Assessing actuators

An actuator is a device that produces motion in a mechanical system, based on signals received. A pneumatic actuator is therefore an actuator that employs pneumatics in the form of pressurised compressible fluids, such as air, to effect this motion. It translates the potential energy of the pressurised air into mechanical motion for actuating plant components. By contrast, the actuators employing incompressible fluids such as oils are known as hydraulic actuators.

Whether using a piston or a diaphragm, a pneumatic valve actuator makes use of compressed air very efficiently. The working principle is such that the compressed air pushes against the piston or the diaphragm, building up force against it, and generating the motion required to open or close a valve. Here, air is usually preferred to other compressible fluids, due to its ease of compressibility and non-hazardous nature.

There are many criteria by which pneumatic actuators can be classified; however, the two categories on which we will be focusing are single acting (or spring return) and double acting.

Difference between a single-acting and a double-acting actuator

As the name suggests, spring return pneumatic actuators feature a loaded spring on one side of the piston that keeps the valve in its natural position. To open or close the valve (move the valve from its natural position), pressurised air is supplied on the other side of the piston. The force of the air overcomes the spring's force, making the valve open or close. When the air supply is cut off, the spring returns, taking the valve back to its original position.

A double-acting actuator, on the other hand, has two compartments for holding compressed air instead of one, as was the case with a single-acting actuator. These two compartments allow the air to enter from one and exhaust from the other, thus manipulating the position of the piston and the actuator valve it operates.

In double-acting actuators, the degree of opening of the valve can be manipulated with the air pressure, thus giving the user control over the flow rate through the actuator valve.

Safety considerations

Because the working principle of pneumatic actuators does not involve combustion or sparking, they are ideal for use in energy-sensitive industries such as oil and gas. Yet, single-acting actuators may be safer in some applications while double-acting in others.

For your application

In industries that need force, pneumatic actuators work wonders. Compare that with electrical actuators, which offset their lower force-applying capacity with higher accuracy. In applications where increased fail-safe considerations may be required, single-acting actuators may be more appropriate.

For your budget

As compared with other types of actuators, namely electric or hydraulic, pneumatic actuators have lower initial costs, in addition to minimal maintenance costs. Due to their simple and compact design, single-acting actuators may have lower initial costs. However, according to their application, double-acting actuators may save operating costs in the longer run.

Five advantages of double acting actuators

Having gone through the basic working principles and characteristics of single and double-acting pneumatic actuators, let us now see the advantages that double-acting actuators have over their sing-acting counterparts.

More control

Double-acting pneumatic actuators provide more control than spring return actuators. To understand this, we go back to their internal mechanisms. In spring return actuators, the force of the spring dictates the air pressure required to compress it, thus limiting each actuator to a single purpose of either fully opening or closing the valve. In double-acting actuators, however, the air pressure difference between the two compartments is what controls the movement of the piston. This allows for more control of the extent of opening or closing of the actuator valve, thereby permitting manipulation of fluid flow rate through the valve.

Energy-efficient

Due to the utilisation of springs in single-acting actuators, higher pressures of air are required to overcome the spring force. Additionally, the constant compression and extension of the spring produces added friction and dissipates energy through heat, further reducing its energy efficiency. By comparison, the two-compartment nature of double-acting actuators allows for lower pressures of air to be employed, thereby reducing the energy consumption required to pressurise the air.

High torque or thrust strength

In single-acting actuators, the return force of the spring acts to reduce its thrust strength. The lack of a spring in double-acting actuators, on the other hand, circumvents this issue, resulting in higher thrust strengths, employing the full energy of the pressurised air. Double-acting actuators also have higher torque, and in both directions, which makes them ideal to be used in the petroleum industry.

Heavy industry application

This brings us to the consideration that double-acting actuators are much more applicable in heavier industries, such as oil and gas or chemical manufacturing. Where controlling the flow rate is more important than simply shutting off a valve, double-acting actuators prove to be ideal. Moreover, single-acting actuators require higher air pressure to function, making them a safety hazard - especially for energy-sensitive heavy industries.

Holds its last position

Again, the spring becomes the bane of a single-acting actuator. Let us suppose that a high-energy, hazardous fluid is flowing through the normally closed actuator valve. Air pressure is applied to compress the spring and open the actuator valve, venting the fluid. In case there is a power shut down while there is still a need for venting the hazardous fluid, the spring will have no way to be held open and will extend, shutting down the valve and preventing the fluid from venting. This, as you can imagine, has the potential for disastrous consequences.

Double-acting actuators, contrarily, may be of the locking cylinder kind, wherein they stop mid-stroke in case there is a power shutdown. This allows them to hold their last position and, in the example we used above, the continuous venting of the hazardous fluid, thereby preventing a potential safety incident.

To summarise, both single-acting and double-acting pneumatic actuators have their advantages. However, for heavier industry applications where safety and thrust strength are of the utmost importance, double-acting actuators prove more advantageous. It all comes down to the application of the actuator and the specific purpose it is providing.