How Do Pneumatic Valves Work?

Post By: Tom Rowse On: 15-06-2023 Read Time: 7 minutes - Guides - Pneumatics

A pneumatic system uses valves to distribute compressed air or gas. Pneumatic valves are used in the circuit to adjust pressure and control the direction of airflow. Working together, the various valve configurations control force, velocity and movement.

Pneumatic Valves

Pneumatic valves manage the airflow, from the source to the actuators. They act as a mediator, guiding and dosing the air supply using various locking and regulating elements. They’re controlled by either pneumatic, mechanical or electromagnetic signals – or sometimes a combination of these.

  • Pneumatically-operated valves work by using the build-up of air pressure to open or close them, using the same air supply that controls the system.
  • Mechanical pneumatic valves require some form of mechanical force to operate them, such as a lever or cam mechanism. They’re usually designed for simple tasks like opening and closing, and can function in different degrees such as quarter turns or 90° steps.
  • Electromagnetic pneumatic valves work with an electric solenoid coil, which generates a magnetic field that moves the valve stem.

Air Preparation Systems

Pressure relief valves prevent over-pressurisation. They control the pressure in things like accumulators or receivers, relieving the pressure at the inlet port by exhausting air to the atmosphere. Pressure regulators are also usually installed as part of an FRL (filter/regulator/lubrication) unit, limiting downstream pressure to actuators by blocking pressure upstream.

How Does Pneumatic Valves Work

Pressure regulators can be used to reduce and distribute pressure over a network of work ports. They’ll also control the air pressure of individual actuators, in the form of an inline regulator or one mounted onto a work port. These units include reverse-flow check valves, like those found in double-acting cylinders. Some manufacturers also offer differential pressure regulators, which establish a pre-set pressure differential that’s then maintained between the two ports.

Pneumatic Valve Types And Design

Flow Controls

Probably the most common type of valve in pneumatic systems is for controlling air flow. The design is quite simple, and you’ll usually find two distinct types used in two different ways. One type is configured as a needle or choke valve, which provides a variable restriction of the flow. With this valve, a variable orifice can be opened or closed by means of a knob or screw adjustment.

The other type is configured as a check valve, allowing air to flow freely in one direction, while restricting the flow in the opposite direction. This type of valve has gradually become known as a flow control valve. It can function on a meter-in or meter-out basis, adjusting either the incoming or outgoing pressure.

Meter-in controls the airflow rate as it enters the cylinder, allowing it to move rapidly with high efficiency and force. Meter-out controls the outflow rate, but reduces some of the cylinder’s dynamic force and efficiency. Despite this, meter-out flow control is used by most pneumatic applications, since it offers more stable and repeatable cylinder velocity. The loss of energy can be compensated by increasing upstream pressure.

How Do Pneumatic Flow Controls Work?

Flow control Flow control is achieved by the locking and regulating elements in the pneumatic system. The main actions required by the valves are a discrete action – i.e. on or off – and a smoothly continuous action where the parameters are adjustable. These actions require two main structural types of valve – piston and spool.

Discrete flow control uses piston valves to provide rapid isolation between the channels. In this configuration, the valve seat travels along the flow line. Their design typically has only two or three outputs, and is best suited to small systems doing simple tasks.

More complex flow control operations use sliding spool valves, which can manage complicated circuits and higher flow rates. They consist of a metal cylinder with metered notches, designed to slide inside a precisely-machined housing. This housing is drilled with port holes, usually from three to five. If the valve is pilot-operated, this number can increase to seven ports.

In this configuration, the spool travels perpendicular to the flow line. Spool valves are very versatile, and can work in simple circuits as well as systems with four or five working lines.

Directional Control Valves

Pneumatic directional valves come in many styles, configurations and sizes. The main functions of a valve that determine its type are:

  • How many flow lines or ports it has.
  • How many possible positions its control mechanism has.

The first figure in a valve description always indicates how many ports there are in the device (usually three to five). The second figure shows how many spool positions are available. For instance, a 3/2 pneumatic valve describes a valve with three ports and two available spool positions. Depending on these factors, pneumatic valves are named by common practice in ascending degree of complexity.

The ports are labelled in a specific order. If the valve has five ports:

  • Port 1 = pressure inlet.
  • Ports 2 and 4 = work ports.
  • Ports 3 and 5 = exhaust ports.

A pneumatic valve configured with three spool positions has an extend condition, a neutral condition and a retract condition. This combination of ports and positions describes a five-way, three-position valve, usually written as a 5/3 (or 5-3) valve.

Pneumatic Valve Configurations

The most common valve configurations found in pneumatics are 5/2, 3/2, 5/3, 4/2 and occasionally 2/2.

  • A diagram drawn with lines shows the number of connecting ports. Each port is designed for a specific operation, primarily to extend or retract the drive medium and to exhaust.
  • A diagram drawn with positions indicates the number of possible options for switching the valve position.

A directional valve’s description also includes its method of positioning and how that operates. The mechanism supplying the force to move the valve between positions is called the valve operator. This can be a spring (e.g. a 5/2 spring-offset valve), or using detents (e.g. a 5/2 detented valve).

  • Spring-operated valves, such as poppet valves, return to their starting position when the operator is deactivated, via the return spring. These are called monostable valves and work using a single external signal.
  • Detented valves stay in the position where they were last activated, held in place by the detent or latch. To return them to their original position, these valves need an additional return signal to release the detent, so two external signals (on/off) are required.

Two types of valve configuration denote the initial position of the spool in regard to the airflow:

  • Normally Open (NO) valves initially allow air to flow freely, and only close when a signal is received to close the port.
  • Normally Closed (NC) valves initially block the airflow, and only open when a signal is received to open the port.

Proportional Valves

Developments in pneumatic technology have seen the introduction of proportional valves. These are a combination of both directional and flow valves. They control the airflow pressure at the same time as controlling its direction. They work electrically, using pulse width modulation (PWM).

PWM maintains a continuous voltage, but varies the amount of current supplied to the valve. This controls the power of the magnetic field that’s generated by the coil, and in turn controls the amount of movement in the valve. Adjusting the distance a spool or poppet travels within its body causes a subsequent adjustment in the size of the airflow channel. This allows the airflow to be increased or reduced, according to demand.

Today’s proportional valves work by means of advanced electronics, using linear differential transducers to monitor the spool position. Rather than old-fashioned metering notches that throttle the airflow, these transducers can adjust the spool movement by microscopic distances. By means of sensors and electronic feedback signals, proportional valves can adjust the PWM output constantly to maintain the desired position.

Thanks to modern advances in valve technology, this type of valve is now achieving the efficiency level of a high-response valve. In the past, only servo valves could achieve this level of performance, but some proportional valves can now match it. These valves can apply different positions or steps to regulate flow output with great precision. They’re much more energy-efficient, as they require minimal power to keep the valve in its desired position.

Variety Of Options

The way that pneumatic valves work is becoming more sophisticated. While they’re still the main airflow and directional regulators in all pneumatic systems, the variety of ways they do this is vastly increased. Each type of valve has its own function and design. With this increasingly wide variety of options, you can find a pneumatic valve for every different control task in a pneumatic system.