10 replies to this topic

[Phani Kanth]

Hi All,

Please share information regarding fixing of Levels(LLLL,LAL,NLL,LAH,HHHH) in kettle type reboilers.

In these escepically fixing of LLLL level in my openion this will be above tube bundle is better but in some references i had found these LLLL level in near bottom row of Tube bundle.

So, Please suggest better one



Regards,
Phani Kanth

[Art Montemayor]

Phani Kanth:

I believe that the best way to discuss this important topic is to rely on a detailed sketch of a kettle reboiler – especially the one type that you are employing (or discussing). It is important to identify what you mean by your nomenclature. I have found throughout the years and many countries I’ve worked in, that there are different ideas and interpretations of what is meant by:

• LLLL;
• LAL;
• NLL;
• LAH;
• HHHH.

I am presently on my way to a family reunion with my 3 grandchildren for Father’s Day and so, haven’t got the time to really make a detailed sketch of what I mean. So I am offering the attached as perhaps an illustration of what I mean.

I will add more detail and comments Monday, when I return. Have a good weekend.

Attached Files

•  Kettle Reboiler.xls   893KB   517 downloads

[Phani Kanth]

Thank you Mr.Art for your reply,

In my earlier post the nomeclature meaning is:

LLLL- Low Level Shut Down

LAL - Low Level Alarm

NLL - Normal Operating Level

LAH - High Level Alarm

HHHH- High Level Shut Down


and further to my earlier post here I'm attached probelm description with sketh form as provided by you with two options that are Option-1 and Option-2.

and thanks for giving good suggestion regarding problem description with sketh.


In Option-1:

I was placed LLLL level above to top row of Tube bundle.


In Option-2:

I was palced LLLL level above to bottom row of Tube bundle.


and my question is which is better option and which option is normal practice and is there any guideline for the same




Regards,
Phani Kanth.

[Phani Kanth]

Sorry...!

Attachment missed for my earlier post and same please find here.


Regards,
Phani Kanth.

Attached Files

•  Kettle Reboiler(With Level Description).xls   989.5KB   361 downloads

[TS1979]

Option 2 is impossible. The level in kettle reboiler is controlled by the height of the weir. The height of the weir is set high enough to keep the tube bundle submerged in the liquid. No alarms or shutdown is required for the kettle reboiler design at the tube bundle side. At the over flow side, a level control is required to control the withdraw of the bottom product.

[Art Montemayor]

Phani Kanth:

I think there is some confusion as to what the intent and purpose of the various instruments around a kettle reboiler are supposed to conform to. I have finished the sketch of the kettle reboiler as I originally intended it to be.

It is not uncommon to get confused with a kettle. What seems to be a simple application can require more thinking and engineering because of the internal design and the control of the heat transfer that is necessary. This is the reason I made it a point to have you identify your nomenclature. And now, because of your reply and explanations on what you mean, I can clearly identify that you have a problem because of the lack of a detailed understanding of what is expected in the operation of a kettle reboiler.

The kettle reboiler is designed to operate as follows:

• Employ submerged heat transfer, ensuring that there is a constant liquid bath around the heating medium and that the reboil vapor is generated in the liquid surface. This is a “gentle” type of vaporization;
• The horizontal orientation of the kettle lends a very large and ample vapor disengagement space above the vaporizing liquid, ensuring that the rising vapor is dry and there is a logical equilibrium established that allows one to count the reboiler itself as a theoretical tray – or a part of one.
• In order to maintain the tube bundle totally submerged, an internal weir is usually employed (although an overflow pipe could also be used. When a weir is used, it establishes TWO different heat transfer and hydraulic chambers within the kettle: one chamber ALWAYS has a constant, fixed, liquid level where there is heat transfer; the other has a constant liquid level that is controlled by instrumentation (not by a weir) and has no heat transfer within it.
• The objective is to keep a constant liquid level over the tube bundle (which the weir accomplishes) and a liquid seal on the outboard side of the weir in order to avoid vapor blow through and control the NPSH on the outlet bottoms pump.
• Level controls and alarms are required to avoid a high level that could flood 100% of the kettle and cause a violent eruption of vapor through the vapor outlet. This could happen is there is excessive liquid feed into the kettle or if the bottoms pump fails.
• Level control is also needed for a low level on both the tube bundle side as well as the outboard side of the internal weir. However, a low level on the tube bundle is normally not expected (in the event of a stoppage of liquid feed) because the internal weir is 100% seal-welded and there is a shutdown in the event of feed failure.
• A low level on the outboard side of the internal weir is possible due to the bottoms pump pumping an excess of bottoms product due to a level control failure on the outboard side of the weir.

