19 replies to this topic

[Pronab]

In a fractionation column where kettle reboiler is used as heat source, to avoid steam blow, a stand pipe is installed in parallel of reboiler. Unfortunately level is not build-up in this stand pipe. However we don't find any symptom that steam is blowing through the reboiler. Anyone can suggest how to build-up this level?

Thanks in advance.

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•  Reboiler stand pipe.jpg   53.57KB   1 downloads

Edited by pronab, 28 December 2020 - 02:53 AM.

[fallah]

In a fractionation column where kettle reboiler is used as heat source, to avoid steam blow, a stand pipe is installed in parallel of reboiler. Unfortunately level is not build-up in this stand pipe. However we don't find any symptom that steam is blowing through the reboiler. Anyone can suggest how to build-up this level?

Thanks in advance.

 

Hi,

 

I think to have build up level inside the stand pipe, per the attached, you should consider small size top connection for the upper stand pipe connection to the steam inlet line.

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•  Stand pipe.png   313.58KB   1 downloads

[katmar]

Why would you expect a level to build up in the line that bypasses the reboiler?  Unless the FCV is changed to a LCV linked to the level transmitter on the bypass line there is no reason for the condensate to back up.

[fallah]

Why would you expect a level to build up in the line that bypasses the reboiler?  Unless the FCV is changed to a LCV linked to the level transmitter on the bypass line there is no reason for the condensate to back up.

 

Dear Katmar,

 

This is a typical configuration to monitor/control the condensate level in the steam reboilers in which a stand pipe (or condensate pot) is being installed in parallel to the reboiler with an equalization line (such as what i proposed to the post initiator in detail) to do so...

[breizh]

Hi,

To add to the previous answers.

Hope this helps

Breizh 

 

[Pilesar]

If I understand correctly, the 'standpipe' acts as a large level bridle to indicate there is some amount of equivalent condensate in the reboiler. Relative elevation matters. Your drawing shows the level transmitter range entirely above the reboiler tubes where it will not register a level. Perhaps your drawing is inaccurate, but the measurement range for the level should span the relevant area and preferably extend below the reboiler elevation.

[fallah]

If I understand correctly, the 'standpipe' acts as a large level bridle to indicate there is some amount of equivalent condensate in the reboiler. Relative elevation matters. Your drawing shows the level transmitter range entirely above the reboiler tubes where it will not register a level. Perhaps your drawing is inaccurate, but the measurement range for the level should span the relevant area and preferably extend below the reboiler elevation.

 

Correct... In my previous responses i assumed the top flange of the stand pipe (condensate pot) is above the top of reboiler tube bundle and the bottom flange well below the bottom of the reboiler itself...

[Bobby Strain]

Many designers make this same mistake in connecting the top of the standpipe to the inlet steam line. If I were you, I would simply close the valve to the steam line and forget about the pot. When you have an opportunity you can follow Fallah's advice about arrangement. And connect the top of the pot to the exchanger at the top of the condensate outlet pass.

 

Bobby

[Pronab]

 

If I understand correctly, the 'standpipe' acts as a large level bridle to indicate there is some amount of equivalent condensate in the reboiler. Relative elevation matters. Your drawing shows the level transmitter range entirely above the reboiler tubes where it will not register a level. Perhaps your drawing is inaccurate, but the measurement range for the level should span the relevant area and preferably extend below the reboiler elevation.

 

Correct... In my previous responses i assumed the top flange of the stand pipe (condensate pot) is above the top of reboiler tube bundle and the bottom flange well below the bottom of the reboiler itself...

 

The level in stand pipe will ensure that reboiler tube bundle is submerged and avoid steam blowing to down stream. The FV,flow control valve works in Casade mode with Tray temperature in the control parameter.

[fallah]

 

The level in stand pipe will ensure that reboiler tube bundle is submerged and avoid steam blowing to down stream. The FV,flow control valve works in Casade mode with Tray temperature in the control parameter.

