what formula (math/physics?) do I use?

What’s the equation/formula/math what have you that deals with liquid flowing through a hose?
I’m trying to figure out what pressure/suction is required to get water up a hose.

Bernoulli’s equation


If you don’t care about flow rate, then you just need the density of the liquid. If you put that in lb/cu. Inch, then required pressure (in psi) will be density * lift (in inches)

It is not clear from the OP what you are trying to accomplish but the first response is quite correct in pointing out that for flowing systems Bernoulli’s equation defines the relationships between flow, elevation, head, etc. However, in situations involving lifting of water through a hose or pipe another issue also arises that may have to be dealt with. Without trying to over-complicate the issue I would offer the following.

If you are trying to pump liquid from a reservoir (tank) that is located below the pump then you also need to be concerned with [ol]
[li]Suction Lift[/li][li]Net Positive Suction Head (NPSH)[/li][/ol]
With regards to suction lift, the maximum theoretical suction lift is one atmosphere and using the relationship that Kevbo has defined above (where water, at room temperature, weighs about 0.036 #/ft3 and one atmosphere equals 14.696 #/in2) this equates to ~406.8 inches of water or 33.9 feet. However, practically the limit (due to friction losses, etc.) is more like 23-25 feet.

For NPSH there is the NPSH(Required) which is a mechanical property of the pump itself and is usually given by the pump manufacturer and the NPSH(Available) which can be calculated or measured and is a function of the system itself (suction lift, water temperature, etc.). In order for the pump to function properly:

NPSH(Available) ≥ NPSH(Required)

Im trying to build a home made pump and Im trying to figure out what sort of size relation I have to have between the hose (diameter and length) going into the water and the reservoir/container/whatsit thats producing the suction. It’s all jury rigged, so absolutely no help forthcoming from the parts and my first attempts failed to create sufficient suction to pull the water up to where I wanted it.

Figured Id try out some math before wasting more time.

Remember, you can’t suck anything. The best you can do is create a partial vacuum so that the atomphere has something to push against. If you have a total vacuum that means you can only get a net 14.7lbs/in[sup]2[/sup] and less if you are above sea level. If you had no atomosphere you couldn’t lift water with a vacuum pump. Then again you wouldn’t have liquid water but that’s beside the point.

I love to mess with people’s heads when trying to explain concepts like vacuum, dark or hole flow (vs electron flow). Take the example of a vacuum accumulator in a car. It’s a tank that is filled vacuum from the supplyu created by the manifold. If you get a leak you’ll hear a hissing sound from the vacuum escaping. Naturally the escaping vacuum will displace atmosphere and push it into the chamber formerly filled with vacuum.

I am a son of a bitch :smiley:

Ho exquisitely (perverse :rolleyes: ) or inverse. :wink:

Could you describe the setup in a little more detail? How big a hose (length, diameter), whatever info you have on the pump?

Is there a target flow rate you’re after, or would just about anything be OK as long as there’s water runnin’?

I’m pressed for time right now, but I think I might be able to be of some help. I’ll check back later this evening when I have some more time. :wink:

Rabbit - I agree with brad_d’s request for more information. Your last post suggests your trying to build a pump which sounds significantly different in terms of objectives from your OP.

I would also make sure, if you are using an old pump, that it is not blocked in any way on either the suction or discharge side and that the impeller can turn freely. Also, when you turn the pump on does the motor start getting warm?

More info would be good.

More specific information is needed to supply an intelligent answer.
Are you looking for a generic equation or for determing details for a specific situation/application? Try a calculator.
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