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Valve
A device which controls or isolates the flow of a liquid or gas in a conduit or pipeline.
Variable Orifice
A small variable profile valve put in a flow line and used with a pilot to restrict the flow into the pilot and make the pilot more or less sensitive to changing conditions.
VDS (Valve Data Sheet)
A data sheet defining the minimum level of a valve design requirements including the materials, testing, inspection and certification.
Vent Plug
A special pipe plug having a small allen-wrench operated vent valve. These special plugs are located at the bottom of most ball valves. With the line valve closed (and under pressure) the body cavity pressure can be vented through this small valve to check tightness of seat seals or to make minor repairs. Having vented the body pressure the vent plug may be removed to blow out debris and foreign material or to flush the body cavity. On some gate valves the vent plug is installed on the bonnet for the sole purpose of venting the body. Such valves have separate drain valves. See Block and Bleed and Drain Plug.
Venturi Valve
A reduced bore valve having a bore smaller in diameter than the inlet or outlet. The flow through a Venturi Valve will be reduced because of the smaller port.
Vickers Hardness Test
A standard method for measuring the hardness of metals particularly those with extremely hard surfaces. The surface is subjected to a standard pressure for a standard length of time by means of a pyramid-shaped diamond. The diagonal of the resulting indention is measured under a microscope and the Vickers Hardness value read from a conversion table.
Viscosity
A measure of the internal friction of a fluid or the resistance of a fluid to flow. Two fluids of identical specific gravity may have quite different viscosities.
Visual Examination Test (VT)
Examination of parts and equipment for visible defects in material and workmanship.
Volumetric NDE
Examination for internal material defects by methods such as radiographic and/or ultrasonic testing.
Voting Systems
Applied to high integrity control systems where a valve closure requires more than one command signal to operate. Typically 2 out of 3 to alleviate a single component failure.