We have a pumpskid controlled by an outlet pressure transmitter. We wish to charge the line against a closed valve, stop the pump, and have it waiting until needed. When the closed valve is opened, the pressure will fall, and the pump will kick back on and be controlled to the target pressure. My question is, how can we use the functions of a vfd to achieve the three conditions?
1. Inlet pressure say 5 bar pump starts and charges up
2. Pump charged pressure say 15.5 bar and pump will stop and wait.
3. Target pressure once valve is opened say 15 bar.
How Can we achieve point 2 to stop the drive if point 3 is set lower using the functions of a vfd?
Yes, with many VFDs internal programming will allow you to program this logical sequence. Not all VFDs are the same, so you will need to know which VFD is being used and then get the instruction manual from the VFD manufacturer. The pressure transmitter would be wired to the analog input terminal of the VFD.
That's great thanks. So we could use the internal controls of the drive so that two of the conditions don't fight against each other. Ie 15.5 would never be reached as the drive would always want to keep to the 15 Bar point.
Don't forget, not all pumps hold pressure after shut off, especially if you have a recycle loop. If you are counting on a check valve to hold pressure, not all check valves offer reliable tight shut-off.
Most such systems require a holding tank for liquids or a pressure reservoir for gases.