Submersible Pumps Fed from VFD's 1

[powersoff]

Hello,

I wandered over here from the Electrical side. I am looking for help with understanding the control of submersible pumps, in order to possibly modify plc code or recommend something else. The system I am working with has 5 pumps all supplied by vfd's. The pumps are trying to maintain system pressure. The problem I am seeing is excessive pump starts(+100 a day)and low flow(which results in pump shutting off). The VFD speed reference is coming from PID's running in the plc code. The PID is trying to maintain system PSI(setpoint) and ramps the vfd speed accordingly.
Any insight would be appreciated

 

[jcarrr]

The overriding control issue here is to preserve the life of the aquifer and wells. Pumping should be done according to a plan created by a reservoir (underground) engineer or experience designer. A good plan probably includes the goal of even and consistent draw down of all wells and steady level control of each well to minimize rushing water after pump turn ons and repeated scrubbing of the screens by rapid water rise after the pumps turn off.

There are lots of considerations, but if this goal is met many of them are taken care of

The best way to accomplish this uses some sort of level sensing in each well. If the well heads are vented to the atmosphere a pressure sensor at a known elevation in the well can control the pump with a simple PID loop.

A more simple system is to use a float switch at the desired level to control the run/not run control of the drive. Set the acceleration deceleration rates with some experimentation. After the correct values are found, the controls will converge the the Hz on the right value and pumping elevation in a couple of hours operation.

If the desired product is natural gas released from coal by reducing the water pressure, more controls are necessary.

It is proper to pump wells this way. The plan was correct, execution is bad. Given my experience, the likely problem is that there is not nearly as much water available as the original plan assumed. The controls are trying to work past one end of the spectrum of possibilities.

The formation is probably too tight to make shutting some pumps off if you need all the water that is available.

The pumps are probably grossly over-sized for the water supply. Remember the rules. Pressure is proportional to angular velocity. If a particular speed will just not quite overcome the pressure head requirements, a little bit faster will deliver an amazing amount of water to an open discharge. Because the pumps are so inefficient, friction adds a lot of buffering to this tendancy. The pumps should probably just achieve head pressure and minimum flow at 45 Hz.

 

[jcarrr]

Oops, I know, square of angular velocity

 

[itsmoked]

Thanks jcarr for the education.

Keith Cress
kcress -

http://www.flaminsystems.com