5 replies to this topic

[Radionise]

Hello guys,

Imagine an LCV at the produced water outlet stream of a separator for an upstream offshore service, which means that inlet flow rate will vary from time to time...

It just occured to me that how can an LCV which is choked due to big pressure differential across it, say a DP = 70 bar across the control the level, permits any level control at all?

Correct me if I'm wrong but I suppose the flow rate out of the control valve has to vary in order to maintain the water level in the separator. So am I right to say that an the flow across any LCV must not be at choked condition to allow for level control purposes.

Thanks in advance.

[latexman]

You are correct.

To continuously control the level in a steady-state manner it is best to have the ability for the output_water to equal or exceed the largest expected instantaneous input_water, so the accumulation term in a mass balance goes to zero. If not, then you have to have the ability for the average output_water to exceed the average input_water, but this gets a little dicey and complicates your control. If these are not possible, you simply cannot control the water level.

[Art Montemayor]

Radionise:

The following follows the way I interpret your two questions.

First, which “inlet flow rate will vary from time to time...” – the inflow to the Liquid Control Valve? I presume so, if the inlet to the production separator is also varying.

Secondly, engineers normally are only interested and deal in gaseous fluids achieving sonic velocity. I see no practical reason or purpose to impose sonic velocity on a liquid fluid inside a pipe (assuming you could design for, and handle, a sonic-level liquid hammer in your piping). Therefore, the only way that you are going to tolerate sonic velocity (“choked” flow) inside your drain pipe is for the drained liquid to “vaporize” – actually, an isenthalpic flash – across the drain valve (LCV). This may be true in your case, but it depends on the liquid fluid we are talking about. Production separators can handle a variety of hydrocarbon liquids as well as water – it depends on your production characteristics. Only you can tell us that.

Based on a material balance, the liquid fluid that enters the separator must exit through the LCV. I believe that is a basic premise for your continuous operation. There is NO liquid accumulated in the separator. In my opinion, the vapor (or gas) flow at choked condition CAN EXIST at the LCV outlet – if you want it that way or if there is enough gaseous volumetric flowrate through a fixed, small pipe diameter. Normally, this is not what you want – and, therefore not what happens. I can safely control the separator’s liquid level by expanding the liquid content down to whatever pressure I need to drain down to and handle it accordingly. If I find that my LCV will be actually flashing, I need to take this into consideration immediately at its sizing and design stage. If this is true you will find that you probably will also inherit other problems: the isenthalpic expansion will cause cooling – perhaps down to the sub-zero range - due to the hydrocarbon refrigerants being flashed. This is exactly the same principle and technique employed in mechanical refrigeration. The expansion (“refrigeration”) valve is nothing more than a fancy LCV acting in exactly the same manner that you describe – except that it’s signal is reversed: it opens more on liquid level depletion in the evaporator. For many years, before the advent of turboexpanders, that’s the way we produced liquid hydrocarbons at low temperature in order to subsequently apply de-ethanizers, de-propanizers, de-butanizers, etc., etc.

I believe that answers your two basic questions.

[djack77494]

Guys,
I have never heard of anyone being concerned with (strictly) liquid phase sonic (or choked) flow. That is probably because sonic velocities in a condensed phase would be very high indeed. Common acceptable velocities in liquid lines, and even in the minimum flow area of a partially open control valve, should be VERY much lower than sonic. Having seen a number of posts in several forums referring to choked liquid flow, I believe that there is a misunderstanding of this condition. For all practical industrial conditions, I believe that liquid sonic velocity will not exist. Period. Taking a pressure drop through a device of more than 50% of the upstream pressure does not mean you will have choked flow, but I think that "rule of thumb" is perhaps being applied to liquid flow. That would explain the large number of questions I've seen. This "rule" was never intended to be used for liquids, and does not apply to liquids.

If the vapor phase is also present, then the above does not apply, as vapors might easily reach sonic velocities in high dP situations. This could be true even if vapor is not expected at the inlet or outlet conditions, but might form in the vena contracta of (say) an orifice or control valve.

If you are truly refering to a choked flow situation, let me apologize for suggesting otherwise.

Art and other jedis - I would enjoy hearing your opinions on this topic.

[Art Montemayor]

Doug:

Thank you very much for interceding and putting the emphasis on what is a very simple, but important point: The definition and complete understanding of what constitutes “choked flow”.

It is unfortunate that this subject is taking precedence over the main theme of this thread, which is “Level control in a 3-phase separator”. Nevertheless, the subject of “choked” (or sonic) flow is one that is not only important, but one that I thought we had thoroughly expounded AND pounded into the ground on many prior threads. It seems that this subject will not go away and a stake through the heart won’t kill it. I believe I know why the subject won’t die; it’s because young engineers are still laboring under the mis-understanding of what choked flow is and why it is important. I trace this all back to the basic Fluid Mechanics that don’t seem to be taught with practical emphasis at the university level. I also attribute a weakness in the Unit Operations courses being taught to ChEs in university. Perhaps we should not discuss this here in this thread for several reasons:

1. We will lose the continuation of the main theme;
2. We will inherit a very long thread that will have an erroneous title;
3. We will lose the ability to index or search the subject matter in the future.

Let me know if we should move or transfer these posts to a new thread that can be created for the purpose of fully explaining the principles, uses, errors, mis-conceptions, and mis-applications of “choked” flow. I have a lot to say and critique about this subject, but I am fearful that we will lose the effort if we don’t give it the importance it carries on its own at the very outset.

In the meantime, anyone who is interested (and I hope all Chemical Engineers – whether professional or students) can visit and read the following websites:

• http://en.wikipedia.org/wiki/Choked_flow
• http://www.eng-tips....d=174985&page=5
• http://www.eng-tips.com/viewthread.cfm?qid=180311

I have a lot I can write about the problem described in this thread and how to resolve any liquid level control application in a 3-phase separator – and it has little to do with choked flow and more to do with cavitation and flashing problems. However, the subject of choked flow has already been introduced into this thread as if it is a proven reality. This creates another front to attack and diverts us away from the basic engineering application. The answer to Radionise’s level control dilemma is very simple. However, explaining choked flow (or better yet, how it doesn’t apply here) is going to take us on a lengthy subject that is going to take a lot time to explain simply because a lot of engineers out there have not had a good, hands-on explanation of what it is and how it is used in real, practical process design.

Let me know your thoughts on it, and then let’s all attack the common foe: an understanding of what Choked flow is and how it is used and respected.

[djack77494]

Thanks for your response, Art. I concur - this topic has been thoroughly discussed here and elsewhere, and no further comments are needed. For any still interested, follow Art's advise and the included links.
Doug