Union.
Union. A fitting designed to unite the two screwed ends of a pipe, with a single nut to secure them.
Vacuum. A void space; an inclosed chamber from which the air (or other gas) has been very nearly removed, as by an air pump.
Valve. Any device or appliance used to control the flow of a liquid, vapor, or gas, or loose material, through a pipe, outlet or inlet; the term includes air, gas, steam and water-cocks of all kinds; water-gates, air-gates, etc. One hundred and fifty of such devices are named by Knight in his “Mechanical Dictionary.”
Viscosity. Glutinous, adhering, or sticky, as tar, gums, molasses. Internal friction or resistance to change of shape.
V thread. A thread on a rod or bolt cut in the form of a letter V.
Washer. A circular piece of leather, rubber, metal, or other material with a hole in its center, through which a rod or bolt may pass.
Water Arch. A chamber of plates or of pipes over the furnace door of brick set boilers to take the place of the usual cast iron or fire brick arch, and connected with the boiler to supply it with water. The feed water is often introduced through the water arch.