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Pressure head is a component of hydraulic head, in which it is combined with elevation head. When considering dynamic (flowing) systems, there is a third term needed: velocity head. Thus, the three terms of velocity head, elevation head, and pressure head appear in the head equation derived from the Bernoulli equation for incompressible fluids:
Hydraulic head or piezometric head is a measurement related to liquid pressure (normalized by specific weight) and the liquid elevation above a vertical datum. [ 1 ] [ 2 ] It is usually measured as an equivalent liquid surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer .
The meter is "read" as a differential pressure head in cm or inches of water and is equivalent to the difference in velocity head. The dynamic pressure, along with the static pressure and the pressure due to elevation, is used in Bernoulli's principle as an energy balance on a closed system.
Pressure head is the difference in pressure between the suction point and the discharge point, expressed as an equivalent height of fluid. Velocity head represents the kinetic energy of the fluid due to its bulk motion. Friction loss (or head loss) represents energy lost to friction as fluid flows through the pipe.
In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation: The Available NPSH (NPSH A): a measure of how close the fluid at a given point is to flashing, and so to cavitation. Technically it is the absolute pressure head minus the vapour pressure of the liquid.
Vessel dished ends are mostly used in storage or pressure vessels in industry. These ends, which in upright vessels are the bottom and the top, use less space than a hemisphere (which is the ideal form for pressure containments) while requiring only a slightly thicker wall.
The minus signs, in front of the right-hand sides, mean that the pressure (and hydraulic head) are larger after the pipe expansion. That this change in the pressures (and hydraulic heads), just before and after the pipe expansion, corresponds with an energy loss becomes clear when comparing with the results of Bernoulli's principle .
Constant head is important in simplifying constraint when measuring the movement of water in soil. Several measurement techniques employ the Mariotte's bottle to provide constant head. The Guelph Permeameter measures unsaturated hydraulic conductivity in the field and uses this principle to create a constant head. [3]