9 replies to this topic

[Marco78]

Dear All,

I need your opinion about the system described in the attached drawing.
It is part of an existing plant for the temperature control of hot/warm water.
Nobody has been able to explain to me how to configure it (they are using it in manual mode...)and I thought it could work as following:

Valve 1 and 2 are in split range mode: 0-100% and 100-0% or something intermediate like 0-60% and 40%-100% in order to guarantee a "constant" flowrate.
Valve 3 opens only when valve 1 is closed

I would like to know your opinions
Many thanks

Marco

Attached Files

•  t control.pdf   17.83KB   57 downloads

[Zauberberg]

Marco,

There is no such controller description like "something intermediate or 0-100%". In order for us to understand how the process works (or how it has been envisaged to work), we need accurate and complete controller description.

At what ranges of TIC controller output, the valves V-1, V-2, and V-3 are open or closed? What is the position of each of these valves as the controller output changes from o to 100%?

[Marco78]

Marco,

There is no such controller description like "something intermediate or 0-100%". In order for us to understand how the process works (or how it has been envisaged to work), we need accurate and complete controller description.

At what ranges of TIC controller output, the valves V-1, V-2, and V-3 are open or closed? What is the position of each of these valves as the controller output changes from o to 100%?

Dear Zauberberg,

please consider the situation in which the controller has still to be configured. What I would like to know what would be the best philosophy for the temperature control using the heat echangers described in the drawing.
I perfectly understand that my description is not complete but as first step I would like a general opinion/explanation.
Thanks

Edited by Marco78, 12 November 2010 - 06:12 AM.

[Marco78]

Marco,

There is no such controller description like "something intermediate or 0-100%". In order for us to understand how the process works (or how it has been envisaged to work), we need accurate and complete controller description.

At what ranges of TIC controller output, the valves V-1, V-2, and V-3 are open or closed? What is the position of each of these valves as the controller output changes from o to 100%?

Dear Zauberberg,

please consider the situation in which the controller has still to be configured. What I would like to know what would be the best philosophy for the temperature control using the heat echangers described in the drawing.
I perfectly understand that my description is not complete but as first step I would like a general opinion/explanation.
Thanks

[breizh]

Hi Marco ,
As suggested by Zauberberg , we need more input from you about your process .
My understanding is that the controller is managing the flow of water passing through the valves V1 and V2 and if the measure of the temperature is below the set point then V1 will be partially closed , V2 will be more open with V3 also open to compensate the drop of temperature.

Breizh

[Marco78]

Hi Marco ,
As suggested by Zauberberg , we need more input from you about your process .
My understanding is that the controller is managing the flow of water passing through the valves V1 and V2 and if the measure of the temperature is below the set point then V1 will be partially closed , V2 will be more open with V3 also open to compensate the drop of temperature.

Breizh

Dear Breizh,

the system should work as you have described.
The Tset is 65°C, cooling water is 14°C and stem is 150°C. Normally temperature of the water from the process should be below 65°C but recently (since the we have changed some services) the temperature coming from the process is 70-75 °C.
My idea, but i don't know if i'm right is:
V1 and V2 in split range 100-0% and 0-100% like a three way valve. V3 opens only when V1 is totally closed and V2 totally open. (in alternative v3 opens a bit before)
Could it work? I would like to avoid excessive consumption of steam

[Zauberberg]

In my opinion, this scheme will never work in reality - particularly if you expect temperature swings of inlet water (process side) stream, and if you need tight and accurate temperature control. There's no such a smart temperature controller around.

For example, if the inlet water temperature is e.g. 75 degC then you have to cool it down for 10 degC, and how will you do it? There is no control of the cooling water flow, and you would need to transfer some portion of process water through the V-2 line while keeping the steam valve closed.

A great thing would be to confirm process inlet temperature once for good, and see if you would really need alternative heating and cooling. If that is the case, I would simply put the exchangers in series, and provide a bypass line + a three-way valve downstream of the cooler.

- For inlet temperature lower than the set point: route the entire stream through bypass line of the cooler, and act on the steam control valve in order to heat the process water;

- For inlet temperature higher than the set point, keep the steam valve closed and manipulate the three-way valve in order to reach the desired outlet temperature. I believe this can be done.

Attached Files

•  Control Scheme.pdf   72.05KB   34 downloads

[Marco78]

In my opinion, this scheme will never work in reality - particularly if you expect temperature swings of inlet water (process side) stream, and if you need tight and accurate temperature control. There's no such a smart temperature controller around.

For example, if the inlet water temperature is e.g. 75 degC then you have to cool it down for 10 degC, and how will you do it? There is no control of the cooling water flow, and you would need to transfer some portion of process water through the V-2 line while keeping the steam valve closed.

A great thing would be to confirm process inlet temperature once for good, and see if you would really need alternative heating and cooling. If that is the case, I would simply put the exchangers in series, and provide a bypass line + a three-way valve downstream of the cooler.

- For inlet temperature lower than the set point: route the entire stream through bypass line of the cooler, and act on the steam control valve in order to heat the process water;

- For inlet temperature higher than the set point, keep the steam valve closed and manipulate the three-way valve in order to reach the desired outlet temperature. I believe this can be done.

Dear Zauberberg,

very kind as always! After I had seen your scheme I remembered that we have something similar in one other our plant .
I would like to ask you:
- why put the three-way valve downstream the 1st exchanger and not upstream?
- would there be any difference if I first put the "heater" and then the "cooler"?

Many thanks
Marco

p.s. is there anybody amongst you that have seen something similar to my scheme(I mean the two excanger in parallel)? The plant is 15 years old and I'm trying to figure out why they designed the plant in such way.

[breizh]

Hi Marco ,
In my previous plant we have similar set up on Hot oil , 1 line going through an HX to cool down the H.O and a by pass line with auto valves on both lines managed by a split range .



Hope this helps.

Breizh

[Zauberberg]

- why put the three-way valve downstream the 1st exchanger and not upstream?
- would there be any difference if I first put the "heater" and then the "cooler"?

Many thanks
Marco

1. It's preferred to have the valve downstream of the exchanger, in order to create sufficient backpressure on the exchanger and suppress vaporization of process stream due to pressure drop. If there is a control valve anywhere downstream of both exchangers, you can put the three-way valve on the upstream side as well.

2. Sequencing the exchangers is a matter of preference. As long as the TIC is configured to control the system in the desired way, both the heater and the cooler can be arranged as the first or second exchanger in series.