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Thread: Questions on deep well submersible pump

  1. #1

    Default Questions on deep well submersible pump

    Hi. I'm building a house in the mountains of Colorado and am trying to figure out how to size and set a submersible pump in my well.

    First, the well is 420 ft. deep, with a static water level of 105 ft. , and produces 8 GPM (all this according to the well yield test). Other relevant details are: The house is off-grid, so I'm very power conscious. I do have 220V available to run the pump. Also, I need 75 PSI of pressure (min) at the inlet filter to the house.

    I've decided (I think) that I'd like to use a Grundfos 10SQ15-330 pump, mostly because it's the only brand I can find that has a 'soft-start' feature, which will benefit my power system. Anyone know of another brand with this feature? I think the Grundfos model will produce enough pressure and flow, but that's where my questions begin.

    1) The above pump has an outlet size of 1.25", but I'm told that 1" poly pipe is 'standard' for running to homes from wells. If this is the case then I'll have to adapt it down to 1", which I'm assuming will have some effect on the flow rates and pressures the pump can supply. Is this correct?

    2) I realize that if my static water level is 105 ft, and I'm pumping at or below the well yield, I'm only pumping the water 105 ft. (even if my pump is set deeper). The house pressure is equivaltent to (75psi * 2.31) = 173 ft. of head, which gives me a total head of 105 + 173 = 278 ft. . According to the Grundfos pump curve the above model should deliver approx 13 GPM under these conditions, which is more than enough. Am I approaching this correctly? If so, then the next question ...

    3) If I mount the pump at a depth of 410 ft in the well (10 ft from the bottom) and the static water level drops to 410 ft., then I'm pumping the water the entire 410 ft.. Add this 410 ft. to the 173 ft. of head required at the house and I get 583 ft. of head required by the pump. The pump will not supply this. According to the curve the pump will produce aprox. 9 GPM at 420 ft. of head, dropping to 2 GPM at 500 ft of head. I think this means I'll still get water from the well, just at really low pressure. Is this correct? If so, should I mount the pump at the bottom of the well, or only as deep as the pump will produce the required pressure and flow?

    4) Speaking of pressure, in case #2 above where the pump is working properly, I calculate the pressure output by the pump to be approx. 120 PSI (278 / 2.31). Some of the poly pipe I've run into has a pressure rating of 100 PSI. Does this mean the pipe will burst under these operating conditions? If so, in case #3 above, the pump will be outputting over 250 PSI to get the water out of the well. Do I need to find poly pipe with a pressure rating beyond this? Does this exist? Is my well just too deep to use poly pipe?

    I realize I could pay someone to do this and forget about it. However, I'm a curious DIY'er and I'd like to give it a shot. Any help you can provide would be appreciated.

  2. #2

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    For 1) you are correct. To decid whether the friction losses are important to you or not you should calculate the friction losses. There is a calculator here: Tool Box. At 13 gpm the difference in head due to friction is about 10 psi between your 1.25" and 1" pipes.

    2) is true, neglecting the friction loss above, which isn't a bad approximation.

    3) since the Grundfos is a turbine pump you might exceed the shutoff pressure here.

    4) Yes, if the well's dry the static head alone will go over the poly pipe's rating.

  3. #3
    Moderator valveman's Avatar
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    "9 GPM at 420 ft. of head, dropping to 2 GPM at 500 ft of head. I think this means I'll still get water from the well, just at really low pressure. Is this correct?"

    Yes, you will be getting 2 GPM at 38 PSI. However, when you close the faucet, the pump may not be able to build enough pressure to shut itself off. This could cause the pump to overheat and be destroyed.

    When the water level drops and you are producing 2 GPM at 38 PSI up top, the pipe in the well will be seeing 216 PSI.

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    If the well produces 8 gpm, you will never see the two gpm your discussing. If you try to over pump the well at more than 8 gpm you will certainly draw the water down to the pump if it's capable of producing that much water at that depth. The important thing is that the pump can't produce 8 gpm at close to the 400 foot mark. This way it will be pumping plenty of water to stay cool and not nuke itself.

    If you think you will need more than 8 gpm for extended periods, you may want to consider a Cistern.

    bob...

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    Moderator valveman's Avatar
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    Also the soft start feature may be good for the off grid power system but, if you have any sand or grit in the well, it can also keep your pump from getting started.

    According to Franklin, using the longest length of the smallest wire possible will give you a natural soft start from a standard pump. In other words, if you use 400' of #12 wire with a standard pump, it will result in about a 20% reduction in starting current and about a 36% reduction in starting torque.

    The Grundfos SQ spins 10,600 RPM as compared to a standard pump that spins only 3450 RPM. According to the Grundfos engineering manual, every time you double the RPM of a pump, you quadruple the wear rate. Works out good for Grundfos, not so good for the end user.

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    Another pump suggestion. A company named Lorentz makes a pump designed for off-grid use. Google and you will find dealers. They have two kinds. The one I am talking about is a helical rotor unit. The system is designed to run from solar or batteries. They also had a 120V version of the controller. It uses the moving contact point between the helical rotor and a molded rubber surround. It is like a screw lifting water.

    I have one at 5oo ft. Static water level above the pump has gone to zero when I have pumped faster than recharge rate. I have never seen the power to the pump go above 500 watts. It is inherently soft start. With a normal pressure tank and pressure switch it can serve as your supply. Or you can pump to a storage tank. It is capable of generating serious pressure. There is no slipping mechanism like a centrifugal pump.

    I run mine from a 48VDC supply I built for it.

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    Helical rotor is a good idea for a solar pump. It can pump a little even there is very little power or voltage going to it. More power equals more water. With regular centrifugal impellers inside a submersible pump, there has to be a certain RPM before the pump starts moving any water. So it can't work well at low power levels, even with an inverter.

    They use to make helical style submersibles back in the 60's. They would really build the pressure. These type pumps don't have a point where they will just quit building pressure like am impeller pump will. Close a valve and if the motor is strong enough to keep turning the pump, something is going to blow. Be sure and use a couple of good pressure relief valves in case something malfunctions. It was a long time ago but, I still remember seeing a couple of pressure tanks that had gone through the well house roof when a pressure switch stuck.

    Other than that the only problem with those old helical pumps, is that we couldn't keep the top bearing in the motor. It is hard to balance a helical shaft, and the little vibration would take out the top motor bearing. Hopefully the slower RPM of the solar design will help with both of these problems.

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    These pumps use a water bearing. I don't know if this fixes the bearing problem, but it seems to work OK. There is normally no solid to solid material contact. There are no active devices in the well; as opposed to some designs. They use a permanent magnet for the rotor and an epoxy embedded three phase stator. Not a whole lot to fail down the hole. All the electronics are in the controller. It generates three phase AC to drive the motor.

    Overall, I have been very pleased by the performance.

    A high pressure relief valve is indeed a good idea. These pumps can easilly exceed the bursting pressure of components if the pressure control fails.

    But they are a slick solution.

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    Nearly all pumps have a "water bearing". A stainless steel shaft inside a rubber bushing with water flowing over it makes a really good bearing. However, standard pump shafts are straight. I have spent many an hour with a torch and water quench line trying to make pump shafts as straight as a gun barrel. It is very hard to balance a helical shaped shaft. If you can get it to last at least 5 to 7 years, I would say they have gotten better at it.

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