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Butterfly&Ball Valve Automation Pneumatic actuator selection
- Aug 08, 2018 -

Once the safety factor is incorporated into the torque rating of the valve, the next step is selecting a pneumatic actuator for valve control. Two types are available: double acting and single acting. A double-acting actuator requires air to open and close the valve. If a loss of air pressure occurs at any point, the valve will hold its last position. A single acting actuator uses air to rotate in one direction and a set of springs to move in the opposite direction. Whether the springs are used to open or close the valve depends on how the two pieces are assembled. This will also be the fail position when air pressure is lost.

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Sizing the actuator depends on the available air pressure. The lower the pilot pressure, the larger the size required. The actuator’s torque rating must be greater than or equal to the final valve torque calculated for the application. For ease of automation, the process valve should have a direct-mount pad so that the two components can be assembled without a mounting kit. Most valve and actuator suppliers use the International Organization for Standardization (ISO) 5211 interface standard, which consists of a bolt pattern and a square stem for simple assembly.

Without this ISO Standard 5211, valve assemblies require a mounting bracket and stem coupler that can be not only expensive, but also space consuming because it increases the overall height of the unit. For the greatest flexibility, many valves and actuators come with more than one flange pattern when permissible. Table 1 shows the bolt patterns and stem sizes that are typically found on components using ISO Standard 5211.

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The flange pattern of the process valve will need to be compared to that of the selected actuator. Because this interface standard is based on specific torque ranges, the two components will likely have a matching bolt pattern for direct mounting. If they do not and the actuator’s flange pattern is smaller than the valve’s, the actuator can be upsized to achieve a match. Often the cost of moving up by one or two actuator sizes is still less than the cost of a mounting kit. If the actuator’s flange pattern is larger than the valve’s, then a mounting kit is needed to connect the two pieces.