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The Hazen–Williams equation is an empirical relationship that relates the flow of water in a pipe with the physical properties of the pipe and the pressure drop caused by friction. It is used in the design of water pipe systems [ 1 ] such as fire sprinkler systems , [ 2 ] water supply networks , and irrigation systems.
Allen Hazen (August 28, 1869 – July 26, 1930) was an American civil engineer and an expert in hydraulics, flood control, water purification and sewage treatment.His career extended from 1888 to 1930, and he is, perhaps, best known for his contributions to hydraulics with the Hazen-Williams equation.
These three pressures, the static pressure, the stagnation pressure, and the residual pressure, along with a specified design pressure of 20psi taken from the American Water Works Association (AWWA) or the NFPA are input into the Hazen-Williams formula to calculate the available flow for fire protection.
Most design standards require application of the Hazen-Williams method for determining frictional pressure losses through the piping network as water passes through it. Tree and Loop systems are simple enough that the hydraulic calculations could be performed by hand.
The most common equation used to calculate major head losses is the Darcy–Weisbach equation. Older, more empirical approaches are the Hazen–Williams equation and the Prony equation. For relatively short pipe systems, with a relatively large number of bends and fittings, minor losses can easily exceed major losses.
n is 1.85 for Hazen-Williams and; n is 2 for Darcy–Weisbach. The clockwise specifier (c) means only the flows that are moving clockwise in our loop, while the counter-clockwise specifier (cc) is only the flows that are moving counter-clockwise. This adjustment doesn't solve the problem, since most networks have several loops.
The Hardy Cross method is an application of continuity of flow and continuity of potential to iteratively solve for flows in a pipe network. [1] In the case of pipe flow, conservation of flow means that the flow in is equal to the flow out at each junction in the pipe.
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