PSV Discharge Line

[gord1]

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.

 

[Latexman]

Pcf exists at the exit plane of the flow nozzle (0.785 in^2) of the PSV.

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

 

[gord1]

Thanks Latexman:

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.

 

[Latexman]

Is this an existing installation, or is it being designed? "The existing discharge line is 2in sch 80 (65 ft long) to header" sounds like existing. "BB PSV is selected" sounds like design. Or, is it a hybrid?

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

 

[Latexman]

What Code applies to your installation? ASME Code requires at least a 3" tailpipe on a 2H3.

Good luck,
Latexman

 

[lizking]

The ASME Code now allows discharge piping to be smaller than the PRD outlet size provided a detailed pressure drop calc is performed. However, "good engineering practice" is to have the piping equal to or greater than the PRD outlet size.

 

[Latexman]

lizking,

Is this change in the 2007 Edition?

Good luck,
Latexman

 

[lizking]

I don't know what edition had the discharge size requirement, but I have a 2000 technical inquiry to API 520 that was answered by noting there is no ASME size requirement for discharge (UG-135-f) as there is for inlet (UG-135-b(1)). The answer goes on.."The practice of keeping the discharge piping equal to or larger than the relief valve outlet flange is recognized by API as well as Appendix M Of the ASME Code usually as good engineering practice. There could be technical reasons to deviate from this recommendation, such as to keep the fluid velocity in the line above a certain minimum value. It is our opinion that if the user chooses to use discharge piping smaller in size than the relief valve outlet, then calculations must be completed and documented showing that the back pressure generated does not adversely affect the operation of the pressure relief devices or the protected equipment."

 

[gord1]

Thanks both Latexman and lizking:

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!

 

[Latexman]

gord1,

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

 

[Latexman]

lizking,

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

 

[lizking]

A point of clarification: The ASME Code has had its current wording in UG-135 and Appx. M for eons. It is API 520, Part 2 that that changed from "discharge pipe should be as large as or larger than the nominal size of the RV outlet flange" (in th third edition) to deleting this phrase in the fourth edition. It was replaced with wording quoted in previous post.

 

[4Pipes]

News to me also. I would be more than surprised.

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.

 

[Latexman]

lizking,

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

 

[pleckner]

Here is a section out of ASME Section VIII, Div 1, UG-135, Installation (2006 edition):

(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.

 

[lizking]

pleckner,

That's how I see it (and I think is API's position).

 

[Latexman]

Thanks lizking and pleckner. It's clear now.

Good luck,
Latexman