How Pneumatic Actuators Work

What powers them?

Pneumatic Actuators are powered by compressed gas from an external source, most commonly dry clean air. Usually, a single air compressor can drive several pneumatic actuators at the same time which gives them an advantage over other types of actuators especially if the system has a large number of valves.

For example, in an electrical valve actuator a source of power (motor) has to be installed on each valve.

Why consider a Pneumatic Actuator?

Deciding which actuator is the best for a certain application depends on several factors like environment, required accuracy and closure/open speed. However the main difference between actuators is the thrust or torque they can produce. Here are general applications for the main types of actuators:

Hydraulic actuators: this type of actuator is powered by compressing a fluid like oil, it’s the most powerful and reliable type and recommended for heavy-duty and high load work environment due to the non-elasticity property of the fluid use to power them.

Pneumatic actuators: this type of actuators is powered by compressed gas. It’s suitable for medium-duty and average load work environment and recommended for fast moving, high accuracy applications. Due to the low operating pressures, pneumatic actuators are limited as to the amount of thrust or torque they can generate. Therefore, they are often used to automate smaller valves. High thrust or torque requirements can lead to large actuators.

Electrical actuators: this type of actuators is powered by electric current. It’s suitable for medium-duty and light load work environment and recommended for accurate control. They can be found in industries like food & beverage, pharmaceutical and in heavier industries like mining and power generation.

How do pneumatic valve actuators work?

A Pneumatic actuator typically consists of piston(s) fitted inside a hollow cylinder. Pressure is applied to a side of the piston inside the cylinder. As a result of pressure and the area of the piston, a force is generated that moves the piston along the axis of the cylinder, transferring such energy to the valve to be automated.

The piston then returns to its original position by a spring (fig1) also known as a spring-return actuator. Or compressed air being applied to the other side of the piston (fig2) known as double-acting actuators.

(fig1)

(fig2)

What valve types does a pneumatic actuator work with?

Generally, there are two types of motions that can be provided by a pneumatic actuator.

• Linear motion which is suitable for linear moving valves such as gate and globe valves. These actuators are usually spring-return. It moves back to its original position once air pressure is vented. The valve is either normally closed or normally open (as shown in fig3). For continuous control applications double-acting actuators are used. In those cases, compressed air is applied on both sides of the piston.

(fig3)

• Rotary motion which is suitable for quarter-turn valves such as ball and butterfly valves. These actuators can be spring-return. The actuator moves back to its original position once air pressure is vented. The valve is either normally closed or normally open (shown in fig4). For continuous control applications double-acting actuators are used. In those cases, compressed air is applied on both sides of the piston.

(fig4)

How to select a pneumatic actuator and size it for a certain valve?

Selection and sizing of a pneumatic actuator depends on three main factors

1. Type of motion required
2. Required control
3. Torque or thrust required

The flowing table shows selection and sizing factors

Factor

Application

Recommendation

Type of motion

Globe valve / gate valve

Linear motion actuator

Butterfly valve / ball valve

 Rotary motion actuator

Required control

Open / close valve position, isolation, or shutoff valves

Spring loaded single acting actuator for shutoff valves that need a failsafe action or double acting for non-critical isolation valves

Continuous variable valve position, control or modulation applications

Most commonly Dual port double acting actuator, single acting if a failsafe position is needed. High duty cycle

Torque

Liner along opening / closing movement

Rack and pinion actuator

High torque on start and end of motion

Scotch-yoke actuator

Torque values required for sizing

Operating torque alone is not enough for sizing actuator correctly, the following table shows torque values required.

Torque

Definition

Break torque (BTO)

Torque required to start opening a completely closed valve

Running torque (RTO or RTC)

Torque required to keep valve moving at a constant velocity between closed or opened positions

Seating torque (ETC)

Torque required to close the valve completely (sealed position)

Maximum shaft torque (MAST)

Maximum allowable torque on valve stem ( for safety )

Note that even valves of the same type and identical PN and DN values can have different torque values due to sealing type and materials.

Applications for pneumatic valve actuators in industry:

• Oil & Gas: drive valves for isolating and regulating flow in pipes
• Mining: drive valves for isolating and regulating pressure to nozzles in rocks washing lines
• Water and wastewater: drive valves for isolating and regulating drinking water feed lines and control sand filters and tanks levels

Internal parts and common malfunctions

(fig5)

Usually malfunctions in pneumatic actuators occurs due to two main reasons

• Loss of power due to pressure loss
• Wearing due to lack of lubrication

Following figures showing types of failures and causes:

(fig6) Wear on piston rings and groove due to lack of lubrication

(fig7) Wear on rack and pinion due to lack of lubrication

(fig8) Hollow cylinder of pneumatic actuator showing signs of wear

(fig9) Scotch and yoke pneumatic actuator Cowan series CSY, complete with controls