Pneumatic Valves and Actuators
What are pneumatic valves?
Also called directional control valves, pneumatic valves are used to control the flow of gas or air within a system (switch it on, switch it off or increase or reduce the flow). They are also often used to decrease pressure in a system with the help of pneumatic actuators, positioners and controllers. Pneumatic valves can be opened and closed with motorised or solenoid actuators, manually or electrically. They are commonly used in the construction and mining industries. Different pneumatic valves have different functions.
- Directional control valves manage flow direction in pneumatic circuits. They can stop, start and regulate airflow.
- Non-return valves let air flow in only one direction and block it from flowing back upstream. Pneumatic non-return valves include shuttle valves, check valves, two pressure valves and quick exhaust valves.
- Flow control valves maintain a set volume of airflow.
- Pressure control valves control air pressure and include pressure sequence valves, pressure limiting valves and pressure reducing valves.
What are pneumatic actuators?
Also called air cylinders, pneumatic cylinders and air actuators, pneumatic actuators convert compressed air or pressurised gas into mechanical motion. Pneumatic actuators are made up of a number of parts including a cylinder, piston or diaphragm and pneumatic actuated valves.
There are linear pneumatic actuators for linear mechanical motion and rotary pneumatic actuators for rotary mechanical motion. Linear pneumatic actuators are designed to work with angle seat control valves. Meanwhile, rotary pneumatic actuators are designed to work with quarter-turn valves.
How do pneumatic actuators work?
Pneumatic actuators use compressed air or pressurised gas to create energy. Once the pressure in a chamber is built up to a certain level, it moves a piston or a diaphragm, creating linear or rotary mechanical motion. Pneumatic actuators tend to be more reliable, durable and safe than other types of actuators since they do not need electricity or ignition. As such, they are used in a wide range of industries and applications including aviation, rail, combustion engines and production machinery.
Piston and diaphragm actuators
As their name suggests, piston actuators feature a piston that is pushed upward once enough compressed air enters a chamber. When this air is removed, the piston is forced back with a return spring. Diaphragm actuators come with a membrane instead of a piston.
Valve actuators use compressed air (pneumatic valve actuators), oil flow (hydraulic valve actuators) or electricity (electric valve actuators) to operate a valve. Valve actuators are either linear or rotary. They are also either direct acting or reverse acting. Valve actuators are direct acting if they are closed by air pressure and opened by spring action. They are reverse acting if they are opened by air pressure and closed by spring action.
Selecting a valve actuator should depend on a number of factors including valve type, operational environment and the available power source. Other things to take into account when selecting a valve actuator include cost, the required speed of actuation, life cycle and whether there is a need for a manual override function.
Pneumatic solenoid valves
Also called electrically-operated valves, pneumatic solenoid valves run on electromagnetic force. Pneumatic solenoid valves come with a solenoid coil, which uses electricity to create a magnetic field. This, in turn, moves a ferrous metal rod and opens the valve. Pneumatic solenoid valves are either open or closed when the solenoid is uncharged. They do not require manual or air control.
Also called electrically controlled regulators, proportional valves deliver proportional input and output flow or pressure. This means that if the input increases so does the output. If the valves are interconnected, the output from one proportional valve can act as the input for another.
Used to control high flows in piping systems, pilot valves open when they are placed under a certain level of fluid pressure. Pilot actuated, pilot valves are more durable than mechanically actuated valves. Pilot valves are often used to lower pressure within systems. Plus, they are safe to use in flammable environments since they do not create sparks or increase heat.
Often utilised in industrial automation systems, inline valves feature several ports. This means that multiple valves can be fitted into one base or manifold (inline valves are known as manifolds when used as bases). There are certain considerations that need to be taken into account when selecting an inline valve including the rate of flow, operating environment, valve function and mode of operation.
Quick exhaust valves
Quick exhaust valves are used to speed up the cycle of pneumatic cylinders. This can be achieved because quick exhaust valves release air from pneumatic cylinders into the atmosphere so it does not have to make its way through the valve again. Quick exhaust valves are often used in the oil and gas and offshore industries.
Pneumatic Fitting Materials
Pneumatic fittings are made from a variety of materials, including metals, plastics and composites.
- Aluminium: Corrosion-resistant, light weight
- Steel: Strong and heat resistant, often alloyed with other materials, such as zinc, for better corrosion resistance.
- Stainless Steel: Strong and corrosion-resistant, usually more expensive than steel.
- Brass: Strong and corrosion-resistant, great conductivity, often used in threaded pneumatic fittings.
- Polytetrafluoroethylene and Polyvinylidene Fluoride: Polymers resistant to extreme temperatures of up to 250 degrees Celsius, resistant to chemicals.
- Polypropylene: Strong and compatible with a variety of materials, UV-resistant, affordable.
- Composites: Made from a blend of materials with high melting points including fibreglass, carbon and graphite, low thermal and electrical conductivity, resistant to chemicals.
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Two-way pneumatic valves come with only two ports. The air can only move through the valve if both ports are open. Three-way pneumatic valves are very similar to two-way pneumatic valves with the only exception being an extra port that is used as an air exhaust (the other two are used to connect the valve to compressed air and an actuator). Three-way pneumatic valves can be used to control both spring-return cylinders and single-acting cylinders.
Four-way pneumatic valves feature four ports — one that connects to compressed air, one that acts as an exhaust and two that connect to actuators. This configuration makes it possible for the valve to reverse the motion of a motor or a cylinder. If dual pressure is required, an additional port can be added to four-way pneumatic actuators as an extra exhaust port.
Pneumatic actuators are powered by compressed air. While they do not need a motor, they can be dependent on electricity when coupled with pilot valves or integral solenoid valves. Electric actuators are powered by motors and are ideal for environments without access to compressed air. Pneumatic actuators are durable and resistant to moisture and overheating. They also have a faster cycle time than electric actuators. Pneumatic actuators are often used in the power processing and the automotive, oil and gas and pharmaceutical industries. Meanwhile, electric actuators are commonly used in irrigation systems, manufacturing and food-service equipment.
Double-acting pneumatic actuators use compressed air to move a piston in both directions. Spring-return pneumatic actuators use air to move a piston in only one direction. An in-built spring then pushes the piston in the other direction. Spring-return pneumatic actuators are usually larger than double-acting pneumatic actuators.
A number of factors need to be taken into consideration before selecting a pneumatic solenoid valve. These include:
- The pressure of the line and the power required.
- The speed at which the valve needs to open and close (direct-acting valves are faster to open and close than piloted valves).
- What should be the default position of the valve (will the airflow be mostly opened or closed).
- Port size and the number of ports.
A number of factors need to be taken into consideration before selecting a pneumatic actuator. These include:
Whether you require linear or rotary motion.
- Should the actuator be single-acting or double-acting.
- The level of pressure and flow.
- Operating environment and actuator size.