Electric motors have long been used to actuate large valves, especially valves operated as on/off (“shutoff”) devices.
Advances in motor design and motor control circuitry have brought motor operated valve (MOV) technology to the point where it now competes with legacy actuator technologies such as pneumatic in actuating throttling valves as well.
Electrical Actuator
Most electric valve actuators use a worm gear set to reduce the high rotational speed of the electric motor to a slow rotation suitable for moving a large valve mechanism.
An illustration of a worm gear set appears here:
The worm screw looks much like a threaded fastener, with its “threads” properly pitched to engage with the teeth of the worm wheel gear.
As the worm screw turns, it slowly pushes or pulls the circumference of the worm wheel, resulting in a large gear ratio (i.e. many turns of the screw are required to produce a single turn of the wheel).
This slow-turning wheel may then be used to move a sliding-stem valve by means of a threaded shaft (another screw) or used to directly turn a rotary valve (e.g. butterfly, ball, plug).
Also Read : Motor Trip Logic using PLC
An electric actuator appears in the next photograph, providing on/off rotary actuation to a ball valve. This particular electric actuator comes with a hand crank for manual operation, in the event that the electric motor (or the power provided to it) fails:
A small lever to the left of the hand crank actuates a clutch mechanism to engage or disengage the valve mechanism from the electric motor and the hand wheel.
This clutch “selects” either the motor or the hand wheel as the prime mover for the valve, to avoid having the hand wheel spin as the motor turns. Unless this lever is first moved to the “manual” position, turning the hand wheel accomplishes nothing.
The next photograph shows an electric valve actuator coupled to a large butterfly valve.
Although nothing visible in this photograph betrays the nature of this actuator’s signaling, it happens to be digital rather than analog, receiving position commands through a Profibus digital network rather than an analog 4-20 mA current signal:
The shape of the actuator’s metal casing reveals the final gear drive of this actuator as a worm gear set. Note the round shape of the casing where the open/close indicator is located (at the centerline of the butterfly valve stem): this contains the worm wheel.
Note immediately to the left of that round casing is a vertically-oriented cylinder shape: this contains the worm screw, which engages with the teeth of the worm wheel.
Above the worm screw is a parallel-shaft spur gear set which acts to further reduce the speed of the actuator’s electric motor: the motor shaft terminates in a small gear, which meshes with a larger gear that turns the worm screw shaft.
Above the parallel shaft gear set is the main casing of the actuator, which actually contains its own internal worm gear set. This multiple-stage gear reduction means that the motor spins very fast in comparison to the butterfly element inside the valve, and that the butterfly is capable of exerting a fantastic amount of torque in comparison to the torque rating of the electric motor.
Electric motors require no external fluid power system to function, unlike pneumatic or hydraulic actuators. All they require is a source of electrical power (often 480 volts AC, three-phase).
Some electric valve actuators even have the capability of operating from the power of an electric battery pack, for reliable operation in the event of a power system outage.
Virtually all electric valve actuators require some form of feedback to indicate the valve’s position.
At minimum, this consists of limit switches to indicate when the valve is fully shut and fully open. For throttling services, an electric actuator requires an actual valve position sensor so that it may precisely adjust the valve to any desired state. This sensor may take the form of a potentiometer, or a variable differential transformer (LVDT or RVDT), or pulse encoder.
This Rotork brand MOV has a digital display at one end showing its closed status both in text (“Closed Limit”) and symbolically (by the vertical line, which is supposed to represent a closed butterfly element):
In addition to visual indicators of status, electric valve actuators commonly provide auxiliary electrical contacts signaling fully-open and fully-closed positions, which may be used to energize remote indicator lights or discrete input channels of control systems (e.g. PLC).
Throttling-service MOVs also provide analog (and/or digital) signaling of valve stem position for remote indication or feedback to an electronic control system.
Also Read : Dome Valve Principle
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