Air valves allow metered flow of fluid in one or both directions. Many of them allow for free flow in one direction and reduced or metered flow in the reverse direction. They are used in pneumatic circuits to regulate the rate of activation or exhaust of cylinders and other pneumatic devices.

Classification

There are several methods that can be used to classify valves, including the control mechanism and valve function. 

Valve Function

Valves are a part of many daily- used machines and can perform a variety of functions. The three common valve functions include stopping and starting flow, throttling (control) flow, and acting as a non-return check for flow (check).

• Stop/Start valves are used for systems that do not need the flow throttled. The valve opens to allow the flow and closes to stop flow.
• Throttle or control valves control the speed and capacity of flow through the system.

• Non-return or check valves control the direction of flow. Flow in the desired direction opens the valve, while flow in the opposing direction forces the valve closed. These valves are important for preventing back-flow to systems in applications such as wastewater management. 

Method of Control

The mechanism to control flow can vary based on the application of the valve. In general, there are two means of controlling flow through a valve.

Linear motion valves use a closure member that moves in a straight line to allow, stop or throttle the flow. The closure device could be a disc, slat, or flexible material like a diaphragm. The closure device can be used to:

• Move a disc or plug into or against an orifice
• Slide a slat, cylindrical, or spherical surface across an orifice
• Move a flexible material into the flow passage

Rotary motion valves rotate a disc or ellipse about an angular or circular shaft extending across the diameter of an orifice.

• Quarter turn valves will be in their fully open or fully closed state after a 90° turn of the stem. 

Types of Air Valves

There are many types of air valves.

• Ball valves provide tight shut-offs, but are not suitable for sanitary applications.
• Butterfly valves permit flow in only one direction.
• Check valves are self-actuating and prevent the reversal of process flow.
• Diaphragm valves separate the flow of water from the closure element.
• Directional valves steer flow through selected passages.
• Float valves open or close automatically as the level of a fluid changes.
• Gate or knife valves are linear motion valves in which a closure element slides into the flow to shut off the stream.
• Globe and pinch valves are other types of linear motion devices.
• Needle valves have a slender, tapered point at the end of a valve stem.
• Poppet valves open and close ports with a sealing device and spring.
• Plug or stop-cock valves are designed for both on/off and throttling functions.

• Other types of water valves include pressure relief valves, shut off valves, solenoid valves, and toggle valves.

Valve Construction

There are many types of valves which can be used for air applications. Each valve type has a unique construction to ensure proper operation in its specific application. For a more extensive description on the components of a valve please see the Industrial Valves Selection Guide. Basic components are listed below.

The body of the valve holds the parts together. Air valve bodies can be configured in-line or at right angles with the flow system. In an in-line configuration the input and output ports are on the same axis and in a right angle configuration the input and output ports are perpendicular.

The bonnet is the cover for the opening in the body. The bonnet can also support internal valve parts, such as the stem, disk, and actuator.

Trim is a term used for the replaceable internal parts such as the disk, seat, stem, and sleeves used to guide the stem. The trim is responsible for the basic motions and flow control features of the valve.

The disk and seat provide the capability for permitting and prohibiting fluid flow. The system is under full pressure when the disk is closed. The seat provides a surface for the disk to seal to in order to stop the flow.

The stem is responsible for the movement of the disk, plug or the ball for opening or closing the valve. It is usually forged and connected to the valve hand-wheel, actuator, or the lever by threading. The stem moves the disc in a linear or rotary movement to open or close the valve.

Stem packing is used to form a tight seal between the stem and the bonnet. The packing is fitted with one of several components: a gland follower, a gland, stuffing box, packing material, or a backseat. Packing is important in preventing damage to the stem and fluid or gas loss.

The yoke and yoke nut are used to connect the body with the actuating mechanism. The yoke must be strong enough to withstand the forces, movements, and torque developed by the actuator. The nut is used to control the movement of the stem.

Air Valve Actuation 

The valve actuator operates the stem and disk to open and close the valve. There are several types of actuators depending on the needs of the system such as the torque necessary to operate the valve, speed, and the need for automatic actuation. Some water valves are actuated manually, by a hand wheel or crank, or with mechanical devices such floats and cams. Others are actuated by electric, pneumatic, electro-hydraulic, or hydraulic methods.

