Valves, mechanical devices able to control flow or pressure in a process or system, are as ubiquitous as any industrial process control element. As essential components of piping systems conveying liquid, gas, vapor, or slurry, valves are a component with which almost every industrial process and control engineer will require more than entry level familiarity. They are the controlling element in almost any fluid handling system. What are some of the very basic knowledge points for specifying and selecting a control valve?
There are numerous types of valves available, including butterfly, ball, check, globe, gate, diaphragm, plug, and control valves as the most common. Particular valve types can be better suited to the medium being controlled, or have functional capabilities making them a better selection for your process application. Within each type there will be a wide range of options and features that allow for close tailoring of the complete valve assembly to match the application requirements. Some valves can be self-operated, while others require manual operation. A pneumatic, hydraulic, or electric actuator can be employed on certain configurations to provide for remote control of the valve by a human operator or automatic controller.
General valve functions include:
Flow start or stop
Flow rate increase or reduction
Diversion of flow in another direction
Regulation of a flow or process pressure
Industrial process control valves are often classified according to their mechanical movement. Some common examples include:
Linear motion valves, in which the closure element moves in a straight (linear) direction to control the flow. Gate, globe, and diaphragm valves are in this category.
Rotary motion valves have a closure that follows an angular or circular path. Butterfly and ball valves are in this group.
Quarter turn valves, a subset of the rotary motion class, traverse from the open to closed state when the closure element (for example, the ball in a ball valve) is rotated through a quarter of a full turn. This type is best suited for operations calling for either fully open or closed regulation, with no need for control at points in between those two states.
Each industrial control valve application and installation will have its own set of very specific requirements. The goal of the specification and selection process should be to provide safe operation, low maintenance requirements, robust and accurate operation. A manufacturer’s sales engineer can be a useful source for application and specification information and assistance.