How to Select a Valve Actuator for a Rotary Valve?
Sometimes process engineers need to choose between pneumatic or electric valves for a system. Both styles have advantages, and it is valuable to have the data available to make the best choice
When selecting an actuator type (pneumatic or electric), the first step is to determine the most efficient power source for the actuator. The main points to consider are:
- Power availability
- Stem torque
- Failure modes
- Control accessories
- Operating speed
- Frequency of operation
- Factory environment
- Valve size
- System component costs
- System maintenance
The most practical pneumatic actuators use an air supply of 40 to 120 psi (3 to 8 bar). Typically, they are sized for supply pressures of 60 to 80 psi (4 to 6 bar). Higher air pressures are often difficult to maintain, and lower pressures require very large diameter pistons or diaphragms to generate the required torque.
Electric actuators are typically used with 110VAC power, but can also be used with a variety of single and three-phase AC and DC motors.
Temperature Range: Both pneumatic and electric actuators are available over a wide temperature range. The standard temperature range for pneumatic actuators is -4 to 150℉ (-20 to 70℃), but can be extended to -40 to 250℉ (-40 to 121℃) with optional seals, bearings, and grease C). If control accessories are used (limit switches, solenoid valves, etc.), they may have a different temperature rating than the actuator and this should be taken into account in all applications. In low-temperature applications, the quality of the supplied air in relation to the dew point should be considered. The dew point is the temperature at which condensation occurs in the air. Condensation may freeze and block the air supply line, rendering the actuator inoperable.
Electric actuators can be used in temperatures ranging from -40 to 150℉(-40 to 65℃). When used outdoors, electric actuators should be isolated from the environment to prevent moisture from entering the inner workings. Condensation may still form inside if drawn from the power line, which may have collected rainwater prior to installation. Also, since the motor heats the inside of the actuator housing when it is running and cools it when it is not running, temperature fluctuations can cause the environment to "breathe" and condense. Therefore, all-electric actuators used outdoors should be equipped with heaters.
It is sometimes difficult to justify the use of electric actuators in hazardous environments, but if compressed air is not available or a pneumatic actuator does not provide the desired operating characteristics, an electric actuator with an appropriately classified enclosure can be used.
The National Electrical Manufacturers Association (NEMA) has established guidelines for the construction and installation of electric actuators (and other electrical equipment) for use in hazardous areas. The NEMA VII guidelines state:
VII Hazardous Location Class I (Explosive Gas or Vapor)
Meets application requirements of the National Electrical Code; meets Underwriters' Laboratories, Inc. specifications for use in atmospheres containing gasoline, hexane, naphtha, benzene, butane, propane, acetone, benzene, lacquer solvent vapors, natural gas.
Nearly all electric actuator manufacturers have the option of a NEMA VII-compliant standard line version.
Pneumatic actuators, on the other hand, are inherently explosion-proof. When electrical controls are used with pneumatic actuators in hazardous areas, they are often more cost-effective than electric actuators. Solenoid-operated pilot valves can be installed and powered in non-hazardous areas and are piped to the actuator. Additionally, hazardous area-qualified solenoid valves are readily available at reasonable prices. Limit switches for position indication can be mounted in NEMA VII enclosures. The inherent safety of pneumatic actuators in hazardous areas makes them a practical choice for these applications.
Spring Return: Another safety accessory widely specified for valve actuators in the process industry is the spring return (fail-safe) option. In the event of a power or signal failure, a spring return actuator drives the valve to a predetermined safe position. This is a practical and inexpensive alternative to pneumatic actuators and a big reason why they are widely used throughout the industry.
If spring cannot be used due to the size or weight of the actuator, or if a double-acting unit is already fitted, an accumulator tank can be fitted to store air pressure.
Spring return versions of electric actuators are not widely available; however, battery backup systems are an excellent solution. For a spring return function, an electrohydraulic actuator is usually a good choice. Electro-hydraulic actuation is accomplished by energizing a hydraulic pump, which pressurizes a spring-return cylinder. When de-energized, spring action drives the actuator to its home position. Because this self-contained unit requires only one power source, it is a practical approach to fail-safe electric valve actuation.
Performance Characteristics: Before specifying a pneumatic or electric actuator for valve automation, it is important to consider some key performance characteristics of each actuator.
Duty Cycle: Pneumatic actuators have a 100% duty cycle. In fact, the harder they work, the better they work.
Electric actuators are most often equipped with 25% duty cycle motors. This means that to prevent overheating in high-cycle applications, the motor must be rested frequently. Because most on-off automatic valves remain idle 95% of the time, the duty cycle is usually not an issue. Electric actuators can be upgraded to 100% duty cycle with optional motors and/or capacitors.
Stall: A pneumatic actuator may stall indefinitely without overheating.
Electric actuators should not stall. Stalling an electric actuator draws excessive current, which generates heat in the motor and can cause damage. Torque switches or thermal and current sensors are often installed in electric actuators to protect the equipment.
Speed Control: The ability to control the speed of a pneumatic actuator is an important design advantage. The easiest way to control speed is to install an actuator with a variable orifice (needle valve) at the exhaust of the air pilot.
Since electric actuators are gear motors, it is not possible to make them cycle faster unless a gear change is made. For slower operations, there is an option to add a pulse circuit.
Modulation Control: In modulation service, electric actuators interface well with existing electronic control systems, eliminating the need for electro-pneumatic controls. Pneumatic or electro-pneumatic positioners are used with pneumatic actuators to provide a means of controlling the valve position.
Torque-to-Weight Ratio: Electric actuators have a high torque to weight ratio of over 4,000 lb. in. (450 Nm) pneumatic actuator has an excellent torque-to-weight ratio of less than 4,000 lb. in.
The above introduces the factors that need to be considered when selecting a valve actuator for a rotary valve. If you want to buy an actuator, please contact us.
UG Controls is a custom valve actuators and accessories manufacturer. Its main business is the import and export of various automation equipment and technologies. UG Controls is proprietary business solution integrating chemical engineering, water, oil and gas, power plants, manufacturing, food and beverage, and more. We also have very eminent engineers who have broad and in-depth knowledge in their assigned fields to provide ideal solutions to get the most out of our products.
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