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A valve actuator is a mechanical device that uses a power source to operate a valve. This power source can be electric, pneumatic (compressed air), or hydraulic (the flow of oil). There are two main types of actuators, one for each of the two main types of valves that require them. They are rotary and linear.
Selection of the best actuator type for any application is dependent on many factors including:
We have tools for helping you select the best actuator for your application. All valve configurators allow the quick and easy comparison of all applicable actuators.
Rotary Valve Actuators
As the name implies, rotary valve actuators produce the rotational motion needed to operate roatary valves such as ball, plug, and butterfly valves. Rotary actuators are available in many different styles, each with its own benefits.
Linear Valve Actuators
Linear valves such as globe, gate, and pinch valves - due to their distictly different operation - require actuators that are drastically different from the rotary type. These actuators must produce linear motion to operate the valve. They are also available in many styles.
Valve Actuator Power Sources
Both linear and rotary style actuators are available with the various power sources mentioned earlier: pneumatic, hydraulic, and electric. In addition to these, there are also several distinct types of manual operators.
Use this decision tree to decide
whether to use an
electric or pneumatic actuator
The most common manual operator on smaller quarter-turn valves is the basic lever. A long handle is attached to the stem and provides the leverage needed to rotate the valve.
On very small valves, where leverage is not needed, these may be replaced by ovals, tees, and various other shaped knobs.
There are also some specialty manual operators such as:
Larger valves that require high torque to operate often incorporate geared hand wheels. These operators have a large hand wheel with a gearbox that further increases the mechanical advantage. The radius of the wheel and gearing ratio determines the amount of mechanical advantage provided. These wheels will require multiple revolutions to turn the valve only 90 degrees. These type of operators are commonly found on larger butterfly valves, typically 8 inches and above.
If the valve is located high enough to be out of reach, a chain wheel will be added to the same type of gearbox so that the valve can be operated from the ground. The chain is in a long loop that hangs down to where it can be easily reached. The operator would simply pull down on one side of the chain loop to open the valve, and the other side to close it.
Manual Valves with Limit Switches
Many times manual valves perform a function that does not require automated actuation, but the system still needs to know what postion they are in. Manual valves with limit switches (position indicators) are used in these cases. While the valve is still operated manually, the switches communicate the valve's current position to the control system.
Rotary Actuators: Different Mechanical Styles
There are 2 main styles of rotary pneumatic actuators for use on ball, butterfly and plug valves: Rack-n-pinion, and Scotch Yoke. While at first they may appear drastically different, they are actually quite similar. Both operate by converting the linear motion of a piston in a cylinder into rotational motion of the stem drive.
Both are available in dual (shown above) and single versions and both can be spring return or direct acting (sometimes referred to as single acting and double acting).
There is also a 3rd style of pneumatic rotary actuator that is older and far less common - Vane. In this type of actuator the piston is replaced by a vane, which is fixed to the stem drive. The vane is enclosed in a wedge shaped chamber where the stem center is located towards the point of that wedge. Air is forced into this chamber on either side of the vane in order to move it, thus rotating the stem drive.
Double acting pneumatic actuators require air to be used to move the piston in either direction. A solenoid valve is used to control the air flow into and venting of the actuator. Since there is no spring force to overcome, a smaller actuator can often be used.
Both Vane and Rack-n-pinion style double acting actuators have an constant, linear torque output throughout the stroke in both directions. Scotch yoke actuators, due to their mechanics, produce curved torque outputs that are identical on one stroke direction vs. the other.
Spring return pneumatic actuators use compressed air to move the piston in one direction, and a spring to push it back in the other when the air is stopped and allowed to vent. This requires a larger actuator since it has to overcome the force of the spring in addition to providing enough torque to operate the valve.
The most important aspect of a spring return actuator is that it offers a "fail-safe" position. This means that if the air supply is lost, or electric power to the solenoid that controls the air is lost, the valve will travel to this position using the force of the spring(s). This could be either open or closed, whichever offers a safer outcome.
