Valve Actuators Information
Valve actuators mount on valves and, in response to a signal, move a valve to a desired position using an outside power source. Selecting the correct valve actuator will increase uptime, reduce maintenance costs, and increase plant safety. Most valve actuators come permanently lubricated and will operate best with instrument-quality air. They can also be packaged with position sensing equipment, digital communication capacity and motor protection.
Valve Method of Control and Function
Valve motion and operation style are important specifications to consider when selecting valve actuators.
Valve Method of Control
Rotary motion valves (rotary valves) such as ball, plug, and butterfly valves rotate a quarter-turn or more from open to close.
Linear motion valves (linear valves) such as gate, globe, diaphragm, pinch and angle-style valves have a sliding stem design that pushes the closure element open or closed. The valve stem may rise during rotation, or may rise without rotation.
There are two basic operating styles for valve actuators.
Start/stop valves, also known as on/off or isolating devices, limit actuator motion to preset open and closed positions.
Throttling or control devices provide controllable motion so that valves can be throttled as necessary. This type of actuator is paired with a positioner so the actuator can move to the required position accurately.
There are several basic types of valve actuators: manual, electric, pneumatic, and hydraulic.
Manual valve actuators do not require an outside power source. They use a handwheel or lever to drive a series of gears whose ratio results in a higher output torque compared to the input (manual) torque.
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Electric valve actuators use a single-phase or three-phase alternating current (AC) or direct current (DC) motor to drive a combination of gears to generate the desired torque level.
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Pneumatic valve actuators adjust valve position by converting air pressure into linear or rotary motion.
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Similarly, electrohydraulic valve actuators and hydraulic valve actuators convert fluid pressure supply into linear or rotary motion.
Below are specifications for rotary and linear valve actuators.
Rotary Actuator Specifications
Rotary actuators produce rotary motion or torque. The mechanical device produces motion in one direction to cause rotation. Electric versions of the rotary actuator have continuous rotation, while servo or step motors are used to move the actuator to a fixed angular position.
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Rack and pinion actuators consist of a housing to support a pinion, which is driven by a rack with cylinder pistons on the ends. Racks are available in single, double, or multiple designs. The overall efficiency of rack and pinion actuators averages 85-90%. They are able to cover a wide range of torque outputs and rotations range from a few degrees to five revolutions or more. The equation for calculating torque is:
M = Aprp
M - output torque,
A - cylinder piston area
p - operating pressure
rp - pitch radius of the pinion
Rotary actuators can be used in working pressures of up to 18 bar for pneumatic and 210 for hydraulic actuators with rotations of 90°, 180°, or 360°.
As the media moves it creates dynamic torque, which results from non-uniform static pressure distribution on the closure (rotating) member of a quarter-turn valve. Since the pressure is unevenly distributed, it is equal to the resultant force acting at the same distance from the stem axis. Dynamic torque acts on the valve stem; it is the sum of the product of each resultant force and its offset distance. It can either aid or hinder the valve actuator. If the friction torque is less than the dynamic torque, it will cause rotary motion if unchecked by the actuator.
The most common cause of rotary actuator failure is the introduction of shock and surge pressures beyond the maximum rated working pressure of the unit. Failure often occurs in actuators that have rotational speeds in excess of 10 RPM, control of a large mass in the horizontal plane or moving over the center, or operation of a long lever arm.
Linear Actuator Specifications
A linear actuator is an assembly that creates motion and force along a straight line. Linear actuators use an external energy source and various methods to achieve this motion. Mechanical, hydraulic, pneumatic, and electric actuators can be designed as linear actuators. Hydraulic and pneumatic actuators inherently produce linear motion, while other types provide linear motion from rotating motors.
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Performance specifications for linear actuators include valve stem stroke length, actuation time, number of turns, and actuator force or seating thrust.
Valve stem stroke length- Stroke is a term used to define the travel required by the valve from fully open to fully closed. The stroke of an actuated valve is determined by the actuator if the actuator selected has a stroke that is less than the stroke of the valve. Using an actuator with fewer strokes than the valve will "short stroke" the valve and the full CV rating of the valve will not be realized.
Actuation time- The time it takes to fully close the linear motion valve.
Actuator force or seating thrust- The actuator must supply enough force to overcome the pressure in the system to close the closing element and keep it closed.
Load capacity- Actuators handle static and dynamic loads. Static load capacity is how much load the actuator can support when the device is not moving. Dynamic load capacity is the load the actuator can support while it is in use.
Cheap. Repeatable. No power source required. Self-contained. Identical behavior extending or retracting.
Manual operation only. No automation.
Cheap. Repeatable. Operation can be automated. Self-contained. Identical behavior extending or retracting. DC or stepping motors. Position feedback possible. Can be remotely controlled.
Many moving parts prone to wear.
Simple design. Minimum of moving parts. High speeds possible. Self-contained. Identical behavior extending or retracting.
Very small motions possible.
Requires position feedback to be repeatable. Short travel. Low speed. High voltages required. Expensive. Good in compression only, not in tension.
Very high forces possible.