 

 

Absolutely correct...

 

And, furthermore, it's worth to mention the relevant FIC is normally overridden by a LIC when a low level is detected in the stand pipe...

[katmar]

I have never seen this arrangement before, and I cannot find any such diagram in Kister, Kern, Perry or the document posted by Breizh. The OP stated that the purpose of the stand pipe was to avoid steam blow but I cannot see how it will help with that. Unless there is some other low level switch that will close off the FCV how will it prevent steam blow? As I said, I have not seen this arrangement and I would be very grateful to Fallah or anyone who can explain how it is intended to work (seeing that it is commonly used).

The FCV on the condensate outlet will cause the condensate to back up in the reboiler, thus varying the heat transfer area. I can now see that this will cause the stand pipe in parallel with the reboiler to back up as well. But there will be very little flow through the stand pipe (assuming it is well insulated) and the frictional pressure drop through the stand pipe will be low. Of course the overall pressure drop across the stand pipe and the reboiler has to be the same and if the frictional pressure drop through the stand pipe is less than for the kettle then the static pressure drop (and therefore liquid level) has to be a bit higher in the stand pipe than in the kettle. Using this level as a confirmation that there is a liquid level in the tube bundle is the only purpose I can see for the stand pipe.

The fact that the OP has not observed any steam blow from the reboiler is really just a confirmation that the design engineers got the exchanger area correct and the FCV is causing the correct degree of condensate back up - but it seems that the height of the level indicator on the stand pipe is wrong and the back up is not in its range (as pointed out by Pilesar). Or maybe there is a blockage in the upper part of the stand pipe?

However, Bobby Strain's comment indicates that my understanding is completely wrong that the stand pipe is being used to confirm the level. If the top of the stand pipe is connected to the condensate outlet pass of the reboiler it would never see any steam/condensate interface and only condensate would flow through the stand pipe.

Pronab - I can see that if there is a level in the stand pipe (and therefore also in the tube bundle) there will be no steam blow. But what causes the level in the stand pipe independently of the the level in the reboiler?  The level in the stand pipe will confirm that you have a level in the reboiler but it will not ensure it.

This whole arrangement has got me completely confused and I look forward to being educated on how it is intended to work.
 

[fallah]

The OP stated that the purpose of the stand pipe was to avoid steam blow but I cannot see how it will help with that. Unless there is some other low level switch that will close off the FCV how will it prevent steam blow? As I said, I have not seen this arrangement and I would be very grateful to Fallah or anyone who can explain how it is intended to work (seeing that it is commonly used).
 

 

The stand pipe (or condensation pot) per the modified arrangement i provided, actually has two function as follows:

 

- To avoid steam blowing to down stream (by using a LT at stand pipe which relevant LIC overridden the FIC on steam inlet line at low level inside the stand pipe hence in the condensate outlet line from the reboiler, leading to the FCV close till the condensate level again build up).

- To ensure that reboiler tube bundle is submerged (by using an pressure equalization line between the stand pipe and the reboiler itself).

[breizh]

https://www.vega.com...emical/reboiler

 

The link above describes a technology suitable for reboiler .

I've experience with this technology to follow level on Thermosyphon reboiler , about ten columns equipped.

 

Breizh 

Edited by breizh, 29 December 2020 - 07:28 AM.

[katmar]

Thanks Fallah, this makes it much clearer.  So it is the instrumentation that prevents the steam blow and not the stand pipe itself.  The LLA and switch could in theory be mounted on the reboiler steam and condensate headers to measure the level, but I can see that having a parallel standpipe will probably make the measurement more robust.

 

But I'm not so clear about the second point regarding keeping the bundle submerged.  Are you referring to the process side or the steam side?  My understanding is that the steam side is only partially submerged so that there is still some heat transfer surface for the steam to condense on.  The process side is surely not connected to any steam balance line?

