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Thread: Well runs out of water

  1. #1

    Unhappy Well runs out of water

    I have a 3/4 HP Submersible pump that I installed in 1996. It has always worked very well. However, when we use a lot of water, say running it for about 15 minutes, water flow stops. The pump continues to run. For a long time I figured it was just pumping out water faster than the well was able to fill up. Recently, I discovered that when this problem occurs, if I shut off the power to the pump for about a minute & turn it back on, it starts to flow water again. What could be the problem?

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default pump

    Probably what you originally suspected. The pump draws air and then, since it cannot pump air, it spins until you turn it off and the air can escape and water fills the pump.

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    You would do good to either use less water or get some protection for the pump like the Subgard or the Pumptec.

    Running dry is very damaging for a pump or motor.

    bob...

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    DIY Senior Member rshackleford's Avatar
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    A flow control would work well. You can get them in 1 gpm, 2 gpm, and so on. They use a rubber diaphragm to let just that amount of water through.
    rshackleford

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    In the Trades Bob NH's Avatar
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    I don't like to put a flow control on a submersible because of the steep pressure/flow characteristic of submersibles. Some of them will put out a lot more head than you want in the up-pipe. You could put the flow control at the outlet of the tank but that would also reduce the pressure available to the user.

    In most applications the user would just let the pump run longer and still run the well down to the pump. And they will be wasting electricity because of the head loss in the flow control and the fact that the pump is not operating at its most efficient operating point.

    If the pump is running the well down in 15 minutes as the user said, then it will probably run it down in 30 minutes while the lawn is being watered.

    I think the Pumptec is the best solution. You could check on that famous auction site to get some idea of the price to compare with local suppliers. Last I saw they were $128 including shipping.

    Another possibility is that the water table has receded so much that it is below the head capability of the pump, though that is unlikely with a properly selected submersible that has been set at the proper depth.

    If I owned the system I would want to know the model of the pump, the flow/head characteristic, the depth of the well and the depth to the aquifer, and the level that the pump is set at.

    It might be possible to get more water if the pump is set at a lower level. You would probably have to ask the driller/installer if you don't already have that information. If the well can be pumped to a lower level then there is more head available for the aquifer to deliver water to the well.

    The make/model of the pump should be on a sticker somewhere. In 1996 most submersibles were installed with control boxes which may contain the information. Can you post that information if you find it?
    Last edited by Bob NH; 11-17-2005 at 12:37 PM. Reason: Get rid of icon that is showing up

  6. #6
    DIY Senior Member rshackleford's Avatar
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    Water wells typically have a linear type drawdown. This is not an exact statement, but for all intents and purposes this is true. For example, take a well that has a static of 50’, a total depth of 150’, and the pump is set at 100’. Let’s also say that the driller performed a pumping test on the well and determined that the well would pump 10 gpm at a drawdown of 100’. Do the division and you would get a specific capacity of 1 gpm per 10’ of drawdown. This specific capacity is basically a linear function through the depth of the well. So, you could say I want 5 gpm, this would mean 50’ of drawdown.

    Okay, on to my point. I stated above that the example pump is set at 100’. If I am trying to pump this well at 10 gpm, I can probably get that. I might get 10 gpm for 5 minutes, 15 minutes, or even 15 hours, but eventually the 10 gpm pumping rate is going to lower the water level in this well below the pump and I will run out of water at the surface. For this well, if I want to pump continuously and not run out of water I can only pump at 5 gpm or 50’ of drawdown. According to the drillers test pump results I can pump at this rate forever.

    Alright, how does this apply to your well? You can continue to let your pump run out of water and only get 15 minutes worth of water or you can let it pump at 5 gpm and pump water forever. For example sake lets say it takes 15 minutes for the well to recover. In on hour you would pump 10 gpm for 30 minutes or 300 gallons. If you pumped 5 gpm for 60 minutes you would also get 300 gpm.

