Valves are used in pneumatic systems to control airflow by blocking or directing it to meet the needs of the system. A pneumatic valve can have many different configurations. Knowing how to specify which valve is necessary in a schematic is an important skill for any engineer working with pneumatic systems.
Symbol standards for Pneumatic valves are found in the standard ISO1219-1:2012 Fluid power systems and components – Graphical symbols and circuit diagrams. The ISO Standard replaces ANSI Y32.10 Fluid Power Graphic Symbols.
Common Types of Pneumatic Valves
Much like specific electrical components are used to control and protect the circuit, pneumatic systems utilize a variety of mechanical components in order to control and protect the pneumatic system. There are many different types of valves used to control air flow by preventing flow, directing flow, controlling velocity, or relieving excess pressure. A few of the most common types are described below:
Check valves allow free flow of air in one direction but block flow in the opposite direction. They are passive because they require no external input to function. In a sense, they function like a diode in an electric circuit.
Spring-Assisted Check Valve
Basic Check Valve
A pressure relief valve is used to relieve excess pressure in a pneumatic system. Their purpose is to open automatically at a specific pressure and to continue to remain open until the volume pressure drops below the set point. These valves are used as a fail-safe measure to prevent pneumatic component failure resulting from excess pressure due to a control valve malfunction, temperature increase, etc.
A shuttle valve allows fluid to flow through it from two different sources, one at a time. It functions as a logical OR gate. When the flow is applied to one input the other input is closed.
Quick Exhaust Valve
A quick exhaust valve is similar in form to a shuttle valve but instead of two inputs the quick exhaust valve has one (1) input, one (1) outlet, and one (1) exhaust. When pressure is applied to the input, the air flows to the outlet. When the pressure is removed, air flows from the outlet to the exhaust.
Flow Control Valve
Flow control valves are used to control the velocity of air through a pneumatic system. These are often used to control the speed of a pneumatic cylinder to meet a specific application. Flow control valves can be either Fixed Orifice or Adjustable depending on the application. The adjustable version uses a needle valve to control the flow of air.
Fixed Orifice Flow Control Valve
Adjustable Flow Control Valve
Adjustable Flow Control Valve
Directional Control Valve
Symbols representing Directional Control Valves contain information about the valve that they represent. They show the number of positions, the methods of actuation, the number of ports, and the paths that the air can take.
Directional Control Valves
The remainder of this post will concern directional air control valves. These are available in many configurations. The function of a valve is given by two numbers (e.g. 3/2). The first number indicates the number of ports, NOT including pilot feeds or signal ports. The second number indicates the number of valve positions (e.g. on, off, etc.). Because directional control valve symbols communicate more information than the previously mentioned valves, they are necessarily more complex.
Symbols representing Directional Control Valves contain information about the valve that they represent. They show the number of positions, the methods of actuation, the number of ports and the paths that the air can take.
The number of points entering or exiting the flow box represents the number of physical ports on the valve. The number of flow boxes in a valve symbol represents the number of positions of the valve, and each box MUST have the same number of ports. A directional control valve is designated as shown in the example below:
Note: directional control valves are often designated by the number of “ways” in the valve. This term is often misapplied to the last number in the valve designation (i.e. 3/2 way valve). This use is INCORRECT. Ways refer to the number of paths the fluid can take through the valve. For example, a 3-port valve has 2 ways or 2 paths that the fluid can follow (i.e. from the source to the output and from the output to the exhaust). Because the number of “ways” and the number of “valve positions” happens to be the same, the term gets confused. Regardless, it is often used incorrectly by manufacturers and distributors, so it is wise to be aware of what the terms may indicate, right or wrong. THE SAFE APPROACH is to determine the number of required ports and the number of required positions then, using the symbols associated with specific valves (manufacturer specification sheets), find the valve that performs to your specification.
Below are some examples of 2, 3, 4 and 5 port flow boxes:
Flow Lines (Key)
Ports are labeled on the “Normal” position flow box. The remaining flow box ports match the corresponding port location on the “Normal” flow box. Port labels can be letters as well as numbers. The following is a general list of what each letter means if used in a valve symbol:
- (P) – Pressure Inlet Port
- (A) – Outlet Port
- (B) – Outlet Port
- (R) – Exhaust Port
- (S) – Exhaust Port
Using Directional Control Valves in a Schematic
Valve symbols in a schematic are drawn in their normal or default position. The default position can be either Normally Open (NO), or normally closed (NC). Normally open indicates that the default state of the valve will allow fluid flow. Normally closed means the opposite or that fluid flow is hindered in this state:
Normal State (3/2, N.C. valve)
Actuated State (3/2, N.C. valve)
Flow lines in the schematic must be connected to the ports in the normal position flow box, not the actuated position box. The valve symbol can be visualized as moving from one flow box to another when moving from one state to another. The port connections remain the same from one flow box to another and the flow lines within the flow box change to indicate the actuated flow of the valve.
