Pressure drop calcs in low pressure steam lines

[cavierma]

For a project, I have been involved in the pressure drop calculation of a 48 and 30 inch low pressure steam headers (98.5 quality steam). Many of the simulators I have checked use Begg-Brill & Moody for the two phase calculations. Can anyone kindly advise how well this equation handles the pressure drop calcs? Are they conservative?. To your knowledge does exist a better equation to predict the pressure drop?. For this particular case, these calcs are very sensitive because the available pressure is only 189 KPaa and every Kpa counts for the process. So every effort have to be made to be as precise as possible.

 

[zdas04]

Multi-phase flow correlations all have to assume a specific flow-energy state--nature doesn't cooperate very well with that and will change enegy states with alarming frequency within a flow stream. Beggs and Brill never claimed much better than around 10-20% "accuracy". If you need much better than that, you need to work towards getting rid of the second phase by using drip pots etc. or provide a significant amount of superheat. The dry-gas equations match observed data much closer, but it would be difficult for anyone to have confidence in predictive calculations on something where 1 kPa variance makes a significant difference. I would say you are deeply into the realm of trial-and-error.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[katmar]

Does your simulator plot the Baker flow regime map? If not, it is worth doing by hand. Your simulator will give you all the numbers you need and it will only take a few minutes to do the calculations. This will give you a good feel for what you are trying to calculate. While you are at it, calculate the Lockhart Martinelli parameters as well.

Your conditions are fairly extreme in that you virtually have a gas only situation. The volume ratio for your gas to liquid is about 37000 to 1. On the usual Baker and Lockhart Martinelli charts you will find that you are off the scales.

I doubt whether you will find anything that will give you an accuracy of better than 25%. If I were doing the calculation I would assume gas only, and add 50% to the pressure drop.

I know that we all have to rely on computer packages these days to get our work done, but sometimes they can mask what is really happening. Please don't think I am knocking software (hey, I sell the stuff!) but you need to keep the calcs in perspective.

Good luck
Harvey


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com