10 replies to this topic

[ProcessEng_12]

Hello all.

Hope you all doing well.

I am working on one of the simulation for plant match case which contains columns, pumps, control valves.

When I was going for simulation what I found was that there are temperature readings for various stages in column which I need to match in my simulation.

I was just curious for any particular stage, what  is the location of the temperature measurement? 

Is it in liquid pool above tray or in vapor space above the tray?

Is it just below the where the downcomer of above tray ends or in front of the weir of the same tray or in the side of the active tray area (if we look from top view)?

 

Thank you in advance.

 

[breizh]

https://www.chemical...re-measurement/

 

Hi ,

Consider this schematic.

As built drawings of the column should confirm.

Breizh

Edited by breizh, 02 February 2021 - 02:23 AM.

[SilverShaded]

P&ID's are your friend in this case.

[Pilesar]

You have to review the vessel drawings to know where temperature measurements are located. They may be in the froth area just above the surface of the tray, in the vapor space, or measuring the liquid from the tray above in the downcomer.

[ProcessEng_12]

Thank you Breizh, Silvershaded and Pilesar for your prompt replies.

Even I had discussions with my colleagues.

Their explanation was that mostly thermowell should be in the vapor space above the tray and should not be in the liquid above tray as it may get foul in liquid (if any dirt, foreign particle precipitates)

It is not possible to have just below the downcomer or in front of weir as the thermowell has a protuding inwards. If that happens then it may obstruct the flow liquid below the downcomer or in front of weir.

 

So Answer is : It is on the side of the active tray area (if we look from top view) in vapor space.

 

I hope this is correct. Anyone would like to add or correct me if I am wrong?

[SilverShaded]

Simulations tend to give you the temperature of the liquid on the tray.  In most simulators thats the only temperature it will give as trays are assumed to be in some kind of thermal equilibrium with the vapour (if not actually K equilibrium).  So typically knowing exactly where the thermocouple is doesnt really help.  Tray temperatures for complex multi-component columns are often difficult to match anyway.

Edited by SilverShaded, 03 February 2021 - 03:24 AM.

[Pilesar]

"I hope this is correct. Anyone would like to add or correct me if I am wrong?"

The responses from this forum already gave you different answers than you came up with after discussion with your colleagues. What more do you want from us? I do not choose to put thermowells in the vapor space between trays when designing columns and am surprised to learn that other locations are 'not possible.' In any case, there is often a difference between how equipment should be designed and how it actually exists. Your temperature measurement may be in the vapor space as you claim. Are there no drawings of the vessel?

[ProcessEng_12]

"I hope this is correct. Anyone would like to add or correct me if I am wrong?"

The responses from this forum already gave you different answers than you came up with after discussion with your colleagues. What more do you want from us? I do not choose to put thermowells in the vapor space between trays when designing columns and am surprised to learn that other locations are 'not possible.' In any case, there is often a difference between how equipment should be designed and how it actually exists. Your temperature measurement may be in the vapor space as you claim. Are there no drawings of the vessel?

Thank you for response.

I am sorry my trying to understand it properly has offended you.

Why do you not choose put thermowells in vapor space? What is a 'a difference between how equipment should be designed and how it actually exists'  for thermowells w.r.t. column?

I do not have drawings for the column.

[ProcessEng_12]

Simulations tend to give you the temperature of the liquid on the tray.  In most simulators thats the only temperature it will give as trays are assumed to be in some kind of thermal equilibrium with the vapour (if not actually K equilibrium).  So typically knowing exactly where the thermocouple is doesnt really help.  Tray temperatures for complex multi-component columns are often difficult to match anyway.

Thanks for your response.

[Pilesar]

It is generally best to measure temperature in the liquid near the bottom of a downcomer. (The thermowell would be installed somewhat parallel to the weir, not orthogonal to it.) If you have a temperature measurement in the vapor space between trays, how do you know you are measuring vapor temperature? What if liquid splashed on the thermowell? What if the tray were flooded? What if sometimes liquid splashed on the temperature probe and sometimes the probe were dry? Thermowells in liquid are continually washed which can also help with fouling. Perhaps I came across as cranky, but how can someone you don't know in this forum convince you that your experienced colleagues whom you trust are incorrect? There are lots of reasons why thermowells may not be in the best position. Perhaps the tray internals were altered in a revamp while keeping the existing thermowell. Perhaps one of your colleagues designed the column. There may be external obstructions or lack of maintenance access affecting the thermowell placement. When I can not place a thermowell where needed in the downcomer, I place it just above the tray in the froth area to keep it wetted. I suspect that a drawing of your column exists in the pressure vessel records. Your simulation might not be improved with the knowledge.

[ProcessEng_12]

It is generally best to measure temperature in the liquid near the bottom of a downcomer. (The thermowell would be installed somewhat parallel to the weir, not orthogonal to it.) If you have a temperature measurement in the vapor space between trays, how do you know you are measuring vapor temperature? What if liquid splashed on the thermowell? What if the tray were flooded? What if sometimes liquid splashed on the temperature probe and sometimes the probe were dry? Thermowells in liquid are continually washed which can also help with fouling. Perhaps I came across as cranky, but how can someone you don't know in this forum convince you that your experienced colleagues whom you trust are incorrect? There are lots of reasons why thermowells may not be in the best position. Perhaps the tray internals were altered in a revamp while keeping the existing thermowell. Perhaps one of your colleagues designed the column. There may be external obstructions or lack of maintenance access affecting the thermowell placement. When I can not place a thermowell where needed in the downcomer, I place it just above the tray in the froth area to keep it wetted. I suspect that a drawing of your column exists in the pressure vessel records. Your simulation might not be improved with the knowledge.

Thank you for the apt explanation.