Pneumatic valves control the rate, pressure, and flow direction of compressed air and gasses in industries. These valves control the air or gas at the source and regulate its movement into hoses, tubes, or devices as needed in an automated pneumatic system. Pneumatic valves are commonly available in various housing materials like stainless steel, brass, aluminum, and industrial-grade plastics like PVC and have seal materials like NBR, FKM, and EPDM. Selecting the suitable material for a valve is crucial for its working. This article discusses the various types of materials used in pneumatic valve

What is a pneumatic valve?

Pneumatics is the technology that makes use of gas or pressurized air. A pneumatic system utilizes a compressor to reduce the air volume to increase its pressure. The air (or gas) is filtered and passed through pneumatic hoses, pipes, or tubes. During this transit, the air is controlled by valves, after which it reaches an actuator like a cylinder or a device that performs a function, for example, moving, lifting, or gripping.

A pneumatic valve controls the pressure, flow rate, and direction of air and gasses. According to these applications, a pneumatic valve is classified into the following categories:

• Pneumatic pressure relief valves: Pressure relief valves keep the system pressure below a preset maximum value.
• Pneumatic flow control valves: Flow control valves are used in pneumatic systems to govern the rate at which a media (liquid or gas) is allowed to flow. Flow control valves can typically be unidirectional or bidirectional.
• Pneumatic directional control valves: Directional control valves like solenoid valves are used to start, stop, or direct airflow in a pneumatic system. By directing the flow of air, these valves control the action of other pneumatic devices like motors, cylinders, pumps, or other valves.

Know your process media

Pneumatic valves have two distinct contexts of use in general:

1. A pneumatic valve is a device used to control the flow of air or other gasses in a pneumatic system. The valve can be actuated manually, electrically through a solenoid, motorized, or pneumatically. In this case, the media being controlled is air or gas, and hence the materials chosen for the valve body and seal should be compliant with these media. Possible options include aluminum, stainless steel, brass, and industrial-grade plastics
2. Air can be used as a control mechanism for the valve, but in this case, the media flowing through the valve may be something other than air like oil, water, or other fluids. Here it is necessary to check how compliant the valve housing material is with respect to the various media. Read our article on the chemical resistance of materials to check the compatibility of various housing and seal materials with the media and environment used.

Driving pneumatic media or gas

The pneumatic media or gas (like an inert gas or CDA air) used in an automation control process is high conditioned, dried, and filtered. Therefore the valve design need not consider the aggressive media nature or account for the high purity level of the media. Industrial grade plastics (like PVC) can be used for these highly conditioned media.

Process fluids

Pneumatic valves for process fluids deal with highly acidic, corrosive, alkaline, or high purity media. Therefore, it is mandatory to select the construction materials suitable for protecting the media and the valve. Stainless steel is a great option for the valve housing material for aggressive and corrosive media. For neutral and non-corrosive media, brass is commonly used as the housing material.

Pneumatic valve housing materials

Always select the housing material suitable for the medium and the working environment. The housings of pneumatic valves for automation control are typically manufactured with a mix of different materials discussed below:

Industrial grade plastics (like PVC and nylon)

Plastic valves are lightweight, durable, and cost-effective. The material is suitable for pneumatic applications involving air and corrosive chemicals. But plastic valves have a low pressure and temperature rating compared to brass and stainless steel valves.

Brass

Brass is an alloy of copper and zinc, and it is an excellent forgeable and robust material. Brass is used in pneumatic applications involving non-corrosive gases. The material can withstand more heat compared to PVC but comes at a higher price. Brass can be easily welded, and it is more versatile when compared to stainless steel. Brass is commonly used to construct pneumatic valve bodies and end pieces.

Stainless steel (304/316)

Stainless steel is a very durable and resilient material, but it is more expensive than brass. Valves made out of stainless steel effectively resist leaks, and these valves can be operated at very high temperatures when compared to brass. Stainless steel is corrosion-resistant and lasts much longer compared to brass, and it is an ideal choice for high temperature and high-pressure pneumatic applications. Stainless steel is commonly used in the pneumatic valve bodies and trim materials like seats, discs, and wedges.

