3 replies to this topic

[marthin_was]

Hi,

 

Our existing facilities process heavy oil crude with 90% water cut becoming 1% BSW (BSW: base sediment water, the clean oil is transfered to Refinery), the  process is very simple, is to separate oil from water.

 

The water is taken out via "water leg" from bottom of the tank, with a "water box" to control oil-water interface. The water is transfered with gravity flow to atmospheric pit.

 

The oil is spilled over to another tank with gravity flow, the outlet goes to bottom of secondary tank which has static head.

 

The problem occurs when we'd like to determine line capacity. 

 

 I have read some discussion about gravity flow design (article written by Hills). Key points from the thread, to have a self venting flow, the line should be sized so that the Froude Number (Fr < 0.3).  This is to avoid vapor entrainment to liquid line which may bottleneck the flow.

 

But the problem is, if we follow the rules, in reality, our plant capacity may exceed the Fr = 0.3 without any significant vapor entrainment / flow bottleneck.

 

My question is : for my application, is it necessary to evaluate line capacity with self venting flow criteria? or for my process we may assume the pipe will always run full ?

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[katmar]

What do you mean by "But the problem is, if we follow the rules, in reality, our plant capacity may exceed the Fr = 0.3 without any significant vapor entrainment / flow bottleneck."?

 

If you are saying that it is all working well then there is nothing to worry about.  The Fr < 0.3 rule is a way to ensure that a new design is safe, but it is not the absolute limit of operation.  It is quite possible to work successfully at higher flowrates, especially if a bit of entrained air and slightly fluctuating levels are acceptable.  Sometimes the "if it aint broke, don't fix it" motto is the right advice.

[marthin_was]

Katmar,

 

Thank you for your reply.

 

Yes,  our operations working well eventhough the Fr > 0.3

 

Another question, if we follow Fr<0.3, we will come up with large vertical pipe size, 

 

Let say for building new facilities, we'd like to save money with installing smaller pipe size, can we do that with designing certain liquid depth (above pipe) to ensure no vortices or vapor entrainment?

Edited by marthin_was, 18 April 2013 - 05:00 AM.

[katmar]

Yes, the Hills article discusses the submergence required.  Just bear in mind that such a design is done for a specific flowrate and that at lower flowrates (if they can occur) the pipe might not be submerged.  But hopefully at the lower flow rate the pipe would be big enough to be self venting.

 

If the Froude number in the vertical section is > 0.65 then air will be safely drawn down with the overflowing liquid, and if Fr > 1.0 then the air flows at virtually the same velocity as the liquid.  If your overflow is short and the frictional pressure drop through it is low enough to ensure Fr > 1.0 with the available head above the overflow point then again you have a safe design - provided of course the entrained air is no problem downstream.