Pressure control of pump

aand74

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Join Date
Dec 2005
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Deinze
Posts
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I will have to start up frequency controlled pump that has pressure feedback.
In the prepared plc program a pi-controller that has a pressure setpoint and pressure feedback directly gives the setpoint for the frequency controller.
This looks easy, but I was wondering if that easy approach will give a good result. Especially if you startup the pump and the PI-controller still has find a stable state. Are there procedures (like f.i. first shortly running the pump at low speed) that assure a stable startup. Or otherwise said, helping the pi-controller to smoothly reach a stable state?
 
It is time for drbitboy DMD.
pt1.jpg
 
aand74 said:
I will have to start up frequency controlled pump that has pressure feedback.
In the prepared plc program a pi-controller that has a pressure setpoint and pressure feedback directly gives the setpoint for the frequency controller.
This looks easy, but I was wondering if that easy approach will give a good result. Especially if you startup the pump and the PI-controller still has find a stable state. Are there procedures (like f.i. first shortly running the pump at low speed) that assure a stable startup. Or otherwise said, helping the pi-controller to smoothly reach a stable state?
Click to expand...

Control your ramp rate of the pump, this will help limit hammer.

Tuning of the PI is critical, I use controlstation software LoopTuner Pro (its soooo good, feels like you are cheating).

You can control the Ramp rate at your output, or in the VFD config.

Changing ramp rate will impact your tuning parameters.
 
We do these all the time, tuning is also dependent on type of pump, line size, viscosity of fluid.

Centrifugal pumps don't start pumping until they reach around +/- 50%.
But positive displacements start pumping straight away.
 
We do use both PD, and Multistage Centrifugal pumps.

I usually do not have a problem tuning either, they both provide some sort of process response that can be measured from 1%to 100%. The pump curve will be something you want to ask for and examine to know for sure. Luckily I do not have to rush in our process.

Where I have a challenge is Control Valves controlling pressure or flow, they typically do not have any effect ( on our systems ) on the PV from 0-60%, PV reaction is measurable from 60% to 90%, that is where all the action happens, and in those situations, I usually limit the PID with CV Min and Max to avoid winding up the PI.
 
Usually you can start from 0% and let the PID do the rest. A PD pump is has better control and is easier to tune because the flow rate is nearly linear with pump speed.

Tuning is the key, as mentioned.
 
If the pressure set point is unlikely to change much then one solution is on start up set the PID control to manual & force the SV to a known value (approx. the flow you want), then switch it into auto with bump-less control.
Have done this many times especially if the ramp up has to be a little slow.
Even had a lookup table so depending on the set-point it would select the approximate value (we had 3 set points that were used depending on the process).
 
parky said:
If the pressure set point is unlikely to change much then one solution is on start up set the PID control to manual & force the SV to a known value (approx. the flow you want), then switch it into auto with bump-less control.
Have done this many times especially if the ramp up has to be a little slow.
Even had a lookup table so depending on the set-point it would select the approximate value (we had 3 set points that were used depending on the process).
Click to expand...




Ooh, nice.
 
drbitboy, you are going to lose you DMD status.
aand74 provided us no information about his system.
How are we supposed to answer his question?

There are pump curves and system curves and where the two intersect determines the pressure and flow. The system curve is usually not linear so having separate gains as a function of flow or pressure is a good idea.

Positive displacement pumps make controlling the flow a lot easier but if the pressure is going up with the flow squared, then what?

Another problem with positive displacement pumps is that the flow is not continuous. There are ripples caused when each of the pistons in the pump pushes out the fluid.

Controlling the pressure is much easier if the system is "compliant". This means having an accumulator down stream of the pump will reduce the variations in pressure.
 
LOL, I am filing petition to revoke both of your DMD status.

The proper way to control pressure is to add a Back Pressure regulator (BPR ) and if automatic mode is desired then add E/P regulator.
 
In many process applications, Back Pressure regulators (BPR ) are not acceptable, because of pressure drop and they cause shearing of materials, which adversely changes viscosity.
 

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