Single acting (spring return)quarter turn actuators

General operating principle of a single acting actuator

Air to port A forces the pistons outwards causing the springs to compress.

The pinion turns counterclockwise whlie air is being exhausted from port B.

Loss of air pressure on port A, the stored energy in the springs forces the pistons inwards.

The pinion turns clockwise while the air is exhausted from port A

Air t o por t A forces the pistons outwards,causing the springs to compress.
The pinion turns clockwise while air is being exhausted from port B.
Loss of air pressure on port A, the stored energy in the springs forces the pistons inwards.
The pinion turns counterclockwise while the air is being exhausted from port A.

Sizing a single acting actuator

The most frequent application:Closing with spring force

When the valve is closed, the springs in the actuators are pre tensioned. This means: A single-acting actuator will always have a lower maximum air torque than the identically sized double-acting actuator(same piston,same design)

When the process valve opens, the actuator works against the spring force. If the springs are compressed,the force in the springs increases and the opening force of the air decreases proportionally. This means that the actuator must overcome the torque generated by the spring force as well as
the normal breakaway torque. The air torque decreases in accordance with the increasing spring force.

The most important criteria for the sizing of single acting quarter turn actuators are, with the exception of the valve's closing torque, the same as for double acting actuators.

-Breakaway torque of the process valve
-Closing torque of the process valve What torque is required to move the shut-off device(ball, valve …) securely back into the seal?
-Compressed air supply
-Process valve type

As with double acting actuators, the main operating conditions must be known before the breakaway torque can be specified by the valve manufactures or correctly derived from existing tables.

These conditions also apply for the closing torque. As the lubricating properties of the medium remain almost incalculable here, this is more difficult to
determine.
For that reason, most process valve manufacturers do not specify a closing torque.

The Solution

The breakaway torque is used instead of the closing torque as it is always greater than the closing torque of a process valve. It can generally be assumed that the breakaway torque specified by the process valve manufacturer can be used without a safety factor. The suggested safety factor for the breakaway torque is a factor of at least 1.3 to 1.4, 30% to 40% safety should be included when sizing the single acting quarter turn actuator. Single acting quarter turn actuators can close or open with spring force as a safety function.

The ideal quarter turn actuator in this case is shown in Figure 9.3. The breakaway and closing torques of the actuator are above the torque values incl. safety factor(40%) calculated for this valve when both opening and closing. This guarantees a reliable actuator function.