Some fire sprinkler design authorities such as EN 12845 limit the velocity through pipes and valves in fire sprinkler systems; this is the case with EN 12845 however NFPA and FM do not have any restriction.  The case for limiting velocity is that the Hazen-Williams formula is less accurate outside its normal range and equivalent pipe lengths for fittings, which are generally used, start to lose their validity.  Some authorities believe that velocity is self-limiting as pressure losses increase exponentially as velocities increase, so pipe sizes must be increased to make use of available water supply pressure.

EN 12845 limits velocity to 6 m/s through valves and flow switches and 10 m/s at any other point in the system.

Velocity in pipe can be calculated using the following formula:-

When:  

v = Velocity m/s
Q = flow of water in L/min
d = internal diameter of pipe in mm

The following table lists the maximum flows in litres per minute which can be obtained through steel pipework to EN 10255 specifications for both 6 m/s and 10 m/s.

Maximum flows through pipes for EN 12845 fire sprinkler systems