Both Vane and Rack-n-pinion style spring return actuators have a descending, linear torque output throughout the air stroke and spring stroke. The slope of the line is caused by the spring force increasing and decreasing as it compresses and expands. Scotch yoke actuators, due to their mechanics, produce curved torque outputs that are inverse on one stroke direction vs. the other.
Common Characteristics of Rotary Pneumatic Actuators
There are some aspects of pneumatic actuators that are common across all of these types.
Linear Actuators: Different Mechanical Styles
For linear valves such as gate and globe valves, the linear motion is achieved directly. Air is used to push the "rising stem" up and down. There are 2 main styles of linear actuators, they are diaphragm and cylinder (or piston). Both of these styles are also available as spring return or double acting.
Rotary Actuators: Motorized Valve Actuators
Most electric actuators use a reversing motor that travels in both directions. Most also use limit switches to turn the motor off when the actuator reaches its final position (end of travel). These switches are activated by cams that are mounted on the stem drive shaft. The cams can be adjusted to change or fine tune when the actuator stops in the open and closed positions.
[ image of stem and cams ]
Many lines of actuators have various sizes, each having a range of speed/torque options that differ inversely based on the gearing. The same size actuator body could provide:
This is based on different gear ratios in the drive chain.
Other aspects of electric actuators that are important to consider include the voltage, duty cycle, wiring entry and connections, and the electrical protection rating according to IP (ingression protection) and/or NEMA (National Electrical Manufacturer's Association) standards.
Electric actuators are available from most manufacturers in the following common voltages: 12, 24, 48 VDC, and 24, 48, 120, 240 VAC. There are also multi-voltage or "Universal Voltage" actuators such as our S4 series that accept any of these voltages. These units sense the voltage and react accordingly.
Many electric actuators have a less than 100% duty cycle. This means that they cannot be run continuosly or the motor will burn out. This is not typically a problem since valves are not actuating that frequently. An actuator with a 50% duty cycle that takes 8 seconds to cycle requires 8 seconds of recovery time before it should be actuated again.
Wiring Entry and Connections
There are many different methods of making the electrical connections to the actuator. This can largely be a matter of preference and is typically not that big of a factor in selecting an actuator. The most common types of electrical connections are:
Standard NPT Conduit Entry
This is a simple threaded port to feed a wire through to connect to an internal wiring screw terminal strip or that has wire leads or a cable emerging through it so that the housing does not need to be opened to wire it. This port can have either conduit fittings or cable glands threaded into it for the wires to travel through.
Permanent Cable Glands
Cable glands include a rubber seal that contracts around the wires or cable when it is compressed by tightening the head.
DIN Plug Connectors
These are industry standard plugs that have blades on the male connector that fit into slots on the female connector. Actuators will typically have the blades emerging from a standard seating plate for the female connector to attach to.
These are short lengths of cable or wires emerging from the housing through either a conduit port, cable gland, or permanently sealed exit.
Electric actuators need to comply with NEMA or IP ratings based on the environment in which they are used. The most common ratings for valve actuators (and solenoid valves on pneumatic acutors) are NEMA 4/4X and NEMA 7.
NEMA 4/4X "weatherproof"
Type 4X Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow, splashing water, and hose directed water); that provides an additional level of protection against corrosion; and that will be undamaged by the external formation of ice on the enclosure.
NEMA7 "explosion proof"
In Hazardous Locations, when completely and properly installed and maintained, Type 7 enclosures are designed to contain an internal explosion without causing an external hazard. Type 7 Enclosures constructed for indoor use in hazardous (classified) locations classified as Class I, Division 1, Groups A, B, C, or D as defined in NFPA 70.
Common Characteristics of Rotary Electric Actuators
Electric actuators have basically the same common characteristics as pneumatic with the exception of the NAMUR mounting for solenoids since they are not used on electric actuators.
Linear Actuators: Motorized Valve Actuators
For rising stem valves there are electric actuators that operate with a screw drive. The obstructor of the valve is moved up and down by the rotation of a threaded rod that is connected to the stem. The need for many revolutions between up and down allows for precise control.