Solenoid Actuated Valves
Solenoid Actuated Valves
Solenoid actuated valves provide for automatic open-close valve positioning as illustrated in Figure 35. Most solenoid actuated valves also have a manual override that permits manual positioning of the valve for as long as the override is manually positioned. 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 to provide for both opening and closing by applying power to the appropriate solenoid. Single solenoid valves are termed fail open or fail closed depending on the position of the valve with the solenoid 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.
One application of solenoid valves is in air systems such as those used to supply air to pneumatic valve actuators. The solenoid valves are used to control the air supply to the pneumatic actuator and thus the position of the pneumatic actuated valve.
Speed of Power Actuators
Plant safety considerations dictate valve speeds for certain safety-related valves. Where a system must be very quickly isolated or opened, very fast valve actuation is required. Where the opening of a valve results in injection of relatively cold water to a hot system, slower opening is necessary to minimize thermal shock. Engineering design selects the actuator for safety-related valves based upon speed and power requirements and availability of energy to the actuator.
In general, fastest actuation is provided by hydraulic, pneumatic, and solenoid actuators. However, solenoids are not practical for large valves because their size and power requirements would be excessive. Also, hydraulic and pneumatic actuators require a system for providing hydraulic or pneumatic energy. The speed of actuation in either case can be set by installing appropriately sized orifices in the hydraulic or pneumatic lines. In certain cases, the valve is closed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep the valve open.
Electrical motors provide relatively fast actuation. Actual valve speed is set by the combination of motor speed and gear ratio. This combination can be selected to provide full valve travel within a range from about two seconds to several seconds.
Valve Position Indication
Operators require indication of the position of certain valves to permit knowledgeable operation of the plant. For such valves, remote valve position indication is provided in the form of position lights that indicate if valves are open or closed. Remote valve position indication circuits use a position detector that senses stem and disk position or actuator position. One type of position detector is the mechanical limit switch, which is physically operated by valve movement.
Another type is magnetic switches or transformers that sense movement of their magnetic cores, which are physically operated by valve movement. Local valve position indication refers to some visually discernable characteristic of the valve that indicates valve position. Rising stem valve position is indicated by the stem position. Nonrising stem valves sometimes have small mechanical pointers that are operated by the valve actuator simultaneously with valve operation. Power actuated valves typically have a mechanical pointer
that provides local valve position indication. On the other hand, some valves do not have any feature for position indication.
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