# Meaning of flow rate in water pump

Rated flow in the following is in conjunction with the rated lift meaning it can lift 15 meters at flow of 12 Liter/ minutes from a well? But what if its sucking water from the water company with existing flow rate. So the total flow rate is additive and is more than 12L/ minute, right? Or wrong? I plan to buy this pump and it confused me for days.

• No pump can suck water up more than about 10 meters. You can push water up 15 meters. Sep 25 '20 at 13:53
• If it cant suck 10 meters. Why is there a max lift of 20 meters and rated lift of 15 meters.
– Jtl
Sep 29 '20 at 1:11
• You can lift by pushing or pulling. A pump can’t suck any lower than a perfect vacuum which will lift water only about 10 meters. Sep 29 '20 at 14:17
• What is the diameter of this 10 meters limit? Is it like 4inches or even say 1 meter diameter pipe or no limit and any diameter?
– Jtl
Sep 30 '20 at 4:53
• Diameter doesn’t matter. When sucking water up, you can’t create more than one atmosphere differential pressure. Sep 30 '20 at 11:36

To fully answer we need a diagram of how this pump will be connected to the water company. I will speculate on a few configurations:

If the city water flows into a bucket at atmospheric pressure, and the pump pulls out of that bucket, it will just be limited to the performance of the pump as expected.

If the city water flows into the inlet of this pump; the users (sprinklers etc) on the outlet will see an increase in pressure. (~15m of water more pressure for your 12L/m) This configuration is typically called a "boost pump", because it boosts the pressure.

Increased pressure can mean increased flow, but the amount the flow increases depends on the the downstream user. For a sprinkler or some sort of orifice type restriction, the orifice equation gives us some idea. If the pressure at your sprinkler/user goes up by a factor of 4, the flow rate through that sprinkler/user will double.

Note that if the flow from the city water is higher than this 12L/m rating of the pump you will see less pressure boost, and at some point (say 24L/m) this pump will actually reduce the pressure (and the resultant flow) delivered.

• It will be related to the geometry internal to the pump; requiring experimentation. I can estimate it though. What flow do you want to achieve? (or back up and tell me what the user is, what the status is with the existing water, and what your end goal is). Sep 25 '20 at 2:17
• A centrifugal pump can certainly suck (within the limits of atmospheric pressure /cavitation), but it certainly wont double your flow rate. Lets say your city water pressure is 72psig or 500kPa at zero flow. At the full 12L/min into the bucket it is at 0psig or 0kPa. So it is taking 72psi differential to push that water from the water company to your entry valve at 12L/min. 15m of water is 21psi or 145kPa, but on suction your are limited by cavitation (atmospheric pressure is the hard limit at 14.7psi), so lets say the pump will do 10psi/69kPa suction.... Sep 25 '20 at 2:41
• ...so with the pump in series you now have 72psi + 10psi or 82psi differential from the water company. (I'm going to back track on the equations because units in terms of absolute pressure probably matter and don't have time to get all that straight). If you instead just look at a linear increase (which it in fact will be less than this), you new flow would be flow= 82/72 * 12L/min = 13.7L/min Sep 25 '20 at 2:46
• In this case since you are pushing you get the full 21psi. Since you doubled the flow we know your pressure on the nozzles went up by a factor of 4. So P + 21psi = P * 4, so your pressure without the boost pump is 7psi, and your pressure with the boost pump is 28psi. (this is pretty sloppy since the pressure in you pump would be a bit higher at the lower flow, but you get the idea). Sep 25 '20 at 3:12
• Sorry pushing was slang, just saying that you are not sucking or limited by atmospheric pressure / NPSH. When you said "it seems the booster pump can double the flow" I assumed you tested this scenario, so I extrapolated the data based on your 3 to 6 double information. This scenario is much more likely and viable so I went with it. So in that hypothetical scenario where your shower head is getting 7psi and 3L/min from the water company, this pump would boost the pressure up to 28psi and the flow out the shower heads would increase to 6L/min. Sep 25 '20 at 3:33