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Valves on chemical process equipment may be a long distance from control rooms, or high in the air on towers or elevated tanks. An electric valve actuator can be used to remotely control them without the need for manually turning the valves. Although all of these actuators run on electricity, the designs can vary greatly, and they come in a variety of sizes to fit different valve applications. Power can be direct current (DC) or alternating current (AC) depending on the plant design. A key factor in all electric actuators is available torque, or the force available from the electric motor for opening and closing the valve.
When valves are seldom used, or handle sticky or abrasive materials, the valve may initially be hard to turn. Plant designers will specify valve actuators with extra torque to prevent these control problems. Manual control wheels or handles can be added to actuators to help free stuck valves.
Sticking valves can cause the motor to overheat and burn out if it tries to move the valve. There are different ways to protect the motor, including over-torque, motor overheat, or electric current limiting sensors. These monitor for excessive valve opening force, motor winding temperature, or electric flow respectively, and will shut off the motor to protect it. Regardless of the design, actuators should be purchased with overload protection to prevent stuck valve failures.
An on-off electric valve actuator will turn a valve completely open or on, or the reverse, when a signal is sent to it. They typically have a limit switch that shuts the actuator motor off when the valve is either fully open or closed. On-off actuators do not provide any flow control, and are used primarily on storage tank or delivery piping where simple controls are needed.
Proportional control is another option for an electric valve actuator. An actuator connected to a process controller can be wired to open and close slowly or more rapidly based on the signal sent from the controller. This allows the valve to control raw material flow to reactors, vary heating or cooling liquid flow for temperature control, or control filling rates to tanks or trucks to prevent overflow.
Heating and cooling equipment can use an electric valve actuator for both damper and valve control. Dampers are metal plates hung inside the vent piping leaving a furnace. As the furnace turns on and off, the actuator can open and close the damper to control the vent gases exiting the furnace. Similar dampers can be installed on fresh air vents entering the furnace to control air used for burning gas or fuel oil, typically connected to sensors that monitor gases leaving the furnace. Adjusting inlet air and flue vent flow rates can provide maximum combustion efficiency.
Electric actuators on furnace valves can control gas or oil flows to the burner, or water flow to boilers that circulate hot water through a building for heat. Heat pumps have a reversing valve that switches the refrigerant flow direction when changing from heat to air conditioning. An electric valve actuator can control the valve based on the heat/cool selection switch at the thermostat inside the building.
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