The handbook of Technical irrigaTion informaTion - Hunter Industries

Handbook of Technical Irrigation InformationFriction Factor Short CutsThe Friction Factor Method of sizing lateral line pipe is based on the premise that theoperating pressure of all heads on a circuit should not vary by more than 10% to 20% of their designed operatingpressure. This will provide the most uniform application of water.The following is a simplified way to determine the maximum flow (GPM) allowed in lateral lines using the FrictionFactor pipe sizing method. The example is based on lateral lines of Class 315 PVC for ½" and Class 200 PVC for ¾"- 3". Additional charts for lateral lines of Class 160 PVC and Polethylene are on pages 54 and 55.Determination of appropriate pipe size is dependent upon 1) the sprinkler operating pressure, PSI, 2) theacceptable pressure variation between sprinklers on the circuit 10% to 20% and 3) the distance from the valve to thefarthest sprinkler head on the circuit (note: in some cases this distance can be the distance between the first and lastsprinklers on a circuit).The Friction Factor is the allowable PSI loss per 100 feet of pipe. With the Friction Factor, pipe can be sized toavoid excessive pressure losses. The friction factor is calculated by the following formula:Friction Factor = Sprinkler Operating Pressure x Allowable % of PSI VariationCritical Length (from valve to farthest head) in 100’s of feetF f = P o x P vL c/100'1) Determine the Friction FactorExample:Electric Remote Control ValveFull Circle Sprinkler 2.0 GPM at 35 PSI125 feetSprinkler operating pressure = 35 PSIAllowable pressure loss = 10%Critical Length = 125 feetFriction Factor = 35 x 0.10125 / 100Friction Factor = 3.51.25Friction Factor = 2.8 allowable PSI loss from valve to farthest headRound off the Friction Factor to the nearest ¼ PSI.Example: 2.8 is rounded off to 2.75This represents an allowable rate of PSI loss from the valve to the farthest head of 2.75 PSI per 100 feet of pipe.50