9 replies to this topic

[fallah]

Dear Mr. Art Montemayor
I believe the fact that when we require for liquids to flow by gravity from one vessel to another, there must exist an equalization of the vapor pressures in both vessels,but i have some questions as follows:
1-If there is no "entrained gas" what is your mean about "vapor displaced" in your previous statement dated june 7 2007 :"In order to freely drain liquid from one vessel into another using gravity flow, you must take into account that the vapor displaced in the lower target vessel has to be either removed or transmitted back to the original, upper source vessel"?
2-Would you please explain about the difference between your two statements as follows:

-"No, an equalization line can be substituted with a self-venting dip pipe. However, the diameter of the dip pipe will tend to be very big in order to facilitate self-venting." dated june 12 2007
- "The same principle is applied in a self-venting nozzle. As I mentioned, you can also employ a direct drain nozzle from the condenser into the reflux drum (without a dip pipe) – but you must ensure that the nozzle is large enough to furnish self-venting (this is another way of equalizing the pressures in both vessels." dated june 23 2007
I want to know which is correct? "self-venting with dip pipe" or "self-venting without a dip pipe"
3-when we have an equalization line along with a simple nozzle rather than dip pipe (for liquid gravity flow),do we have any "entrained gas" along with the draining liquid?

Warm Regards
Fallah

[fallah]

Dear Art.

Hope, replying to my questions have not been forgotten.

Warm Regards
Fallah

[Art Montemayor]

Fallah:

Answering your questions, as you can see by the attached, is not a simple task. The problem of communicating in writing is what I believe is keeping you and many others from understanding what I mean by gravity flow and drain.

Please do not confuse english terms - such as "entrainment" - as meaning something else. Entrainment does not (or should not) occur in a properly designed system if one is trying to drain only the liquid portion - as was expressed in the original thread of draining the condensate from a partial condenser.

Please study my examples and sketches very carefully and also read P. D. Hills' article which is included in my workbook.

I hope this helps you and other individuals who are trying to understand why we place such a strong emphasis on proper draining design and understandings of the basic hydraulic principles.

 Gravity_Flow.zip   1.03MB   1210 downloads

[fallah]

Dear Art.
Excuse me for quickly follow-up and thanks a lot for your valuable answers to my questions,but I have not still convinced in the case of my 2nd question.In your spreadsheet a part of my 2nd question (I want to know which is correct? "self-venting with dip pipe" or "self-venting without a dip pipe") is deleted and therefore your answer is not compatible to my question.I think one of your below statements,and in my opinion the second one,could be correct and both of them could not.
1-"No, an equalization line can be substituted with a self-venting dip pipe. However, the diameter of the dip pipe will tend to be very big in order to facilitate self-venting." dated june 12 2007
2- "The same principle is applied in a self-venting nozzle. As I mentioned, you can also employ a direct drain nozzle from the condenser into the reflux drum (without a dip pipe) – but you must ensure that the nozzle is large enough to furnish self-venting (this is another way of equalizing the pressures in both vessels." dated june 23 2007

Please explain and correct me if I am wrong.
Warm Regards
Fallah

[Art Montemayor]

Fallah:

I’m sincerely sorry if I have failed to explain specifically what I have been trying to state: Although I don’t understand your meaning of “correct”, I believe you mean to query which of two methods is preferred in a given situation: a simple, direct, draining nozzle or a dip pipe. I believe I’ll repeat myself one more time and state: neither is “correct”. Both are applicable – depending on what you want to accomplish and how accurately you design the system so that there is no entrainment of vapors or “slugging” (intermittent) flow – which is what I have stated you require for a distillation system under vacuum.

Please understand that I have no interest in proving that I am right and that you are wrong. I believe that logic will always determine that outcome. I am simply offerng and detailing out to you and others what I have learned in over 47 years of carrying out my engineering work and projects. If you disagree with what I recommend, then we disagree. You do not have to believe in what I explain. I have tried to explain in both writing and detailed sketches, trying to show how gravity influences the flow of liquids down a vertical duct. I have even copied out the entire article of P.D. Hills – complete with illustrations and graphs – in order to reinforce the facts backed up by experience and observations of the hydraulics of gravity on liquids.

Perhaps if you state exactly what it is that you don’t understand, or believe is “correct”, then we can discuss the specific details of your argument. So far, what you’ve said essentially is that I’m not correct; you have yet to state what it is that you prescribe or identify as “correct”. What I continue to expound is that I’m not interested so much in being “correct”. I’m an engineer and I know, from experience and hydraulic studies, that what I prescribe works. It has worked for me, and it has worked for many others in the past. If you know something else that works better, than let us know in writing and in detailed drawings so that we can share that experience.

