Sec 7 Basic Irrigation Hydraulics

Basic Irrigation Hydraulics Important facts to remember: A column of water 1 foot high = 1 foot of head = .433 psi 1psi of pressure is exerted from a column of water 2.31 feet high (1psi = 2.31 feet of head) Pressure Engineered Software describes pressure as “the amount of force acting on a surface per unit area” Pressure = force area Static Pressure Is the pressure of liquids at rest (all valves closed) Is affected only by elevation change (feet of head) The shape of the water vessel is unimportant – depth of water is important Water 100 ft 6” pipe 2” pipe Water 100 ft 6” pipe 2” pipe Water tank Water tank What is the pressure at the bottom of the 2” pipe? _______________ What is the pressure at the bottom of the 6” pipe? _______________ 36 ft125 ft 99 ft 65 ft 60 ft 120 ftM 36 ft 125 ft 99 ft 65 ft 60 ft 120 ft M B B If the static pressure at the water meter (M) is 75 psi what is the static pressure at point B? Velocity The speed at which water is moving inside a pipe SHOULD NOT EXCEED 5 FT PER SECOND (fps) on supply line Friction loss The loss of pressure in a pipe due to the friction of water on the surface of the inside of the pipe Different types of pipe will have different friction loss The same velocity in a smaller size pipe will increase friction loss As velocity increases friction loss increases Calculating Friction loss Friction loss can be calculated or charts can be used to determine friction loss. Charts are found in most high end irrigation manufacturers’ catalogues or websites. 1. Find the flow of water in GPM in the column on the left 2. Find the size of pipe you are using along the top of the chart ( Note: there are 2 columns under each size of pipe – one for psi loss and one for velocity. Make sure you are following the correct column for the information you require) 3. intersect the 2 columns to find the psi loss per 100 feet of pipe 4. Divide this number by 100 to determine psi loss per foot of pipe 5. Multiply this number by the length of pipe in feet. Try these (use chart for Class 160 PVC – IA friction loss charts under moodle) What is the pressure loss in 100 ft of ½” pipe if the flow is 10 gpm? _________ What is the pressure loss in 100 ft of 3/4” pipe if the flow is 10 gpm?_________ What is the pressure loss in 50 ft of 1” pipe if the flow is 12 gpm? _________ What is the pressure loss in 780 ft of 1½” pipe if the flow is 30 gpm? ________ Use the charts for PE and Class 200 PVC from the IA friction loss charts in Moodle under Resources What is the minimum Class 200 PVC pipe size you would recommend for a flow rate of 30gpm? _____________________ What is the minimum PE pipe size you would recommend for a flow rate of 14 gpm? __________________ What size PVC pipe is best suited to carry 150 gpm? ___________________ Which type of pipe has the lowest friction loss at 20gpm? Use smallest staying within 5 fps _______________________________________ Which type of pipe has the lowest friction loss at 190 gpm? Use smallest staying within 5 fps. ________________________________________ Dynamic Pressure Is the pressure of liquids in motion (valves are open, water is moving) Is affected by elevation change (feet of head) friction loss in the pipe, pressure losses through valves, meters, and fittings. (sometimes fittings are calculated by adding 10% to the other pressure losses) 100 ft 100 ft 75 ft 50 ft 100 ft 100 ft 75 ft 50 ft B B A A In the figure above all the pipe is 1 ¼” Class 200 PVC and the flow is 18 GPM. At point A, a pressure gauge reading indicates 85 psi. In order to determine the dynamic pressure at point B 1. Find pressure change due to elevation 75 ft x .433 psi/ft = 32.48 psi 2. Find pressure loss due to friction in the pipe 18 GPM in 1 ¼” Class 200 PVC pipe = 1.33 psi /100ft 1.33/100 = .0133 .0133 x (50+100+100) = 3.33 psi 3. 85.00 psi pressure at Point A - 32.48psi loss due to increase in elevation 52.52psi subtotal - 3.33psi loss due to friction in the pipe 49.19psi dynamic pressure at Point B Sample Dynamic problemAB A B In the diagram above the length of pipe from point A to point B is 125 ft. The difference in elevation is 15 feet. All pipe is 1” Class 160 PVC. Flow is 16 GPM. If the Pressure at point A is 80 psi what is the dynamic pressure at point B? ________________________ Using the scenario at the top of page 2 what would the dynamic pressure be at Point B if all the pipe is 1” Class 160 PVC, the pressure at the meter is 65 psi and the flow is 12 gpm ? _________________________________________ What is the dynamic pressure at a sprinkler that is 75 ft horizontally from the POC? All pipe is 3/4 “ PE. The flow in the pipe is 10 gpm. Pressure at the POC is 50 psi. _____________________________________________________ If this sprinkler was a spray head designed to operate at 40 psi would there be enough pressure for the sprinkler to operate properly. YES NO