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3 Phase Separator With Weir

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#1 bernath


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Posted 29 November 2010 - 12:12 AM

Hi All,

I'm still newbie in 3 phase separator design. There were few questions arise in my head. Hope you guys can help me.

I realized in designing the 3 phase separator with weir, there are two possible design,

1. The normal liquid level (NLL) of oil is below the weir level at the oil compartment (see pic I)
2. The normal liquid level (NLL) of oil is above the weir level (see pic II)

In "Successfully Specify Three-Phase Separators" by Monnery & Svrcek, the oil NLL should be below the weir level, but in reality I have found one case whom oil NLL is above the weir level(pic II). IMHO, this configuration is due to the small flow rate of gas. Now that I face the task to design a new separator with small gas flow rate, I'm kind confuse whether to use the model and configuration on the paper (by Monnery & Svrcek) or not.

the new 'smaller' design spec are as follow:
gas flow rate: 6 MMSCFD
oil flow rate: 6,000 bopd
water flow rate: 6,000 bwpd

any answer is appreciated


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Edited by kanankiri, 29 November 2010 - 12:16 AM.

#2 Zauberberg


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Posted 29 November 2010 - 12:32 AM

Just by following common sense, you can conclude that it is not possible to have the oil phase NLL (I assume this stands for "Normal liquid level") neither above, nor below the overflow weir:

1) Oil simply overflows the weir, it cannot 'float' higher than that. The additional height on top of the weir is a so-called overflow crest which cannot be used for level measurement and/or control.

2) If oil phase NLL is constantly below the weir, there will be no separation between aqueous and hydrocarbon phases and there will be continuous carry-under of oil with the outflowing water.

A simple explanation of the phrase that made you confused - about the oil phase NLL - is that the "oil level" actually stands for interphase level between aqueous and hydrocarbon phases. And that makes sense, as it is applicable for both sketches you have uploaded.

#3 bernath


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Posted 29 November 2010 - 01:31 AM

yes, you're right Zauberberg. It's not the Oil NLL. Still I haven't found my answer yet.

the above 2 pics, are they actually is one separator with different condition and operating flow rate (i.e. min, max or normal) or indeed a two different kind of separators?

what i mean by oil NLL below the weir level is at the oil compartment(right side of weir), not water compartment (left side of weir). Please be clear about it.

As far as i know from the paper by Monnery and some references across the net, the High High Liquid level (HHLL) of oil at the right side of weir cannot exceed overflow weir level but the 2nd picture clearly doesn't follow that rule. Btw, I got the 2nd pic from 'pipingguide.net'

any answer are highly appreciated

thank you

Edited by kanankiri, 29 November 2010 - 01:32 AM.

#4 csp_process


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Posted 29 November 2010 - 03:05 AM

@ Kanankiri- You are just confused, what Zauberg said was more then enough to clarify your query. Better then googling, you should have rescanned principles of phase separation from Chemical engg. reference books(Ludwig n so many)
  • NLL- level of liquid in vessel at normal condition
  • HLL/LLL - High/level of liquid in vessel at abnormal conditions
There is nothing wrong with both figures, fig.1 represents a low liquid level(less than normal level). Fig 2 represent normal level ( as it looks like 50% of vessel volume).
There is no such rule I have heard or read anywhere , which says level of oil cann't go above weir height. Please don't say 'High High liquid level' again and again to liquid level you see in fig.2. Generally It is normal liquid level at which Level controller / safety will be happy.
For high level it has to cross the Normal level, and of course the weir height by which your are confused.
No author will write it, please post that paper on forum if it is written. Probably what you have misinterpreted is Interface level( water) shouldn't cross the wier height, which is common sense more than any engineering.

Back to your intial post-
You refer 6 MMScfd as small gas flow rate, Can you imagine how small is 6 MMSCFD ?
It is around 7000 Std. m3/h which is not at all a small amount of gas for liquid 6000 bopd(~ 39-42 m3/h of oil or ~39 m3/h of water).

Refer a good chemical engineering handbook.

All the best

Edited by csp_process, 29 November 2010 - 03:21 AM.



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Posted 09 December 2010 - 11:56 AM

Sorry everyone, but I think he will be more confuse with all the explanation

Dear kanankiri

First you must know that the working principle of 3 phase separator, the working principle is to separate all the three phase based on residence time inside the separator.
This will allow gas, water and oil to separate.

