Velocity in pipe

 

Some design authorities limit the velocity through pipes and valves in 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 and EN 16925 limit 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