Protecting a submersible pump using flowmeter

Discussion in 'Pumps and Tanks Well Forum & Blog' started by Michaelco, Jul 26, 2014.

  1. Michaelco

    Michaelco New Member

    Messages:
    13
    Location:
    California
    I have a well with a 3/4 HP submersible, and the difference in current when under load and the "run dry" case is too small to allow a good set point to trip the controller. If I ever have to replace the pump, I'll size it correctly so the contrast between normal load and run dry is more reasonable. Right now, the difference is ~ .2 amps (from 6.5 -> 6.3).
    Does anyone know of an existing device that works on the actual flow where once flow drops off, it signals a shutdown timer which keeps the pump turned off for the selected time delay? I can buy a flow meter, and build a circuit that does this, but would rather buy something off the shelf where others have already worked out any issues.
  2. Boycedrilling

    Boycedrilling In the Trades

    Messages:
    42
    Location:
    Royal City, WA
    It's been years, but I have set up centrifugal pumps up with a paddle type flow switch that would shut the pump off under a no flow situation. It was tyed into a timer . There had to be no flow for a certain amount of time, before the pump would shut off.

    I sure that you could make a circuit that used a 4-20 milliamperes signal off of a flow meter to shut the pump off. I've seen vfd's that used the flowmeter signal to control the pump speed
  3. valveman

    valveman Moderator Staff Member

    Messages:
    4,495
    Location:
    Lubbock, Texas
    You just haven't tried the right controller. A Cycle Sensor can easily be set for 6.4 amps. It is hard for me to believe there is that small a difference when pumping air, but the Cycle Sensor was designed to see the small difference between restricted flow and no flow, so it should work.

    I hate flow switches. Can't seem to keep them calibrated and you have to bypass them somehow to get the pump started.

    [​IMG]
  4. Michaelco

    Michaelco New Member

    Messages:
    13
    Location:
    California
    I had a camera recording a clamp on meter as I was watching the flow into a tank, which allowed me to see "no flow condition" at the same time as I "saw" the current draw - that is, with a little verbal queue from the ladder. I had looked at the Cycle Sensor, but was concerned that either drift in the system, or noise would give me false trips, or miss the zero flow event. Is this a potential problem using the Cycle Sensor in my situation?

    I understand you'd have to bypass the flow switch, but that wouldn't be difficult. Surprised about the need for frequent calibration of the flow switch - I had more concern about the alternating surge and lull of water that I see during cavitation. Figured someone had made a device that worked that part out - in other words electronically smoothed out the flow sensor signal. Thought this would be the area which would require a lot of "tinkering".
    I was hoping to find a company who already went through this effort and figured this out. Are there any mechanical devices which could reduce the surge/lull cycle seen during cavitation? You mentioned needing to recalibrate the flow sensors - what changes in the sensor outputs have you seen? Drift?
  5. valveman

    valveman Moderator Staff Member

    Messages:
    4,495
    Location:
    Lubbock, Texas
    If you are going from 20 GPM to 0 GPM in 1” pipe there is a big difference and a flow switch works fairly well. But when going from 5 or 3 GPM to 0 flow, there is not much difference in the position of a flow switch, and I find them sensitive. The larger the pipe the worse it gets. You would probably want a time delay also to make sure flow has stopped for at least a few seconds before shutting off the pump.

    I usually see more of a difference in amperage between low flow and zero flow condition. The thrust bearing in the motor may not be good.

    It has been years since I looked at any of this stuff but I think the Cycle Sensor looks at amps times the power factor, which should give enough of a different reading to make it work. The Cycle Sensor also has a 10 second time delay before shutting off the pump. So if the amps are bouncing, they have to stay below the current adjust setting for at least 10 seconds before it will shut off the pump.

    Normally you can even prevent the pump from running dry by setting the Cycle Sensor to 6.5 amps. A 3/4HP should start out closer to 7 amps, and by the time the water level drops to where the motor is only pulling 6.5 amps, you can shut the pump off before any surging occurs.
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