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Gravity Vertical Flow Line

seawater overboard line

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#1 Afshin


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Posted 17 January 2020 - 06:00 AM

Dear Experts,


Currently I am checking adequacy of existing seawater overboard pipe due to increase of seawater flowarte. The seawater flowing

by gravity to overboard caisson via a 20 inch GRE pipe.


A 10 inch vent line already installed in highest point of overboard pipe which I believe there is a continuous air flow via that vent to inside of the vertical pipe to  prevent vacuum in vertical pipe.


Some questions came in my mind as per below:


1- Is this pipe should be checked based on single liquid phase or two phase (water + air) criteria?


2- In case of two phase flow :


  •       How air flowrate can be estimated?
  •       Is this pipe  should be sized based on maximum erosion velocity? If yes, what C-factor should be considered for GRE pipe  in API 14 E formula?

Waiting for your valuable response.

Edited by Afshin, 22 January 2020 - 07:43 PM.

#2 Technical Bard

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Posted 30 March 2020 - 04:41 PM

It should be checked based on a gravity flow equation for a non-full pipe.  It isn't two-phase flow in the traditional sense, because the vapour phase isn't moving the same direction as the liquid.  You should check that the Froude number is lower than 0.3.

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