3 replies to this topic

[Kryz]

I think I have similar problem like chcockin is small segment of weldolet. If you can read my memo post on 13 April by Kryz.

In regards of Arrow program. This software is good in sonic chocking detection. It is worth to contact AFT support regarding hydraulics model, or even Trey Walters, AFT President, he his good expertise in compressible flow. Reading his paper is good source of reference "Gas Flow Calculations. Don't Choke". It can be download from AFT web site, or I can also send PDF.

Perhaps if you discharge to atmosphere you dealing with pressure when gas is less compressed having higher velocity at the end. When connect to header the gas is more compressed at the end of pipe, because of header backpressure, therefore velocity is smaller (not sonic). I think if you change pipe size for outlet (increase) it should reduce backpressure. Just applying reducer and larger pipe.

In regards of chocking on the flange. My colleague from work asked similar question, saying how PSV with 2 inch flange can discharge such capacity, or something similar. This is something to ask the PSV vendors like Farris for example. It looks like we doubt if PSV is properly designed by saying that PSV without any connected discharge pipe may have problem in discharging rated capacity. They must have answer why they selected such outlet flange size for given orifice size.

I noticed in my case when I have backpressure too high I looked at Farris program for bellows option PSV. I could not find the same outlet flange size. My conventional PSV was 2 inch size, but corresponding model in bellows family for increased backpressure 3 inch. It was something which give me to think. The inlet PSV was the same for both 1.5 inch.

[Art Montemayor]

Three good and instructive articles are to found free and downloadable at the Applied Flow Technology website: http://www.aft.com/news/

They are:
Applied Flow Technology Article Featured in Chemical Processing (06/15/2002)
Optimizing Pumping Systems to Minimize First or Life Cycle Cost (03/18/2002)
Chemical Engineering - Gas Calculations: Don´t Choke (01/31/2000)

[JoeWong]

Mr Montemayor,
Thanks for your links.

Links as follow for ease of others...

Applied Flow Technology website: http://www.aft.com/news/
1) Applied Flow Technology Article Featured in Chemical Processing (06/15/2002)
http://www.aft.com/n...hemProc6-02.pdf

2) Optimizing Pumping Systems to Minimize First or Life Cycle Cost (03/18/2002)
http://www.aft.com/n...ing_Systems.pdf

3) Chemical Engineering - Gas Calculations: Don´t Choke (01/31/2000)
http://www.aft.com/n...low Reprint.pdf

Kryz,
This phenomenon is normally called secondary choke. Once process engineer checked and found that secondary choke occured at the outlet of PSV, one may take the following measures :

• Select next larger outlet flange size available within the same orifice size (API STD 526)
• Select a type of PSV with a higher allowable back-pressure
• Divide the relief requirement among two or more valves
• Recheck after above actions

Hope above helps...

regards,

JoeWong