Control Valve
Application
Guide
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
5.4 PRESSURE CYCLE INDUCED BY THE ELASTICITY OF HOSE AND TUBING
Micro-irrigation zone pressure control valves can experience pressure cycling due to the elastic nature of the tubing. In the
event a large zone of micro/drip tubing is empty, the valve will fill the tube using full flow. When the tubes are full the pressure
expands the tubes much like a balloon. The valve will respond late to over pressure situation. Due to the elasticity of the tube
the valve may even shut-off until the pressure in the tube drops below the set point at which time the cycle again starts.
The problem is best handled by slow filling of the zone. (Many times slow filling is not an option because of the need to
have frequent irrigation.)
Use a design which keeps the zones small to help reduce the problem.
An excellent way to optimize the filling time and reduce the pressure cycle potential is by use of a R
ATE
-
OF
-F
LOW
control option.
6.0 ELECTRICAL APPLICATIONS
Valves can be equipped with optional electric solenoids for remote operation. Controllers and electric remote control valves
must be properly wired together, otherwise operation of the system will be affected and become unreliable. Controllers
are furnished standard with a nominal input voltage rating of 115 volts AC, 60 Hz (Hertz or cycles) and an output of 24
volts AC on the secondary side for powering electric valve solenoids. There are many optional input voltages in both 50
and 60 Hz. Most solenoids will work with both 50 and 60 Hz. Start by understanding the needs for the controller. Some
controllers will only operate one valve per station. Select a controller which not only has the number of zones required but
also can operate the number of valves required per station.
The current draw will vary with make and model, depending on the amount of current required to operate the controller itself
and the number of valves that the controller is designed to operate per station. A qualified licensed electrician should be
employed to install and connect the controller to an adequate power source in compliance with local codes. For the average
system, wire size for the controller power supply is governed by local codes. The proper wire size must be calculated for the
controllers at remote distances from the power source. This application guide will not address the controller input wire size
requirement but will restrict the discussion for sizing of wire to the wire size required from the controller to the valve solenoids.
6.1 VALVE WIRING (FROM THE CONTROLLER TO THE VALVE)
The most common NELSON 800 Series
solenoid used on a zone control valve is the
24 VAC (part #7510-015). Two wires are
required to provide the secondary low
voltage power supply for these valve
solenoids. The common wire serves all
solenoids. A power or hot wire from
the controller to each solenoid completes
the circuit. A typical wiring schematic for
one valve operating is shown in Figure 8.
6.2 METHODS OF WIRE SIZING
The following gives two methods of wire sizing which will result in minimum wire size and therefore selection of the wire size
of least cost. Tables can be used to select wire size for simple systems. When the wire run distances exceed those given in the
tables or for more complicated systems, the wire sizing must be calculated using formulas. Formula usage will be explained later.
6.2.1 WIRE SIZING WITH TABLES
Table 4 has been prepared for the Nelson 24 VAC solenoid (part #7510-015). This table makes selection of wire size simple.
The calculations for Table 4 are made to assure that adequate voltage (20.4 VAC) and inrush current (~0.3 amps) will be
available to the solenoid. Note other assumptions listed at the bottom of the table.
EXAMPLE: Find the maximum length of run for a system with 150 PSI maximum line pressure using 12 AWG size wire.
SOLUTION: Enter the 150 PSI chart of Table 3, follow the 12 AWG row to the 12 AWG column and read the answer
for maximum length of run of 3497 feet.
ELECTRICAL APPLICATIONS
13
Figure 8
