Commonly referred to as spring-return actuators, a single-acting pneumatic actuator uses air to open the valve and a mechanical spring (or springs) to close the valve. The actuator can also be configured inversely so that air closes the valve and the spring is used to open the valve; this is called a normally-open configuration. The most common reason single-acting actuators are chosen is for their reliable fail-safe design. This means that if power or air pressure is lost, the valve will quickly move to the normal position. This is useful in applications where serious consequences may occur if the valve remains open (or closed) when power and/or air pressure is lost. Three drawbacks to single-acting actuators are:
- The inclusion of mechanical springs in the design adds additional cost to the valve assembly. Cost difference depends on the size and style of the valve.
- The springs are susceptible to fatigue and will usually be the first component in the actuator to fail. Therefore, single-acting actuators are generally considered to be less durable than double-acting actuators.
- The spring-modules give the actuator a larger form factor that may not be compact enough to fit into some applications.
When to Use A Single-Acting Actuator
Single acting actuators are common in low to moderate cycle applications where the valve failing in the wrong position could present a serious safety hazard and/or cause damage to nearby equipment. If power or air pressure is lost, single-acting actuators ensure that the valve will fail to the normal position. For this reason, single-acting actuators are the best choice when the simplicity and reliability of a safety system are of the utmost concern.
