Key Considerations for Choosing Pneumatic Actuators in Industrial Applications

Picture this: You've carefully selected a pneumatic actuator, only to discover it either applies too much force—snapping the valve stem—or too little, leaving the valve sluggish or stuck midway. This frustrating scenario underscores why proper actuator selection is crucial for valve operation.

At its core, a pneumatic actuator's primary function is to drive valves. For smooth operation, the actuator must overcome various resistances, making torque—the rotational force—the most critical consideration in selection.

• Break Torque: The initial force required to move a stationary valve from its closed position, analogous to the effort needed to push a stationary car.
• Running Torque: The continuous force needed to maintain valve movement during opening or closing, similar to maintaining a car's speed.
• Seating Torque: The final force required to fully close the valve and compress sealing surfaces, comparable to applying brakes to stop a vehicle.
• Maximum Allowable Stem Torque (MAST): The maximum force the valve stem can withstand without damage, serving as the valve's load limit.

Different valve types exhibit distinct torque characteristics. Metal-seated butterfly valves, for instance, demand high break and seating torque but require less during mid-travel. In contrast, metal-seated ball valves maintain high torque requirements throughout their operation.

To ensure reliable operation, engineers typically multiply the valve's required torque by a safety factor (often 25%). This margin accounts for unexpected variables, similar to engineering safety margins in automotive design.

Modulating valves require precise, continuous flow adjustment, demanding actuators with superior performance characteristics. On-Off valves, used for simple open/close operations, present less demanding requirements.

Accurate torque data from valve manufacturers forms the foundation of proper actuator selection. Most manufacturers provide minimum opening torque values under specific conditions, typically for water service. However, these values change with different media, necessitating appropriate safety factors for varying applications.

• Rack and Pinion: Delivers consistent torque throughout the stroke.
• Scotch Yoke: Exhibits reduced torque during mid-stroke.
• Spring Return: Torque decreases as the stroke progresses due to spring compression.

Understanding each type's torque curve—available from manufacturers—is essential for proper matching to valve requirements.

Actual pressure at the valve often differs from compressor output due to system losses. Accurate pressure measurement at the installation point is crucial for proper actuator sizing, much like accounting for pipe resistance in hydraulic systems.

• Single Acting (Spring Return): Uses spring force for return motion, available in "fail-open" or "fail-closed" configurations.
• Double Acting: Requires air supply for both opening and closing, maintaining position during air loss.

• Start of Air: Initial movement torque
• End of Spring: Initial position spring torque
• Minimum Air: Stroke's lowest torque point
• End of Air: Stroke-end pneumatic torque
• Start of Spring: Return stroke initiation force

When using these tables, ensure valve torque requirements represent minimum values, while MAST must exceed all other torque values in each row.

Proper actuator selection requires careful consideration of multiple factors. This systematic approach helps prevent operational issues and ensures optimal valve performance.