Deciding what is the best actuator type to use on a dry bulk processing gate or diverter needs significant consideration. Selecting the correct type of actuator can eliminate costly mistakes and provide optimal performance over the life cycle of the valve. Acquisition costs, running expenditures, maintenance frequency, spare parts, safety, environment, torque, and accuracy all play into the decision-making process.

Valve manufacturers need to carefully review each application to ensure the actuator used on any valve is fit for purpose. This is accomplished assessing the following:

1. How much force and in what directions (push, pull, vertical, and/or horizontal) will the actuator need to move?
2. What stroke length is required for a given valve design and bore size?
3. How fast will the actuator need to move the valve blade or flap?
4. How many actuations are required in a given time period or “duty cycle”?
5. What is the desired life cycle for a particular actuation package?
6. How will the actuator be mounted to the valve?
7. Does the application suggest particular safety mechanisms, such as "manual overrides" for use in a power supply failure?
8. Will environmental factors (temperature variations, moisture, vibration, or product degradation) impact the performance of the actuator?
9. Is there space available within the installation area to accommodate the actuator?
10. What type of power supply is available at the plant location?
11. Is feedback required for speed and/or position?

Depending on the answers to these questions, it is necessary to decide the best actuation package for application. There are three primary type of linear driven actuators used on dry bulk solid processing valves: pneumatic, hydraulic, and electric. Here are some of the general advantages and disadvantages of each.

Pneumatic Actuators

Double acting pneumatic linear actuators consist of a piston inside a hollow cylinder. Air pressure from an external compressor moves the piston inside the cylinder. As pressure increases, the cylinder moves along the axis of the piston, creating a linear force. The piston returns to its original position by air pressure being supplied to the other side of the piston.

Advantages:

• The design of a pneumatic actuator is quite simple, which usually translates to easy maintenance. Ring seals and seats are generally reliable and can be easily replaced if needed.
• In case of electrical supply failure, a pneumatic air cylinder can use solenoid valves to default to closed or in the last position. An electric actuator must include a manual override or have alternative power sources for safety issues.
• Torque requirements needed for most dry bulk material handling applications can be covered through the use of pneumatic actuators.
• By utilizing the correct configurations of solenoids, sensors, and PLC programming, pneumatic actuators can provide accurate intermediate positioning.
• By using compressed air, pneumatic actuators do not require hazardous materials to operate.
• The unit cost of pneumatic actuators is significantly lower compared to other actuators.

Disadvantages:

• In the case of an air supply failure, there is no way to move the blade or flap position of a valve unless there is a Fail Safe compressed air tank in place.
• Pressure losses and maintaining clean dry air through the system can make pneumatics less efficient than other linear-motion methods. A compressor will run continually at operating pressure even if nothing is moving.
• Even though air is readily available, it can be contaminated by dirt, oil, water, or lubrication, leading to downtime and maintenance.
• To operate efficiently, pneumatic actuators must be sized correctly for a specific application. Improper bore size selection can result in slow actuation and potential failure to close off material flow.
• Accurate positioning control can be achieved; however, the necessary components to accurately hit intermediate positions can raise costs and complexity.
• Pneumatic actuators are not practical on large equipment that require large bore cylinders due to compressed air consumption.

Hydraulic Actuators

Hydraulic linear actuators operate similarly to pneumatic actuators, but an incompressible liquid from a pump rather than pressurized air moves the cylinder. The diagram below outlines the basic design and function of a hydraulic actuator.

Advantages: 

• Hydraulic actuators are robust and suited for high-force applications. They can produce forces up to 25 times greater than pneumatic cylinders of equal size.
• Hydraulic motors have higher horsepower-to-weight ratio than pneumatic actuators.
• A hydraulic actuator can hold force and torque constant without the pump supplying more fluid or pressure.
• Hydraulic actuators can have their pumps and motors located a considerable distance away with minimal loss of power.

Disadvantages: 

• Hydraulic actuators and the power packs required to operate them can have a higher unit cost versus pneumatic or electric actuators.
• No matter how much prevention is used, hydraulics will leak fluid. The loss of hydraulic fluid leads to less efficiency, cleanliness problems, potential environmental contamination, and safety issues.
• Hydraulic actuators require many companion parts, including a fluid reservoir, motors, pumps, release valves, and heat exchangers, along with noise-reduction equipment. This makes linear systems large and difficult to accommodate.

Electric Actuators

Electric actuators are driven by a motor that is connected to rotate a lead screw. A lead screw has a continuous helical thread machined on its circumference running along the length. Threaded onto the lead screw is a lead nut or ball nut with corresponding helical threads. The nut is prevented from rotating with the lead screw (typically the nut interlocks with a non-rotating part of the actuator body). Therefore, when the lead screw is rotated, the nut will be driven along the threads. The direction of motion of the nut depends on the direction of rotation of the lead screw. By connecting linkages to the nut, the motion can be converted to usable linear displacement.

Advantages: 

• Linear electric actuators can be modified to work in lower temperature environments, where pneumatics may have issues with elastomer gaskets and potential frozen water in the lines or cylinder.
• Electric actuators offer the highest precision-control positioning. Their setups are scalable for any purpose or force requirement, and are quiet, smooth, and repeatable.
• Electric actuators can be networked and reprogrammed quickly. They offer immediate feedback for diagnostics and maintenance.
• Electric actuators provide complete control of motion profiles and can include encoders to control velocity, position, torque, and applied force.
• In terms of noise, they are quieter than pneumatic and hydraulic actuators.
• Because there are no fluids leaks, environmental hazards are eliminated.

Disadvantages:

• In case of an electrical power failure, an electric actuator requires a type of manual override or alternative power source to shut off the gate or “fail close”.
• The initial unit cost of an electric actuator is higher than that of pneumatic actuators.
• A continuously running motor will overheat, increasing wear and tear on the reduction gear. The motor can also be large and create installation problems.
• Electric actuators are not suitable for all environments, unlike pneumatic actuators, which are generally safe in hazardous and flammable areas.
• The chosen motor limits the actuator’s force, thrust, and speed to a fixed setting. If a different set of values for force, thrust, and speed are required, the motor must be changed.

Which Actuator Should I Choose? 

Talk to your valve manufacturer who can help select if a pneumatic, hydraulic, or electric actuator is right for you.  By covering all relevant questions and discussing the advantages and disadvantages of the choice actuator, your valve manufacturer can help select the best actuator for your specific situation.