial mode of operation requires (by more than just selecting the first size with an area bigger than required) then i would say its time to step up to the plate as an engineer!

Best regards, Morten

 

[PeterIgg]

The 'Impact' of oversizing is just that - impact! In an over-pressure situation, the disc will likely chatter on the seat and could well destroy itself. As MortenA says, a correctly sized PRV is the way to go.

 

[washuchem]

Thanks MortenA and PeterIGG for your kind answers. I wonder what a recommended value for over design might be advisable for valve selection to avoid such problem as chattering. Anyone knowing if calibrating over designed PSVs might be an issue? Thanks!

 

[georgeverghese]

One would not deliberately choose an oversized PSV, but this may often occur in cases when BOTH nonfire (due to operational emergency or systems failure)and firecase relief loads are applicable, and the firecase load is much lower than the non fire case load.

In such cases, I wouldnt get too hung up over chattering during firecase - I've yet to come across designs where another dedicated RV for firecase was provided purely due to this reason. The RV will still relieve and prevent loss of containment though the trim may be damaged - there is probably much more extensive damage done to other equipment during a local fire than this RV. Also the relieving temp reached during a firecase relief scanario is not taken into consideration when setting the design temp for the vessel.

The same criterion would apply for thermal liquid expansion relief loads, where the installed capacity is almost always many times more than the calculated load due to heat gain from ambient.