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Separator Sizing And Selection, And What Is Slug?

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#1 Doneisbetterthenperfect


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Posted 26 January 2024 - 06:55 AM

 Hi, I'm sizing 2- phase separator.


I have many problems now, 


First, While designing the vessel, a question arose.

What is a slug when selecting a vessel? I have known slug flow as a type of flow.


Why does the presence or absence of a slug affect the design of a vessel?

I don't understand the correlation. Why does the presence or absence of a slug affect the selection of vessel type? And what is a slug?


I don't know what it means because I've never seen it.

I googled it, but only snails came up. Please help me ㅠㅠ


Second, I tried to set the level to calculate the height of the separator. I found out that in order to set the level, the following conditions must be followed.


"Low level pre-alarm should be either at least 0.10 m above LL or, if required, should be located so that there is sufficient liquid hold-up time - typically 1~2min for control room action - between the two levels for operator intervention."
A question arosed. How I can Calculate liquid hold up time. 
Is it possible to calculate the outlet volume flow rate calculated through simulation by dividing it by the calculated vessel cross sectional area?
However, since this is only my opinion, I doubt whether this is correct. I need your help.

#2 Pilesar


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Posted 26 January 2024 - 08:51 AM

A slug is the slug in a slug flow regime. This may be a much larger volume of vapor entering the separator for a short period of time. Slugs can have a very large volume in very long pipe lines. The separator must be large enough to handle the burst of extra vapor. [see my correction posted 28 January below. Liquid slugs are the concern, not vapor slugs.]

Consider liquid hold-up as the amount of time it would take for the liquid to exit if the feed were stopped. It is the volume of liquid in the vessel divided by the liquid flow rate. Volume is not the same as cross sectional area. For a vertical cylinder, the liquid volume is the cross sectional area times the liquid height. For a horizontal cylinder, the volume calculation is more complicated but still just a simple formula. The outlet liquid volume flow rate should be nominally the same as the inlet liquid volume flow rate for a separator.

Edited by Pilesar, 28 January 2024 - 03:52 PM.

#3 Bobby Strain

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Posted 26 January 2024 - 01:35 PM

You can find software at my website to size a vertical separator. And helpful links, too.



#4 breizh


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Posted 28 January 2024 - 03:45 AM


Consider this ppt and calculator.

CheCalc ‐ Vapor Liquid Vertical Separator Sizing


Note ; Get a copy of GPSA>


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#5 Pilesar


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Posted 28 January 2024 - 03:48 PM

Bobby Strain sent me a message that troublesome 'slugs' are slugs of liquid, not vapor as indicated in my previous post. Thanks Bobby! 

The concern in design calculations is that the average vapor and liquid flow rates do not represent the high variability that can be seen with slug flow. Separator equipment should be designed to handle the entire expected range. As Bobby's correction indicates, it is a surge in liquid that generally causes the problems.

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