Pressure relief header design 1

[weirweb]

Can anyone point me in the right direction for correct piping design on relief headers? Specifically, the preferred method for entry into the header. Top entry versus 45 degree lateral entry? I do not need specifics on back pressure or relief valve design. Thanks

 

[Montemayor]

weirweb:

I have made field adjustments in how a PSV discharge best enters a collection header. I can also state that what you are zeroing in on is a very important detail in the design and operation of PSV's - a subject that is all too often left to others without a firm knowledge of fluid flow (like pipe fitters). I believe any time you spend on making sure you have the most appropriate design in this area is a well-justified effort.

I have tried to stay away from straight, 90 degree entries into the header and I always try to install 45 degree entries. The reason is that I have found it makes a difference in the PSV discharge pressure drop it has to overcome to get into the header. This is especially true for 2-phase flow systems. Additionally, depending on the quality, the design, and the flow rate, the 90 degree entry will tend to errode the header more so than the 45. I realize this is obvious horse sense and I can't explain why many designers haven't considered this - other than they haven't gone thoroughly into the details of fluid flow as you have. Also, I have to admit that it is more economical and space-saving to use the direct, 90 degree entry AND the other factor (as you probably well know) is that construction designers and contractors don't have to worry about running your unit continuously around the clock - they won't be around.

Using a 45 degree entry will use up more real estate and call for a bigger header - at least one to two sizes higher. I don't see a big economic problem with a bigger header (within reasonable length designs) since there isn't much difference in total installed cost between a 10" and a 14" size; however, the 45 degree entries can be a bother and take up a lot of room and pipe fitting. Nevertheless, the benefits from an operating point are there in a 45:

less pressure drop;
less errosion;
less turbulence;
easier flushing action.

I hope this discussion helps you analyze your application.

Art Montemayor
Spring, TX

 

[weirweb]

Art,
Thanks for your time. Your keen piping sense is a hard trait to find. My pipefitters will be challenged, but they need it anyway.

 

[EGT01]

weirweb,

You may also want to review API RP-521, 4th edition, March 1997 (or other edition). Look at section
5.4.1.3 Design of Relief Device Discharging Piping

In particular, check section 5.4.1.3.7 which discusses "Many design details and features merit particular emphasis with respect to relieving systems" and reiterates those comments by Montemayor.

Though I'm not sure if it was clarified I believe 45 degree top entry would be preferred over 45 degree lateral entry.

Montemayor,
What is the issue with a 45 degree entry requiring a larger header pipe size?

 

[Montemayor]

EGT01:

My reason for mentioning the possible need for a larger header was primarily for the case where the 45 degree lateral is introduced in the side of the header (at the centerline) and thereby reduces the available expansion space in the header if the header is running with a liquid level inside of it due to other fluids discharging into it.

If the header is running "dry", there should be no penalty paid by increasing its size. And this applies to the side (lateral) entry or the top entry of the 45 degree nozzle.

Thanks for bringing this point back at me; the way I wrote it could easily be misinterpreted. Communicating correctly means everything.

Art Montemayor
Spring, TX