Nelson Irrigation Corp. 848 Airport Rd. Walla Walla, WA 99362-2271 USA Tel: 509.525.7660 Fax: 509.525.7907 E-mail: [email protected] Web site: www.nelsonirrigation.com
Control Valve
Application
Guide
Graph 1
PROPER VALVE SIZING
3.1.2 VALVE PRESSURE LOSS
The reason to consider pressure loss through the valve is to assure adequate pressure will be available to the irrigation system.
Pressure loss is defined by the following basic equation:
D
P =
(Q/Cv)
2
P
1
= Pressure on the upstream side of the valve
P
2
= Pressure on the downstream side of the valve
D
P = Pressure loss across the valve in PSI
Q = Flow through the valve in GPM
Cv = Valve sizing coefficient (flow rate at which one psi DP occurs.)
NOTE:
Cv provides an index for comparing the capacities of different size valves. The Cv coefficient is defined as the flow
(GPM) which will pass through the valve with a pressure loss of 1 PSI (the larger the coefficient the greater the valve capacity).
The Cv for the Nelson 800 Series control valve is:
8" size Cv = 2000 GPM
6" size Cv = 1100 GPM
4" size Cv = 310 GPM
3" size Cv = 240 GPM
2" size Cv = 115 GPM
The following graph can be used to determine pressure loss for a simple ON/OFF valve. GRAPH 1 applies to a fully open
valve. The valves were tested with adequate pressure to be fully open as listed in Table 2. Pressure control valves typically
operate in a partially closed condition. See the section 3.2 on sizing other reducing and sustaining valves for more information.
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