7 replies to this topic

[R_Gowtham]

Hi all,

 

Is any good engineering practice or standard available regarding the distance limitation between the vessel to its respective PSV? 

 

Is 40m of PSV inlet line acceptable if its clear the inlet 3% criteria?

Edited by Gowtham Rajendiran, 13 May 2022 - 05:52 AM.

[breizh]

Hi,

Consider the document attached from Leser .

Edit : Let you take into account the comment in the link underneath:

https://www.e2g.com/...rop-the-3-rule/

A sketch of your system will help together with your calculation . 

 

good luck.

Breizh

[latexman]

At present, the standards (ASME, API 520 and 521) do not limit inlet line length.  The 3% criteria is the standard, HOWEVER there is documented discussion on inlet line length in API 520 Part II:

 

 

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.

 

The attached document may be useful to you.

Attached Files

•  Smith-Burgess.-Relief-Device-Inlet-Piping.pdf   387.71KB   46 downloads

[jayari]

From the viewpoint of my experience, 40 meters to reach the PSV seems like a long distance and considerations other than pressure loss may cause improper PSV function. Maybe others that work with larger systems than I do don't think this is an issue.

 

You should give us details on the system and the situation the device is sized for to get more input.

Edited by jayari, 13 May 2022 - 01:51 PM.

[fallah]

 

Is 40m of PSV inlet line acceptable if its clear the inlet 3% criteria?

 

Hi,

 

Although looks like 40 m is much higher than normal for PSV inlet line if the 3% criteria is met, no problem...

[latexman]

I'm curious, please describe why 40 m PSV inlet pipe is needed.  Thanks.

[fallah]

I'm curious, please describe why 40 m PSV inlet pipe is needed.  Thanks.

 

I guess that's due to low elevation of the vessel in comparison to that of the flare header which is routed in much higher elevation...

 

Then having free drain from PSV discharge toward the flare header, they have to install the PSV in an elevation higher than that of the flare header...

[astro]

Further to the various comments (especially noting latexman who sensibly refers back to API STD 520 Part II), it's not uncommon for relief calculations to overlook the importance of "total nonrecoverable pressure loss" when assessing the 3% inlet loss criteria. Non-simple piping configurations can give rise to pressure recovery, so it's important to assess losses accurately that are consistent with the criteria definition.

 

The Smith, Burgess and Powers paper that latexman has uploaded in Post #3 is a worthwhile read but it's effectively superseded by the more complex methods cited in API STD 520 Part II. I've personally used the method proposed by Melham, "Analysis of PRV Stability in Relief Systems, Part II - Screening" to do a PSV force balance assessment for brownfield work in cases where the 3% rule was violated. The Melham paper provides a clear method that is not too difficult to put into spreadsheet form.

 

The paper (and its other parts) can be sourced from the ioMosaic website.

 

I can see a future where the 3% rule will be consigned to history and a force balance check will become standard practice. Until then, if you're less than 3% on the upstream side of the PSV, the design is compliant with the current standard.