I am just guessing here but I think the Relief valve is not In-Line with the control Valve. The PSV is in a branch off the main line leaving the CV. Is that correct?
If I am correct then the number of pipe diameters down stream from the CV does not make a difference. The Pressure Loss from the Main Line to the PSV is the critical issue
Sometimes its possible to do all the right things and still get bad results
A sketch and/or more explanation would eliminate the need to guess, because I'm wondering if the following applies, "When a pressure-relief valve is installed on a normally flowing process line, the 3 % limit should be applied to the sum of the loss in the normally non-flowing PRV inlet pipe and the incremental pressure loss in the process line caused by the flow through the PRV".
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
Thanks for the posts. Latexman, the process is as follows: boiler, control valve to reduce pressure, relief valve, then equipment.
The relief valve is protecting the equipment in the event that the control valve fails open. The RV is attached directly to the piping.
I am not so concerned about the RV inlet dP. This is strictly a practical, or rule of thumb, question. I was wondering if the RV should be located a certain distance away from the control valve to insure proper operation of the CV. And conversely, if having a device generating high pressure drop (in this case the CV) might impact the operation of the RV. Perhaps this isn't a problem.
Maybe some study of API 520 Part II section 7.3, "PRV Inlet Pressure Drop Limitations" would be of benefit. An excerpt of section 7.3.5, "Background on PRV Inlet Pressure Loss Criteria":
"Limiting the inlet pressure drop to a specific value may not be sufficient to guarantee PRV stability. Recent research
and experience indicate that PRV instability is complex and cannot be attributed to just pressure loss in the PRV inlet
piping. Limited testing has shown that in many cases PRVs did not chatter when inlet losses exceeded 3 % of set
pressure while in some tests PRVs chattered when inlet pressure losses were less than 3 %. Industry experience
has shown PRV failures due to chatter are rare. Many existing PRVs in vapor service with inlet losses greater than
3 % of set pressure have not resulted in loss of containment while performing their function [19]. Inlet pressure loss
criterion alone is not sufficient to predict PRV stability. There are additional factors that also need to be considered as
shown in literature [13], [27], [28], [32]. Consequently, due to the complex nature of PRV instability behavior, further
research is needed before changes to the inlet loss criteria in 7.3.4 can be justified."
Also, API 521 section 5.5.12 speaks to "Acoustic Fatigue".
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
