advertisement

BASIC OPERATION AND FUNCTION OF CONTROL VALVES

What is Control Valve?

A control valve is a valve that controls fluid flow by adjusting the size of the flow passage in response to a controller signal. This allows for direct control of flow rate and, as a result, control of process quantities such as pressure, temperature, and liquid level.

advertisement

Operation of Control Valve:

Automatic control valves are often opened and closed by electrical, hydraulic, or pneumatic actuators. Normally, valve positioners are used to ensure that a modulating valve, which can be set to any position between fully open and totally closed, achieves the required degree of opening.

Because they just require a compressed air supply, air-actuated valves are often employed, whereas electrically-operated valves require additional cabling and switch gear, and hydraulically-actuated valves require high pressure supply and return lines for the hydraulic fluid.

Terminologies of Control Valve

advertisement

A pneumatic hydraulic or electrically powered device that provides force and motion to open or close a valve is known as an actuator.

A regulator that controls the supply pressure to the valve actuator and its auxiliaries is known as an airset.

An angle valve is a valve design having one port collinear with the valve stem or actuator and the other port at a right angle to the valve stem.

Trim, anti-cavitation: See trim, anti-cavitation.

Trim, anti-noise: See "trim, anti-noise."

A bellows stem seal is a thin-walled, convoluted, flexible component that forms a seal between the stem and the bonnet or body, allowing stem movement while maintaining a hermetic seal.

Benchset: The calibration of a control valve's actuator spring range to account for in-service process stresses.

The body of the valve serves as the major pressure barrier, as well as the pipe connecting ends, the fluid flow channel, and supports the seating surfaces and the valve closure member.

The section of the valve that includes the packing box and stem seal and may guide the stem is known as the bonnet. It may also serve as the main entrance to the body cavity for the assembly of internal parts, or it may be an inherent element of the valve body. It may also include instructions for attaching the actuator to the valve body. Bonnets are typically fastened, threaded, welded to, pressure-sealed, or integrated to the body.

Butterfly valve: A valve having a circular body and a rotary motion disc closure component held pivotally by its shaft.

Cavitation is a two-stage liquid flow phenomena. The first stage is the formation of vapour bubbles within the liquid system as a result of the fluid's static pressure at the vena contract falling below the fluid vapour pressure; the second stage is the collapse or implosion of these cavities back into an all-liquid state as the fluid decelerates and static pressure is recovered.

flow, characteristic: See also "characteristic, intrinsic flow" and "characteristic, installed flow."

- distinguishing feature, equal percentage: An intrinsic flow characteristic that, ideally, results in equal percentage changes in the existing flow coefficient for equal increments of rated trip (Cv).

- intrinsic characteristic: The relationship between the flow coefficient (Cv) and the closure member travel when the valve is moved from closed to rated travel with a constant pressure drop across the valve.

advertisement