This equation is used for the design of pipes under pressure.
Typical values of C are:
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Extremely smooth and straight pipes 140
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New, smooth cast iron pipes 130
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Average cast iron, new riveted steel pipes 110
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Vitrified sewer pipes 110
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Cast iron pipes, some years in service 100
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Case iron pipes, in bad condition 80
(Taken from: Schaum's outline series, Fluid Mechanics & Hydraulics 2nd Ed.)