solar pump head loss from non-return/check valve?

mzungumzungu Registered Users Posts: 1
Hi Everyone,

I've been browsing this forum for a while, but this is my first post, so please excuse any errors. I've recently installed a solar water pump on my farm outside of Nairobi, Kenya and have been having a few issues. Any guidance you guys/gals can offer would be greatly appreciated!

Here is my setup:

Pump Power: 210 Watts
Pump Max Head: 80 Meters (260 feet)
Pump Max Q: 1.25 M^3 per Hour (330 GPH)
Pump Outlet: 0.75"
Pump Voltage: 36 V
Controller Type: MPPT
Solar Wattage: 100 Watts
Solar Voltage: 17.5 V
Solar Configuration: 4 panels, wired in series

Water Source: Hand-dug well (capacity around 20,000 liters per day (5,200 gallons))
Well Depth: 40 Meters (from static water line to surface) (130 feet)
Total Dynamic Head: 45 Meters (up to elevated 5,000 Liter tank)
Pipe Diameter: 25 MM (1 inch)
Pipe Material: HDPE

A non-return/check valve has been installed about 2 meters above the pump along the 25mm HDPE line. The non-return/check valve has been connected to the HDPE using two male pressure connectors (like this one http://nbevergreen.en.hisupplier.com/product-528014-Male-adaptor.html).

When the pump is tested at the surface, pumping from the house water tank to the elevated 5,000 liter tank, water is flowing at about 1,000 liters per hour (262 GPH) consistently, so the controller and pump seem to be working fine. However, when the pump is put down the well, barely any water (10 LPH) is flowing up to the tank.

Could the problem be with the non-return valve or does this suggest that there's a problem with the pump? We're going to try it without the non-return valve, but that can cause issues during cloudy days when the water will reach 80% of the way up the pipe and then descend when the power drops. Could the issue be with the placement of the non-return valve too close to the pump outlet?

Thanks!

Charlie

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,058 admin
    Re: solar pump head loss from non-return/check valve?

    Charlie, welcome to the forum!

    Regarding your pump, is this a submersible pump or does the motor/pump sit above the surface of the water?

    pumps use a "foot valve", a check valve with very low opening resistance installed at the inlet to the pump. This keeps the pump primed with water at all times (once you get air out of the system).

    With pumps that have have a check valve on the output, they can get lose prime and get an "air lock" between the pump rotor and the output check valve.

    I do not know about well pumps, but for sump pumps (smaller pumps installed basements/below ground rooms), you would drill a small diameter hole between the pump output and output check valve to allow air to be vented out of the pump and let it prime correction. If the hole is too small, it can be blocked by debris. If the hole is too large, you lose a lot of water out the hole as it sprays back into the sump area.

    If this is a submersible pump, is it always located below the dynamic water level (i.e., are you sure the pump is always below the water level)?

    Are you seeing any air in the water output? It would sound like an air leak (pipe threads leaking?) between the check valve and the pump (assuming the check valve is installed above the well water level.

    Trying without the output check valve would be nice (you are just pumping to a cistern/tank, so as long as the output is above the tank's water level, it will not siphon back down). However, pulling 40+ meters and a well pump is not fun either.

    I am certainly no pumping expert, but that is where I would start.

    -Bill

    PS: Is there a screen/filter on the inlet to the pump? Could be be clogged with silt/debris/etc.?

    Some pumps will work with a little bit of sand in the water--Others will be quickly destroyed by sand/grit... If the pump used to work and now does not--grit in the well may have damaged the pump.

    And a clogged inlet filter/screen will cause all sorts of pumping problems. And can cause damage to the pump in the long term (cavitation).

    Cavitation - Wikipedia, the free encyclopedia
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: solar pump head loss from non-return/check valve?

    Hi Charlie,
    Could you provide a make and model for your pump so we can see how it works?
    "Standard" everyday deep well pumps that operate on mains are almost without exception of the centrifugal type, having a stack of impellers, each one adding to the pressure supplied to it by the impeller below, until there is finally enough pressure to drive water to the surface. With these types of pumps, it is totally normal for the flow to drop off as the height from the surface of the well water (regardless of how far below the water the pump is located) to where the water will be delivered, increases. Eventually a height will be reached where the water will slow to a dribble, and eventually stop, leaving the pump beating it's head against a brick wall and getting nowhere, so to speak. If your pump is of the centrifugal type, this could well be your problem, it's near it's limit pressure wise. Very roughly, it takes one PSI to lift/push water up 2 feet, so in your case, with an overall head of roughly 145 feet, your pump would have to be delivering a pressure at it's outlet of roughly 75 PSI just to get the water to the surface, and even more if you wanted a decent flow rate.
    Re the check valve, unless it's defective, it won't allow drain back, and if it's under water, airlock wouldn't be a problem. Generally check valves require very little pressure to open, less than one PSI. However I have seen some designs where when opened, the plunger is driven so high that it partially blocks the outlet from the valve. This however would show up real fast in your above ground tests. I personally often use oversized check valves because the internal openings are smaller than I like to see them.
    We'll have a better idea once we have a look at what type of pump you have. It's also possible that the long wire run may be offering too much resistance to the pump, thus reducing the power available to it. My best guess at the moment however is that the pump is near it's limit with pressure output.
  • BB.BB. Super Moderators, Administrators Posts: 30,058 admin
    Re: solar pump head loss from non-return/check valve?

    If Wayne is correct about the pump being near its output pressure limit... You could test that on the surface with a valve and a pressure gauge. Close the valve opening enough so that the pressure on the pump is near a 40 meter head (PSI, Bar, kPa, whatever units work for you). And see how much flow you get.

    Pure centrifugical pumps are interesting in that they usually use less power with reduced water flow. In some cases with limited solar arrays, etc... a valve is needed to reduce water flow to get the pump operating efficiently (don't think that is the case here though).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: solar pump head loss from non-return/check valve?
    BB. wrote: »
    If Wayne is correct about the pump being near its output pressure limit... You could test that on the surface with a valve and a pressure gauge. Close the valve opening enough so that the pressure on the pump is near a 40 meter head (PSI, Bar, kPa, whatever units work for you). And see how much flow you get.

    Pure centrifugical pumps are interesting in that they usually use less power with reduced water flow. In some cases with limited solar arrays, etc... a valve is needed to reduce water flow to get the pump operating efficiently (don't think that is the case here though).

    -Bill
    Bill is dead on the money! This will most certainly check your pump!
    Many "standard" 1/2 hp sub pumps top out at 100 PSI when tested this way.
  • westbranchwestbranch Solar Expert Posts: 5,182 ✭✭✭✭
    Re: solar pump head loss from non-return/check valve?

    I see a couple of things right away, and they are interdependent.
    1 what is the flow rate for the pump (Make/ Model please) at its maximum rated lift? The same table should tell you the expected pressure at 45m...
    2 If you take the 400W STC panel rating and derate it by ~70% you get 280 Watts, at the panel, now you have to get that power to the pump. (You need at least 210 plus surge to start that pump.) What size wire are you using? Too small a wire will cause too much line loss. I assume the 36 Volts for the MPPT pump is the minimum.
    3 What is the rated output (gals / Liters) of the pump at higher Voltages than 36V?

    hth
     
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: solar pump head loss from non-return/check valve?

    45 meters total lift = 150 feet.
    In a standard install this would require a 1 HP submersible minimum.
    Second check valve should not be necessary; only certain instances require them.

    As the others have said, the particular pump is key to the answer. For solar install it should be something like a Grundfos 16 SQF-10 and have about 800 Watts of panel.
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