Gravity Flow with Frode > 0.3

[ChemEngSquirrel]

I have an existing overboard dump caisson which sends produced water from an offshore platform overboard.

I'm increasing the water rate to this caisson. The new rate will exceed the Froude No. 0.3 limit as per Norsok P-001 but the line size meets all other sizing criteria typical for pressurised piping. I'm going to install a vent to prevent vacuum formation & unstable flow in the caisson hence i'm not worried about Froude No. being > 0.3. The actual Froude No. is 0.8.

Does anyone see any problems with my proposed way forward here.

Thanks.

 

[BigInch]

Sizing for pressurized piping is not the same as for pipe with gravitationally driven flow. Without seeing an actual configuration, I wouldn't be so hasty to diminish my concerns about your F number and pipe diameter, or offer any other advice for that matter.

I hate Windowz 8!!!!

 

[katmar]

Depending on your sizes and flowrates, but because these offshore installations are generally large, the quantity of air drawn in could be high - but is difficult to calculate. This could be noisy and dangerous. You might need to consider installing a silencer and/or protective cage on the vent depending on the location. There will also be air bubbling up around the discharge point in the sea which may result in questions.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[MortenA]

I think that a Fround no. of .8 is high - and if i was to approve it i wouldnt.

 

[ione]

Frankly I'm not aware about the existence of a quantitative theoretical analysis which has lead to establish 0.3 as a recommended upper threshold for Froude number in order to ensure self venting occurs in gravity flow scenarios (vertical lines). From a qualitative point of view the goal is to keep the downward fluid velocity such low to allow counter-flow (rising) of air, or gases generally speaking, to happen. ChemEngSquirrel you've mentioned that you've approached the problem adopting criteria typical for pressurised piping. Particular attention should be paid to the fact you will have to deal with a mixture of produced water and air (two-phase flow). As noted above by others, surge and pulsations are quite likely to happen.

 

[katmar]

ione, the best review I have come across of the theoretical and experimental work that has been done on self venting and siphon flow was written by Larry L Simpson in Chemical Engineering, June 17, 1968 pgs 192-214. It has always surprised me how little attention is given to proper design of gravity piping in standard fluids text books, especially in the light of my experience over the years that this is one of the most troublesome areas of piping design and operation. Simpson's article was written nearly 50 years ago, but I am not aware of anything else nearly as good written in the interim. The PD Hills article from 1983 is often quoted, but it is mainly a set of recipes based on the results quoted by Simpson.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[ione]

Thanks Harvey. The work of Larry L Simpson is quoted among references of many very good books (Ludwig's Applied Process Design for Chemical and Petrochemical Plants and John McKetta's Piping Design Handbook just to mention a couple), but I've never been able to put my hands on it.

 

[Latexman]

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Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.