Controlling pump pressure

[checonbill]

I have a machine running on a chilled water system that has a booster pump to feed the water at 70 psi from a water system that runs at 60 psi I am currently controlling the water system pump by a pressure transducer and VFD. My problem is that when the return pressure goes up in the system my flow in the machine goes down too much, I can't bring the booster up any more. Should I be controlling the system water on return pressure or pressure differental as I need to keep the machine at a 50 psi drop across the machine pressure and the system return pressure. I can make adjusments to the system pressure if needed.Thanks for any ideas

 

[MintJulep]

What is the requirement that you are trying to control to?

50 psi presure drop?

70 psi input pressure?

Flow rate?

Assuming that this is cooling water, it would seem to me that temperatures should be important, but you don't mention that at all.

Establish a requirement, then it will be a simple matter to design a control system to maintain that condition.

 

[DesignerMike]

I would GUESS you would only be concerned about the flow rate?

You SHOULD be able to drop the pressure transmitter all together, and just slap in a flow meter and have more precise control over cooling rates as long as your water temp remains constant.

 

[checonbill]

My cooling water temperature is very constant. I am trying to operate one piece of equipment @ a 50 psi pressure drop off a system that can operate @ a 30 psi drop. The piece of equipment has a booster pump with a manual pressure regulator and I can only operate up to 70 psi. As other equipment goes on and off my return pressure flucuates so I cannot maintain the 50 psi drop at the 1st piece of equipment

 

[MintJulep]

Ok, if you need to maintain a pressure differential then you will need a differential pressure sensor as the control input.

However, with a manual pressure regulator in the system, I don't see how you can accomplish any pressure control.

I would replace the manual regulator with a controlable regulator, and control that to maintain the desired 50 psi delta-P. I might also be tempted to control the booster pump output to hold a constant pressure at the regulator input - however there would be a very real possibility of oscillations in the system. A diaphragm pressure accumlator between the booster and the regulator should probably smooth things out.

The other possibility - and possibly the better choice - would be to move the booster pump to the return side of the equipment.

 

[Chris0164]

Checonbill,

if I understand you correct, you have a cooling water system operating at 30 psi and you need 50 psi for only one piece of equipment. Therefor you ave added a secondary loop with variable speed operated booster pump over that one piece of equipment. The booster pump will give the flow corresponding with pressure drop over the equipment plus the pressure of you return system. If the prssure in your return system goes up then the pump tends to give less flow until the frequency drive corrects it again at the required flow (it will speed up). You say that the flow of the booster pump drops to far so that means that the responce time of your controller is to large (overshoot to big). What type of controller do you have ?? PD) Otherwise I would suggest to install a PID type of controller which limits the overshoot. Flow regulation is indeed an option.



Kind Regard,

Chris

 

[katmar]

It sounds to me like your return piping is too small. The return pressure is fluctuating as your other equipment cuts in and out. If you can deliver 70 psi, and you require a differential pressure of 50 psi then your back pressure in the return line must stay less than 20 psi. Size the return line back to the chiller for the maximum flow rate and a pressure drop below 20 psi and then your controller should manage just fine.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Assumptions]

AS you wish to maintain a 50 PSI drop across the particular system the best approach is to use a feedback loop and measure that D/P. Thus a D/P transmitter across the system or two pressure transmitters.

It would appear that the system has a fixed resistance and by maintaing the 50 PSI drop one is actually maintaing a fixed flow rate through the system. Thus if the D/P across the system is not practical it would appear that a flow loop would acheive the same result.