PSV Discharge Line
PSV Discharge Line
(OP)
A scenario for fire case.
Pset = 525 psig
Relief Pressure = 1.21*525+14.7 = 649.95 psia
Relief Temp = BP@Relief Pressure = 220 F
Latent Heat: 95 BTU/LB
BB PSV is selected.
Max rated capacity is 64000LB/HR
Calculated area from Safety Engineer is API 2H3 (0.785in^2). Now the existing discharge line is 2in sch 80 (65 ft long) to header (constant superimposed pressure 10psig). I am checking if it is OK for discharge line to discharge the flowrate.
Now I suppose that the calculation is correct from safety engineer. First see if it is choked in PSV, based on API 520,
Pcf/P=(2/(k+1))^k/(k-1) = 360 psia
Obviously Pcf is much more than back pressure (10psig), so the discharge pressure should be Pcf , that is, the inlet pressure of discharge pipe is Pcf . Then I run aspenplus pipe unit operation (2” sch80 65 ft pipe (inlet stream/outlet stream). Based on simulation the pressure drop is 56 psi. That is the outlet pressure is 304 psia. In fact header is 10 psig.
If I work backwards starting from header 10psig, choked flow is reached. I am confused for the case. Any suggestions or comments?
Thanks for your inputs.
Pset = 525 psig
Relief Pressure = 1.21*525+14.7 = 649.95 psia
Relief Temp = BP@Relief Pressure = 220 F
Latent Heat: 95 BTU/LB
BB PSV is selected.
Max rated capacity is 64000LB/HR
Calculated area from Safety Engineer is API 2H3 (0.785in^2). Now the existing discharge line is 2in sch 80 (65 ft long) to header (constant superimposed pressure 10psig). I am checking if it is OK for discharge line to discharge the flowrate.
Now I suppose that the calculation is correct from safety engineer. First see if it is choked in PSV, based on API 520,
Pcf/P=(2/(k+1))^k/(k-1) = 360 psia
Obviously Pcf is much more than back pressure (10psig), so the discharge pressure should be Pcf , that is, the inlet pressure of discharge pipe is Pcf . Then I run aspenplus pipe unit operation (2” sch80 65 ft pipe (inlet stream/outlet stream). Based on simulation the pressure drop is 56 psi. That is the outlet pressure is 304 psia. In fact header is 10 psig.
If I work backwards starting from header 10psig, choked flow is reached. I am confused for the case. Any suggestions or comments?
Thanks for your inputs.





RE: PSV Discharge Line
The pressure at the PSV outlet flange/tailpipe inlet flange is 10 psig + frictional pressure drop working backwards from the header. This assumes there is negligible frictional pressure drop in the header such that there is no built-up backpressure due to the header.
Good luck,
Latexman
RE: PSV Discharge Line
The key is that discharge pipeline reaches choked flow when back from 10psig to PSV discharge point. This is the reason why I can not figure it out. Thanks again for your input.
RE: PSV Discharge Line
What is BB in "BB PSV"?
The tailpipe is too small. Enlarge it to eliminate choking and so backpressure meets the backpressure restrictions of the "BB PSV" (conventional, bellows, or remote).
Good luck,
Latexman
RE: PSV Discharge Line
Good luck,
Latexman
RE: PSV Discharge Line
RE: PSV Discharge Line
Is this change in the 2007 Edition?
Good luck,
Latexman
RE: PSV Discharge Line
RE: PSV Discharge Line
It is an existing one and BB (Sorry for that, Latexman, we use CONV, BB(Balanced Bellows) and Pilot for simplicity) is used for relief. I am following a rule without choked flow in tailpipe and 6" should be fine(back from 10 psig to discharge line of PSV). However I have some questions for the from PSV inlet to tailpine discharge):
If I increase tailpipe to 6" without choked flow, the DP should be 5 PSI, it means that discharge pressure of PSV should be 15 PSIG. My question is that the PSV absorbs all DP (620.25 PSI)from 649.95 PSIA to 29.7PSIA? However the critical pressure Pcf for PSV is 360 psia based on specific heat ratio. I am confused with the pressure distribution. What is the relationship between Pcf and back pressure (in my case, it is 15psig or 29.7psia) we calculate from header backward). Any comments will be welcome, if you can analyse it from theory, it is great, thanks!
RE: PSV Discharge Line
The pressure at the outlet plane of the nozzle of the PSV will be set by the pressure that gives sonic flow, Pcf. Outside the nozzle there will be expanding jet(s) and shock wave(s). Across the shock there are step changes in entropy and pressure (loss) to the pressure level of the surroundings. The pressure downstream of the shock equals the pressure drop in the tailpipe plus superimposed backpressure, in your case, 10 psig. A shock wave is very, very thin and this irreversible pressure loss happens extremely fast, most likely before reaching the outlet flange of the PSV though I don't think I've ever seen specific data on this. I recommend reading Shapiro's The Dynamics and Thermodynamics of Compressible Fluid Flow.
Good luck,
Latexman
RE: PSV Discharge Line
2000 huh? That's news to me. The company I work for has subject matter experts that interpret these things and change the design manuals, if needed. I don't have Codes to refer to. For whatever reason, our relief SMEs maintain that the discharge piping equal to or larger than the relief valve outlet flange is mandatory by using keywords like shall and must.
Anyone else have any insight here?
Good luck,
Latexman
RE: PSV Discharge Line
RE: PSV Discharge Line
I would doubt any manufacturer would agree to anything less than outlet flange size. It is no surprise that the API 526 valve orifice size - outlet flange size combinations are such that I believe all valves will work with a single outlet sized elbow a couple of foot of outlet size pipe pipe. (See ASME B31.1 App II).
By definition, reducing the outlet size below flange size is not going to work in general if at all for conventional valves. Balanced bellows will improve things but I'd be surprised if that by that much.
RE: PSV Discharge Line
I'm beginning to think that API's response to your technical inquiry to API 520, specifically "there is no ASME size requirement for discharge" is in error. It has been my companies interpretation and my understanding for many years that the discharge piping equal to or larger than the relief valve outlet flange is mandatory. If someone with the Code handy could quote the pertinent passages, it would shed more light on the subject.
Good luck,
Latexman
RE: PSV Discharge Line
(f) Discharge lines from pressure relief devices shall
be designed to facilitate drainage or shall be fitted with
drains to prevent liquid from lodging in the discharge
side of the pressure relief device, and such lines shall
lead to a safe place of discharge. The size of the discharge lines shall be such that any pressure that may exist or develop will not reduce the relieving capacity of the pressure relief devices below that required to properly
protect the vessel, or adversely affect the proper operation of the pressure relief devices. [See UG-136(a)(8) and Appendix M.]
UG-136(a) talks about liquid accumulation so it doesn't apply to this discussion. However, there is a section in Appendix M (2005 edition) that talks about the discharge line being the same size or larger than the PSV outlet flange. However, Appendix M is non-mandatory (but I consider it "good enginering practice" and will not deviate unless I have a pretty good reason to do so). It obviously looks like there is a slight contradition that exists within ASME (what else is new?!?). And this is where API usually tries to help out.
Therefore, I can only conclude that it is not mandatory (at least by ASME) to maintain the discharge line equal to or greater than the PSV outlet flange. BUT I would also conclude that it is still "good engineering practice" to do so.
RE: PSV Discharge Line
That's how I see it (and I think is API's position).
RE: PSV Discharge Line
Good luck,
Latexman