For gas and liquid separation you will size the separator so that the gas velocity inside the separator is less than the critical velocity, or the velocity which is required for the specified droplets size to settled. While the separation of water and oil is based on the difference of density, if we give enough time to a mixture of water and oil to the water will tend to be on the lower side of the separator while oil will tend to be on the higher side

Based on your sketch, both are the same type of three phase separator, it is a 3 phase separator with weir

The different is on how it work for the water and oil separation, in the first drawing the mixture of water and oil will be separate in the area before the weir, means the overflow from the weir will be mostly oil and the total liquid level inside the separator will be control by a liquid level before the weir to be only slightly above the weir, while the liquid level after the weir will control the liquid level of the collected oil from overflow. This is an overflow weir type

While in the second drawing, this is a submersible weir type, means the weir will always be under the liquid. The control of liquid level also will be from both side of the weir. But, the level control after the weir will control the total liquid level inside the separator, while the liquid level before the weir will control the interface level between water and oil before the weir.

Hope this can help


#6 fallah


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Posted 10 December 2010 - 02:33 AM

Dear Ervin,

I think you certainly mean in the first drawing ,same as second one, the interface level between water and oil before the weir should be controlled to prevent any risk of water overflow from the weir.


#7 Art Montemayor

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Posted 10 December 2010 - 11:55 AM

I also have labored in the past with understanding what the basic design principles behind the fabrication of a 3-phase separator are. However, I was fortunate to be born in Texas and I was exposed to and received a personal explanation on the detailed workings of how the process is supposed to work.

I mention that I was fortunate, because I have never found a text book or manual that fully explains what I have outlined and commented on in the attached Excel Workbook. Everyone in oil and gas production has assumed that the workings of a 3-phase separator are simply intuitive and should be understood by recent graduates or rookie engineers out in the field or in the design and fabrication of these vessels. I don’t believe that the intuitive process is that simple because nothing really is “simple” until one has had the basics and scope of design fully detailed out to you – no matter how “simple” that may be.

Please note that there are some errors or misunderstandings throughout this thread regarding how the separator should operate. I point this out not to criticize or embarrass individuals, but rather to finally reveal to all what was revealed to me many years ago. I apologize and regret not having made mention of these basic principles and scope of work in prior threads or discussions, but like all other elderly engineers I too am getting old, tired, and forgetting to mention these important points that were so helpful to me in my younger days.

I believe the original poster (OP) has made some erroneous or misunderstood statements and has included two different sketches that are not related in keeping with the basic topic. To simply state basics, I make mention that THE OIL LEVEL IN THE DOWNSTREAM PORTION OF THE FIXED WEIR SHOULD NEVER EXCEED THE HEIGHT OF THE WEIR. To allow this to happen is to defeat the principles of the basic design. The only reason for the weir is to create a chamber within the separator to control the small volume of oil that is retained there as a control LIQUID SEAL that ensures that HP gas will not exit with the produced oil. If you insist on raising the oil level higher than the weir, you are defeating the reason for the weir and could operate simply without a weir in the first place. It is perfectly possible to fabricate a 3-phase separator without a fixed weir. All you need to do is to put in an overflow pipe within the oil layer. However, if a fixed weir was installed in the separator, then the original scope and design of the vessel was to operate in the simple manner that I show in my sketch that I include in my workbook.

Why couldn’t the original OP take the time and effort to create the same sketch that I did? It would have been so much simpler because it allows us all to make our comments (with call outs) directly on the sketch and be fully understood right away.

I hope this experience helps you out.

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#8 bernath


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Posted 21 December 2010 - 04:24 AM

@erwin and art montemayor: many thanks for the explanation, u guys have been been very helpful.

sorry art, for not make my question clearer

Edited by kanankiri, 21 December 2010 - 04:46 AM.

#9 trishark


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Posted 04 January 2011 - 05:42 PM

I also have spent a bit of time in Texas and Oklahoma in the oil patch, both onshore and offshore. I would like to compliment Art on his excel diagram and explanation of the working of an "oilfall" type separators. I would like to add a little info about the other type of separator, a uniform oil level.

In both of the above versions, the weir is critical to the function of the separation as is the length of time required for the oil in the 3 phase mixture to flow over the weir. Not having the weir would allow the water to "flow" along the bottom of the vessel and escape thru the oil outlet. Also, residence time for the water is as critical as for the oil, the water outlet would be as close to the weir as possible. The oil is floating up in the water as the water sinks from the oil. In Art's diagram, the mixture at the left would be almost consistent from top to bottom with whatever the water cut is for the mixture. As the mixture flows toward the right, liquids separate and the % of other liquid is gradually reduced. If due to lack of residence time, there is still 1% water left in the oil when it passes over the weir, it does not mater if the oil level is at the weir in an oilfall or several inches above with uniform oil NLL across the entire vessel. The suspended water will still pass into the oil chamber.

