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I've read some similar questions (i.e. this one) but still I need some help. I have a water barrel in the garden:

  • capacity 120 L
  • 50 cm diameter
  • 80 cm height

I want to feed a water hose, placing the joint and a valve on the top of the barrel. The hose has:

  • a connection of 1/2"
  • an inner diameter of 12 mm
  • a length of 30 meters

The use is for watering my greenhouse without having to mess with the can inside the barrel for few liters per time...

Here my math:

  • I assume to have an initial pressure of about 3 bar (or something less), since my hose works fine with the house taps

  • using the Bernoulli formula:

    H = (h2 - h1) + (P2 - P1) / γ + (v2^2 - v1^2) / 2g + y

where

  • h1 = 0 m (the pump will be placed at the bottom of the barrel)
  • h2 = 1 m (height of the barrel plus some margin)
  • γ = 9800 N/m^3 (water density)
  • P1 = 1 bar = 100000 N/m^2 (atmospheric pressure)
  • P2 = 3 bar = 300000 N/m^2 (the pressure I want to the junction with the hose)
  • v1, v2, y = 0 (they should not matter for this application)

I get:

H = 1 m + 200000 N/m^2 / 9800 N/m^3 = 21,4 m => 20 m => 2 bar

Using this calculator, for a standard 12 mm hose @ 3 bar I need a pump that can achieve at least 5 L/m of flow rate.

  1. is my math correct?

  2. I found this product that should match the specification: https://www.amazon.it/dp/B078SZ3VNJ/, do you agree?

  3. do you recommend any kit? I'm aware I can buy the pump, pipes, hose joint and valve separately, than make a frame by myself to place the valve and the joint on top of the barrel, but if there is a kit (with or without the pump) it would be easier!

Mark
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  • A minor thing, but h2 would be the height of the water in the barrel, not the barrel itself. So you would drop ~0.8 bar of pressure from a full barrel to empty. I'm sure you've already considered it, but do you have the room to get the bottom of the barrel 3m off the ground? – MackM Aug 23 '23 at 16:38
  • @MackM, not sure to understand the first advice. Should not `h2` the final height of the water? I mean, the height where the pump is supposed to move the water. hence 1 m is where I'd put the frame with the valve and the hose joint. Of course in the worst case (barrel almost empty) – Mark Aug 23 '23 at 16:44
  • @MackM yeah, it was the first thought but I cannot do that for several reasons – Mark Aug 23 '23 at 16:44
  • I think I'm the one not understanding. I did my math wrong, you would lose about 0.08 bar over 80cm, not that much. But I do believe `h1` and `h2` are the start and end points of the water, meaning the water surface height in the barrel, and the height the end of your hose is at. Your pump will need to be able to lift the water out of the barrel since you're coming out of the top, but pressure-wise it gets that energy back as it falls the same distance in the hose. – MackM Aug 23 '23 at 17:00
  • Actually it does not. Keeping the nozzle in hand, it will be still at about 1 m from the floor. Not super precise, but it's not a physics homework, indeed :) – Mark Aug 23 '23 at 17:12
  • I'm just trying to say, I believe the only water heights you need to know are the open surface in the barrel and the hose end. But since the hose will be 20-100cm above the water level in the barrel (do I understand this time?), that will be a 0.02-0.1 bar contribution to the pressure. So... negligible. Thanks for taking that walk with me :) – MackM Aug 23 '23 at 17:31
  • Oh yes, now I understand. I've just simplified this detail, in fact – Mark Aug 23 '23 at 17:35
  • Using the hose calculator you link to with a 12mm bore (from your hose), 2.1 bar (from the linked pump), and the "Quantity Fluid Flow vs Hose Length" graph, I think you can see what flow rate you can expect at different hose lengths from your linked pump. For example, I'm seeing you can expect 9.7L/minute out of a 5m hose, and 7.0L/minute out of a 10m hose. But what do I know? I'm out of my depth here. – MackM Aug 23 '23 at 17:40
  • In any case they are less than the maximum flow rate of the pump, hence it should be ok – Mark Aug 23 '23 at 17:42
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    This belong in a physics category , not gardening. I have syphoned for 60 years and never needed to calculate a volume. – blacksmith37 Aug 24 '23 at 16:01
  • Notwithstanding all the fancy calculations, unless you live in an area where rain is substantial & daily, the limiting factor in all of this is that you have a tiny water barrel with only 120 litres when full and unless you upgrade your water collection & storage to at least 1000 litres, you will be depleting the contents of the barrel too quickly to worry about fancy calculations. All you need to do is work out how long it would take for any pump to empty the barrel - if it was a large greenhouse, only a few hours. Collect rainwater and put it in a barrel inside the greenhouse. Job Done. – Nikki Aug 24 '23 at 16:19
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    Guys, the math was just to select the right pump. If I asked: "which pump do I need?" without showing my effort you could tell me: doing your homework first. But the main question is about any "kit" to avoid me to buy or make every single piece separately. – Mark Aug 25 '23 at 06:22
  • @Mark No answers, but lots of advice on how you *ought to* have asked anyway ;) – MackM Aug 28 '23 at 12:37

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