Vessel Relief Valve Inlet Piping Questions

[PrudhoeBayAK]

Looking over ASME BPV Section VIII Div 1 UG-135 I am slightly confused.

If I have 2 50% capacity relief valves protecting the vessel (i.e. both combined as 1 level of overpressure protection), each with 2" inlets, can I use a common 2" supply piping run?

Subpart (c) seems to imply the cross sectional area of each PSV inlet must be added, and used to calculate the NPS of the inlet line. Or, is it more an issue of the combined area of the API orifice size of the relief valves?

Is it ever ok to have a reducer in the discharge piping of a PSV?

Thanks!

 

[TD2K]

In looking at UG-135, I would say you have to take the area of the PSV inlet(s) (not the orifice) and have the supply line at least equal to the sum of the 2 PSV inlet area. So, if you have 2" by 3" PSVs, you need at the very least a 3" supply header AND ensure that the pressure drop through that 3" header is no more than 3% of the PSV set point (unless you are using a remote sensing line for pilot operated valves).

Reducers on the discharge are fine if they are going larger . Regardless if you calculations show that the PSV capacity isn't impacted by a reducer going down in size on the discharge, I bet you'd have problems getting it approved by everyone.

 

[deanc]

Please refer to ASME SecVIII Div1 Appendix M. Also NBIC RB-3500 Always read the entire paragraph in the Code. Your answer is there.
Check with your AIA,Insurance Inspector or Jurisdiction.