    So what is the advantage of 5 gpm? The toughest thing on a motor is starting and stopping. This is the case for any electric motor. When a motor starts it builds up a lot of heat and works on destroying the insulation. On the 10 gpm pump you will start the motor 48 times in a day and you will only start the 5 gpm motor once in a day.

    There is also a well development method called "rawhiding." This involves pumping water from the well until the well draws way down. Then the pump is stopped until the water returns. Then the pump is turned on again and this cycle continues. This is called “rawhiding” and it encourages sand to come out of the formation and into the well water. On a well screened well this will not cause sand in the water, but on a poorly screened well or a well with very find sand you will get sand in the water. Also, some wells will pump sand at higher flow rates and not at lower flow rates. This is another advantage to slowing down a well.

    The best protection you could actually get would be to install a flow control and Pumptech or whatever Goulds calls it in the lower horsepowers.

    I do agree that you should keep some records of your well and pump information.
    Last edited by rshackleford; 11-17-2005 at 06:03 PM.
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    Good description of drawdown Bob,

    I think Shack is talking about a Cycle stop or Smart Tee, but your right, with the wrong pump this could be disasterous.

    bob...

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    DIY Senior Member rshackleford's Avatar
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    Nah, I was just talking about a good old fashion Dole flow control. Pump the well and monitor drawdown. Find a pumping rate that keeps the water level above the pump and choose a flow control in that range. We use these quite a bit for low flow wells. We are in an area where people are happy if they get 3 gpm and 10 gpm is a smokin’ well. The flow control will be a simple, economic, and well friendly solution.
    rshackleford

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    Is that the coupling with the insert that has a little tiny hole on the middle? If so, the first one I saw was when I was about 15 years old ( a few years back) and couldn't figure what the hell it was. Especially since it was installed on a perfectly good well. I think someone put it in accidentally.

    I think I would rather have something with a little more adjustment available if needed like an old fashioned backpressure control from a jet pump maybe.

    bob...

  10. #10
    In the Trades Bob NH's Avatar
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    Default You Can Find the Problem and Fix It

    We all agree that the flow rate should be reduced. I think it is better to do it at the pump than with a flow control.

    The problem with a flow control on a submersible is that it drives the pump to very high pressure where you are wasting energy by operating the pump off its best efficiency point and you are wasting energy at the flow control. For 3/4 HP Goulds 4" models that a homeowner might use:

    7GS07 Min/Nominal/Max flow rates are 1.5/7/10 GPM and the pressure at 1.5 GPM is 173 psi and 169 psi at 3 GPM.

    10GS07 Min/Nominal/Max flow rates are 3/10/16 GPM and the pressure at 3GPM is 130 psi.

    13GS07 Min/Nominal/Max flow rates are 4/13/20 GPM and the pressure at 4 GPM is 93 psi.

    18GS07 Min/Nominal/Max flow rates are 6/18/28 GPM and pressure at 6 GPM is 78 psi.

    Pressures given are AT THE PUMP, not at the tank.

    If you can't find the model number of the pump, and maybe if you can, you can get a rebuilt residential size water meter ($31, stk #62276) and two unions ($5 ea, stk #48816) from USA BlueBook (800-548-1234).

    Find out what your pump rate is and how much water you use, and you can put a smaller pump head on your existing motor (If it's a Franklin motor). Depending on what you now have for pump and well capacity, you might go to the 5GS05, 7GS05, or 10GS05.

    Depending on your well capacity (which you can determine with the water meter), you may want the Pumptec that is discussed in previous posts. You can probably size the pump so you don't need it. If you match the pump to the well capacity you can save the cost of the Pumptec which would be a good bit of the price of a new pump head.

    While you are replacing the pump head you may want to put on a longer down pipe if the well suits (For reason, see first paragraph of Post #6 by rshackleford). Ask your driller if he is still around.

    If you lower the pump, you need to account for the required head from a greater drawdown. That is usually not a problem with a submersible because you will get about 30% more head at the minimum flow than you will at the nominal flow.