Can leak. Requires position feedback for repeatability. External hydraulic pump required. Some designs perform well in compression only.
Strong, light, simple, fast.
Precise position control impossible except at full stops
Chart Credit: Wikipedia.
General specifications for all types of valve actuators include:
Power source -- In general, actuators are powered with either electricity or fluid (air-pneumatic, liquid- hydraulic). The electricity required for actuators depends on their size; large actuators require three-phase supply and small valves can be operated on a single-phase. Occasionally a DC supply is available as an emergency backup. Fluid powered actuators have more variations; the type of media, available pressure of media, and cylinder size must all be considered when selecting an actuator powered by fluid.
The type of valve is also an important consideration. Proper actuator sizing can only be done when the user knows the type of valve it will be fitted to, if the valve is multi-turn or quarter turn, whether it has a rising or non-rising stem, and the power requirements of the valve. Knowing the type of valve is also important in order to calculate the torque requirement of the valve. This data can generally be requested from the valve manufacturer.
Control signal input type -- There are three basic types of control signal inputs: milliampere, voltage, and pressure.
- Voltage - Devices that use AC voltage or DC voltage are commonly available. The signal is typically a discrete voltage supply of 120/240 VAC or 12/24 VAC.
- Supply pressure- Supply pressure is the input pressure needed to achieve a desired torque or thrust output. Companies specify air supply pressure for pneumatic actuators and fluid supply pressure for hydraulic actuators. Compressed air in pneumatic actuators normally ranges from 60 to 100 psig.
Valve stem diameter -- The valve stem diameter can be combined with the lead and pitch of the valve stem thread in order to size the automation required for the valve. It can also be used with the valve size and the pressure drop across the valve to calculate torque demand.
Number of turns --- The number of turns applies to multi-turn actuators. It defines the number of turns preformed as the rotating valve stem moves from the fully closed to fully open position.
Location type -- Valve actuators for hazardous locations are designed for environments with atmospheres that contain combustible or potentially explosive mixtures. Devices for non-hazardous locations are designed for environments without the risk of combustion or explosion. Electric actuators are not recommended in outdoor applications or hazardous locations since condensation can form inside the actuator, instead compressed air should be used if possible.
Operating temperature -- The full range of ambient operating temperature. Pneumatic and electric actuators can be used in a wide temperature range.
Speed of Power Actuators
Actuators can vary in operating speed. The speed should be selected based on the speed and power requirements of the system and availability of energy to the actuator.
Fast-acting actuators are best used when a system must be quickly isolated or opened. Fast action is provided by hydraulic, pneumatic, and solenoid actuators. The speed of actuation is set by installing the correct orifice in the lines and the valve is closed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep the valve open. Electrical motors can also provide fast actuation when the speed is set through the motor speed and gear ratio.
Slow-acting actuators are best used when cold water is injected into a hot system or slower opening is needed.
There is a direct relationship between the speed of the actuator and the power needed. An increase in valve/actuator speed requires an increase in horsepower. Three-phase electric motor operators have a fixed speed, while smaller DC motors may have adjustable speeds. The speed of fluid powered actuators can be controlled using a fluid control valve.
There are several fail-safe methods for valve actuators. Devices can open or close valves in case of power failure, or in case of loss of control signal.
- Close at no power
- Close at no control
- Open at no power
- Open at no control
Double-acting actuators, those that need an actuation method to move (i.e. air to open, air to close), will fail in their last position if there is a loss of power, while a spring return design will return to its initial position when there is a loss of power.
Spring return actuators are often chosen for fail-safe critical requirements since operators can select whether the valve would be left open or closed in the case of a power failure. Spring return is common in pneumatic actuators and not widely available for electric actuators.
If the actuator is going to be spring-return, the failure mode (i.e., fail closed or fail open) must also be determined, using the following guidelines:
- Double-acting operation- The torque output at the minimum air supply pressure should exceed the calculated torque requirements of the valve.
- Spring-return operation, fail closed- The torque output at the minimum air supply pressure at the end of the spring stroke should exceed the torque required to close the valve.
- Spring-return operation, fail open- The torque output at the minimum air supply pressure at the end of the air stroke should exceed the torque required to open the valve.
The leading cause of actuator failure is poor air quality as defined by ANSI/ISA - 7.0.01.
Due to the wide variety of and variations in valves, the actuator must be sized to the specific valve in the system. Actuator sizing is best done after gathering information on the type of actuator desired, and the torque requirements calculated. Actuator sizing is usually done using a manufacturer's sizing chart. Additional specifications to consider are the required speed of operation since speed has a direct relationship to the power requirement.
If the actuator is undersized, it will be unable to overcome the forces against it. This will cause slow and erratic stroking. If the actuator is not stiff enough to hold the close position, the closure element will slam into the seat, causing a pressure surge. If the actuator is oversized, it will cost more, weigh more, and be more sluggish in terms of speed and response. Larger actuators may also provide a higher thrust that will damage internal valve parts. Actuators tend to be oversized because of safety factors but smaller sizes function just as well when the built-in safety factors are considered.