Calculate PSI based on GPM?

[mikebuild]

Am designing an irrigation system that will be supplied by a tank and pump. The distance between the supply and valve manifold is ~100’ via 1 ¼†pvc with ~5’ of lift. I need to know gpm and psi to properly design system. I ran pump to check volume at this distance/grade. Pump produced ~34gpm. Question ... with above data, can I calculate PSI?
Thanks.

 

[Cass]

Go to Low*s and buy a $10.00 pressure gauge, attach it to the system and you will know exactly.

In fact if you leave it on the system you can monitor it.

 

[Sponsor]

 

[Bob NH]

You need to know the pressure and flow at the end of the pipe.

Put a valve on the end of the pipe with the gauge on the pressure side of the valve. Then, operate the pump without any other water demands on the pump, and close down the valve until the pressure reaches a steady value. Then measure the flow rate (GPM) using a large bucket and timer such as a watch with a sweep second hand.

Measure the flow and pressure at several points such as at 30, 40, 50, and 60 psi. Plot those measurements on a graph with GPM on the horizontal line and the pressure on the vertical line.

That defines the characteristics of your pump and system for those conditions.

Then determine the pressure needs of your sprinkler heads. That should be available from the manufacturer. Add about 5 psi to that pressure to account for pressure losses in your irrigation system pipes and control valve. That is the pressure that you need to deliver at the end of your pipe.

Look at the graph that you just made and find the GPM corresponding to that pressure requirement.

EXAMPLE:
Suppose you got 30, 22, and 10 GPM at 30, 40, and 50 psi.

Your sprinklers require 40 psi at the head (per specification).

Add 5 psi for system losses to get 45 psi required at the end of your pipe.

Look at the graph and you will find that 45 psi corresponds to about 15 GPM. That is the flow for which you should design your system.

All of the above is an example. You need to do the work to find the capability of your system. You can try the analysis with the pressures and flows that I have suggested.

You should design each zone of the system so the pump will operate without cycling (turning on and off) while the system is running.

 

[hj]

pump

With a closed system the psi will be whatever the deadhead pressure the pump can produce. Once water starts flowing psi is produced by whatever volume the system does not use. With a lot of high volume heads the pressure will be essentially zero, with one low volume head the pressure will be the pump's maximum. You do not have enough information to calculate the psi, and are going at it backwards. First you have to calculate the gpm the heads will consume then go to the pump's flow curve and see what its specifications are for that volume.

 

[Bob NH]

You do not have enough information to calculate the psi, and are going at it backwards. First you have to calculate the gpm the heads will consume then go to the pump's flow curve and see what its specifications are for that volume.

If you already own the pump and plan to use it, then you need to find the characteristics of the pump and supply system.

Then select the number and size of sprinkler heads to limit the GPM demand on the pump so the pump can deliver the pressure required by the heads.

 

[comp1911]

If you already own the pump and plan to use it, then you need to find the characteristics of the pump and supply system.

Then select the number and size of sprinkler heads to limit the GPM demand on the pump so the pump can deliver the pressure required by the heads.

This man has it!