Flow switch for dry running protection

Discussion in 'Pumps and Tanks Well Forum & Blog' started by Bob1000, Mar 28, 2008.

  1. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    My pump system for boosting city pressure ( pump , pressure tank , pressure switch ) is now working fine after I fixed a pressure regulator at the house entrance
    However my pump dead head pressure is 58psi ( that is the pressure it can generate when the city pressure is zero ( theoritically) i.e water is just at the pump entrance .
    Most of the time the city pressure is in the range of 20 to 28 psi so I would set the pressure switch to 50/75 psi but in very few times the pressure drops dramatically and so far I have been lucky to find out that and switch off the pump before it burns out

    Of couse I know that - to be in the safe side - I should set the pressure switch cut out pressure to not nore than 58 or slightly less , but it is a real petty not to be able to make full use of the available value of the city pressure which is a reasonable value most of the time

    The question now is , if I fixed a flow switch before the pump in the upstream side would it switch the pump off not only at the full dry running condition but also if the down stream side pressure remained constant for some time while the pump is running but can not achieve the cut out pressure setting of the pressure switch ?

    i.e If the cut out pressure is 75psi but the pump can not achieve more than 70 or 65 psi , would the flow switch sense that the water in the upstream is stagnant and practically NO flow is happening then it switches off the pump?

    If the answer is NO , is there any other way to protect the pump at these 50/75 settings in stead of having to reduce down the pressure switch settings to say 35/55 psi?

    In advance , I appreciate very much your help .
  2. valveman

    valveman Moderator Staff Member

    Messages:
    4,485
    Location:
    Lubbock, Texas
    A reverse acting pressure switch, called a loss of prime switch or a low suction pressure switch would be better. It senses low pressure on the city side and won't let the pump come on. You could also use what is called a Hot Stop. This attaches to the pump and shuts it off if the water temperature goes up, which it will if the pump cannot shut off as it should.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,811
    Location:
    Cave Creek, Arizona
    booster

    Unless the pressure in the city main dropped to zero, and there was no way for water to flow, or be pumped out of it, your pump should pressurize your system to the switch's setting. If you are really worried about it a reverse acting pressure switch, which would open at some minimum incoming pressure would keep the pump from running until the pressure was restored. But it seems counterintuitive to have a booster pump, but only run it when it should not be needed.
  4. speedbump

    speedbump Previous member

    Messages:
    4,540
    Location:
    Riverview, Fl.
  5. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    Temperature

    Ambient temperature reaches very high level in summer at the place of the pump , would that thermostate be effective ? or would it switch the pump off just because of the very hot weather in mid day in July ? the steel body of the pump would be very hot at that time.
    Any idea about the price?
  6. speedbump

    speedbump Previous member

    Messages:
    4,540
    Location:
    Riverview, Fl.
    The switch goes off at 110° and back on again at 90°. This is the pump casting or a pipe very close to the pump that will be sensed. If your ambient temp gets over 110° then NO it won't work.

    bob...
  7. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    High ambient temperature

    Yes it goes over that because the pump location is in direct sun and ambient temperature reaches 50 degrees C .
    It would not work !

    Unfortunately I can not find locally a pressure switch with reverse action ( contacts open when pressure is low) to fix it in the upstream .
    If you think that would disconnect the pump when it is needed then how I can protect the new expensive pump against DRY RUNNING?

    I got a Watts flow switch and was intending to fix it in the upstream but I wonder if it would work or not at very low city pressure .
    Any comments?
    Last edited: Mar 29, 2008
  8. speedbump

    speedbump Previous member

    Messages:
    4,540
    Location:
    Riverview, Fl.
    Where are you that the temps get to 122°?

    I have a Hot Stop Thermostat on a pump on the West side of my house in Riverview, Florida and this spot is the hottest on my 5.5 acres and it works fine. The sun radiating off my house in the hottest hours of the day won't stop the pump from running. You do however have to keep the unit shaded.

    bob...
  9. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    The pump that needs this protection is in the middle east
    However I feel that letting the pump heats up before any protection device disconnects it is not a good way because eventually it would just shorten its age
    I would very much prefere another way ..like sensing flow or pressure
    I wonder which is better , a flow switch or a low pressure switch?
    I also heared that there is some kind of sensor which senses the presence of water in the pipe and disconnects the pump when the pipe is dry .
    Any comments?
    Last edited: Mar 30, 2008
  10. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    What about using a flow switch as a simple latch? The pump would start with a momentary switch or contactor that remains engaged long enough for the flow switch to come up to speed and then latch the pump on. If the flow drops off the power drops off to the pump. I use this in lighting control schemes in home automation to prevent time outs from inputs influencing events which are more current. I have used this already on my new well which I am trying to clean the sand out of but I can't watch it all the time and my sprinkler pump has a warning with it about running with no flow.
  11. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    What you mean by " simple latch" ?
    The pump starts by a pressure switch when the pressure in the pressure tank drops down to the cut in pressure
    How you would the flow switch work with that system?
  12. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    Well it would probably require a small PLC (Like an mini IDEC) or in it's simplest form a timer module that is activated by the pressure switch. This makes it direct power to both the flow switch and the pump just enough time for the flow switch to close it's contacts directing power to the pump. The timer contacts drop off and power to the pump is only delivered by the flow switch, thus latching the power but providing cutoff if the flow drops below the flow switch's "on" position threshold. This would require a manual restart spring loaded push button or a timed restart after a certain period by another timer or the PLC. I know it may sound complicated but it is one of the most basic control schemes (it would have to be as I am a total amateur). Here is a simple timer module that I use for a number of like purposes

    http://www.ieib.com/product-details.html?id=105

    and here is a PLC that requires basic programming

    http://www.cmcontrols.com/info/smart_relay.html
  13. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    Why the flow switch would need some momentary pump working? I am intending to fix it in the upstream side before the pump so as soon as the you open a faucet the water flow ( if enough ) would push the flow switch latch arm and would provide power to pressure switch then to pump etc.. or this is the way I imagin it

