Controlling a Variable Speed Pump off of Pressure Differential

[apprentice3]

My company is wanting me to set up a system to control off of pressure differential. I was wondering how can I determine a good setpoint for this? When controlling off pressure differential does the pump need to slow down when the pressure differential is high?

Thanks

[CB_Matt]

My company is wanting me to set up a system to control off of pressure differential. I was wondering how can I determine a good setpoint for this? When controlling off pressure differential does the pump need to slow down when the pressure differential is high?

Thanks

The Balancer will give you the setpoint, and yes it needs to slow down.

[nsokchea]

Yes you are right. When you PD get higher VSD should slow down. and PD get lower VSD should speed up.

You need to write the LOOP for this operation. For Loop statement you need to carefully set the parameter.

If you set wrong then VSD Variable is too fast and will make your entire system get problem. so you need to set Loop that VSD can change the speed slowly. and you can accept.

yes correct the Balancing Team will tell you the setpoint of the PD. Cause when they do the balancing they will run the VSD manually. after they finish they will go to measure the differential pressure where you install PD sensor. and this set point you need to put in your loop statement in (BIAS). So mean the set point and your BIAS is almost the same.

[crab master]

Just to throw out there are you sure this is an application that a VSD with diff press control is worthwhile?

I've had more than one job where such was done/planned, but it'd never work as the piping arrangement/valve setup wasn't correct - 3 way valves on all the valves, deadheading the pump, etc... Many it ended up just being a nice soft start/stop setup with a bit of 'motor protection'.

[swei]

How big is the pump?

[fixitman]

You don't tell us enough to even guess a set point. Something in the piping needs the flow through it controlled. What is it?

Typically, this control loop would include a VFD or a bypass valve. Bypass is the old-fashioned way, since it tends not to reduce the pumping energy when flow through the load is not required. VFD's are the new-fangled way, since they actually slow the pump down to reduce the the flow, so also reduce the required pumping energy. Typically, you might be controlling the pressure across, say, a heating/cooling coil with a variable valve which is performing proportional cooling. Maybe even a gob of such coils, each with its own valve. Give us a clue ...

[jlmus1]

Measure your pressure at the end of line and start out low. As previously stated we need to know an application. In my experience Trane wshps for example can operate 6-10 psid ...just depends. TAB would be beneficial here.

[ctrlguy]

Assuming there are 2-way valves on the job and the pumps were designed for a 100% call on all valves, the proper procedure would be to open all the valves, run the pump at 60 Hz, measure the differential pressure and that's your setpoint.