Multiple PSVs and Pipe area

[UTBKC]

I have 4 4x6 PSVs located on a 4" nitrogen line, and none of them are spares. As I understand it, all of these PSVs don't actually protect the line and downstream equipment. This is because the process piping area is less than the total area of all of the PSVs. Can credit be taken for just one of the PSVs?

I also want to make sure I'm understanding the area requirement correctly. I'm looking at it as a mass balance. You can't expect more to flow into the 4 4" PSV lines than you have flowing from just one 4" line. However, it would seem as long as the 4" process piping capacity was large enough for the relief load, then flowing throuh just one line to a single PSV (assuming the PSV is large enough) should be fine.

Thanks in advance for the help.

 

[MortenA]

With regards to multiple-device installations you should refer to API 520 (part I) sec. 3.5.2.2

Im not sure if the API allowance for 116% of MAWP is legal in the EU where the PED clearly states max. 110% for all other than fire case (a bit unclear to my mind but the guidelines that can be found at

http://ec.europa.eu/enterprise/pressure_equipment/ped/guidelines/guideline5-2_en.html

). Anybody got a comment on this?

The max. flowrate of nitrogene is most likely not limited by the pipe but by a control valve or an orifice or similar. This capacity will be the limiting factor. Then comes the dP in the line from the nitrogen source to the PSV that must be added so that the pressure upstream wont go above MAWP+10%

Best regards

Morten

 

[halizo]

halizo

In the real world when a PSV (installed) opens ¿what is the flow trough this valve? is the one calculated by a contingency (fire for example)or is the calculated by the actual orifice with the upstream pressure (assuming critical flow).

thanks for the help

 

[UTBKC]

My issue is with the area of the piping. The area of the process piping is D^2pi/4, and the total piping area to the PSVs is 4 times that since all of the piping has the same 4" diameter.

 

[JAlton]

YOu need more information. There are several orifice sizes which can be installed in a 4x6 PSV. For example a 4L6, 4M6, 4N6 or 4P6. Each on larger than the previous, but none having the same area as a 4" Pipe ID. Sch 40 Pipe has an ID of 4" and a area of 12.5 sq in. L orifice area 3.2 sq in., M orifice 4 sq in, N orifice 4.9 sq in & a P orifice 7.2 sq in. You would need 2 @ 4P6 or 4 @ 4L6 to equal or surpass the area of a 4" pipe.

JAC

 

[Latexman]

UTBKC,

For one PSV, the net free flow area of the inlet piping components between the protected equipment and the inlet flange of the safety valve must be at least as large as that of the nominal safety valve inlet flange. Inlet line frictional pressure drop should not exceed 3% of set pressure. Refer to the applicable Code.

Without knowing the set pressure of the four PSVs, the equipment and piping layout, the credible scenarios for sizing these four PSVs, whether it’s a Code application or not, and other details, it seems reasonable and conservative that the line shall have 64 sq. in. of net flow area. This would be an 8” or 10” line. It also seems reasonable that an analysis of this entire system could find that most of those PSVs are redundant. And, the final answer could fall in the middle of these two extremes. We don’t have enough detail to give a better answer.


Good luck,
Latexman

 

[pleckner]

Latexman touched upon what needs to be done.

You need to stop trying to reconcile your piping area with the relief valve areas by qualatative or inspection analysis. There must be calculations that go along with the sizing criteria of these PSVs. Find them and review them. If they are not available, you will need to generate them (easier said then done but they are need whether you are in the U.S. or in Europe). Once you have the calculations, you will have your answer, it is as simple as that.