    Does every flow switch need all that control system? I do understand your explanation but it seem too complicated for a domestic application
    Last edited: Mar 31, 2008
  14. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    The momentary action is the momentary electrical connection to the main pump to get it started. After that, the power to the same (and only)pump is supplied by the flow switch since it is now conducting current. My current setup for my sprinkler pump is simple, just a electrical wall switch in an box wired in parallel to the flow switch. Once water starts flowing I turn off the switch but the pump keeps running.(my flow switch is rated for the steady motor current, and the switch handles the initial high inrush motor current)I can then do other chores and not have to worry about the pump burning out. I'll draw you up a simple ladder diagram (similar to a washing machine schematic) sometime today. If the pressure switch is on the supply side it could reactivate the timer when it comes back up to the kick-in point. This would re-start the whole cycle again. The low volt timer I linked would have to activate a 12 volt or 24 volt contactor for the pump as it's contacts are not rated high enough for the motor. You are right most well pumps rely on direct control from the pressure switch it seems. I am going to have separate adjustable sensors for high and low points since I have a commercial toilet to deal with in my guest/garage I am building.
  15. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    I appreciate what you have said but my set up is different
    My pump system is in continouous use and the wall switch is indoors but the pump / tank and pressure switch are outdoors in the garden.

    I hardly use the wall swich unless there is emergency

    The pressure switch is permanently connected to the power and it switches the pump on and off as appropriate as long as the city water is available

    The suggested flow switch should be connected in some way to the pressure switch with the use of the suggested timer without me having to do any initial switching on of any thing like you do with your sprinkling system !

    I think you lost me some where there loll !
  16. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    Yer right I definitely lost you. I think you gotta be pullin my leg about the wall switch!
  17. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    I read your post once more.......in my scheme of things, and I could be way off, your pressure switch would have 12 volts wired to it instead of 120 volt motor voltage. When the pressure switch is at it's low cut-in it sends 12 volts to both the flow switch and the timer module. The outputs of both the timer module and the flow switch are connected to the relay (contactor) that controls the pump. No electrical current is passing through the flow switch yet but the timer module has a direct connection to the pump relay. This starts the pump. The flow starts and now the flow switch is conducting electrical current and voltage to the pump relay. The timer contacts disengage after a few seconds but the pump keeps running until the flow switch opens the circuit in a no flow situation. This is called a latch circuit. On the supply side when the pressure builds up again (I am assuming this pressure is supplied from elsewhere) the whole thing starts over again, without you having to GO TO THE WALL SWITCH.
  18. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    very simple diagram

    Attached Files:

  19. Bob1000

    Bob1000 New Member

    Messages:
    114
    Location:
    Egypt
    That makes sense now , thank you !

    However I got some questions

    How reliable is that electronic components?

    Do you think that that stage of the timer kicking in the pump for the flow switch to contact is necessary?
    Wouldn't the city water flow move its arm when you open a faucet ( of course assuming that there is a reasonable flow there ) .

    Let me assume the following case :-

    Flow switch cuts off the circle because there is no water in the city supply side , the pressure switch is still in the cut in position and that would make the timer kiks in the pump at the designed intervals but process would not continue cos the flow switch would not contact
    The pump would then cycle untill the city water is back and the pump can complete its operation circle
    Assume the city water is cut for some hours , what would be the case?

    I think the timer would need a long programme
    Last edited: Apr 1, 2008
  20. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    Yeah I haven't thought about your whole scenario. The timer powers up, drops off at the thumbwheel programmed time, and will not trip again until power has been interrupted to it. So the pressure would have to build up to cut-off before the timer could cycle again. If flow stops from lack of water to the booster pump, the pressure switch is still calling for the pump, but the relay remains open until city flow is restored enough to operate the pump via the flow switch. I wonder if a 2nd timer on the flow switch output side would be needed here to prevent too many starts and stops of the pump if the city pressure is fluctuating near the on/off threshold of the flow switch (to act as a dampener)? It probably depends on if the flow switch has similar curve characteristics of a pressures switch that would prevent that. If the pressure reaches the cut-off setting then the booster pump stops since the flow has stopped, interrupting the electrical path thru the flow switch to energize the pump relay. Now when you operate a faucet, if the pressure switch is still at or below cut-in, yes the booster pump will come on. Is this a problem?
    As far as the ct-70 timers go, I have some that are still working in alarm panels after 20 years! They were used on older alarms to engage entry delay shunts for motion detectors looking at entry doors so they have gotten quite the workout too. This has been interesting, I am curious how they have solved this in all-in-one pump protection products. I am not crazy about the hot-stop idea, seems like it would be hard on the pump but I really have no experience. I know that with a plc micro you can try different timing scenarios and monitor the whole process on your laptop screen while it's operating. Once it's tuned in you disconnect and just let it run. I have two of the Amtrol booster pump and tank setups I got on E_bay, an RP-10HP and an
    RP-15HP, not sure if I will use them but they were both steal deals.
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