20 replies to this topic

[tdeb]

Dear all, I would like to ask for your help with the following concern I have. I've read in posts in this forum, that a condensate pot equalization line should be connected to the top part of the condensate outlet.

 

If that is the case, it would mean that the pressure in the pot and in the steam outlet would be the same. But if the top of the pot is below the level of the reboiler, I conclude that the operation would be one of the following alternatives:

- The pot is flooded with liquid up until the level of the condensate in the kettle reboiler, or

- The steam in the reboiler condensates partially, and some steam remains available in the condensate outlet.

 

I would guess that if the equalization line connects to the steam inlet instead of the condensate outlet, then this problem would be solved, as the pot would be at a higher pressure, so the liquid level would be lower than the reboiler level.

 

Am I wrong in any of these statements?

 

Thanks in advance

[Bobby Strain]

A sketch, please.

 

Bobby

[Pilesar]

If the condensate pot is 1) lower than the exchanger and 2) the same pressure as the exchanger, then the condensate will drain by gravity into the condensate pot. The equalization line contains vapor. The exchanger drain contains the denser liquid so the condensate flows through the drain line.

[breizh]

Hi,

Take a look at: http://www.separatio.../POT_Chp03a.htm

Breizh

[shvet1]

para 14.11

https://www.amazon.c...&s=books&sr=1-9

[Pilesar]

[...] if the top of the pot is below the level of the reboiler, I conclude that the operation would be one of the following alternatives:

- The pot is flooded with liquid up until the level of the condensate in the kettle reboiler, or

- The steam in the reboiler condensates partially, and some steam remains available in the condensate outlet.

 

I wanted to address your conclusions directly.

1) Your first alternative (pot flooded) is not the design because the level in the pot is controlled. As the level gets high, the liquid drain valve for the pot opens more to let more water discharge and not allow steam to discharge.

2) Steam condensation is not caused by the condition of the pot, but by the surface area of the reboiler where the steam transfers heat to the process. If there were no heat transfer to the process or to the walls of the shell and pipe, then steam would remain available in the condensate outlet. This would not be a problem because the level control of the pot prevents steam from leaving the pot with the condensate. Normal operation will have steam condensing and building a liquid level higher than the liquid level in the pot in order that the additional head will offset pressure drop in the drain piping due to friction.

  The equalization line is connected between the pot and the region in the exchanger above the liquid level of the condensate in the exchanger so that the pressure at the condensate outlet of the exchanger is close to the pressure of the vapor in the pot so gravity flow would occur. If the pot were somehow a higher pressure than the exchanger with no equalization line, then vapor would travel upward in the drain line against the liquid downward flow causing much loss of capacity for the exchanger drain.

  I am not sure I understand what you mean by the reboiler having a steam outlet. The outlet for steam is after it condenses. There should be no other steam outlet for the system. Reboilers may have a vent for non-condensibles, but any steam exiting that vent is unintentional. The flow in the equalization line would only be enough to maintain pressure in order to make up for steam condensing in the line itself or on vessel surfaces.

[Bobby Strain]

Still no sketch.

 

Bobby

[tdeb]

 

[...] if the top of the pot is below the level of the reboiler, I conclude that the operation would be one of the following alternatives:

- The pot is flooded with liquid up until the level of the condensate in the kettle reboiler, or

- The steam in the reboiler condensates partially, and some steam remains available in the condensate outlet.

 

I wanted to address your conclusions directly.

1) Your first alternative (pot flooded) is not the design because the level in the pot is controlled. As the level gets high, the liquid drain valve for the pot opens more to let more water discharge and not allow steam to discharge.

2) Steam condensation is not caused by the condition of the pot, but by the surface area of the reboiler where the steam transfers heat to the process. If there were no heat transfer to the process or to the walls of the shell and pipe, then steam would remain available in the condensate outlet. This would not be a problem because the level control of the pot prevents steam from leaving the pot with the condensate. Normal operation will have steam condensing and building a liquid level higher than the liquid level in the pot in order that the additional head will offset pressure drop in the drain piping due to friction.

  The equalization line is connected between the pot and the region in the exchanger above the liquid level of the condensate in the exchanger so that the pressure at the condensate outlet of the exchanger is close to the pressure of the vapor in the pot so gravity flow would occur. If the pot were somehow a higher pressure than the exchanger with no equalization line, then vapor would travel upward in the drain line against the liquid downward flow causing much loss of capacity for the exchanger drain.

  I am not sure I understand what you mean by the reboiler having a steam outlet. The outlet for steam is after it condenses. There should be no other steam outlet for the system. Reboilers may have a vent for non-condensibles, but any steam exiting that vent is unintentional. The flow in the equalization line would only be enough to maintain pressure in order to make up for steam condensing in the line itself or on vessel surfaces.

 

 

Dear Pilesar,

 

Thanks a lot for your thourough response. Clearly, I'm having some trouble understanding how the system would be operating. Firstly, I didn't mean to say there was a steam outlet, I meant the condensate outlet (I said steam referring to the stream in which the steam was, but that might have been misleading).

 

What I don't quite get is: if the outlet has a vapour fraction higher than 0, what would happen with the steam that constantly comes out of the reboiler? Would it condensate in the pot? And how would that happen?

 

As for my first alternative, what I was imagining was a case in which the outlet valve's control signal would be saturated, with the valve completely open (forgive me if I don't make myself clear, English is not my mother tongue)

 

Thanks again

Edited by tdeb, 04 July 2023 - 12:19 PM.

[tdeb]

Still no sketch.

