Flowrate Control with VFD and Electropnuematic Control Valve

The flow rate seems very low for use with a centrifugal pump. In order to meet the head requirement, the pump may need to run at a substantial fraction of full speed, in which case the flow may be excessive. This could result in the high hysteresis.

If the pump is reasonably matched to the desired flow rate, I think your option of PID controller interfaced to VFD would be best.

If I am only using VFD with PID controller, then the issue I am facing are

1. Once the flow requirement is 15 lit/hr then the motor is running at very low frequency around 5 Hz, my concern is that if I go onto implement this motors bearings might wear down or related issues might come up

2. Secondly, during the process of achieving set point if the PID controller send command signal (say 4mAmps) to the drive, it shuts off the motor getting the flow back to zero and then starts it as command signal (mAmps) goes higher. Thus the SP is not achieved.

3. To counter the scenario in '2' I am keeping the lowest frequency of drive to 5Hz for min command signal of PID i.e. 4mAmps still massive hysteriss observed.

This suggests that the pump is the wrong type, or way too oversized, for the application.

I do agree and was to an extent aware of this fact but unfortunately I would have to live with it. The customer won't be willing to spend extra bux.

1) Can you control the loop in manual? 2) Do you have a flowmeter with an electronic output?

1. Yes the loop can be controlled manually via PID controllers interface.

2. Yes the flowmeter with electronic output is present, upon the PV provided by this flow meter the PID controller regulates the flow.

PID controller type: Siemens SPIRAT20.

Flowmeter: KOBOLD variable area

I am actually looking for high precision as the flowrate of water is critical for the fuel purification in this scenario.

Well, currently I don't have anything for pressure sensing I am just doing test runs for controlling VFD (SAMCO) and SAMPSON's electropnuematic valve via PID(Siemens SPIRAT20) whose sensing is based on flow rate sent by KOBOLD variable area flowmeter.

Variable Area is not a "high precision" technique!

+/-5% is typical.

Go for a fixed setting for the pump and vary the flowrate through some sort of small aperture actuated valve with PI control on the flowmeter's signal to achieve the desired set flow. Derivative control is not needed and the integral setting will be slight.

Some method of getting rid of the surplus flow is needed so as to keep the pump happy. How about a pressure sustaining valve downstream of the controlled flow take-off and sending the surplus liquid back to the day tank?

Don't use cost as an argument. Go for the best technical approach.

Based on your statements regarding obviously oversized pump, your best choice- for both flow control and pump life- is to abandon the VFD and use only a flow control valve (sized appropriately for the desired flow) and install a pressure-activated by-pass valve to recirculate MOST of the flow back through the pump.

This assumes that the pump heating due to efficiency issues is not a problem. If that could be an issue, use the VFD but set it at a fixed level, maybe 25% of maximum flow, and still use the flow control/by-pass valve assembly.

If vfd control takes it to 5%...

Could you post the specifications of the pump you have?

If change of pump is not an option, I agree that recirculation to tank seems the better way.

Salu2 cordiales.