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Oil-Gas Separator retention time design criteria (API 12-J spec)Helpful Member! 

devaxrayz (Chemical) (OP)
9 Sep 06 7:21
Hi all,

currently i'm looking at a liquid gas separator and need to evaluate it for additional load case.

based on API 12-J, the liquid capacity of a separator is primarily dependent upon the retention time of liquid in the vessel.

API shows some typical value of retention time for some oil gravities:
oil gravities above 35 degree API is 1 minute
oil gravities between 20 - 30 degree API is 1 - 2 minute
oil gravities between 10 - 20 degree API is 2 - 4 minute

My OIL gravities (free of water) is above 35 degree API, however, the liquid entering the separator is DOMINATE WITH WATER with total specific gravity 0.937 (arround 19 degree API).

My question is :
1. what gravities i should use for criteria? the oil (>35 API) or the total liquid (19 API). It will determine how much i can add the separator load.

2. There is also some provision for surge time and hold-up time. What is this term of time? and what it is used for?? it is surge time and hold-up time value is absolute?

thanks and regards,

-Rayz-
Helpful Member!  Montemayor (Chemical)
11 Sep 06 12:48
Rayz:

You really have 4 questions:

1. What gravities I should use for criteria?
2. What is this term of time?
3. And what it is used for??
4. It is surge time and hold-up time value is absolute?

The answers are:

1. What you are seeking is the size (height) of the liquid-containing sump volume at the bottom of the vertical separator (“liquid collection section”; page 10, API 12J) – here, I’m presuming you are talking about a vertical unit since you haven’t told us if you have a vertical or horizontal unit.  Based on the expected liquid input, the total liquid inventory in the separator represents a “liquid retention time”.  Because you are proposing to use the separator in varying load services, you would be wise to design it for the worse case and use a liquid density that is conservative – i.e., use the lowest liquid density.  But bear in mind that it is actually two prime factors that determine the correct vessel size with respect to the liquid collection section:
      a. The time required for your instrumentation or operator actions during the process changes or procedures – both normal and extraordinary.
      b. The need to have a HH and LL liquid level instrument set points for operational and shutdown procedures.

These two factors have much more importance than any “general” rule of thumb or typical value given by anyone – including the API.  Note that API states that these are “typical” times – not those specific to your application.  As is the usual case, you must design to your specific needs and requirements, using good and logical engineering decisions.  Do not depend on the API (or anybody else, for that matter) to give you a “magically” specific equation to your application or problem.

2. If you are to reasonably have liquid surges into the vessel and must account for some hold-up of the same, then you must design for that event as well.  In other words, you must add more liquid collection volume to the separator.  This is not only common sense, it is mandatory good engineering judgment.

3. The additional liquid collection volume is used to handle the additionally imposed liquid volumes under the surge conditions.  If it is reasonable to expect surge conditions, you must design for it.

4. This question is unintelligible.  I don’t know what to make of it.

You are basically applying the simple Brown-Souders equation to a 2-phase separator and possibly allowing for surging liquid flows.  This classical 1930's equation was derived by both venerable gentlemen for determining the proper distances between distillation trays.  The diameter of the vertical vessel is dependent on the densities of the gas and the liquids involved.  If you allow for a conservative and low superficial vapor velocity the only items that can cause you problems are the disengagement distances above the sump’s liquid level, the distance above the demister pad (if one is used), and the amount of liquid inventory designed into the unit.  These factors are decided mainly by considering process needs and requirements set by you or the nature of the operation.  Only you can furnish the real basic data and, consequently, the basis for design.  Neither API, text books, or this Forum can do that.  But I hope this helps you out in assessing your application.

devaxrayz (Chemical) (OP)
13 Sep 06 6:58
montemayor,

First of all, your post is very helpfull, so thanks alot.

Sorry for not giving a detail (clear) picture of the case. But you were right it was a vertical vessel. The vessel is there in field and the liquid-containing sump volume is already fix, i just need to know how much fluid i can add to this vessel from current operating flow.

The vessel is a gas boots intended for early separation of gas and liquid (oil-water) coming from oil producing wells. However, beside of two prime factors that you stated above, we are considering the time required for gas to break-out from the liquid. Unfortunately we dont have any data of this "break-out time" for basis, so we are referring to API.

Anyway, i got the point from your post, so once again thanks alot.

I always got something from sending thread to this forum.... :)

regards,

-Rayz-

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