[fallah]

Dear art.
It is surprising that my simple question about your previous statements is interpreted in another manner.My recent question, leading to complicated discussions, was limited to self-venting gravity [/b[b]]flow.I had studied the P.D. Hills article four years ago and in this article it is stated that(as one of approaches to the design of gravity flow) "For self venting, with the liquid velocity in the outlet pipe kept low enough to allow gas to flow countercurrently to the liquid".
It is obvious that(based on the a.m. fact from P.D. Hills article and other principles of gravity flow) the self-venting gravity flow can not be designed with dip pipe because if we suppose that the dip pipe establishes the conditions for liquid seal, then prevents the gas to flow countercurrently to the liquid (the most important condition for self-venting gravity flow).
Therefore,it seems, we shall consider the design of self-venting gravity flow without dip pipe.
Regards
Fallah

[Parmindar Singh Rana]

Dear Art,
I have a specific doubt about the scenario where we use an equalizing line between the source vessel and the target vessel.
As per my understanding since we are using an equalizing line the pressure in both the vessels is same, so the head gained by the liquid while flowing down from source vessel to target vessel and the friction losses in the pipeline will be equal, and hence we can calculate the flow from given size of pipeline or vice versa. But what if we need to operate the system during turndown as the drain pipe will be of same diameter. Will it still work satisfactorily.
For instance, in my case a mixture of C4 liquid having Density of about 550 Kg/Cu m and mass flow rate of about 26000 kg/hr. The drum pressure is operated at bubble pressure which is around 2.5 Barg.
An extraction column is used to wash away the water soluble impurities from C4 liquid feed.
C4 feed is fed at bottom of the extractor with wash water at top. Top C4liquid and Water interface at top of extractor is maintained by control valve on waste water outlet from bottom of the column.
The top C4 Outlet nozzle is at elevation of about 40 meters and the line leading to a surge drum which is at elevation of about 8 meters, the nozzle is submerged in the drum liquid. An equalizing line is provided between the drum top and the column top. The column top also to be submerged.
The process line from Extractor to drum is sized to be of 6" and Equalizing line is also of 6". Can you suggest if this system will work for turndown case of 50%.

[Art Montemayor]

Parmindar:

Your explanation is not clear to me. Please facilitate us with a detailed sketch showing what you are trying to describe - very much like I have tried to do in the attached Excel workbook.

I may not have depicted accurately what you have because I may have misunderstood you. Correct whatever I have not depicted accurately. A picture is worth a thousand words.
 Extractor_Sketch.xls   895.5KB   261 downloads

[Parmindar Singh Rana]

Hi Art,

The Sketch of the system is as depicted in the attached excel file.

The C4 liquid flows from extractor to the Surge drum via a 6" line (Line Sized per gravity flow criteria per standard), the elevations of the equipment have been shown on the Excel file. The drum is maintained at bubble point of C4 liquid and an equalizing line (6") is used as shown.

As per my understanding since we are using an equalizing line, the pressure in Surge drum and the Extractor is same, so the head gained by the C4 liquid while flowing down from Extractor (source vessel) to Surge Drum (target vessel) and the friction losses in the pipeline will be equal. Please correct me if I am wrong.

Will this system work if I need to operate it 50% throughput. The process condition I have mentioned earlier.

Regards,
Parmindar
 Extractor_Sketch_Rev1.xls   930KB   170 downloads

[Art Montemayor]

Parmindar:

Thank you for the revised workbook. I have renamed it with a “Rev1 suffix in order to keep it in an identifiable and organized state with respect to this thread. You did a great job in depicting what you have and I am sure you can appreciate the clarity and accuracy of communicating with these type of sketches and ensuring that the engineering message conveyed is detailed and accurate. No one reading this thread should be left with only an assumed idea of what you mean. I have taken the liberty of adding some details and comments to your revision and am attaching it as Rev2.

I can now comment that that I see no problem in visualizing approximately 200 gpm of saturated liquid Butane flowing by gravity through 38 vertical meters of height in a 6” pipe – as long as this flow is equalized between the target drum and the tower overflow source. I haven’t done any hydraulic calculations on your system; I can only give my comments based on my experience. I suspect the 6” equalization line is much too big in diameter for what it is intended to do. Hydraulic calculations will, I hope, bear me out.

You have not stated that this system is existing actually as depicted and in successful operation at the capacity stated. I presume it is. You state: “the pressure in Surge drum and the Extractor is same, so the head gained by the C4 liquid while flowing down from Extractor (source vessel) to Surge Drum (target vessel) and the friction losses in the pipeline will be equal. Please correct me if I am wrong”. The difference in static head between the Butane liquid level at the top of the column and the drum liquid Butane level is the driving force that allows the liquid Butane to flow through the 6” line. As I stated, I haven’t done the hydraulic calculations (I suspect you have). However, I feel the frictional resistance in the 6” transfer pipe (plus the entrance and exit losses) should be less than the hydrostatic head – not equal. Thus, flow is free to fall into the drum.

Additionally, if you intend to reduce (“turndown”) the Butane flow rate by 50% (down to 23 m3/h – 104 gpm - , approx.) you should have no problems doing so. Any fixed, gravity flow system instrumented to work as overflow should work even better at a reduced flow rate. You haven’t described your downstream Butane pump setup, but I assume that you are controlling the flow rate through the pump by simply recycling excess flow capacity in the pump back to the drum. If that is the case, then the pump should also not be a problem as long as it doesn’t overheat.

I hope I have responded to your questions and/or concerns.
 Extractor_Sketch_Rev2.xls   1MB   300 downloads