Thursday, January 16, 2014

Some Important Valve Terminology

valve terminologyAccessory: Device(s) mounted on an actuator to complement the actuators’ function, such as positioners, solenoids and limit switches.

Actuator: A pneumatic, electric, hydraulically powered “motor” supplying motion to open or close a valve.

Automated Valve: A valve and actuator package capable of responding to a remote pneumatic or electrical signal.

Capacity: Rate of flow through a valve under stated conditions.

Control Valve: A valve and actuator package responding to a variable pneumatic or electrical signal, providing modulation of the valve position and regulation of the process media.

Control Characteristics: The relationship between flow rate through a valve and the valve travel from full closed to full open. Control Characteristic can be described as inherent or as installed. The inherent characteristic exists under a normal tests where constant pressure differential is maintained. Installed characteristic occurs when the valve is installed in a live system, and where pressure differentials can vary depending on the overall system.

Conventional Disc: A symmetrical plate-like part that controls flow in common types of butterfly rotary valves. Very dynamic torques limit the opening percentages and controllability in throttling service.

Dead Band: The minimum change in signal change required to provide any detectable movement of the valve.

Eccentric Disc: A valve design where the valve/disc takes a slightly eccentric path on opening, allowing disengagement of the disc from the seat, reducing wear and friction.

Double Acting: An actuator in which power (typically air, gas or oil) is supplied to provide movement direction.

Fail Closed: A condition where the valve moves to the closed position upon power loss.

Fail Open: A condition where the valve moves to the open position upon power loss.

Fail Safe: A condition where the valve moves to the full closed position, fully open position, or remains in the last position necessary to protect the process. Fail-safe action may use auxiliary controls in association with the actuator.

Feedback Control: A control loop monitoring the process output and makes changes to the process input to maintain a desired process output.

Final Control Element: Typically the control valve.

Flow Characteristic: Relationship of flow through a valve and percent of open/closed travel from 0-100 percent. Should be designated as either the “inherent” flow characteristic or “installed” flow characteristic.

Flow Coefficients: Most often referring to Cv, defined as the number of U.S. gallons per minute of water at 60°F that will flow through a valve with a 1 psi pressure differential.

Head (pressure): The height of a body or column of liquid above a given point of reference. Used to express pressure as in “feet of water”. One foot of water is equal to .433 psi.

Hydrostatic Testing: A test on a valve body using a liquid (such as water) to verify structural integrity of the part.

Manual Valve: A valve using a handle or gear operator to change the valve position.

On/Off Control: A control action by a controller providing output only to fully open or fully close a valve. Of course, a controller need not be present to have on/off actuation of a valve.

Seat Leak Test: Seat leak testing confirms pressure rating of the valve disc and seat.

Step Response: The measurement of the ability of a valve and actuator to respond to a rapid change in control signal (air or electrical).

Tight Shut Off: In general, this means no visually detectable leakage will occur past the valve seat during test or after installation.

Torques: Rotary motion required by valves to open and close. Also the force provided by actuators to open and close valves. Required torque and available torque are important considerations when sizing actuators.

V-Ball: The most common type of ball control valve. The v-ball includes a partial sphere that rotates against the vale seat or seal. The v-shape ball permits broad rangeability and provides equal percentage flow characteristics.

Water Hammer: When valves closed quickly, water lines can cause a pressure surge in the line. When the amount of liquid and its velocity are high, combined with a fast closing speed, the pressure waves create shock in the pipe which may result in damage to pipe, fittings and valves.

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