  11. #11
    DIY Senior Member rshackleford's Avatar
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    Bob,

    Do you know of any 4” submersible curves that also show pump efficiency? It would be interesting to compare actually difference in HP for your example of flow versus pressure.

    Also, do you know how much energy is actually used by a submersible pump? I have always considered it inconsequential, but it would be interesting to actually see some per month costs of running a submersible pump. I often hear people that live in town say that it must be nice not to have to pay a monthly water bill, but I disagree. I think if you added up all the costs of a well most people would realize their monthly water bill is okay.
    rshackleford

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    I'm not sure what Bob you mean Shack, but submersible pumps don't use much energy and for my money and my experience, I'll take the well any day over the water bill. I am sure it verys from one location to another, but for the most part the water bill will far out weigh the cost's of a normal water well system including maintenance and repair.

    If you want to see the effeciency curves for submersibles, look at a distributors book. Instead of the graphs they show homeowners, the curves are far easier to use and more accurate. They also show an efficiency slot down the middle of the curve. If you like, I'll scan one and post it here.

    bob...

  13. #13
    In the Trades Bob NH's Avatar
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    Default Efficiency vs Flow

    The smallest submersible for which I have efficiency curves is the Goulds 33GS. I also have curves for up to the 80GS and for the Dempster Submaster Y and V series which are rated at 45 and 60 GPM.

    For the 33GS30 the maximum efficiency is 68% at 32 GPM as near as I can pick it off the curve. It is producing 2.13 water horsepower and is using 3.13 shaft horsepower.

    At 10 GPM, which is 31% of the flow at peak efficiency, the pump is operating at 40% efficiency, producing 0.97 water horsepower while using 2.43 shaft horsepower.

    The efficiency curve is pretty flat near the design operating point of the pump so the pump operating without restriction tends to be self-adjusting for variations in pressure at the tank.

    But the efficiency numbers by themselves are deceiving. If the lower flow is achieved with a flow control valve, much of the energy is wasted. The flow-controlled pump will take 3.2 times as long to pump the same water while using 2.43/3.13=0.776=77.6% as much shaft power. Therefore, the total energy to deliver the same amount of water to the tank is 0.776x3.2=2.48 times as much energy.

    So throttling the pump requires 2.5 times as much energy to get the same result. The energy ratio goes to 3.8 if the flow of the same pump is throttled to 6 GPM, or about 20% of the design flow. That corresponds to operating a 10 GPM pump at 2 GPM.

    Besides using more power, the pump is being operated 3.2 times as long at 1.5 times the head. And for every gallon delivered, the impellers are forced to work against another 2 gallons of internal bypassing that can't be discharged because of the flow restriction. That has to diminish the life of the pump.

  14. #14
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    Then there is the debate on whether it is better to backpressure a sub and keep it running or cycle it and take out the motor.

    The best thing is to have all the well stats to start with, pick out the best pump for the job, install some safety equipment just in case (most homeowners won't spend the money) then go away feeling pretty good about your install. Too bad this isn't a perfect world. God knows I've tried to sell the protection many times and the home owner just knows it couldn't happen to them. Just like the teenager in his fast new car.

    And I think we have lost Kondas.

    bob...

  15. #15
    DIY Senior Member rshackleford's Avatar
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    I would definitely be interested to seem some efficiency on the smaller 4” pumps if they exist. I have sold a lot of 5, 7, and 10 G’s and GS’s but I don’t really know how efficient they are.

    I don’t disagree about finding the right pump. A designer should absolutely pick a pump that matches the well discharge.

    Sometimes, though, a well might be produce less than 5 gpm and that is when a choke might be used given certain head conditions. The other case where a choke is good is on shallow stock water well. The well is only going to be used a couple months out of the year the 10 gallon series pump is the cheapest available.

    My calculations tell me that operating a ½ hp pump at $0.07 per kW would only cost $0.026 per hour. This isn’t much.
    Last edited by rshackleford; 11-20-2005 at 05:11 PM.
    rshackleford

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