How Pneumatic Actuators Work

Pneumatic actuators are driven by compressed gas sourced externally, most frequently dry clean air. Commonly, a single air compressor can simultaneously power several pneumatic actuators, endowing them with an advantage over other actuator types, particularly when a system has a large number of valves. For instance, in an electrical valve actuator, a power source (motor) must be installed on each valve.

Why Consider a Pneumatic Actuator?

Determining the most suitable actuator for a specific application hinges on several factors such as the environment, required accuracy, and closure/open speed. Nevertheless, the main disparity among actuators lies in the thrust or torque they can generate. Here are the general applications for the main types of actuators:

Hydraulic Actuators: Powered by compressing a fluid like oil, this type is the most potent and reliable. It is recommended for heavy-duty and high-load work environments due to the non-elasticity property of the fluid used to power them.

Pneumatic Actuators: Driven by compressed gas, this type is suitable for medium-duty and average load work environments. It is recommended for fast-moving, high-accuracy applications. Owing to the low operating pressures, pneumatic actuators are limited in the amount of thrust or torque they can produce. Consequently, they are often employed to automate smaller valves. High thrust or torque requirements can result in large actuators.

Electrical Actuators: Powered by electric current, this type is suitable for medium-duty and light load work environments and is recommended for accurate control. They can be found in industries such as food & beverage and pharmaceutical, as well as in heavier industries like mining and power generation.

How Do Pneumatic Valve Actuators Work?

A pneumatic actuator typically comprises pistons fitted inside a hollow cylinder. Pressure is applied to one side of the piston within the cylinder. As a consequence of the pressure and the piston's area, a force is generated that moves the piston along the cylinder's axis, transferring such energy to the valve to be automated. The piston then returns to its original position by means of a spring (known as a spring-return actuator) or by having compressed air applied to the other side of the piston (known as double-acting actuators).

What Valve Types Does a Pneumatic Actuator Work With?

Generally, a pneumatic actuator can provide two types of motions.

Linear Motion: Suitable for linear moving valves such as gate and globe valves. These actuators are usually spring-return and move back to their original position once air pressure is vented. The valve is either normally closed or normally open. For continuous control applications, double-acting actuators are used, where compressed air is applied on both sides of the piston.

Rotary Motion: Suitable for quarter-turn valves such as ball and butterfly valves. These actuators can be spring-return. Once air pressure is vented, the actuator moves back to its original position. The valve is either normally closed or normally open. For continuous control applications, double-acting actuators are used, with compressed air applied on both sides of the piston.

As an example, HEARKEN's Pneumatic Actuator products offer a diverse range. These include Quarter Turn Pneumatic Actuator, Scotch Yoke Pneumatic Actuator, inear Pneumatic Actuator, and Pneumatic Cylinder.

How to Select a Pneumatic Actuator and Size It for a Certain Valve?

1. Type of Motion Required

The first crucial factor in selecting a pneumatic actuator is the type of motion needed to operate the valve. There are two primary types of motion: linear and rotary.

* Linear Motion: If the valve requires a linear motion, such as in the case of gate and globe valves, a linear pneumatic actuator is the appropriate choice. These valves typically move up and down or in a straight line to control the flow of a fluid. Linear actuators work by translating the pressure - induced piston movement directly into a linear displacement. The actuator's stroke length must match the required valve travel distance. For example, in a water treatment plant, a gate valve that controls the flow of water into a filtration tank might require a specific linear displacement to open and close properly. The linear pneumatic actuator selected should have a stroke length that enables this precise movement.

* Rotary Motion: For valves that operate with a rotary motion, like ball and butterfly valves, a rotary pneumatic actuator is essential. These valves turn a quarter - turn (90 degrees) to open or close. Rotary actuators convert the linear motion of the piston into a rotary movement. The torque output of the actuator must be sufficient to overcome the resistance of the valve during rotation. Consider a ball valve in an oil pipeline. The actuator needs to provide enough torque to turn the ball quickly and precisely to either allow or block the flow of oil.

2. Required Control

The level of control needed over the valve operation is another vital consideration.

* On - Off Control: In some applications, simple on - off control of the valve is sufficient. For example, in a basic water supply system, a valve might only need to be fully open or fully closed to control the flow to a particular area. In such cases, a spring - return actuator can be a good option. Spring - return actuators use a spring to return the piston to its original position when the air pressure is removed. This allows for a basic two - position (open/closed) control. The spring force is designed to ensure reliable return of the valve to its default state.

* Proportional Control: However, in more complex systems, proportional control is necessary. This requires an actuator that can adjust the valve position precisely according to a control signal. Double - acting actuators are often used for proportional control. In a double - acting actuator, compressed air can be applied to both sides of the piston, allowing for more fine - tuned control of the piston's position and, consequently, the valve's position. For instance, in a chemical process where the flow rate of a reagent needs to be carefully regulated, a double - acting pneumatic actuator can adjust the valve opening in small increments to achieve the desired flow rate.

3. Torque or Thrust Required

The torque (for rotary valves) or thrust (for linear valves) requirements are critical for proper actuator sizing.

* Torque Calculation for Rotary Valves: To size a pneumatic actuator for a rotary valve, the torque needed to operate the valve must be determined. This torque depends on several factors such as the valve size, the pressure differential across the valve, and the friction in the valve's mechanism. Manufacturers usually provide torque specifications for their valves. For example, a butterfly valve with a certain diameter and operating pressure might require a specific amount of torque to open and close smoothly. The pneumatic actuator selected must have a torque output that meets or exceeds this requirement. It's also important to consider any safety factors or potential increases in torque due to factors like fouling or wear over time.

* Thrust Calculation for Linear Valves: For linear valves, the thrust required to move the valve element is calculated. The thrust is a function of the pressure applied to the piston and the area of the piston. The weight and friction of the valve components also play a role. For example, a large gate valve with heavy moving parts will require a higher thrust to open and close. The pneumatic actuator's thrust capacity must be sufficient to handle this load. Additionally, any changes in the operating conditions, such as an increase in the pressure differential across the valve, might affect the thrust requirement and should be taken into account during sizing.

The following table provides a more detailed breakdown of the selection and sizing factors to assist in making an informed decision about the pneumatic actuator for a specific valve:

Torque values are required for sizing. Note that even valves of the same type and with identical PN (Pressure - Nominal) and DN (Diameter - Nominal) values can have different torque values due to sealing type and materials. The sealing material can affect the friction and, consequently, the torque needed to operate the valve. Different materials might have different coefficients of friction, and this can lead to variations in the required torque even for valves that seem similar in other respects.

Applications for Pneumatic Valve Actuators in Industry:

In the oil & gas industry, they drive valves for isolating and regulating flow in pipes. In mining, they drive valves for isolating and regulating pressure to nozzles in rocks washing lines. In water and wastewater treatment, they drive valves for isolating and regulating drinking water feed lines and control sand filters and tank levels.

In conclusion

Pneumatic actuators find extensive applications across different industries. In the oil and gas industry, they perform isolation and flow regulation functions in pipelines. For isolation, they quickly shut off flow in emergencies, preventing leaks and disasters. For flow regulation, they adjust the rate to meet process demands. In the mining industry, they drive valves for pressure regulation in rocks washing lines and control material flow in various processes. In water and wastewater treatment, they isolate drinking water feed lines and regulate sand filters and tank levels. Overall, pneumatic valve actuators are essential for ensuring efficient operations, safety, and proper process control in these industries.