 

Bobby

Dear Bobby, I'm having trouble uploading the sketch, for some reason I can't upload an URL from google, nor a sketch made in Paint. Please, apologize the inconvenience

[Pilesar]

Steam will condense on any surface it contacts that has a temperature lower than the steam dew point. This is why your steam piping has traps to remove the condensate. The condensation also happens on the outer shell of your reboiler and also on the wall of the equalization line and also in the condensate pot above the liquid level if the steam is higher temperature than the vessel wall. These areas for condensation are very small when compared to the surface area of the tubes inside the reboiler. Process engineers tend to concentrate their design efforts on the action taking place at the tube surface since the main goal is to transfer heat to the process stream. The steam condensation that occurs elsewhere will take care of itself.

  The steam will remain in the reboiler shell until it contacts a colder surface and condenses. Steam will not leave in vapor form with the condensate. There is liquid in the way. The condensate transfer to the condensate pot is not instantaneous and is subject to line diameter, line length, pipe roughness, and the other parameters that affect hydraulic flow. The liquid in the line to the condensate pot forms a barrier to the steam.

  Steam flows through piping to the reboiler. Inside the reboiler, the steam does not flow the same way. As soon as steam contacts the surface of a colder tube, it will condense and its volume will be greatly reduced. New steam molecules rush in to fill the missing volume and the cycle repeats. Steam contacting a colder surface and condensing is the reason the steam continues to flow into the reboiler. There is no other normal exit for steam from the system.

  The condensate pot functions with its liquid level controlled. The liquid level in the condensate pot also forms a barrier to the steam. Steam is free to transfer through the equalization line and will flow enough to replace the volume lost by the condensation described in the first paragraph.

  Reboiler duty is usually either controlled by 1) changing the steam pressure in the exchanger which changes its dewpoint temperature and affects the driving force for heat transfer. or 2) changing the surface area of the reboiler available for condensation by adjusting the level of condensate in the reboiler. In either method, no steam leaves the exchanger except as condensate.

[Bobby Strain]

Piesar,

     The query is about a kettle. I presume that the steam is on the tube side. Most kettles use U-tubes. The query seems unrelated to any actual installation.

   The pot serves one of two functions. If the condensate flow is controlled to provide the proper duty, the pot level is only an override on low level. The other configuration is to remove condensate from the pot to maintain a normal level in the pot. This is used in conjunction with control on the inlet steam to maintain the desired duty. And, the pot and piping are different for these two functions. So, the query is incomplete, and leaves our response to chance.

 

Bobby

[Pilesar]

     The query is about a kettle. 

 

Thanks, Bobby. I read kettle correctly initially but did not reread as the thread got longer so I got off track. It is not obvious how to add attachments in a forum post. A drawing would help and it is likely discouraging to the original poster not to be able to show a pic.

   If I understand your description of pots with kettles, either of the functions you describe could be handled by a steam trap. Is a pot chosen instead of a trap only for reliability reasons?

[tdeb]

Sketch:

https://docs.google....it?usp=sharing 

[tdeb]

Ok, I was able to upload a Sketch I made in Paint (excuse the horrible drawing). The system would be like the one in the picture (same control scheme, pot in a lower elevation). The image on the left would be the case with the pot flooded (option number 2 of my original post), the light blue curve represents the level liquid. The image on the right would be the case with steam not fully condensing. Of course, as there is flow in the pipe that goes to the pot from the reboiler, there wouldn't be a liquid column in the pipe, rather a two-phase flow pattern until the point in the line where the steam fullly condenses, but for representation purposes, I drew it as a liquid column.

Edited by tdeb, 05 July 2023 - 07:12 AM.

[Bobby Strain]

You are using the pot as a substitute for a steam trap. The pot is more reliable. There is always steam present at the tube outlet. So, the sketch on the right represents how the pot is operating.

 

Bobby

[tdeb]

Thanks Bobby. Best regards

[fallah]

Dear all, I would like to ask for your help with the following concern I have. I've read in posts in this forum, that a condensate pot equalization line should be connected to the top part of the condensate outlet.

 

If that is the case, it would mean that the pressure in the pot and in the steam outlet would be the same. But if the top of the pot is below the level of the reboiler, I conclude that the operation would be one of the following alternatives:

- The pot is flooded with liquid up until the level of the condensate in the kettle reboiler, or

- The steam in the reboiler condensates partially, and some steam remains available in the condensate outlet.

 

I would guess that if the equalization line connects to the steam inlet instead of the condensate outlet, then this problem would be solved, as the pot would be at a higher pressure, so the liquid level would be lower than the reboiler level.

 

Am I wrong in any of these statements?

 

Thanks in advance

Hi,

 

The main points are: 1) The equalization line should be connected from the top of the pot to the steam inlet, as you said, and 2) The top flange of the pot (or upper tangent line) to be installed at a minimum height, say 300 mm, above the top of the reboiler tube bundle...

[tdeb]

Thanks Fallah.

I was wondering, does anyone know any source/book that addresses this topic? I think it would help my understanding.

Thanks a lot

[breizh]

Hi,

Consider reading this old post and replies from Fallah, Bobby and Art.

https://www.cheresou...google_vignette

 

Good luck

Breizh

[fallah]

Thanks Fallah.

I was wondering, does anyone know any source/book that addresses this topic? I think it would help my understanding.

Thanks a lot

 

Hi,

 

At first, thanks Breizh for providing the link of old post for more clarification....

 

There is no specific source for arranging the pot and the reboiler and levelling them with respect to each other...and just a simple hydraulic study can dictate such levelling to specify the elevation of the pot beside the reboiler in order to have easy releasing of steam condensate to downstream leading to a smooth and proper operation of reboiler...

[tdeb]

Thank you all! Forgive the delay in the response