Irrigation Basics for Landscape Contractors –LSU AgCenter

Water Fundamentals: Variables and Irrigation Hydraulics Units

y Cross sectional area of a pipe A = π * r2 Dr. Ron Sheffield LSU Ag Center, Biological & Agricultural 2 [email protected] A = π * d /4

Water Fundamentals: Weight, Water Fundamentals: Weight and , and Volume Pressure

y Water….. y Water exerts a force on it’s surrounding ◦ 8.34 lbs/gallon y Pressure: psi (lbs/in2), kPa, bars ◦ 7.48 g/ft3 ◦ 2.31 ft of water provides 1psi ◦ 62.4 lbs/ft3 ◦ 23.1 ft of water provides 10 psi ◦ 1000 kg/m3 ◦ 1ft of water provides 0.433 psi

y Elevation (ft) = pressure (2.31 ft/psi) y Metric: 1m of water = 9.81 kPa

Water Fundamentals: Weight and Water Fundamentals: Flow Pressure y Static Pressure y Velocity is the speed of water as it moves ◦ Pressure in a system when water is motionless through a pipe system. We use “average” ◦ Will vary at different locations in system due to velocity (feet per second, fps, ft/s, m/s). differences in elevation 1 ft/s = 0.305 m/s y Dynamic Pressure y Flow (flowrate) is a measure of the ◦ “Operating” or “Working” pressure amount of water moved during a period ◦ Pressure exiting any point in a system when it is of time (gallons per minute, gpm, ft3/s, operating m3/s). ◦ DP < SP due to resistance and friction losses 1ft3/s = 449/gpm

1 Irrigation Basics for Landscape Irrigation Contractors –LSU AgCenter

Water Fundamentals: Flow Water Fundamentals: Flow

y Relationship between velocity, flow, cross- y Example: Schedule 40 pipe, 2” nominal sectional area of a pipe: pipe size (NPS), 3 feet per second. What is the flowrate in gpm? q = va q = va q = 3 ft/s * 60 s/min * π* (2/2)2-in2 * 1ft2/144in2 2 q = 2.448 * v * D q = 3 * 60 * 3.14 * 12*0.00694 q = gpm ; v = ft/sec; D= in q = 3.92 ft3/min q = 3.92 ft3/min * 7.48 gal/ft3 q = 29.3 gpm

Water Fundamentals: Flow Water Fundamentals: Energy

y Class Problem: Schedule 40 pipe, 4 feet per y Energy Head = the amount of energy second. What is the flowrate in gpm at associated with the combination of the following pipe sizes? elevation, pressure, and velocity

y 1” = y Neglecting velocity head (small y 1¼” = contribution): y 1½” = H (energy head) = p (pressure) + E (elevation) y 2” = H(ft) = p (psi) * 2.31 ft/(1 lb/sq.in.) + E (ft)

Water Fundamentals: Friction Loss Water Fundamentals: Friction Loss

y Water flowing in pipes looses energy y Velocity Affects y Any change in flow, restriction, causes of ◦ Maximum? ◦ Minimum? additional turbulence, etc., will result in a y Recommended ranges of velocities in plastic pipe? decr ease in eeenergy

y Factors affecting friction loss y Costs? ◦ velocity 1 in. diameter PVC Schedule 40 (450 psi) $7.00/20 ft ◦ pipe diameter Thin wall (315 psi) $3.60 /20 ft ◦ pipe roughness (type) ~ 49 % reduction in cost ◦ length

2 Irrigation Basics for Landscape Irrigation Contractors –LSU AgCenter

Friction & Velocity Charts Pressure loss due to pipe length Every shape and style!!!

Height 92.4’ 90.1’ in feet 87.8’ 85.5’ 83.2’ 80.9’

FLOW 100 ft 100 ft 100 ft 100 ft 100 ft

40 psi 39 psi 38 psi 37 psi 36 psi 35 psi

What’s the effect of 1 psi loss per 100 feet?

Friction & Velocity Charts (Choate, Page 388) 1 Water Hammer 1- /2 Sch 40 at 15 gpm = 0.669 psi loss/100 ft y Water hammer occurs when the flow of water in a pipe is abruptly changed or stopped y When water h amm er occurs, a hig h intensity pressure wave travels back through the piping system until it reaches a “point of relief” ◦ valve, sprinkler, elbow, poor glued joint, stressed pipe

Causes of water hammer Avoiding water hammer

1. Valve closure y 5 ft/s maximum design 2. Uncontrolled flow velocity in empty y Thrust blocking pipes y Air relief valves 3. Trapped air in long runs of pipe y Check valves 4. Reverse flow when pumps stop

3 Irrigation Basics for Landscape Irrigation Contractors –LSU AgCenter

Irrigation Hydraulics

Dr. Ron Sheffield LSU Ag Center, Biological & Agricultural Engineering [email protected]

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