Water flow in the pipeline is important to us. We know that water flow in a pipeline can be calculated by multiplying the pipeline cross-sectional area times the water velocity. One can imagine a cross-sectional slice of a pipeline and the water passing through it at some velocity as if it is filling an imaginary pipe from that point on. There are charts that give you this information for many sizes and materials of pipes. For a given pipe type (PVC Schedule 40, PVC Class 200, etc.) the size (cross-sectional area) is fixed but the water velocity can vary to meet the water demand of sprinklers or other uses, thus the flowrate may vary.
Okay, now let’s put sprinklers onto our irrigation system. Now we have introduced openings for the water to escape from the pipelines at intervals along the pipeline. If each sprinkler nozzle is the same size we would expect the same amount of water to escape/flow through each one, the pressure also being the same at each. If there are 20 nozzles that allow 1 gpm of water flow each at 60 psi, then at a water pressure of 60 psi, one would expect 20 nozzles x 1 gpm/nozzle = 20 gpm of water to escape the pipeline. The pump would have to move/push 20 gpm of water into the pipeline to maintain the pressure and satisfy this demand.
While mechanical pumps have not been discussed yet, let it be known that a pump may be able to achieve 70 psi maximum when there is no flow, but at a flow rate of 20 gpm, it may only be able to maintain 60 psi. These are the numbers used here.
Note that the purpose of knowing the amount of water flow is to meet the demand of the outlets – either sprinklers or drip emitters – and to size the pump to supply the required amount of water. Sprinklers or emitters are selected at a given operating pressure and discharge rate, gallons per minute (gpm) or liters per minute (Lpm) to apply the required volume of water. For sprinklers, the selection is based on delivering water to some diameter of coverage and depth of application. One multiplies the number of sprinklers or emitters times the discharge rate to get the required flow in gallons per minute (gpm) or liters per minute (Lpm). It is important to understand this.
The flow rate in the pipeline must total the water demand needed to operate the sprinkler nozzles or drip emitters.