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Thread: Flow switch for dry running protection

  1. #31
    In the Trades Bob NH's Avatar
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    You could get a thermal switch that would shut off the pump if it gets too hot, which it will if there is no flow. http://www.pumpsandtanks.com/Motors-...protection.htm

    You could use a reverse acting pressure switch with a wide range setting. You would need to verify the characteristics of the supply to be sure that it would not cycle rapidly by restarting immediately after the pressure dropped. If the pressure returns quickly because the flow stops, then you would need a time-delay relay to protect the pump.

    If you can find a pressure switch with a low pressure cutoff it will automatically shut off the pump if the pressure drops about 10 psi below the cut-in pressure. It must be manually reset before it starts again.

    There are electronic devices like the Pumptec but they must be matched to the pump and may not work with the one you have.

  2. #32
    DIY Senior Member Bob1000's Avatar
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    Quote Originally Posted by valveman View Post
    A couple of years ago there was a city in Texas that had a big fire. The fire trucks were sucking on the fire hydrants faster than the city and water lines could supply. The suction collapsed the city water lines in many places. If you suck it down below 0 pressure, you can collapse your lines. A flow switch doesn't care about the pressure and could still be closed as you were creating the negative pressure. A low suction pressure switch will catch before negative pressure.

    You can give it a wide bandwidth to stop the cycling from the city pressure coming and going. Set it to shut down at 10 PSI and not back on until the city pressure gets back up to 30 or even 40 PSI. You can also install this switch with a small 1 gallon size bladder tank. If you restrict the size of the opening to this tank and switch, it makes a mechanical timer. Ĺ a gallon of water has to come in or go out of this tank for the pressure to change from 10 to 30. So the slower you let this water in an out, the more time you have between off and on, and the switch does not bounce the pump on and off. I have one set up this way with a 1/4" poly line running from the main line to the little tank and switch. No switch bouncing!!
    I think the reverse acting pressure switch together with that small bladder tank is the best idea
    But how you say that I should set it up to shut down the pump at 10psi and restart it at 30 or 40? the city pressure is around 20 psi most of the time that is why I need a booster pump
    I was intending to set it up cut out at 5psi and cut in at 10 psi , does that make sense?
    Because the new pump would give me the pressure I want at any city pressure I want to make full use of that and I want to be able to keep my system working at zero city pressure but in the same time I want it to shut down at dry running when actually there is no water to suck or a negative pressure would happen.

    I want to make full use of the new pump but in the same time protect it from burning in case the city water is completely cut off

  3. #33
    Moderator valveman's Avatar
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    You can't pump water unless it is there to be pumped. If the city drops below 5 PSI, there is no water left to pump. You will need to wait until the city is back up to maybe 20 PSI before restarting the pump. 5/20 is a range your pump may be able to live with. 5/10 is not.

    Otherwise you need to use a cistern tank that can fill when the city does have water, so you can have water to use during the time the City is below 5 PSI. The booster pump can then give you all the pressure you want as long as the cistern has water. Then the cistern can refill later when the city has water again.

  4. #34
    DIY Member MarkHash's Avatar
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    A low level float switch or similar on the cistern could then be your master off.

  5. #35
    DIY Member MarkHash's Avatar
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    Hey valveman, what is the control scheme on the cistern then? It couldn't be a pressurized tank could it? Then it would be essentially another ballast pressure tank right? Would it have to be isolated when it becomes full?

  6. #36
    Sprinkler Guy Wet_Boots's Avatar
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    Just think how much simpler this guy's life would be if he ponied up for a good thermostatic shower valve.

  7. #37
    DIY Senior Member Bob1000's Avatar
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    Quote Originally Posted by Wet_Boots View Post
    Just think how much simpler this guy's life would be if he ponied up for a good thermostatic shower valve.
    That valve might solve the problem of the pressure fluctuations but would not help if the city pressure is too low to let the pump work and deliver water

    Their ideas are smart and deserve to think about , in fact last summer I was thinking about the cestern issue and I would do it if I became sure that the city low pressure periods are too long to avoid that complication

    I think a level control switch together with a mechanical float valve would be the control system for that cestern , And I think it would be an open tank , any comments?

  8. #38
    Moderator valveman's Avatar
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    Toilet type float valve on the incoming city line to feed the cistern. Then suck out of the cistern with the 2 HP pump with pressure tank and pressure switch for control. A float switch in the cistern connected with the pressure switch would keep the pump from running if the cistern were dry. This way you still have a cistern full of water when the city is out, and your pump makes sure you always have enough pressure.

  9. #39
    DIY Member MarkHash's Avatar
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    Valveman, can you point me to a link for such an apparatus? I've installed (previous job) sewage wetwells with hi/low/emer floats, dual pumps, and pump cycling timers, but i've never heard of a mechanical "toilet type" combination level control and shutoff valve on the scale that would fill a cistern! I would have thought it had to be an electric valve of some sort.

  10. #40
    Moderator valveman's Avatar
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    I can get you a mechanical float valve as big as you want. I think a 6" is the largest I have done but, they make larger ones. Cla-Val, Watts, Bermad, and others make these kind. I have also seen 1" and larger at Farm and Ranch supply houses and Country Feed Stores. I can get you some names if you need them.

    You donít want to use a toilet type float coming from a pump. The closer the float gets to the top, the slower the flow and the pump will cycle on and off. These are called modulating type float valves. You can get non-modulating float valves to use coming from a pump. These are either fully open or fully closed.

    The regular modulating type will work fine for the line coming from a city supply.

  11. #41
    DIY Senior Member Bob1000's Avatar
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    Thanks to all of you for the very rich ideas , will decide on one of them , carry it out then get back to you with the feed back

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