high back pressure relief valves

[chemjock]

Recently a colleague asseted that PSVs with high set pressures (about 500 psig plus) and sonic velocity in the tailpipe should have a higher flange rating than 150# on the outlet because the pressure at that point would exceed the lower flange rating. Scanning a vendor catalogue shows most valves have 150# flange ratings. Should PSVs in this situation have 300# flanges?

 

[EGT01]

Chemjock,

There can be any of a number of things that could affect the determination of the pressure at the outlet flange of a relief valve. Just to name a few - type of relief valve, superimposed backpressure, amount of overpressure used, etc. The bottom line is you need to determine what the pressure is and select the appropriate equipment.

If I were to assume for your example that you had a conventional type valve, discharging to atmosphere and sized for 10% overpressure, then the maximum allowed backpressure at the valve outlet can be only 50 psig regardless if the flow was sonic at the atmospheric end. So 150# flanges should be sufficient.

 

[MortenA]

Have you checked your psv for critical flow cd. 3.1 in API 520?

If you have sonic velocity downstream it could be that you dont have critical flow in your psv.

If its sized for critical flow and you dont have that - the the flowrate will be much lower - hence not sonic velocity an so on.

I dont think its good to have such high velocity in the tail pipe.

Best regards

Morten

 

[JimCasey]

ASME Sec VIII stipulates that the discharge piping should be as short and direct as possible. If you have an involved recovery system downstream of the SRVs, you are using them too casually.

Remember that the SRV is there to be the last defense between you and a crater with a plaque that reads "Site of the Great Plant Disaster".
If you routinely overpressure your system, you should have active valves and/or controls to mitigate the situation. SRVs are for when everything else fails. And although it is customary for SRVs to have a larger discharge than the inlet conection, it's not a bad idea to size the downstream pipe large enough so that the flow will be subsonic (<0.7M) anyway.

 

[DavidofLA]

These questions are valid but if you look at the manufactures catalog and sizing programs it will configure the size of inlets and outlets easily. Both Consolidated and Crosby offer good programs for this. If you have any more questions feel free to contact me.

 

[MortenA]

The piping may have an entirely different size than the valve - this is in fact what i have ssen in most systems i have come across (mostly larger).

Best regards

Morten

 

[chemjock]

This situation would occur in a revamp. In a new installation as JimCasey points out the downsteam pipe would be designed for no more than 0.75 mach. When installing a larger PSV in a revamp the velocity in an existing tailpipe may be allowed to go to sonic. Then the downstream pressure is determined by the critical pressure ratio. With a high enough set pressure the downstream pressure during relief may exceed a 150 # flange rating even with the piping code allowable overpressure.There is also the capacity reduction because of backpressure.