Sunday, December 16, 2012

Final Control Element: Control Valve

A control valve is simply a variable orifice that is used to regulate the flow of a process fluid according to the requirements of the process. 

 In a control valve, an actuator that is connected to the valve’s plug stem and moves the valve between the open and closed positions to regulate flow in the process. The valve body is mounted in the process fluid line and is used to control the flow of fluid in the process. The body of a control valve is generally defined as the part of the valve that comprises the main boundary, including the connecting ends. Valves are classified into two general types based on the movement of the valve’s closure part: linear and rotary.
Globe valve.

Types of Control Valves.

Though there are many kinds of valves, the most common types are globe, gate, diaphragm, butterfly, and ball valves.

The globe valve, which is of the linear movement type, is most common of these five types. In a globe valve, the plug is attached to a stem, which is moved linearly in a cavity with a somewhat globular shape to regulate flow (see Figure ).

A flat or wedge-shaped plate that is moved into or out of the flow path to control flow characterizes the gate valve. These valves are widely used for manual on/off service, but a few designs are used in throttling service.

Diaphragm valves are linear-motion valves with flexible diaphragms that serve as flow closure members. Diaphragm valves are mainly used with difficult fluids such as corrosive liquids or slurries. The valve body can be lined with glass, plastic, or Teflon. The diaphragm is normally rubber, but in some cases it is Teflon, which, however, requires a high closure force.

The butterfly is by far the most common rotary-motion control valve. Butterfly valves range in size from one-half inch to over two hundred inches. In the very large pipe sizes, the butterfly valve is the only cost- effective
solution for the control valve application.

The ball valve is also a rotary-motion valve. The part that closes the flow is a sphere with an internal passageway. The ball valve is the most widely used control valve after the globe valve. Advances in seal design and sealing material enable the ball valve to offer tight shutoff. Because of this feature it is now widely used in on/off service for batch processes.