Submersible Pumps Fed from VFD's
Submersible Pumps Fed from VFD's
(OP)
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
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





RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
The typical method is to run the pumps on fully with one modulating. As more is needed another is turned on while the modulating pump is reduced back again. Trying to run them all at reduced speed can cause numerous headaches.
I think there are some fellows who have done this successfully in forum237: Electric motors & motor controls engineering.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Submersible Pumps Fed from VFD's
You probably need to measure flow as well, and by using the curves, determine that if your flow is between x and y, you can control your pressure by running, say, 3 pumps on vfd, but if the flow demand increases to z, you'll need to bring an additional pump on line...
RE: Submersible Pumps Fed from VFD's
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
For flowrates between 10% and 110% of total capacity, you can get within 10% of any flowrate you target without even using any vfd at all, just by turning on 1,2,3,4 or 5 pumps.
You probably shouldn't be running at less than 20% of total capacity anyway; just a guess, anyway certainly not 10%. Below 50% motor load, efficiency for motors and vfds drop off very fast, so there's not much benefit, if any at all, for having vfds at those flow ranges. If you're running submersibles, maybe you have a high static head and vfds might not generate enough static head at those low rpms anyway.
If you run X pumps on rated speed and try to run X+1 on vfd, you have to match the head (discharge pressure) of the other units to properly share load. If you produce less head (pressure) with the unit on vfd then you are producing with the other units, they try to backflow into your vfd controlled unit until your vfd unit brings up its pressure to equalize. So then you wind up running X+1 at rated speed too. That's no good.
So, do you really need flow accuracy to within better than 10% of any flowrate you want to target? If not you're home free without VFDs. If you do, I'd try putting a spill back control valve on a line going from discharge header to suction header that could modulate 10% of the total flow capacity and quickly forget about all that electronic cable and power conditioning and bearing current headaches, etc. etc. etc.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Submersible Pumps Fed from VFD's
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
Yeah it sounds like the Aquifer cant supply the fluid required to maintain the pressure, so it's not really an electrical problem. If you are testing drawdown, and inflow that is the best route. The system may have to be rethought around what your aquifer can supply, and indeed will supply, with VFD's you can lower the frequency which will to reduce drawdown, or else leave, some offline. Or alternatively batch produce and allow the source to recharge. Hope it all goes well rgds.
RE: Submersible Pumps Fed from VFD's
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Submersible Pumps Fed from VFD's
Yes. But nowhere near as informative as finding some point where the pumping rate exactly matches the infill rate.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Submersible Pumps Fed from VFD's
More detail needed, at the moment we are second guessing.
RE: Submersible Pumps Fed from VFD's
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Submersible Pumps Fed from VFD's
Just some thoughts...
If the number of starts is so high, could means that the pump is delivering higher capacity then the capacity pump has been selected/quoted.
One probable reason is that the system curve is very flat and system losses are lower then expected.
I deem that lowering pump speed (all pumps at same speed) could allow to reduce/mitigate the number of starts...
So it looks like :
- The system resistance downstream has changed during pumps lifecycle or there is a strong variations : min/max scenario of the system curve.
- Selecting 5 pumps in parallel is quite unusual. In any case, It seems the pumps have been quoted with the OP shifted on far left (what is impeller type : is it closed/open impeller, screw like for handling fiber, sand, etc? ) fitted for high losses scenario
Just trying some brainstorming ... one could try two control scenario :
(1) max system losses (5 pumps together with OP at left let say 20%)
(2) min system losses (2 or 3 pumps and cyclical swaping to reduce total capacity and then reduce starts)
RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
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.
RE: Submersible Pumps Fed from VFD's
RE: Submersible Pumps Fed from VFD's
Keith Cress
kcress - http://www.flaminsystems.com