MIL-HDBK-1164
Head, Pressure. The head represented by the expression, p/w, where p is pressure
and w is weight. When p is in pounds per square foot and w is the weight of the liquid
per cubic foot, h become head in feet.
Head, Static. The vertical distance between the free level of the source of supply, and
the level of the free discharge surface.
Head, Static Suction. The vertical distance from the free surface level of the source of
supply to the center line of the pump.
Head, Total. The difference in elevation between the surface of water at the source of
supply and the elevation of the water at the outlet at the source of supply and the
elevation of the water at the outlet (static head), plus velocity head, and friction head.
Head, Velocity. The theoretical vertical height through which a liquid body may be
raised due to its kinetic energy. It is equal to the square of the velocity divided by twice
the acceleration due to gravity.
Hydraulic Grade. A line connecting all points representing the head in any system.
Frequently, it is mistakenly called hydraulic gradient, but it is simply a curve
representing line pressures at various points along a line or throughout a water system.
Hydraulic Radius. The right cross-sectional area of a stream of water divided by the
length of that part of its periphery in contact with its containing conduit; the ratio of area
to wetted perimeter.
Hydrogen-ion Concentration (pH). A measure of the acidity or alkalinity of a solution.
A value of seven is neutral; low numbers are acid, large numbers are alkaline. Strictly
speaking, pH is the negative logarithm of the hydrogen-ion concentration.
Hydrologic Cycle. The complete cycle of phenomena through which water passes,
beginning as atmospheric water vapor, passing into liquid or solid form as precipitation,
thence along or into the ground surface, and finally again return to the form of
atmospheric water vapor.
Impeller. A rotating set of vanes designed to produce the rotation of a mass of fluid.
The peripheral speed of the vane tips determines the head produced and the working
pressure of a pump. The rotating unit in a centrifugal pump.
Infiltration. (1) The flow or movement of water through the pores of a soil or other
porous medium. (2) The absorption of liquid water by the soil, either as it falls as
precipitation, or from a stream flowing over the surface. Also called seepage.
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