Velocity Calculator
Velocity limits by design standard
Under EN 12845:2015+A1:2019, velocity must not exceed 6 m/s through valves and flow switches, and 10 m/s at any other point in the system.
FM Global Data Sheet 2-0 imposes a different constraint: the Hazen-Williams equation is valid only up to 9.1 m/s (30 ft/s). Above that, Darcy-Weisbach must be used instead. NFPA does not set an explicit velocity limit but is subject to the same Hazen-Williams ceiling.
These limits exist because Hazen-Williams loses accuracy outside its intended range. In practice, designers increase pipe diameter to maintain acceptable pressure loss. This approach keeps velocity within limits as a consequence rather than as a primary design objective.
Worked Example
A sprinkler range pipe carries 350 L/min at a flow rate through a pipe with a 65 mm internal diameter.
v = 21.5 × Q / d² = 21.5 × 350 / 65² = 21.5 × 350 / 4,225 ≈ 1.78 m/s
Well within the EN 12845 limit of 10 m/s, and the FM Global Hazen-Williams ceiling of 9.1 m/s.
For the full breakdown of limits by standard, see Water Velocity and limits for different design authorities.
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