Irrigation System Valves Introduction

By Fedro S. Zazueta, Allen G. Smajstrla and Gary A. Clark

Reprinted with permission from the University of Florida, Institute of Food and Agricultural Sciences.

Actuators (Automatic Valves)

Valves specifically designed for remote control are an important component of the irrigation control system. These type of valves are constructed using different materials. Typically brass, stainless steel and a wide array of plastics. Different valve actuation designs are used to operate the valves:

1) diaphragm type,

2) piston type,

3) and electric thermal motor type.

Diaphragm Valves

The most common type of automatic valve is the diaphragm type. In this type of valve a diaphragm is used to block the passage of water through the valve. The diaphragm is moved from an closed to open position by using the action of a spring and water pressure differences. The design of the valves is such that pressure differences can be manipulated using a solenoid assembly with an actuator or a hydraulic pressure tubing.

Electric solenoid diaphragm valves

In an electric solenoid valve in the closed position, the pressure of the water in the chamber above the diaphragm (Figure 1) is the same as in the pipe at the entrance of the valve. Because the positive action of the spring and the lower pressure on the outlet, there is a force acting down on the diaphragm that pushes the seating disc against the body seat. If water is allowed to "leak" from the chamber above the diaphragm to the downstream end of the valve, the pressure in the chamber will decrease, allowing the pressure on the inlet side (below the diaphragm) of the solenoid valve to push the diaphragm against the spring with enough force to compress it, causing the valve to open. The valve will remain open as long as there is leakage from the upper chamber into the downstream end of the valve. When the solenoid is deactivated and the plunger obstructs the exhaust passage, water flows into the chamber through the inlet pressure orifice until the pressure in the chamber is equal to the pressure of water at the inlet. Because the spring provides a positive action and the force due to the pressure difference is decreased, the valve closes. As the downstream pressure drains, the pressure downstream decreases even more, providing a pressure difference that further tightens the seal between the seating disk and the body seat.

Figure 1 

Electric diaphragm valves are subject to very little wear and require little maintenance. They are very economical, particularly in small sizes. They seldom leak because the seat disk is molded into the diaphragm and they do not require packing (such as the packing required in valves that have a stem). Operation problems in these types of valves are usually related to blocked pressure inlet and exhaust orifices, sticky plungers, faulty solenoids, and (were very dirty water is used) debris between the seating disc and the body seat.

Hydraulically activated diaphragm valves

This type of valve functions in a manner very similar to the electrically activated diaphragm pump. However, in this type of valve, the pressure in the chamber is controlled by a small diameter line that is in turn connected to a pressure source using a hydraulic switching device. The switching device uses the small diameter line to relieve pressure in the chamber resulting in the valve opening, or to apply pressure, resulting in the valve closing.

Another method used is to connect the chamber to the main supply line using a small tube. Then a pilot valve is used to bleed the chamber allowing the pressure in the line to open the valve. To close the valve the pilot valve reconnects the hydraulic control line to the main supply, pressurizing the chamber and causing the valve to close (See Figure 2 ).

Figure 2

Hydraulic control systems require that water used in the hydraulic system be clean to prevent clogging of the pilot valves and the control lines. Also, hydraulic control lines must be protected where there is a danger of freeze.

Piston Valves

These valves employ a moving piston to open and close valves. Their operation is similar to diaphragm valves in that they are operated by pressure differentials. These valves are problematic because they incorporate O-rings or leather cup washers. Leather cup washers tend to dry out. O-rings tend to wear and stick after long periods of non-use.

Electric Thermal Motor Valves

These valves use a thermal motor to directly operate the valve.

Pump Start

With the exception of those situations where pressurized water is readily available, in addition to activating valves, the control system must have the capacity to activate the pump unit. Electric motor driven pumps can be activated using a pumpstart relay that is activated by the computer. This feature is available in most low-cost irrigation timers. Startup systems are also available for internal combustion power units.

For small or intermediate systems, depending on location of the pump station, an air tank with a pressure switch can be used to start the pumping station. In this way the pump motor will respond to a drop of pressure in the system due to a valve opening. These types of systems are especially useful where the flow demand varies.

Wiring

Wiring of an irrigation control system should be done by a competent professional to insure that safety requirements are met and that the system meets the necessary codes. Most problems with irrigation controllers can be traced to poor electrical installation, particularly lack of adequate grounding. Wherever electronic components are used it is important that attention be paid to signal and powerline protection.


Footnotes

1. This document is SSAGE22, one of a series of the Agricultural and Biological Engineering Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date March, 1993. Reviewed July, 2002. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.

2. Fedro S. Zazueta, Allen G. Smajstrla, Professors, Agricultural Engineering Department; Gary A. Clark, Associate Professor, Gulf Coast Research and Education Center, Bradenton, FL; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.


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