From the above description of how the kettle reboiler operates and how it is designed, you can arrive at the answer(s) to your query: there is no option involved on where to place the Low-Low Level Alarm on the tube bundle side of the weir. You normally do not have any low level alarm there. If you want one (because you don’t trust the seal weld on the weir), then you can install one on the visual gauge level indicator on the tube bundle side.

I hope this help you better understand this operation.

Attached Files

•  Kettle Reboiler Rev2.xlsm   37.32KB   514 downloads

[Phani Kanth]

Dear Mr.Art Montemayor,


Thank you for your reply and sharing valubale points for this post.

Yes, I'm confused earlier and after reading your points i'm clearly understading this reboiler operation and thanks for the same.

Due to curiosity of learning i was not posted my problem clearly,

Actually i'm designing the Glycol Reboiler which is used for Gas Dehyradtion systems.
In this application this reboiler is connected(Case-1,External by using overflow pipe,Case-2,Internally fixed) with Stripping column.
So, in this reboiler internal weir may be not practical then how to control by using level instruments this kind of reboiler?

Here I'm attached detailed sketh for the same.

Please give your guideline for the same and please answere for below points also

1) How to fix height of weir?

2) Lenght between tube bundle end and weir?

3) How to calculate required vapor disengagement space above the liquid surface?

Attached Files

•  Reboiler with Stripping column.xlsx   32.57KB   217 downloads

[Art Montemayor]

Phani Kanth:

Now we hear the rest of the story.

I don’t know why our members resist the need to furnish accurate, complete, and detailed basic data at the very outset of their first posting in order to generate accurate, complete, and detailed comments and recommendations. These queries are not military or government secrets.

Now that I am receiving more details and the true and specific nature of the query, I can offer more detailed and, hopefully, valuable advice.

You continue to be confused and I am inclined to think that you either are not a chemical engineer or you lack considerable design and operating experience. Whichever the reason, my comments on the proposed design are found in my revision of your submitted workbook. My comments on your latest post are:

• You are not correct in stating “reboiler internal weir may be not practical”. It is not a question of being “practical”; the internal weir is simply not required when an overflow pipe is employed on the kettle.
• You ask: “how to control by using level instruments this kind of reboiler?” the answer is simple: the overflow pipe (that acts as a weir) controls the level around the tube bundle. It is physically impossible to have a low level around the tube bundle – unless the kettle ruptures and leaks out the TEG. If that happens, the entire unit is down and you have a catastrophe on your hands. You can have a level higher than the overflow pipe and for that purpose what I have used is an external, visual level gauge that has a high level alarm and/or switch on it that allows for alarm and shutdown.

And, to answer your numbered questions:
1) How to fix height of weir?
This is easy. You have NO weir. What you do set is the height of the overflow pipe at approximately 5-6 inches above the top of the tube bundle. That is my recommendation.

2) Lenght between tube bundle end and weir?
Again, there is NO weir. The distance between the end of the tube bundle and the end head on the kettle should be approximately 12 to 24 inches in my opinion.

3) How to calculate required vapor disengagement space above the liquid surface?
I recommend you allow a minimum of 18 inches between the top of maximum liquid level and the top of the kettle shell.

I hope my comments and recommendations help you out.

Attached Files

•  Reboiler with Stripping Column Rev1.xlsx   32.07KB   318 downloads

[Propacket]

This is again an excellent response from Mr.Art. I just wanted to add that vapor disengagement space above the liquid surface recommended by Mr.Art (18 inches) should be considered as "minimum". S&T exchanger manufacturers recommend a vapor disengagement space of 1.4 to1.5 times the bundle diameter.

[Phani Kanth]

Thank you Art for your more detailed explanation and comments from above posts and i had really learned now

and Thanks to P.Engr for his extended suggestion.

This lecturer is very useful to my career improvement and this was i can't forget and I wish you Art to continue this kind of replies for forum topics.

Regards,
Phani Kanth.

[srfish]

There are two types of equations for calculating the kettle diameter. One type calculates a vapor load (vapor flow divided by the volume of the vapor space). This equation can be found in the Wolverine Tube Heat Transfer Data Book. The other equation type is where you calculate a maximum horizontal velocity. Its basic form is a Souders-Brown vapor/liquid seperator equation where you use an entrainment coefficient. This coefficient has a value of 0.15 in U,S. customary units. HTRI has a more complicated series of equations to calculate this entrainment coefficient.