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Low Flow in Pipes- posted in Ankur's blog

Two Phase Flow - Pressure Drop

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#1 chemks2012


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Posted 11 November 2013 - 10:15 AM

Dear all,


I have calculated two phase release rate and now I want to calculate inlet and outlet pressure drop for the given bursting disc.

Does anybody know how to calculate pressure drop for two phase flow?


Thanks for your help in advance.



#2 Zauberberg


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Posted 11 November 2013 - 10:31 AM

Calculating 2-phase pressure drop is always tricky, if the exact piping/pipeling elevation profile is not known. The results obtained by hand-calculations can be considered as a guide number only. The simplest method is to calculate individual pressure drop for each phase, assuming that the other phase is not present, and then sum up the two results as total pressure drop.


Attached is a spreadsheet that calculates pressure gradient according to Lockhart-Martinelli equation, and below is the link to River City Engineering where another spreadsheet is available.




Attached Files

#3 Bobby Strain

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Posted 11 November 2013 - 11:16 AM

Lockhart-Martinelli equation is quite old. There are more recent correlations. I have used Duckler correlations for plant piping for 30 years, and my programs are still in use by major engineering contractors. Lots of facilities have been designed using this correlation. And they all worked. If you have HYSIS, it has a number of correlations to choose from. Of course, there are websites where calculators ar available, but I wouldn't recommend using them without thorough testing. Buy a spreadsheet from Katmar. If you search this site you can probably find some spreadsheets. You need to verify that they are good, though, if you are going to design piping.



#4 PaoloPemi


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Posted 11 November 2013 - 12:21 PM

if it's for a bursting disc on critical flow condition

you may consider to use methods similar to PSV sizing,

for a two-phase flow HEM usually gives reliable values,

for two phase critical flow I simulate the discs with HEM

model or (for a different case) with

constant energy flash operation,

these procedures are complex to solve  by hand

so I use a library (PRODE PROPERTIES)

which works directly in Excel,


for the frictional losses of the relief piping and all piping components

similar methods should be applied,

see for example "Easily Size Relief Devices and Piping for Two-Phase Flow"

by Leung on CEP 96

Edited by PaoloPemi, 11 November 2013 - 03:40 PM.

#5 Steve Hall

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Posted 12 November 2013 - 11:27 AM



You might find the attached pages from my book to be useful.



#6 khan83


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Posted 13 November 2013 - 01:39 AM

Steve Hall:


Hi, i am looking for Rules of Thumb for Chemical Engineers, 5th Ed., by


Stephen Hall. Butterworth-Heinemann, 2012. if any one has soft copy please share here...Thanks

#7 curious_cat


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Posted 13 November 2013 - 05:03 AM

My advice: Buy it.  Or else, visit your library.  Sharing a copy is unethical and probably illegal too. 

#8 ChemEng01


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Posted 13 November 2013 - 05:30 AM   Best Answer

why not just use HYSYS or FLARENET for your pressure drop.


Inlet and outlet losses are meaningless for a rupture disk. The max 3% inlet and 10% outlet (conventional valve) losses do not need to be considered because once the rupture disk has burst you essentially just have a section of pipe from your protected piece of equipment and the outlet.


Just calculate your equivalent length from protected equipment to outlet (including equivalent length for RD (usually K factor on RD)), including any super imposed backpressure. As long as your pressure drop is less than the RD set pressure its going to be ok. So check for choked flow.

#9 chemks2012


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Posted 15 November 2013 - 06:22 AM

Thank you very much to all for your input.


Special thanks to:


JRudd [Agree what you said - yes it does make complete sense], Zauberberg and Stephen Hall

PS: I am loving this forum more and more! :rolleyes:

#10 khan83


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Posted 03 December 2013 - 04:15 AM

Steve Hall:


Thanks a lot sir for sharing the pages of your book



#11 srfish


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Posted 03 December 2013 - 09:40 AM

I'm late to the game, but the accuracy of the two-phase calculation depends on the operating pressure. For operating pressures of less than approximately 200 Psi use the segregated flow model. For higher operating pressures use the homogeneous flow model.

#12 processengbd


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Posted 09 May 2014 - 04:40 AM

Dear srfish can you please give some reference which specifies which should be followed for different pressure range. 


Thank you

Best Regards


#13 breizh


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Posted 09 May 2014 - 06:22 AM





This resource should support your query.


I've added a spreadsheet based on the reference above . It might help you and others .



Edited by breizh, 11 May 2014 - 01:03 AM.

#14 srfish


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Posted 11 May 2014 - 05:42 PM

One good reference for two phase flow is chapter 2.3.2 in the Heat Exchanger Design Handbook. They do not give exact operating pressures for each type of correlation. They do tend to favor the separated flow model at lower and intermediate pressures.

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