Valve actuation is proven to increase plant up time, reduce maintenance costs, and increase plant safety. In a previous post we discussed 5 Reasons to Install a Valve Actuator that covered the many ways actuators are beneficial. But, what are the different methods of actuation and how is one selected?
Read on to learn what a valve actuator does, valve actuation method examples, and factors you must consider when selecting (and sizing) an actuator.
WHAT DOES A VALVE ACTUATOR DO?
Valve actuators move a valve to a desired position using an outside power source. All valve actuators, no matter which type, have several jobs to do. They must:
- Move the valve closure member to the desired position. The valve closure member consists of the disc, ball, or plug and it's the actuator's responsibility to provide enough torque to move the closure member under the most severe conditions.
- Hold the valve closure in the desired position. Actuators should have the right amount of spring or fluid power to overcome dynamic torque created by throttling applications.
- Seat the valve closure member with sufficient torque to provide the desired shutoff specification.
- Provide a failure mode in the event of system failure, such as battery power or springs. Fail positions of an actuator/valve can be open or closed.
- Provide the required rotational travel. Some pneumatic actuator manufacturers offer a 180° option, but for anything greater than 180°, an electric valve actuator is usually preferred.
- Provide the required operating speed. Any actuator can be regulated in cycle speed.
METHODS OF ACTUATION
Actuation methods include manual, electric, hydraulic, pneumatic and a few others. All of these methods use a single power source. Below, we define a few of the most popular methods of valve actuation:
Manual: As implied, these valves do not require an outside power source. They use a lever, or a hand wheel to drive a series of gears.
Electric: 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.
Pneumatic: Adjusts valve position by converting air pressure into linear or rotary motion.
Similarly, electro-hydraulic valve actuators and hydraulic valve actuators convert fluid pressure supply into linear or rotary motion.
ACTUATOR SPECIFICATION FACTORS
When specifying an actuator for a valve, there are several factors to consider to ensure the right actuation method (and size) is selected for the application.
Power source: Actuators are powered with either electricity, air, or fluid. The electricity required for actuators depends on the size of the valve. Large actuators require a three-phase supply and small valves can be operated on a single-phase. If fluid is your choice of power, consider the type of media, available pressure of the media, and cylinder size.
If a plant has an air supply nearby, it is less costly to operate a pneumatic actuator than to operate an electric one.
Failure mode: Battery and capacitor failure methods are viable "fail-safe" options on electric actuators. However, the failure mode requirements may dictate whether a pneumatic or electro-hydraulic unit is more fitting. For example, pneumatic actuators commonly use springs as a failure mode.
Duty cycle: Electric actuators do not have a 100% duty cycle, where a pneumatic actuator does. This means an electric actuator cannot be operated continuously, 100% of the time, without overheating. An electric actuator with a 25% duty cycle can only operate 25% of the time. If you require a high duty cycle, a pneumatic actuator is the better choice.
Valve type: 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 required.
Control signal input type: There are three basic types of control signal inputs: milliampere, voltage, and pressure. Devices that use AC or DC voltage are commonly available. Supply pressure is the input pressure needed to achieve a desired torque or thrust output.
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 performed as the rotating valve stem moves from the fully closed to fully open position.
Operating environment: Valve actuators can be designed for hazardous locations, such as when the atmosphere contains combustible or potentially explosive materials. If the operating environment is non-threatening, actuators are designed without the risk of combustion or explosion.
Pneumatic actuators are preferred for outdoor applications. If you must use an electric actuator outdoors, make sure it's properly sealed to prevent moisture from getting inside, and has a heater installed. Keep in mind that electric actuators do have a tendency to overheat if placed in hot environments.
Operating temperature: Know the full range of ambient operating temperature. Pneumatic and electric actuators can be used in a wide temperature range.
If you're looking to gain more control over your process, valve actuators may be the way to go. But with so many options, don't go it alone. Be sure to speak with an engineer, experienced with valves and a wide range of actuation.
If you need help with selecting and sizing a valve actuator, contact our dedicated team of Valve experts at Crane Engineering. Someone is always available to help!