Process Control Valves
Overview and Selection Guide

Butterfly valves are similar to ball valves in that they operate by rotating an obstructor 90 degrees from open to closed position. In these valves, however, the obstructor is a thin disc that turns sideways to open. The disc is perpendicular to flow and completely obstructs the flow in the closed position, and when rotated 90 degrees, the flow travels around the thin profile of the sideways disc. These valves are commonly used as basic on/off valves, but are also suited for proportional flow control by stopping the disc at intermediate positions. When controlling processes on larger pipe sizes, weight and cost are lower on butterfly valves when compared to ball and plug valves of the same size.

Manually operated butterfly valves employ a basic lever at smaller sizes. These levers have various intermidiate positions that they "lock-in" to to ensure repeatability as well as prevent the disc from being rotated by the flow of the media. Larger size butterfly valves use a hand wheel and gearbox to ease the task of generating sufficient torque. a graduated dial on the gearbox will indicate the disc position in this case.

High Performance Butterfly Valves

Resilient Seated Butterfly Valves

These valves have rigid body with a soft seat. The seat acts as both the seal between the valve and pipes, and the seal between the valve and disc.

3-way Butterfly Valves

The geometry of these valves make it impossible for them to be 3-way. When processes with large diameter pipes require a valve with 3-way functionality, an assembly of two butterfly valves on a Tee fitting is used. The two valves are synchronized via a mechanical connection. This connection can be a rod, or a chain drive. See an example of a 3-way Butterfly Valve and Tee Assembly

Plug valves are practically identical to ball valves with the main differnce being the shape of the obstructor is a slightly tapered cylinder. They also operate by rotating the obstructor 90 degrees from open to closed position.

There are many advantages of plug valves over ball valves. One of the main advantages is that they can be adjusted while in use to fix stem leakage. Another advantage is the absence of cavities around the plug in which the media can become trapped. For a comprehensive white paper that details the differences between ball and plug valves click here.

Non-Lubricated Plug Valves

Sleeved Plug Valves

Pneumatic Actuators

Pneumatic valve actuators use air to generate rotational force in order to turn the valve stem which is attached to the ball, plug, or disc. The most common air pressures used are 60 and 80 psi. Higher air pressure will generate more torque.

There are 2 different styles of pneumatic actuators: Scotch Yoke and Rack-n-Pinion. Both of these can be either spring return or double acting. Spring return models use air in one direction and a spring in the other. This offers the benefit of a fail-safe position, that your valve will return to in the case of control air loss. Double acting models use air in both directions and remain in their current position upon loss of air.

In order to use a pneumatic actuator, you must have a method to control the supply air to the actuator. This is done using an electric solenoid valve. Well designed actuators have an industry standard NAMUR mounting pad that allows the mounting of the solenoid valve directly to the actuator. This type of mounting simply and accurately lines up the air ports of the actuator with the ports on the solenoid valve. See our NAMUR mount solenoid valves.

By themselves, these actuators are for on/off operation, meaning fully open or closed. They can be made to be modulating for proportional control by adding a positioner. These positioners can be controlled via air (pneumatic) or via electricity (electro-pneumatic). See our pneumatic actuator positioners.

Spring Return Rack-n-Pinion Actuator Operation

Note how the torque reduces from the beginning to the end of each stroke.

Double Acting Rack-n-Pinion Actuator Operation

Spring Return Scotch Yoke Actuator Operation

Note how the torque dips in the middle of each stroke and is a mirror image of itself when comparing the 2 strokes.

Double Acting Scotch Yoke Actuator Operation

Electric Actuators

Electric actuators are available in a variety of common voltages. They typically operate more slowly than pneumatic models. They do not normally have a fail-safe position, however they are available as an option on some models via a battery that will provide for a single actuation to the fail-safe position in the event of power loss.

An important consideration of electric actuators is the protection ratings. There are standards set by NEMA (National Electrical Manufacturers Association) for use of electrical components in different environments. For instance any actuator that is to be used outdoors, where it is exposed to weather, must meet the requirements for NEMA 4/4X to prevent the ingression of rain water. Another situation would be any environment with combustible vapors present. This would require a NEMA 7 rating which means that it would contain any sparks that may occur and prevent an explosion.

Electric actuators can be on/off or modulating. Modulating control is achieved by the addition of a circuit board inside the actuator. The modulation is controlled by an industry standard control signal input of either 4-20mA or 0-10VDC. Typically the low end (either 4mA or 0VDC) will drive the actuator to the closed position, and the high end (either 20mA or 10VDC) will drive it to the open position. Any signal in between will drive the actuator to the corresponding proportional position. This signal can be reversed on most models so that the low end is open, and the high end is closed.