[fallah]

 

But I'm not so clear about the second point regarding keeping the bundle submerged.  Are you referring to the process side or the steam side?  My understanding is that the steam side is only partially submerged so that there is still some heat transfer surface for the steam to condense on.  The process side is surely not connected to any steam balance line?

 

Correct...in fact having low or low low condensate level inside the stand pipe indirectly indicates having no enough submergence at process side to absorb the latent heat of the steam...leading to low condensate level at the reboiler's outlet condensate line....

[Pronab]

 

 

The level in stand pipe will ensure that reboiler tube bundle is submerged and avoid steam blowing to down stream. The FV,flow control valve works in Casade mode with Tray temperature in the control parameter.

 

 

Absolutely correct...

 

And, furthermore, it's worth to mention the relevant FIC is normally overridden by a LIC when a low level is detected in the stand pipe...

 

Yes it is indeed,override the flow controller. However since level is not maintain at all, LIC kept manual and although we know there is no level in the stand pipe. Otherwise Flow control valve will close and there will be no steam supply. We have two locations same type of reboiler configuration with same behavior(no level in the stand pipe).

[Pronab]

Thanks Fallah, this makes it much clearer.  So it is the instrumentation that prevents the steam blow and not the stand pipe itself.  The LLA and switch could in theory be mounted on the reboiler steam and condensate headers to measure the level, but I can see that having a parallel standpipe will probably make the measurement more robust.

 

But I'm not so clear about the second point regarding keeping the bundle submerged.  Are you referring to the process side or the steam side?  My understanding is that the steam side is only partially submerged so that there is still some heat transfer surface for the steam to condense on.  The process side is surely not connected to any steam balance line?

Yes process side is not connected anyway with steam side. Reboiler tube bundle is submerged with process fluid. The level in stand pipe only to ensure that steam is not blowing. The arrangement of instrumentation is such that in case of stand pipe level coming down, Steam flow control valve will start to close and lowest output of two controllers will follow the opening of FCV ( Steam flow cntrol valve).

[fallah]

Yes it is indeed,override the flow controller. However since level is not maintain at all, LIC kept manual and although we know there is no level in the stand pipe. Otherwise Flow control valve will close and there will be no steam supply. We have two locations same type of reboiler configuration with same behavior(no level in the stand pipe).

 

 

 

Yes...but although the level isn't maintained; it's not allowed the level to be lowered than a low low value...as once it happens an process interlock will close the FCV to prevent steam blow out...

[Bobby Strain]

     Interesting discussion. However, I have never seen such a contraption in over 50 years of my career. It is common to use a condensate pot with a level control device to override the condensate flow control, but only for exchangers with shellside steam condensation. So I think Pronab has the only such installation in existence. Often times with shellside steam, designers have tried to control exchanger exposed area by controlling the level in the pot. But it never works. I have fixed many of these installations. So, after more thought, my recommendation still stands. The purpose of the standpipe is to assure that there is condensate, not live steam, but, it seems to be unnecessary.

 

    Pronab's installation won't show a level because the inlet steam pressure is sufficiently higher than the condensate pressure to suppress the level below the level transmitter range.

 

Bobby

[fallah]

     Interesting discussion. However, I have never seen such a contraption in over 50 years of my career. It is common to use a condensate pot with a level control device to override the condensate flow control, but only for exchangers with shellside steam condensation. So I think Pronab has the only such installation in existence. Often times with shellside steam, designers have tried to control exchanger exposed area by controlling the level in the pot. But it never works. I have fixed many of these installations. So, after more thought, my recommendation still stands. The purpose of the standpipe is to assure that there is condensate, not live steam, but, it seems to be unnecessary.

 

 

In fact, despite of the shell side or tube side steam condensation in the relevant reboiler, the main role of the condensation pot and relevant level control is to avoid sending steam to the liquid condensate network...then it's obvious having also such facility in the reboiler with steam condensation in the tube side...