Manual/hand operated actuators use a hand-wheel or crank to open or close the valve. They are not automatic but offer the user the ability to position the valve as needed. Manual actuators are used in remote systems that may not have access to power, however they are not practical for applications that involve large valves. The hand-wheel can be fixed to a stem or hammer which allows for the valve to be pounded open or closed if necessary. Gear heads can be added for additional mechanical advantage and open/close speed.

Electric motor actuators permit manual, semi-automatic, and automatic operation of the valve. The motor is usually reversible and used for open and close functions. The high speed motor is connected through a gear train to reduce the motor speed and thereby increase the torque. The actuator is operated either by the position of the valve or by the torque of the motor.  A limit switch can be included to automatically stop the motor at fully open and fully closed.

Solenoid operated valves use hydraulic fluid for automatic control of valve opening or closing. Manual valves can also be used for controlling the hydraulic fluid; thus providing semi-automatic operation. A solenoid is a designed electromagnet. When an electric current is applied, a magnetic field is generated around the wire. An iron "T" or plunger is put in the center of the coil to concentrate the magnetism. Since iron is a strong magnetic conductor and air is not, the "T" is drawn by the magnetic field into a position where the magnetism can travel 100% through the metal conductor. The moveable "T" acts as the actuator of the valve. Solenoid valves can be arranged such that power to the solenoid either opens or closes the valve. One application of solenoid valves is to supply the air to systems like pneumatic valve actuators. These valves are not practical for large systems because their size and power requirements would be excessive.

Pneumatic operated valves can be automatic or semi-automatic. They function by translating an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. Pneumatic actuators are fast-acting for use in throttle valves and for open-close positioning.

Hydraulic actuators provide for semi-automatic or automatic positioning of the valve. They are used when a large force is required to open the valve, such as a main steam valve. With no fluid pressure, the spring force holds the valve in the closed position. Fluid enters the chamber, changing the pressure. When the force is greater than the spring force, the piston moves upward and valve opens. To close the valve, hydraulic fluid (such as water or oil) is fed to either side of the piston while the other side is drained or bled.

Self-actuated valves use the system fluid to position the valve. These are commonly found in relief valves, safety valves, check valves, and steam traps. Because these actuators use the fluid in the system, no external power is required.

Connection Types

There are many connection types for air valves. Examples include compression fittings, bolt flanges, clamp flanges, union connections, tube fittings, butt welds, and socket welds. Mounting choices for air valves include cartridge, subplate or manifold, pipe or line mount, port mounted, or stacked or sandwich mount. The connection type should be based on pre-existing fittings in the system as well as the seal tightness required.

Performance Specifications

Performance specifications include flow control and flow compensation. Also critical to air valve function in the valve size, pressure rating, media temperature, and valve flow coefficient. For more detailed information on the performance specifications of valves, please see the Industrial Valves Specification Guide.

Air Valve Flow Control

Flow control describes the number of directions the flow will travel as it exits the valve. This is chosen based on the pre-existing system conditions and the requirement for the valve application. Flow control choices for air valves include unidirectional (one-way), bi-directional (two-way) or three-way. 

• In a one-way configuration the valve permits metered (restricted) flow in one direction and free flow in the reverse direction. 
• In a two-way configuration the valve provides for metered (restricted) flow in both directions. 
• In a three-way configuration the control provides flow control in three directions. 

Flow Compensation

Flow compensation is an important consideration when selecting an air valve. Choices include non-compensated, pressure compensated, and temperature compensated.

• In a non-compensated flow control the amount of flow that passes through the orifice and the pressure drop across it are directly related. Flow through the valve varies with the fluid viscosity and pressure across the valve. 
• A pressure compensated flow control maintains accurate output flow regardless of the input pressure, up to rated limits. The orifice is adjusted to compensate for pressure variations and maintain a set flow rate. 
• A temperature compensated flow control maintains accurate output flow regardless of the input temperature, up to rated limits.

Sharp-edged orifices or expansion rates of dissimilar metals are used to maintain a constant flow rate.  

Features

• An integral relief valve, which is pressure limiting.
• An integral bypass, which controls by diverting or bypassing excess flow. 
• A check valve, which offers resistance to flow in only one direction. 
• A meter-in system that controls the flow into the actuator or a meter-out system that controls the flow out of the actuator outlet.
• Tamper-proof controls, which have a feature such as a removable knob or lock to prevent inadvertent or unauthorized adjustment.
• A locking nut on the adjustment stem to lock the metering setting. 
• Swivel air valves, which rotate or swivel for alignment on a cylinder in any orientation. 

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