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The Basic Hydraulic Calculations for Fire Sprinklers

When beginning hydraulic calculations for automatic sprinkler systems, assume in most cases that water is a clear fluid. Water should be calculated with a weight density of 62.4 pounds per cubic foot for basic hydraulic calculations.

  1. Specific Gravity

    • Specific gravity is a calculation used to modify the flow of water up columns throughout the sprinkler system. Calculate specific gravity using the formula Sg = Wa + Wb where Wa is the weight of an additive substance in pounds per cubic foot and Wb is the weight of water at 62.4 pounds per cubic foot.

    Theoretical Flow from an Orifice

    • The foundation flow formula is Q = av where cubed. Q equals the flow in cubic foot per second while a refers to the cross-sectional area in square feet and v equals velocity in feet per second. The value a is determined using a = πD² ÷ 4. The entire flow formula is Q = 60 x 7.48 x πd² ÷ 576 √2gh. The value g refers to acceleration due to gravity at 32.2 feet per second while h is the head in feet as determined by 2.31P where P equals psi or relevant pounds per square inch.

    Flow Conversions

    • Flow conversion calculations are used when other liquids, such as foam or antifreeze, are added to or used in place of water in the sprinkler system. The standard conversion formula is Q = K√P. When adjusted to account for the specific gravity of the new substance the formula becomes Q = K√(P ÷ Sg).

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