This bias towards conservative pipe sizes is necessary because designing with pipe schedules is so imprecise. Perhaps the greatest drawback to using pipe schedules is that the designer has no way of knowing exactly how the fire protection system will perform in the event of fire. He simply must hope that if he uses the pipe sizes specified by the schedules and locates sprinkler heads according to NFPA 13, that the system will perform as required when called upon. Ideally, a designer likes to know the exact flow rates and coverage he can expect for any section of the covered area of operation. It is little solace to the designer that he can defend his design as being in strict accordance with NFPA 13 schedules in the event of failure or inadequate operation.
Hydraulic calculations are the alternative to the use of pipe schedules.
The purpose of these calculations is to determine flow rates, water velocity, and pressure losses through each pipe section. In addition, the flow and residual pressure available at each sprinkler are calculated. Accurate hydraulic calculations show exactly how a sprinkler system will perform if called upon. The primary advantages of using hydraulic calculations are first, that there is much more certainty that the system will operate as designed, and secondly, that the hydraulically designed system will also tend to use smaller, more economical pipe sizes. The only problem with hydraulic calculations is that they are very tedious to perform, and for some systems virtually impossible to do by hand.