Pump Math

[Leaky Boot]

How many PSI are used up lifting water from a 90 ft water level to the surface using 1 inch drop pipe? I have a used pump that can do 70 PSI max in my test tank. I figure the water column has a weight of 25 to 28 pounds and I need 50 to 55 PSI to reach switch kick off. Thanks, LB

 

[Reach4]

39.02 PSI plus a small amount for the dynamic head, which differs with the flow rate. That dynamic part might be 3 or 4 PSI at 10 gpm, and maybe half that at 7 gpm.

http://www.kylesconverter.com/pressure/feet-of-water-to-pounds-per-square-inch

 

[Sponsor]

 

[Leaky Boot]

39 psi used to move a 1 inch water column up 90 ft sure seems like a lot. Then to get 55 psi I need a total pressure of 94 psi at the pump..??

 

[Valveman]

2.31' equals 1 PSI. And yes you add the 55 PSI you need to the 39 PSI it takes to get it up the hill. 95 PSI at the pump will give you 55 PSI at the top of the hill. It would be the same with any size pipe. The size of pipe only matters for the GPM flow. Any friction loss in the pipe would need to be added to the 95 PSI already figured.

Using a pressure tank only you would get 95 PSI average using a 85/105 pressure switch. If you add a Cycle Stop Valve the pump won't be constantly cycling on and off from 85 to 105 to give you an average 95 PSI, the CSV would keep the pump from cycling and give you a constant 95 PSI.

 

[Reach4]

What do you want to do with the water? Use it for the plumbing in a house, fill an open tank, or what?

You can only pull water from about 25 ft down. From 90 feet, you would usually want a submersible pump that is under the water, or a pump where there is part of the pumping system under water. Pushing water is much easier than pulling water.

 

[Leaky Boot]

Interesting. So using these figures if we double the depth to a static level of 180 ft we will need a pump capable of producing 133 psi at the pump to get the 55 psi switch to turn off the pump?

 

[Reach4]

There are pump books that have easy to use tables. Here is an example

Now for your application, I would be considering a 7 gpm 1/2 or 3/4 HP pump. I have a table for a a 1/2 HP 7 gpm handy. This says that from 100 ft down, with a 30/50 switch, the pump would be delivering about 8.1 gpm at 30 PSI, but as the tank pressure is rising, that falls off to about 6.8 gpm. Good deal.

Now the water level drops to 180 due to drought or whatever. The pump delivers 5.6 gpm at 30 psi, which would probably be enough to backwash a big water softener. As the pressure rises, it is only delivering 3.8 gpm as the pump is turning off. So if you were expecting that water level for real, you would probably go to a 3/4 HP pump. On the other hand, the pump delivering water a little slower when you are about to run out of water is not so bad.

If you had a lot of bathrooms, or you had a backwashing iron filter that needed more gpm, then you might go to a 10 gpm pump.