10 replies to this topic

[Processmen]

Dear All,

I am having trouble understanding the control loop a heat exchanger. frankly I think the configuration is bit odd. The goal is to control the outlet temperature of the exchanger. The process liquid is being heated with no phase change, and the steam becomes saturated liquid. I think we can control the temperature of the exchanger just by using the PCV, which is signaled by the TIT on the liquid line. The propose of the PCV is said to be keeping the steam pressure constant, but in this case I think the TCV can not control the temperature. Am I right?

I attach the system configuration, please let me have your comments.

Attached Files

•  Heat Exchanger.pdf   10.79KB   75 downloads

[breizh]

The TCV is there to control the flow rate of steam to heat up your process fluid at the right temperature . As stated the purpose of the PCV is to keep constant the pressure of the steam .



Breizh

Edited by breizh, 21 November 2011 - 01:51 AM.

[Processmen]

Thank you for your reply

can we delete the TCV, and control the temperature of the exchanger by the PCV?

I mean what is the point of having two control valves on a steam line? is it true that from flow rate and pressure of steam one can be controlled? If we control the pressure then the flow rate can not be controlled.

Edited by Uottawa, 20 November 2011 - 05:47 AM.

[fallah]

Thank you for your reply

can we delete the TCV, and control the temperature of the exchanger by the PCV?

I mean what is the point of having two control valves on a steam line? is it true that from flow rate and pressure of steam one can be controlled? If we control the pressure then the flow rate can not be controlled.

Uottawa,

Your concern about interacting two control valves controlling two various process variables could be reasonable. IMO, the concern may be ignored if the piping connection between two control valves having adequate capacitance (e.g. the piping connection to be a steam header with adequately larger size than TCV inlet line (as a branch from the header) size which its pressure to be maintained by PCV). Of course, i think it might actually be this way and because you did submit a simple sketch the details have been hidden.

For improving the temperature control of the outlet process fluid it is better to consider a FCV rather than TCV such that relevant FIC to be cascaded by TIC (in process fluid outlet line) and would modulate the FCV.

Also, it would be better to consider the FCV on outlet condensate line results in lower pressure drop in inlet steam line and higher exchanger performance.

Fallah

Edited by fallah, 20 November 2011 - 07:11 AM.

[Processmen]

Dear Fallah,
The sizes are identical and the PCV is after the main steam header, and there is no note about the piping capacity. what about we just use a TIT to modulate the PCV to control the temperature? The upstream of steam is identical to the operating pressure of steam in the exchanger. The steam header is 4 bar and the the exchanger is design for saturated 4 bar steam.

[fallah]

Dear Fallah,
The sizes are identical and the PCV is after the main steam header, and there is no note about the piping capacity. what about we just use a TIT to modulate the PCV to control the temperature? The upstream of steam is identical to the operating pressure of steam in the exchanger. The steam header is 4 bar and the the exchanger is design for saturated 4 bar steam.

Uottawa,

I have never seen to use a TIC modulates a PCV to control temperature, while this would easily be done by using a FIC cascaded with that TIC to modulate a FCV.

Of course, if you insist on using PCV there (may be due to pressure flactuation in steam header), as mentioned in my previous post, it is suggested to replace the TCV with a FCV and relocate it to condensate outlet line. Obviously, in this new configuration the FIC would be reset by TIC.

Fallah

[kkala]

Following view represents my understanding. Comments would be welcomed to correct the picture.
1. Purpose of PCV is to keep constant pressure upstream of TCV to "facilitate" TCV function. Actually the TCV reduces pressure of downstream steam, thus lowering its temperature and resulting heat transfer rate (see "steam control" option at http://www.driedger...._sh/CE4_SH.html).
2. In this sense PCV had better be removed, if one of the two valves should be to avoid unstable control. See example of "steam control" above, where there is only one control valve at ingoing steam (the valve in condensate pot merely controls condensate exit).
But probability of unstable control because of two valves seems quite remote in the specific case. Fluid is compressible. Response of PCV is quite fast, as concluded from steam headers pressure control; TCV must be slower, taking input from liquid temperature at a different stream. See also http://www.cheresour...alve-in-series/.
Nevertheless flow stability criteria on a line with two control valves in series may exist, advice on this matter would be appreciated. It might involve elements from control theory not so familiar to Chemical Engineers. Concerning an LPG vaporizer (1984), Instrument Dept placed PCV and TCV on ingoing steam, assuring of no problem. Process was reluctant to accept two on line control valves, then project stopped without result.
3. Concerning such control, I have not seen PCV alone, either, probably too "nervous" for the service.
4. If it were existing status, I would leave working system as is. But it seems to be a new design.

Edited by kkala, 22 November 2011 - 12:19 PM.

[pavanayi]

Uottawa,
If this is from an installed/operating system, could you say what is the actual physical distance between the two control valves?

[Processmen]

sorry I was away fore a couple of days. This is the first revision of detail design P&ID. I do not think there is much distance between them. I think this configuration might cause unstable control or at least delay in system stabilization, because if TCV take action then PCV should regulate the pressure, which means a new inlet condition for TCV.

Edited by Uottawa, 28 November 2011 - 09:36 AM.

[Processmen]

Following view represents my understanding. Comments would be welcomed to correct the picture.
1. Purpose of PCV is to keep constant pressure upstream of TCV to "facilitate" TCV function. Actually the TCV reduces pressure of downstream steam, thus lowering its temperature and resulting heat transfer rate (see "steam control" option at http://www.driedger...._sh/CE4_SH.html).
2. In this sense PCV had better be removed, if one of the two valves should be to avoid unstable control. See example of "steam control" above, where there is only one control valve at ingoing steam (the valve in condensate pot merely controls condensate exit).
But probability of unstable control because of two valves seems quite remote in the specific case. Fluid is compressible. Response of PCV is quite fast, as concluded from steam headers pressure control; TCV must be slower, taking input from liquid temperature at a different stream. See also http://www.cheresour...alve-in-series/.
Nevertheless flow stability criteria on a line with two control valves in series may exist, advice on this matter would be appreciated. It might involve elements from control theory not so familiar to Chemical Engineers. Concerning an LPG vaporizer (1984), Instrument Dept placed PCV and TCV on ingoing steam, assuring of no problem. Process was reluctant to accept two on line control valves, then project stopped without result.
3. Concerning such control, I have not seen PCV alone, either, probably too "nervous" for the service.
4. If it were existing status, I would leave working system as is. But it seems to be a new design.

Dear Kkala,
I spoke to the designer, The PCV is meant to control the steam pressure, and the TCV is for controlling the exchanger outlet temperature. I do not think it is a good idea to control the system this way

[kkala]

Other options of control in steam heaters can be read in Norman Lieberman's book "Working Guide to Process Equipment", as reported in http://www.cheresour...h__1#entry51688 probably useful.
It seems, Uottawa, that inlet steam throttling is an applicable method, while two control valves in series (steam service) can be successful on certain conditions (http://www.cheresour...alve-in-series/). I could not specify these conditions, there may be risks, but what is designer's opinion? Designer has to defense its proposal or select another scheme based on its experience, so that design responsibility remains with it. This does not mean blind acceptance on topics that your experience does not recommend, but Designer had better "be convinced" on them.
If scheme remains as is after the discussions, you can try to know situation when PCV is constantly wide open (probable status, if the control scheme fails).
If you have the chance, please advice on the control scheme, after the decision (just to learn).

Edited by kkala, 29 November 2011 - 04:05 AM.