An actuator is a mechanical device that converts energy into linear or rotary motion. Actuators are used in devices such as: motors, pumps, switches, and valves. In this article, you will learn about the different types of actuators, how actuators are specified, and repair and maintenance considerations.
Actuators can be categorized as: manual, manual geared, electric, pneumatic, hydraulic, solenoid, or spring.
Manual actuators enable movement by utilizing gears, wheels, or levers. They are hand-operated and straightforward to use. Manual actuators are ideal for the following conditions: unrestricted, low damper/valve quantity, reasonable operator input requirements, safe environment, and infrequent use. In toxic, remote, and classified environments it is recommended to apply a different type of actuator due to accessibility concerns. Also, specific process requirements may require quicker operations than physically possible via a manual actuator.
Actuation may be accomplished via a handle, hand wheel, chain wheel, or manual gearing. Manual geared actuators provide a mechanical advantage and at times may be characterized independently.
Manual Geared Actuators
Manual gear actuators utilize an additional gear train to allow for easier turning capability. As a result, torque and performance are increased and speed is reduced. Operational life in gear actuators is extensive, due to overdrive prevention. Gears can be configured to fit many partial-turn/multi-turn valves to reduce the amount of work required to cycle a valve. Extra friction, inertia, or sharp load changes may occur when manual geared actuators are used.
Electric actuators apply an electric motor to produce torque. Common electric actuators vary in dimension/size and are non-toxic, quiet, safe, and energy efficient. They require readily available electricity or batteries for performance. Typical uses are industrial valves and technical process plants (e.g power plants, oil and gas plants, wastewater treatment plants).
Pneumatic actuators are powered by air or gas pressure that converts energy into rotary/linear motion. Air pressure acts on a piston/bellows diaphragm that applies linear force on a valve stem. They are highly reliable, efficient, and safe sources of motion control. Common applications include: pistons and ignition chambers in gas-powered vehicles, air compressors, packaging/production machinery, and transportation equipment. Variable speed control and slow movement is difficult with pneumatic actuators. Periodic pressure testing/leak checking is also required.
Hydraulic actuators convert fluid pressure to mechanical energy to produce linear or rotary motion. Hydraulic actuators are generally used for linear movement to control valves. Most hydraulic actuators have fail-safe features that close or open a valve in emergency situations. They operate similarly to pneumatic actuators, but utilize incompressible liquid from a pump instead of pressurized air. Hydraulic actuators provide high accuracy movement/control and are commonly used when high speed and large forces are required. However, they are dirty to operate and leakage is common. Usage is typically seen in industrial process control and transport applications. Hydraulic actuators are also the most expensive option.
Solenoid actuators convert electrical signals into mechanical linear motion. Made of a movable coil and iron core, the pushing and pulling strength of a solenoid is determined by the number of coil turns. A major aspect of the solenoid is a small jolt that produces a high level of operation, with little power required. If the current flows, the coil pushes the core to open the channel/valve. No flow results in a closed channel. Typical solenoid applications include: Linear applications, latching/switching, sorting, fuel injectors.
Spring based actuators hold springs in place that operate a valve when an anomaly is detected (e.g power is lost). These types are desirable where power failure can compromise a system. When an actuator loses power, air supply, or hydraulic pressure, the springs expand to return the actuated component to original position (fail position). These only operate once, without resetting, and are typically used for emergency situations. Spring actuators do not require an electric supply to move the valve and can operate from restricted battery power, or mechanically if all power dissipates.
Specification of Actuators
Determining which actuator fits the application best is a consideration of budgetary requirements and fitting the right type of actuator to the use case. Applications considerations for each type are further explored below:
Manual actuators must be a traveling nut, self-locking type and be designed to hold valves in any position (between fully open and fully closed) without creeping/fluttering. They are usually equipped with mechanical stop-limiting devices to prevent travel of the disc in open/closed positions. Valves close with a (clockwise) (counter-clockwise) rotation. Designed to produce the specified torque with a maximum pull of 80 lb. on the handwheel/ chainwheel. Actuator components withstand input force of 610 Nm. at extreme actuator position without damage.
