Process newbie questions

Can I just measure the input and output pressure across a pump to get the Differiantial Pressure?
I got dropped on a team to control a gearpump. The boss wants to control the DP across the pump and keep a constant discharge pressure but the inlet will vary. If the inlet jumps up or down how do I keep the discharge pressure constant? I assume that I will use a PID to vary the speed of the pump. Is that correct? I usually do machine control so this process stuff is all new.

Can I just measure the input and output pressure across a pump to get the Differiantial Pressure?

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Yes, but you probably don't need the Differential Pressure to control the outlet pressure.

1. Measure Outlet Pressue for sure, the thing that you want to control.

2. A PID instruction should work, if you can determine the proper P, I, and D factors. Other methods will work also.

3. Use Outlet pressure to control speed of pump. Too low >raise speed. Too high > lower speed of pump. Set your PID Control Mode to be "Reverse Acting". If the pressure changes rapidly, you may need to average the pressure over a time period, before feeding it to your PID instruction. Otherwise, the output pressure may oscillate around your pressure setpoint.

Also, note that pump speed versus pressure output is not necessarily a linear curve. The pressure does not usually go up in a straight line. You need to look at the pump output curve to see if you can attain the pressure control that you need, given the pump performance.

4. If your boss still wants a DP readout, then measure the Inlet pressure also, subtract Inlet from Outlet, and display it somewhere to satisfy his curiosity.

I would avoid going PLC way for this application. All Variable speed drives can be easily programmed for this application.

They control the speed using output pressure as feedback.

I have used more than 40 Altivars a Schneider Electric product (ATV61 series) for similar application.

Assuming the discharge side of the pump is relatively constant ie resistance and only the suction varies then controlling the pump speed with VFD seems pretty strait foward especially if you get one designed for pump control. The only question I have is are most "pump VFD's" designed for centrifugal and not gear pumps which will have a different pressure curve related to speed. Come to think of it even if discharge resistance varies the VFD should still be able to hold constant discharge pressure - limited only by max pump speed which would be limited by max load on motor.

Dan Bentler

Hi,
I don't know your application, I don't think that you need the DP, just the input and output pressures, and the input more for information and alarm.
In the case that you really need the DP, you can get it making OutP-InP, but bear in mind that the resolution and error of a real DP gauge is different.

Lancie1 said:

Yes, but you probably don't need the Differential Pressure to control the outlet pressure.

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I started thinking of how to control the inlet that is coming out of a plastic extruder big and slow. I don't want to play witht that for now.

Lancie1 said:

2. A PID instruction should work, if you can determine the proper P, I, and D factors. Other methods will work also.

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How do you go about even guessing a baseline to start from.

Lancie1 said:

. Use Outlet pressure to control speed of pump. Too low >raise speed. Too high > lower speed of pump. Set your PID Control Mode to be "Reverse Acting".

Click to expand...

Yes that is exactly how I want it to work.

Lancie1 said:

If the pressure changes rapidly, you may need to average the pressure over a time period, before feeding it to your PID instruction. Otherwise, the output pressure may oscillate around your pressure setpoint.

Click to expand...

I don’t have to worry about the inlet changing rapidly.

Lancie1 said:

4. If your boss still wants a DP readout, then measure the Inlet pressure also, subtract Inlet from Outlet, and display it somewhere to satisfy his curiosity.

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A simple tag on an HMI looks like a winner to me.