The handbook of Technical irrigaTion informaTion - Hunter Industries

Handbook of Technical Irrigation InformationFriction Loss in PipeThe Hazen-Williams equation can be expressed as follows and is the most commonly usedformula for calculating pressure loss in PVC pipe.h f = 0.00090194( 100 )C1.852 Q 1.8524.866 LdWhere:h f = head loss due to friction in pounds per square inch (PSI)C = Hazen Williams coefficient for roughness of the inside of the pipeQ = flow in gallons per minute (GPM)d = inside diameter of pipe in inchesL = length of pipe in feetExample:A 2-inch class 200 PVC pipe (I.D. = 2.149") 500 feet in length will deliver 50 GPM to an irrigation system. Computethe friction loss in the pipeline.h f = 0.00090194( 100 )150h f = 7.208 PSIStatic Pressure Determination50( )1.852 1.8522.149Static pressure is the measure of pressure when the water is at rest. This pressure is determined by the weight of acolumn of water resting on one square inch and expressed as the pounds per square inch (PSI). The weight of a columnof water one foot high will create 0.433 pounds of pressure over every square inch. Static pressure can be determinedas follows:P s = A ± 0.433 HWhere:P s = static pressure at a given point in the system, in pounds per square inch (PSI)A = static pressure at starting point in PSI0.433 = a constant representing the weight of water in a column one foot high as expressed in pounds persquare inchH = the net vertical change in elevation from the surface of the water to the given point in the systemin feet, increase in elevation, uphill, results in PSI loss (–0.433) downhill results PSI gain(+0.433)4.866500Point “A”60 psi 414 kPa(Static Pressure)ControlWater40 feet (12 m)MElevation View8