PSV Downstream Piping

Firstly, when a relief valve opens there is a very sudden flow and resultant forces with which your pipework must cope.

Secondly, the back pressure created during the operation of the valve must be minimised otherwise it will affect the valve performance - see API 520 and previous threads on this site.

If these are taken into account and satisfied then you can have any angle bend you please. Many relief valves are fitted with very short outlet pipes that take an immediate 90^o bend to point vertically upwards. Nice long radius bends go a long way to satisfy both the above needs.

Drip pan is not for the purpose of draining. It is to isolate the PSV from the forces of the discharge piping. However it is a mute point whether the drip pan will have adequate sealing water in hot climates all the time.

Additional to kaisan's comments - as long as the discharge built-up pressure is lower than 10% of set pressure for conventional valve. But no concern for balanced-bellow and pilot operated.

There are a number of things to consider.

As noted above there is a key parameter of minimising the built up pressure drop in the discharge piping. I do not think using 2 x 45 degree compared with 1 x 90degree would make any significant reduction in pressure drop (although long radius elbows as mentioned are a good idea) and like other respondents have seen numerous systems where there is a 90 degree elbow immediately downstream of the PSV/PRV

Then there is the mechanical forces on the PRV. This is outside my knowledge but in principle as the PSV lifts there is a reactive force on the valve and the piping that has to be accommodated. Why 45 degree elbows would be better for this i do not know you would need to consult a piping stress engineer.

You do not mention the service being considered and one concern I have is are we talking about pipe or ducting. My experience is in refineries etc so have always used piping which is strong enough to withstand the forces but from observation I notice that ducting very often uses 45 and lower angle mitred bends. With circular piping cross section ASME piping I see no problem in using 90 degree bends but for ducting I do not know.

I am not entirely sure what you mean by a drip pan elbow but if the relief flow goes directly to atmosphere (I assume it is a safe service like air or nitrogen or steam) then a 6mm hole is usually drilled in the bottom of the first elbow downstream of the PSV. This is there to allow for rainwater / condensed water to drop out of the discharge piping. If it was not there then rain could fall down the discharge piping and over time build up. If the discharge point is 10m above the relief valve this would impose 1 bar additional pressure! For air conditioning systems operating at low pressures even a 1m build up of water on the downstream side of the valve could be dangerous. This drip line cannot be valved. It is only safe to use this for systems that discharge to atmosphere.

This is steam service and set pressure is around 2 bar and another valve has 22 bar.

Downstream pipe length is around 25 meter.

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