PSV Discharge Size Reduction 1

[Veneratio]

I've been browsing the forums and a few other sources, but cannot seem to find a definitive answer regarding reducing the discharge piping size.

ASME VIII UG-315f states that the size of the discharge line shall be such that any pressure that may exist or develop will not reduce the relieving capacity of the pressure relief device below that required to properly protect the vessel or adversely affect the proper operation of the pressure relief devices.

Our customer wants an ASME stamped PSV. The PSV is for a blocked flow case of the pump. The issue we are having is that the smallest PSV we are able to find that meets their approved vendors, is 1/2" inlet and 1" outlet.

Our piping system is currently a 1/2" inlet, 1/2" outlet and we would prefer to run the PSV discharge back to the suction line instead of outside of the building and back to the tank.

The PSV has a rated capacity of 7.79 L/min, where as our process flow is 10 L/h.

Is it possible to do a calculation and use UG-135f to show that the PSV still has the relief capacity?

Would installing a short length of 1" suction header from where the PSV ties into suction and runs to the pump be sufficient?

 

[Duwe6]

You already did most of the calc:

Required capacity = 10 L/hr
Rated capacity = 7.79L/min = 467.4 L/hr [with no backpressure]

Now derate the flow based on the backpressure in the suction line and the use of a 1/2" discharge line. Done!

 

[Veneratio]

It seems like it should work that way. We are just running into some people who are dead set that you cannot reduce the piping on the discharge of a PSV for any reason. It seems like ASME allows it if you do the calculations though.

Thank you for the response.

 

[Latexman]

Those naysayers are remembering old rules that are now obsolete.

Good luck,
Latexman