The prod water level (interface, often referred to as NILL) must be controlled to be below the top of the weir. This is critical in offshore floating platforms as vessel pitch and roll could allow water to surge and overflow into the oil section under some disruptions. It also disrupts the flow of oil over the weir. I see more uniform level separators offshore.

Many oil separation units consist of multiple separators in series. This allows a small percentage of the water to pass thru into the oil section. It will be caught in the next separator. Additionally, the first or flash separator might have 2000 psi internal pressure. An adjustable choke would be used upstream of the separator to control both flow and separator pressure from the well gathering flowlines. At 2000 psi, only about 20 to 50% of the gas in solution originally with the oil and water would be separated. Think pop the top on a Coke. By taking additional pressure drops thru chokes and separators, the reduced vessel pressure allows dissolved gas to separate and flow out the top of the vessel. Obviously, the more time for the oil and water to remain in the separation chamber of the separator, the less water will still be in suspension with the oil and gas dissolved in both liquids.

Hydrocylones or other technology is used to separate the small amounts of oil from the prod water. This oil will typically be returned to one of the separators to go thru the process again.

Residence time within a separator could be increased so that all oil and water absolutely separate but the vessel would be so large/long and expensive that it would not be practical.

LSLL (level safety low low) levels for both the interface and the oil level are critical as Art mentions for a seal against gas blow thru and to keep oil from flowing out the water nozzle. On the oilfall separator, the oil level in the separation chamber is not controlled directly by a control valve exiting the vessel. It is a result of having sufficient oil in the produced fluid to keep oil on top of the water. If water is removed and not enough oil is present, the oil level could be below the top of the weir. There may or may not be a SD on this low low level. If the interface reaches its LSLL, the SDV on the prod water line would close. For the uniform oil level separator, the LSLL is usually just above the weir top.

Just my understanding of separator design and control and as always, open to discussion.


#10 Erwin APRIANDI


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Posted 13 April 2011 - 03:11 AM

Hi All Just To make it clear

please find the enclosed file that I get from Shell DEP

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#11 Art Montemayor

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Posted 13 April 2011 - 09:14 AM


Without a detailed explanation of how the submerged weir design works, I am afraid you have muddied the waters moreso, rather than "made it clear".

There are so many un-identified lines drawn on the Shell sketch that it is confusing without a detailed explanation. It takes mental work to organize and produce an explanation that makes things clear. Shell Oil doesn't just produce a sketch and throws it at you saying that it suffices to make things clear. Their DEP is extensively written, reviewed, illustrated, checked, and edited to ensure that the technical explanation is clear.

#12 Erwin APRIANDI


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Posted 14 April 2011 - 10:18 PM

Dear Art, Apologize that I just upload the sketch without having detail explanation for the sketch.

Actually DEP has not explain to detail on the process description for the submerged weir.

I just want to said that I have seen this kind of 3 phase separator construct, commissioned, and operate since I have seen one under operation

The main reason why this type of separator is choosen is because the amount of water flow is not to much if compare with condesate and gas flow (how much is the ratio, from what I have check from the balance is less than 15% of Volume Flow)

So what I can explain for the submerged weir is:

When the feed go inside the separator
the gas and liquid will disengage and, the area separation above the liquid level should be able
to ensure the required critical velocity is achived for gas liquid disengagement
and mixture of liquid will be going to the calming baffle, where the flow is calmed so that the separation of heavy liquid and light liquid will occurred. Since the heavy liquid is much less than the light liquid
in the area near to the weir it is expected that the separation has been achieve and most of the upper part of the liquid will be occupied by light liquid therefore when the liquid pass thru the weir,
most of the liquid after the weir will mostly be a light liquid

I'm not chalanging anyone here, just to elaborate that such submersible separator is exist and it is under operation, even if you ask me to choose which one which I preferably used for my design, I would better choose the overflow weir, since from what I know from the operator, the carry over of water in the light liquid for the submersible weir is much higher compare to overflow weir (this is based on their comparison of Production Separator (submersible weir) Performance VS Test Separator (overflow weir)

Hope this has made it clear Art,

#13 smalawi


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Posted 15 April 2011 - 06:10 AM

not related, but Shell Oil is the US company and uses the EGGS, and DEP is used Shell outside US (RDS), recently there was a move to combine these. Also the DEP's are very detailed indeed as Art indicated, yet they are confidential and used with permission

#14 Elizabeth_I


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Posted 21 April 2011 - 07:34 AM

Thanks for sharing this useful information. It's great!

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