MIL-HDBK-1164
Unit Weight. The weight of a unit volume of material, such as pounds per cubic foot or
per cubic inch.
Vacuum. Condition existing in a closed space from which all gas, vapor, or other
matter has been removed. Commonly applied to a closed space in which pressure is
much lower than surrounding atmospheric pressure; any space where negative
pressure exists.
Vacuum Breaker. A device for relieving a vacuum or partial vacuum formed in a
pipeline, thereby preventing back-siphonage.
Vacuum Filter. A filter consisting of a cylindrical drum mounted on a horizontal axis,
covered with a filter cloth, and revolving with a partial submergence in liquid slurry. A
vacuum is maintained under the cloth for the larger part of a revolution to extract
moisture. The cake is scraped off continuously.
Valve. A device installed in a pipeline for the purpose of controlling the magnitude and
direction of the flow. It consists essentially of a shell and a disc or plug fitted to the
shell.
Valve, Altitude Control. A valve which automatically shuts off the flow when the water
reaches a predetermined elevation in an elevated tank, and opens when the pressure
on the pump side is less than that on the tank side of the valve.
Valve, Check. A valve provided with a disc hinged on one edge, a spherical ball, or
other device so arranged that it opens in the direction of normal flow, and closes with
reversal of flow.
Valve, Gate. A valve where the closing element consists of a disc which slides over the
opening or cross-sectional area through which water passes, and fits tightly against it.
Valve, Needle. A valve with a circular outlet through which the flow is controlled by
means of a tapered needle which extends through the outlet, reducing the area of the
outlet as it extrudes, and enlarging the area as it retreats.
Valve, Pressure-Regulating. A valve placed at either end of a pressure-regulating
apparatus inserted in a water main to regulate the pressure in a waterline either
upstream or downstream from the valve.
Valve, Four-Way. A valve constructed with four waterways and with a movable element
operated by a quarter turn to provide passage between either pair of adjacent
waterways.
Vaporization. The process by which water changes from the liquid or sublimation to the
gaseous state.
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