Actuators are used to switch a valve from one position to another. There are many types of actuators, some of which can be combined on a single valve. Each actuator has its own symbol which attempts to illustrate its function. Actuator symbols are attached to the valve flow box that would become active if the actuator were triggered.
Note: The actuators shown below do not represent a comprehensive list, nor does every actuator symbol shown match exactly the symbols used by manufacturers or schematic designers.
The manual symbol means that switching states or positions in the valve is a manual process. This might include a button, a switch, or a lever activated by direct human intervention.
A pushbutton actuator is a specialized manual actuator. It is generally combined with a spring return so that the valve is default in one position and momentarily actuated (changes position) when the button is pressed.
A special case of manual valves is a “latching” valve (generally with a push/pull-button actuator). These are often used as emergency stop switches and require a secondary motion to actuate or “reset” the valve.
A lever actuator is a specialized manual actuator. A lever can have two positions (i.e. on/off) or have multiple selectable positions when paired with a detent feature.
A pedal actuator is a specialized manual actuator intended to be activated with the operator’s foot.
A mechanical actuator utilizes some form of mechanical actuation to operate the valve. The actuator might be a roller, cam, lever, piston, etc. that is triggered without direct human intervention. These are generally used as mechanical switches on pneumatic systems such as factory automation and conveyor systems. An example of a mechanically actuated valve is shown below:
Electrically Controlled Valves
An electrically controlled valve is NOT activated by direct human input. It is tied to an external control system which activates the valves according to the requirements of the associated system and/or program.
A solenoid actuator is a small electrical coil which uses an electromagnet to change the valve position.
An internal pilot actuator uses an internal pilot valve (like a diaphragm) to change or maintain the position of a larger valve. The larger valve remains actuated as long as the main air pressure is present. When the pressure is lost, the internal pilot changes the position of the main valve. This type of pilot actuator can serve as a passive means of actuating an in-line valve, generally as a failsafe.
An external pilot actuator also uses a smaller pilot valve to change the position of a larger valve. The larger valve changes position when a signal pressure or flow is reached. In this case, the pilot actuator is external to the system and can be physically located on or away from the larger valve.
The piloted solenoid is a common combination of actuators. In a piloted solenoid an electrical signal opens and closes the pilot valve which operates the larger valve using the pilot medium as the mechanism for forcing the actuation. This is commonly used to actuate large valves where a solenoid would have insufficient power to actuate the valve.
The following symbols are commonly used in combination with primary actuation symbols to indicate a more specialized function:
A detent feature provides clear physical delineation between valve positions. It is usually paired with a lever actuator.
A spring provides a restoring force to the valve. Generally, the spring side is the normal side, functioning to return the valve to “normal” in in a power-loss situation.
Valve Symbol Examples
Following are a few examples of directional valve symbols. Notice the variation in how portions of the symbols are drawn:
2/2 Valve with Lever Actuation
3/2 Valve with Push-Button Actuation
3/2 Valve with Pilot Actuation
5/3 Valve with Pilot Actuation and Closed Center
Recently Apollo designed a simple pneumatic system for a large prop on a game show. The system was designed to use large bore pneumatic cylinders that, when activated, engages the prop driving surface. The output force is proportional to the input pressure to the cylinders. The pneumatic schematic is shown below:
In the schematic above, there are 4 directional flow control valve symbols. Their purposes are as follows:
- A 3/2 manual lever valve which isolates the system from the compressor when in its normal state. When activated it allows air to flow from the compressor to the cylinders. This valve also exhausts when in its normal state. Because the exhaust feature on this valve isn’t used in this system a 2/2 valve could have been selected. The reason this valve was chosen rather than a simpler 2/2 valve is because of its Lock Out Tag Out (LOTO) feature.
- A 4/2 manual lever valve that, when valve 1 is activated, will supply air to the cylinders. When this valve is in its normal state, it will supply air to the extend side of the cylinders and allow air from the retract side to exhaust. When this valve is actuated, the path for the air is switched, the cylinders are retracted, and the extend lines are exhausted.
- Valve 3 is a simple 2/2 lever valve that is normally closed. Its purpose is to allow a pressure transducer to be placed near the cylinder to monitor any pressure spikes that the system might see. When the transducer is in place, the valve is opened.
- Valve 4 has the same purpose as valve 3 but is near the second cylinder.
This system also features flow control valves, pressure relief valves, and a spring assisted check valves. The function of each of these valve types is discussed earlier in this post.