Relief Valve outlet Pipe size

[jgibbs22]

Hey Everybody:

Does ASME Code require that the outlet of a conventional spring operated relief valve be larger than the inlet? I have seen a few NB marked valves that are 3/4" x 3/4", but the ones I am looking at our 3/4" x 1". My client currently has a coupling reducer on the outlet, turning it into a 3/4" x 3/4". I assume if we show that minimal back pressure occurs (less than the 10% overage rule of thumb) it would be ok to do this.

 

[casflo]

It would be possible to have the inlet and outlet pipes of the same size. It depends of the length of the discharge pipe. If there is not a discharge pipe, the flow of the valve is determined by its nozzle diameter that always is less than the inlet/outlet piping size. If there is a discharge pipe you must calculate that the valve plus the pipe have equal or more capacity than the valve.
casflo

 

[curtis2004]

jgibbs22,

The only size restriction I am aware of for Section VIII, Division 1 vessels ("U" stamped ones), minimum size of inlet of PRV should be not less than 1/2" NPS. Outlet sizes is defined by flow capacity of PRV and could be either the same size or one size bigger.

Regards,
Kurt