well pump pressure

[Bob from Plains]

Hi Guys; I have written earlier about installing a new underground sprinkler system (orbit) and not getting the coverage advertised by orbit. After visiting with our local well/pump man he advised against jumping the gpm up as it might cause more problems as a result, when initially drilled the records say the well produced 40 gpm, when I tested it I came up with just under 20 gpm. When I talked with the orbit rep. he said to pull the pump and raise the pressure to 50lbs. as it only puts out 28 when the sprinklers are running. He said it's possible to raise the pressure and not the gpm's, is this true. Thanks for your help.
Bob

 

[Pumpman]

The higher the pressure a pump runs at, the less water it will pump. However, if your pump is running at 28 psi with the sprinklers on, it will continue to run at 28 psi, no matter what you set the pressure at. If you want the sprinklers to run at a higher pressure, you'll need to use a different pump. One that is capable of higher pressures at the GPM you need. I'm assuming that you are not using any pressure regulators, etc.
Ron

 

[Sponsor]

 

[Bob from Plains]

Thanks for the reply, we arn't using any regulators on the system. The 28 lbs seems to be pretty constant, the rep said it is possible to change just part of the pump to get the pressure up and not worry about the gpm part of it. Not to sure if this is true. I have calculated how much each zone is using and have kept it at around 12.5 gpm per zone so our gpm should not be the problem. The rep says the big problem now is the pressure. Thanks for the help.
Bob

 

[Valveman]

Valveman

If you are running 12.5 GPM per zone and only getting 28 PSI, you will probably need more horse power to get 12.5 GPM at 50 PSI. So you will have to change motor and pump. When you start giving the sprinkler zone 50 PSI instead of 28 PSI, the zone that is now using 12.5 GPM at 28 PSI will probably use more GPM at 50 PSI. The sprinkler company should be able to tell you these sprinklers let out 12.5 GPM at 28 PSI and X GPM at 50 PSI. Then you can size the pump accordingly.

 

[Bob NH]

Flow generally varies about as the square root of the pressure. Square root of (50/28) = 1.336. Multiply that times 12.5 GPM and you get 16.7 GPM.

The formula doesn't hold if there are controls in the line such as a Dole valve.

You can't just "pull the pump and raise the pressure to 50lbs". You need to match pump and motor to the demand.

Sometimes a small pump that is selected for high pressure delivers less pressure at high flows than a larger pump of the same HP pump that better matches HP, flow, and demand (but that only goes so far). What are the make, model and horsepower of your pump? What pressure and flow do you want at the sprinkler? How deep to water in the well, and what size and length of pipe? With that information, someone can suggest a pump that will deliver what you need.

 

[Bob from Plains]

The pump is a Fairbanks Morse 3/4hp. p/n 40039665, not sure how old it is but would imagine it could be the original one with the house which is 24 years old. The records we got with the house state the well is 40 ft. deep as all the wells in this area are, not sure how far down the water level is. The line to the house from the pump is 1 1/4", I have t'd off of this and have supplied the main manifold with 1" as well as all the zones are 1". I don't have a problem with changing the pump & motor as I am sure they are old, is there any validity to what the local well man said about not going to a pump with more gpm output, he says it could do damage to the well as far as bringing in sediment and so on.
Thanks Bob