Control Valves – Functioning and Actuation

In addition to simple tasks of water opening, shut-off and flow throttling, valves accomplish more complicated tasks like: flow-metering, flow and head regulation, backflow prevention, air-release from the pipes, fertilizer injection, filter flushing, etc.

Fig. 7.13. Diaphragm Valve Components ^Adapted

from Valve Types by Valvias

a. Weir Type b. Straight-through Type

Fig. 7.14. Diaphragm Valves

Since the mid-fifties, many valves are controlled by programmed controllers and computers. The operation instructions actuate the valves by means of hydraulic or electric actuators. There are also small diameter control valves that are actuated mechanically by the inherent water flow in the valve

Fig. 7.16. Control Valves Actuators

These control valves are combinations of valves and actuators. The most prevalent valves are of the flexible diaphragm type that is controlled by an electric valve (solenoid) or hydraulic actuator. Piston valves are used in smaller scale. Power to the actuators is transmitted as an electrical current through wires, through wireless devices or as hydraulic pressure through small diameter tubes. The controllers are classified as normally open (NO) and normally closed (NC) groups. Normally Open means that the valve remains open when the power source is off. The normally closed type remains closed when the power source is off. The normally closed type is safer since when the power supply is impaired, the water is closed. On the other hand, this type has greater pressure losses.

7.4.2.1. Electric Control of Hydraulic Valves

In many cases, the driving power of the control element is hydraulic pressure that is activated by the electrical current delivered to the actuator. The flow control element can be in the form of a piston that shuts and opens the flow path in the valve. The piston actuator can also serve as a flow regulator.

7.4.2.1.1. Solenoids

The solenoid valve, which is commonly used in irrigation systems, employs an electromagnetic force to drive the piston directly or to activate the piloting action that opens the valve.

The conversion of electric pulses into mechanical energy by which the actuator closes or opens the water is done by means of solenoids.

Solenoid actuated valves provide for automatic

open-close valve positioning. Most solenoid actuated valves have a manual override that Fig. 7.17. Cutaway of Solenoid Valves

allows manual operation of the valve. Solenoids position the valve by attracting a magnetic slug attached to the valve stem. In single solenoid valves, spring pressure acts against the motion of the slug when power is applied to the solenoid. These valves can be arranged such that power to the solenoid either opens or closes the valve. When power to the solenoid is removed, the spring returns the valve to the opposite position. Two solenoids can be used in one valve to apply both opening and closing by switching power to the appropriate solenoid.

Single solenoid valves are termed fail open or fail closed depending on the position of the valve when the solenoid is de-energized. Fail open solenoid valves are opened by spring pressure and closed by energizing the solenoid. Fail closed solenoid valves are closed by spring pressure and opened by energizing the solenoid. Double solenoid valves typically fail "as is." That is, the valve position does not change when both solenoids are de-energized. Modern solenoid valves offer fast operation, high reliability, long service life, and compact design.

a. Fail-closed (Normally Closed) b. Fail-open (Normally Open) Fig. 7.18. Scheme of Solenoid Operation

Fig. 7.19. Fail-closed (NC) Solenoid Valve – Components and Working Pattern

In one working procedure, the solenoid is acting directly on the main valve. In other cases, a small, fully functioning solenoid valve known as a pilot is used to actuate a larger valve. While the second type is actually a solenoid valve combined with a

hydraulically actuated valve, they are sold and packaged as a single unit referred to as a solenoid valve. Piloted valves require much less power to be activated, but they are noticeably slower. Piloted solenoids usually need full power at all times to open and stay open, where a direct acting solenoid may only need full power for a short period of time to open it, and only low power to hold it open.

a. Blow-out b. Working Pattern

Fig. 7.20. Hydraulic Control Valve

7.4.2.2. Hydraulic Control of Valves

The structure of a hydraulic controller or actuator is similar to that of the electrical control valve but the power is transmitted by a pressure change above the diaphragm or piston actuator. This change of pressure is transmitted through the hydraulic control tubing or by partial diversion of water flowing through the valve. Most of the hydraulic control valves are normally open.

Functionally, hydraulic valves, like the electrical ones, fall into two categories:

Normally Open (N.O.) and Normally Closed (N.C.).

a. Normally Open (N.O.) valve stays open until the control chamber is filled with water under system pressure. When the chamber is full, the valve shuts-off.

b. Normally Closed (N.C.) valve is kept closed by the water pressure in the mainline.

In case of a rupture in the command line, the closure is secured by pressure of a spring. The valve is opened when a tiny valve at the top of the control chamber opens, releasing water from the control chamber into the atmosphere.

Normally closed hydraulic valves have higher head losses, but they are safer to use, as the valve remains closed even if the command tube is torn or plugged.