Aluminum

Aluminum is a lightweight metal that has approximately one-third the weight of steel. Aluminum is corrosion-resistant to atmospheric gases and hence suitable for pneumatic valve applications. The material is mainly used to construct the exterior components of the pneumatic valve, like the identification tags or handwheels.

Pneumatic valve housing material comparison table

Table 1: Comparison of different materials for pneumatic valve housings

	PVC	Brass	Stainless steel	Aluminum
Cost	Low	High	Very high	High
Durability	Average	High	Very high	High
Corrosion resistance	High	Average	Very high	Low-average
Operating temperature and pressure	Low	High	Very high	High
Weight	Light	Heavy	Heavy	Light

Pneumatic valve seal materials

Seals in pneumatic valves help prevent the escape of volatile and hazardous gasses into the atmosphere. The various types of seal materials used in pneumatic valves are the following:

NBR (Nitrile-butadiene rubber)

NBR has good resistance to compression and general wear and tear but is highly sensitive to weather changes. NBR is suitable for air and inert gasses but has poor resistance to ozone, ammonia, and steam. NBR seals can provide continuous sealing for gaseous media only at low temperatures compared to the sealing properties of FKM.

FKM (Viton)

FKM is typically used to manufacture O rings, gaskets, and seals for pneumatic valves. FKM seals have excellent resistance to the media, aging, and ozone. FKM is suitable for medium-high temperature pneumatic applications and has higher thermal resistance than PTFE. Also, FKM has superior strength, sealing capabilities, and flexibility when compared to PTFE. The material has excellent overall chemical resistance making it suitable for gaseous fuels, and the material shows more chemical resistance universally compared to NBR.

PTFE (Teflon)

PTFE is suitable for high temperature and pressure pneumatic applications. The material is non-elastic and has good resistance to wear and tear abrasion and most chemicals. PTFE offers superior resistance to gaseous chemicals as compared to FKM.

Materials for specific pneumatic valves

Directional control valves

Directional control valves are used in pneumatic systems to stop or direct the flow of compressed air or oil to their connected appliances. However, it is not recommended to use these valves for media other than air for most applications. Many pneumatic solenoid valves are piloted internally, and these valves vent a minimal amount of air required to actuate the valve. A small loss of air into the surroundings is acceptable in most applications but not in the case of oil, water, or other types of media. A few examples of materials for these valves that are compatible with the media and environment are:

• Valve body: Aluminium, Plastic, Brass, Stainless steel
• Seal: NBR, FKM, EPDM

Flow control valves

Pneumatic flow control valves are used to control the flow of media like a liquid or gas. These valves consist of a pneumatic actuator part and the valve part like a ball valve or butterfly valve. The actuator is mounted to the valve using a standard flange, allowing both the actuator and the valve portions to be swapped out with another one using the same flange size. Always choose the valve housing and seal materials to be compatible with the media used. A few options are:

• Valve body: Cast iron, Stainless steel
• Seal: EPDM

Pressure control valves

Pressure control valves reduce the pressure of incoming air to a set value at the output port. Brass is a viable choice for the valve’s body material, with a surface coating of nickel for added protection.

The material chosen for the connection cables and plugs used for transmitting the media to and from the valves should be compatible with the media chosen. Example:

• Valve body: Polyurethane (for media like air, carbon dioxide, nitrogen, fuels, and oils)
• Seal: EPDM

Pneumatic valve specifications

Knowing the valve materials is only one part of selecting a pneumatic valve. Some other factors to consider are discussed below.”

1. Operating medium: The media types that the pneumatic valve can effectively control (compressed air in most cases)
2. Flow capacity (Cv): Cv gives a measure of the valve capacity to move air through it
3. Operating pressure: The range of pressure (in Pa, bars, or psi) that the valve is rated to handle
4. Port size: The physical dimensional parameters that define the port sizes on the valve and the thread style
5. Rated voltage of coil: For electrically actuated valves, the required voltage rating may be given in AC or DC volts.
6. Response time: The amount of time required for the valve to switch states or positions once actuated.

Note: The discussed parameters are for general guidance only, and the individual valve suppliers and manufacturers may specify their valves differently.