I am evaluating relief valves in an Ammonia process and having trouble with the API 520 two-phase flow (2PF) "omega" calculation. First, I am having trouble identifying exactly when I might have a 2PF situation. Can someone give me some guidelines that I might apply?
Second, in many situations below the critical point, omega can be calculated based on physical property data based on stagnation conditions. This includes the mass fraction in the vapor, or quality (Xo). The few examples I can find simply state something like "a stream with 5% quality". How would one estimate this? Base on VP/VLE data?
Any help would be greatly apprecialted.
Full Version: Omega Calc - Estimate Quality
Second question first, yes, you perform a flash.
First question: Ammonia is different than a lot of other chemicals. It is so different that industry basically has its own group that studies the unique safety issues associated with it.
In general, if you have a liquid that is non-foamy and non-viscous and non-reactive in a pressure vessel in a fire, for the most part you could size the PSV for all vapor venting. If it is foamy and/or viscous and/or reactive in a pressure vessel in a fire, then you should be investigating two-phase relief.
Reactive mixtures should be investigated for two-phase relief whether there is a fire or not (run-away).
If you have a large surface area to liquid volume ratio in a pressure vessel in a fire (vessel jackets would be an example), then you should be investigating two-phase relief.
At the risk of repeating myself (which I've done many times on this subject), if you intend to be serious in performing process safety calculations then you need a library of resources:
1. API RP520, Part 1 Latest edition currently 7th Edition, January 2000
2. API RP520, Part 2 Latest edition currently 5th Edition, August 2003.
3. API Standard 521, Latest edition currently 5th Edition, January 2007
4. ASME Section VIII, Div 1 (or your location's pertinent Codes)
5. "Guidelines for Pressure Relief and Effluent Handling Systems", Center for Chemical Process Safety (CCPS) for the American Institute of Chemical Engineers (AIChE), (1998) New York
6. Fisher, H. G., Forrest, H. S., Grossel, S. S., Huff, J. E., Muller, A. R., Noronha, J. A., Shaw, D. A., and Tilley, B. J. (1992). "Emergency Relief System Design Using DIERS Technology: The DIERS Project Manual. AIChE, New York.
7. Fauske, H. K. "Revisitng DIERS Two-Phase Methodology for Reactive Systems Twenty Years later", Process Safety Progress, (AIChE; Vol. 25, No. 3), September 2006
8. "Easily Size Relief Devices and Piping for Two-Phase Flow", J. Leung, CEP, December 1996
9A. Darby, R., Self, F. E., and Edwards, V. H. "Properly Size Pressure-Relief Valves for Two-Phase flow", Chemical Engineering Magazine, June 2002
9B. Darby, R. "Size Safety-Relief Valves for Any Conditions", Chemical Engineering Magazine, September 2005
10. Simpson, L. L. "Estimate Two-Phase Flow in Safety Devices", Chemical Engineering Magazine, August 1991
11. Ouderkirk, R. "Rigorously Size Relief Valves for Supercritical Fluids", CEP, August 2002
12. Leckner, P. Six Part Series on Rupture Disks, Chemical Engineers' Resourece Page
Various Websites including:
(1) The Chemical Engineers' Resource Page Forum on Pressure Relief Devices
(2) www.fauske.com
Is this extensive? You bet it is! Will you have time to read and understand all of this? NO WaY!!!! I've been doing this stuff for 20 years and still haven't gotten it all down. But I consider these to be MUST HAVES in any library if you are indeed serious about pressure relief system design. If you only are doing this occasionally, then References 1 thru 4 will be good enough as long as there is someone around that has the rest and is over-seeing your work.
But of course I said that ammonia is different and there are symposiums out there on ammonia safety and relief you should research.
First question: Ammonia is different than a lot of other chemicals. It is so different that industry basically has its own group that studies the unique safety issues associated with it.
In general, if you have a liquid that is non-foamy and non-viscous and non-reactive in a pressure vessel in a fire, for the most part you could size the PSV for all vapor venting. If it is foamy and/or viscous and/or reactive in a pressure vessel in a fire, then you should be investigating two-phase relief.
Reactive mixtures should be investigated for two-phase relief whether there is a fire or not (run-away).
If you have a large surface area to liquid volume ratio in a pressure vessel in a fire (vessel jackets would be an example), then you should be investigating two-phase relief.
At the risk of repeating myself (which I've done many times on this subject), if you intend to be serious in performing process safety calculations then you need a library of resources:
1. API RP520, Part 1 Latest edition currently 7th Edition, January 2000
2. API RP520, Part 2 Latest edition currently 5th Edition, August 2003.
3. API Standard 521, Latest edition currently 5th Edition, January 2007
4. ASME Section VIII, Div 1 (or your location's pertinent Codes)
5. "Guidelines for Pressure Relief and Effluent Handling Systems", Center for Chemical Process Safety (CCPS) for the American Institute of Chemical Engineers (AIChE), (1998) New York
6. Fisher, H. G., Forrest, H. S., Grossel, S. S., Huff, J. E., Muller, A. R., Noronha, J. A., Shaw, D. A., and Tilley, B. J. (1992). "Emergency Relief System Design Using DIERS Technology: The DIERS Project Manual. AIChE, New York.
7. Fauske, H. K. "Revisitng DIERS Two-Phase Methodology for Reactive Systems Twenty Years later", Process Safety Progress, (AIChE; Vol. 25, No. 3), September 2006
8. "Easily Size Relief Devices and Piping for Two-Phase Flow", J. Leung, CEP, December 1996
9A. Darby, R., Self, F. E., and Edwards, V. H. "Properly Size Pressure-Relief Valves for Two-Phase flow", Chemical Engineering Magazine, June 2002
9B. Darby, R. "Size Safety-Relief Valves for Any Conditions", Chemical Engineering Magazine, September 2005
10. Simpson, L. L. "Estimate Two-Phase Flow in Safety Devices", Chemical Engineering Magazine, August 1991
11. Ouderkirk, R. "Rigorously Size Relief Valves for Supercritical Fluids", CEP, August 2002
12. Leckner, P. Six Part Series on Rupture Disks, Chemical Engineers' Resourece Page
Various Websites including:
(1) The Chemical Engineers' Resource Page Forum on Pressure Relief Devices
(2) www.fauske.com
Is this extensive? You bet it is! Will you have time to read and understand all of this? NO WaY!!!! I've been doing this stuff for 20 years and still haven't gotten it all down. But I consider these to be MUST HAVES in any library if you are indeed serious about pressure relief system design. If you only are doing this occasionally, then References 1 thru 4 will be good enough as long as there is someone around that has the rest and is over-seeing your work.
But of course I said that ammonia is different and there are symposiums out there on ammonia safety and relief you should research.
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