Hello,

We have three pumps of different capacities driven by VFDs. A PLC chooses which pump to run based a flow rate. The VFD for the selected pump gets a signal from the PLC based on output pressure. This is for a small domestic water system which includes fire hydrants. We are worried that any failure in the PLC system would require our operators to manually regulate pressure 24/7 until the system could be repaired. We decided to look into a PID controller that could be wired from the main pressure sensor directly to the VFD speed input for the mid-sized pump. But when I started looking into it I saw references to using a VFD as a PID. Is it realistic to think we could simply connect the pressure sensor directly to the VFD? We have MGI inverters with Mitsubishi controls.

 

Yes some VFDs do have built in smarts including PID control. There may be other single points of failure that could take down your whole system as well, so you should evaluate all failures not just the PLC.

 

Water supply especially fire fighting function are quite critical in the Plant. Suggest to delegate one pump to fire hydrant directly without connect to other water distibution with pressure control all the time by Jocky pump ( NFPA it should have one Diesel Pump,Electric pump and Jocky pump). Anyhow,For fail safe design, we should aware that Power electronic ( VSD ) or electronic device ( PLC ) etc. are out of order by any interference. Using Pressure switches to control on-off the Pumps at fix speed is simple, high reliability ease of Service as backup scheme in case either PLC or VSD which are normal control was failured.