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The opening or closing of automatic control valves is usually done by electrical, hydraulic or pneumatic actuators. Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening. [2]
Control valves can also work with hydraulic actuators (also known as hydraulic pilots). These types of valves are also known as automatic control valves. The hydraulic actuators respond to changes of pressure or flow and will open/close the valve. Automatic control valves do not require an external power source, meaning that the fluid pressure ...
A transmission solenoid or cylinoid is an electro-hydraulic valve that controls fluid flow into and throughout an automatic transmission. Solenoids can be normally open or normally closed. They operate via a voltage or current supplied by the transmission computer or controller.
Ordinary valves can have many ports and fluid paths. A 2-way valve, for example, has 2 ports; if the valve is open, then the two ports are connected and fluid may flow between the ports; if the valve is closed, then ports are isolated. If the valve is open when the solenoid is not energized, then the valve is termed normally open (N.O.).
Electrical energy is used to actuate equipment such as multi-turn valves, or electric-powered construction and excavation equipment. When used to control the flow of fluid through a valve, a brake is typically installed above the motor to prevent the fluid pressure from forcing open the valve.
Power-operated valve actuators may be the final elements of an automatic control loop which automatically regulates some flow, level or other process. Actuators may be only to open and close the valve, or may allow intermediate positioning; some valve actuators include switches or other ways to remotely indicate the position of the valve.
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Some process control schemes and final control elements require this reverse action. An example would be a valve for cooling water, where the fail-safe mode, in the case of signal loss, would be 100% opening of the valve; therefore 0% controller output needs to cause 100% valve opening.
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