Controlling chilled water pumps on differential pressure - Help!

[capaula]

I work on a big and brand new chilled water plant of 20K tons.
It is a primary-secondary pumping system and the secondary pumps are controlled by differential pressure. They are 7 pumps, 150 kW each.
At the end of the secondary loop I have 2 booster pumps + 1 stand by, 30 kW each that take part of that chilled water to a taller office building.
These pumps are not controlled by differential pressure, but by supply pressure only.
I hate this supply pressure control and would like to change to a differential pressure control.
This will cost money and I need to justify.
I noticed that that control hardly modulates the pumps on lower loads.
But I need more "ammunition" to justify the change cost.
The system is relatively new and I do not have data for the winter yet.
Thanks

http://files.engineering.com/getfil...e-43b7-803e-ed63fd1281dd&file=PDITControl.png

 

[pretendfarmer]

Capaula,
It does not surprise me that the pump does not change speed very much. Using a differential pressure sensor will enable the VFD to modulate the pump speed to track the 2 way valve position. A pressure sensor has no way to track the valve position as it is only looking at static pressure which can be influenced by fill pressure, expansion tanks, leaks, etc. Therefore you are operating a very expensive variable speed pumping system as a constant speed pump. The pump will not slow down during mild times of the year, the 2 way valve will be trying to close off against excess pressure, and coil(and entire system)operating Delta T is substantially below design Delta T. My recommendation would be to immediately replace the static pressure sensor with a differential pressure sensor to improve system control, slow down the pumps, and save energy. Another piece of this puzzle is what is the set point for the existing differential pressure sensor. A set point that is too high will also not allow the pumps to slow down properly. The set point should be the design pressure drop of the coil + the design pressure drop of it's corresponding control valve for the coil/valve set most hydraullically remote from the pump discharge. You might inquire what kind of sensor was originally specified(static or differential).It is hard to believe that a static pressure sensor was originally specified. No variable speed system I have heard of uses static pressure sensors. Any questions, please respond back.

 

[capaula]

That is I was thinking too. I needed somebody else to double check me.
This supply pressure control was a mistake, I believe. There were 2 different designers and contractors in this project.
Thanks a lot, that is exactly what I needed to hear.

 

[ChrisConley]

A thought for the system (in addition to the excellent points made earlier) would be also to consider the use of pressure independant valves.

These valves really maximize the potential of a variable speed pumping system. 'Over-pumping' a coil results in tiny capacity changes, changes that are too small for a control valve to worry about, a PI valve will ensure that if a coil needs 500gpm, that is all it is getting.

Most of the major manufacturers are offering PI valves up to 12" in some cases.

Irregardless, modify the system to have deltaP, or throw the VFD's away because they're actually costing you energy to run.

 

[pretendfarmer]

Capaula,
Just curious. What is the design Delta T of your system and what is the actual operating Delta T? Delta T is the best overall indicator of how efficient your chilled water system is running. I was just curious how efficient your new chiller plant is operating because if you are not seeing at least the design Delta T, then your system efficiency may easily be improved.

 

[capaula]

Operating delta T right now is 14 celsius, design is 16.
This "bad control" is on only on the chilled water portion that goes to the office building.
I managed to justify this change, and received funds to change this control on delta pressure.
Thank you a million!

 

[capaula]

In the previous message, the correct answer is 14 degrees farenheit, and 16 degrees farenheit