The coefficient of discharge of a fire hydrant when the outlet is smooth and well-rounded...

The coefficient of discharge of a fire hydrant when the outlet is smooth and well-rounded is 0.90. This is the highest coefficient given by NFPA 291 for fire hydrant outlets, and it means that the stream is almost as large as the outlet diameter. The coefficient of discharge is a factor that accounts for the difference between the actual flow and the theoretical flow from an orifice, based on the shape and condition of the outlet1. A lower coefficient means that the stream is more restricted and has a smaller cross-sectional area than the outlet. NFPA 291 provides three coefficients for fire hydrant outlets, depending on how the outlet projects into the barrel: 0.90 for smooth and well-rounded outlets, 0.80 for outlets with a hexagonal or square projection, and 0.70 for outlets with a long projection or adamaged edge2. The coefficient of discharge is used to calculate the flow rate from the pitot pressure measured at the centerline of the stream3. References:

Fire Fighting Foams - Chemguard

Guide to Hydrant Flow Testing | WSRB

New Fire Hydrant Flow Test Charts (PDF) - MeyerFire