Manual Geared Actuators
Gear actuators limit pull force to 80 lbf (360 N). Maximum lever length or handwheel diameter is further limited by industry specifications. There are a variety of gear actuators with different functions. Utilizing the following steps can determine the ideal gear type for the project:
- Know the valve type that is operated
- Evaluate environment effects on gearbox
- Consider frequency of automation
Electric actuators must be sized to guarantee valve closure at the specified pressure and temperature. They are capable of operating temperature ranges from -22°F to 158°F. The motor requires low inertia, high torque design, and class F insulation. Recommended torque setting of 40-100% rated torque. Torque output ranges from 10 Nm to 2260 Nm. Electric motor voltages typically range 12 to 24 volts for DC and 24, 120, 220 volts for AC.
Pneumatic actuators primarily utilize painted aluminum alloy for end caps and a stainless steel drive shaft. Temperature ranges from -4°F to 176°F with dry air as media. Supply air pressure ranges from 40-120 psi (spring return) and 20-120 psi (double acting). Torque output varies per model, from 400 Nm. – 6700 Nm. Service life provides up to 1 million cycles.
Hydraulic actuators designed to carry linear movements for forces from 10-250 kN. Supplied with multi-grade oil as standard. Allows operation at temperatures of – 13 °C to 50 °C. Push or pull force must be lower than 980 N in any case. Acceptable leakage within min=0.05 Litres/min and max=2 Litres/min. Must be able to tolerate the pressure of 4270 psi for 15 minutes (minimum) with no leakage.
A solenoid actuator typically has one of the following power requirements: 120 VAC, 240 VAC, 24 VAC, 12 VDC. Supply pressure varies from 20-120 psi and operates in temperatures between -40 to 140°F. Standard features include manual override and mechanical spring safety position.
Spring actuators have a standard working temperature of -22°F to 212°F and are sized for 80 psi minimum for pistons and 35 psi for control valves. The body is primarily aluminum and could be hard anodized and epoxy coated. Due to its role in preventing power failure, specifications of this vary for each actuator type.
Repair and Maintenance Considerations
Valve issues can affect the functionality of the actuator and aggravate underlying problems. Typical valve issues that cause actuator problems are listed below:
- Worn out valve stem
- Seized up packing
- An obstruction
- Too much torque
Before attempting to fix an actuator, it is crucial to check the valve for any issues. To do so, put the unit into manual override and try to operate the valve manually (for electric). If the valve remains stagnant it is a valve issue, but if it operates in manual mode the actuator requires repair.
An actuator has four major components that can break down: the center column drive, connection to the valve, the contactor, and the motor.
Center Column Drive
The center column drive is responsible for closing and opening the valve. Rare occurrences can cause this drive to break. Generally, it is not the main cause of actuator issues. Replacement of center column drive requires removal of actuator from service before repair.
Connection to the Valve
The drive nut serves as the main connection to the valve and is common to fail. A broken actuator drive nut will be unable to move the valve stem properly. This is viewable by removing the center column cover and looking down the center to the valve stem.
The motor contactor is an electric actuator internal electrical part that controls the open/close motion of the valve when given an input signal. Failure of the contactor prevents actuators from operating. Fuses around the contactor should be checked prior to a full inspection.
In electric actuators, the motor provides torque to open and close the valve or other piece of equipment. If the motor fails, no movement will occur. In this case, the duty cycle and insulation class must be validated as sufficient for the application.
Regular maintenance can increase reliability, lower maintenance costs, reduce chance of potential major shutdowns, and prevent operability loss. The following maintenance procedures are suggested:
- Check and replenish oil level
- Conduct inspection of all external surfaces
- Check mounting bolts, nuts, washers and screws for damage/tightness
- Confirm correct hand wheel operability
- Signs of deterioration
- Check connections for tightness
- Inspect the motor for ingress of moisture
- Replace ‘O’ ring seal
- Replace all cover screws and apply grease to prevent corrosion
- Validate local and remote operation