NFPA Journal®, September/October 2009

The International Fire Sprinkler Association recently teamed up with the Society of Fire Protection Engineers to promote worldwide use of hydraulic calculations for fire sprinkler systems. The effort was more difficult than expected, because hydraulic calculations are now taken for granted. While training materials are readily available on the subject of "how" to calculate a sprinkler system, few address the issue of "why."

In a day when the green movement and sustainability have become priorities, the use of hydraulic calculations is more important than ever. Reducing pipe sizes saves materials and the energy needed to produce those materials. While the savings are usually not enough to justify the inclusion of a pump where not otherwise needed, hydraulic calculations are almost always economical in situations where a pump is to be provided anyway.

Much of the world still uses pipe schedule systems, where each pipe size is permitted to serve a specific number of downstream sprinklers. For example, the original 1896 edition of what is now NFPA 13, Installation of Sprinkler Systems, allowed a single sprinkler to be supplied by ¾-inch (2-centimeter) steel pipe, but required 1-inch (2.5-centimeter) steel pipe to serve two sprinklers on a line, 114-inch (3-centimeter) pipe to serve four sprinklers, and so on. Pipe schedules were developed over the years for various occupancy hazard classes, but in all cases were intended to ensure that the flows from sprinklers generated a reasonable degree of pressure loss through the piping, based on typical water supply pressures.

Of course, available water pressures are different for every system based on the strength of the municipal water supply or the choice of pumping arrangements. These differences are not taken into account with pipe schedule systems, but are considered through the use of hydraulic calculations.

Hydraulic calculation of sprinkler systems has actually been around for more than a century. John R. Freeman, one of the founders of the NFPA, tested sprinkler systems from 1893 to 1895 and established some of the basic concepts. But the complications associated with hydraulic calculations were not initially believed to be worthwhile. Although hydraulic design was briefly addressed in the 1955 edition of NFPA 13, the 1967 Handbook of Industrial Loss Prevention noted that "detailed calculations of the performance of sprinkler systems are ordinarily limited to infrequent instances where the occupancy may require greater than normal water density or where the adequacy of the available water supply is in question."

In 1964, however, Clyde Wood of the "Automatic" Sprinkler Corporation of America wrote a Study Guide for ‘Automatic’ Sprinkler Hydraulic Data, which he prefaced with this simple statement: "The purpose of fire protection hydraulic calculations is to enable the fire to be extinguished with the maximum efficiency, with a minimum cost of piping, apparatus and supply of extinguishing medium." Some detailed rules for accomplishing hydraulic calculations were added to the 1966 edition of NFPA 13, and density/area curves establishing system design criteria were added in 1972, making hydraulic calculations fully possible for the first time.

For the next two decades, hydraulic calculations shared the stage with pipe schedule systems, until the NFPA Sprinkler Committee observed that pipe schedule systems were only being used where hydraulically calculated systems were not economically viable, where water supplies were so poor that even the pressure losses associated with the pipe schedules were unacceptable. Beginning with the 1991 edition of NFPA 13, the use of pipe schedules was effectively limited to new systems of 5,000 square feet (1,524 square meters) or less, or for additions or modifications to existing pipe schedule systems.

Today, hydraulic calculations are essential to the use of new types of piping and sprinklers. They allow consideration of the flow characteristics of various piping materials and ensure the minimum pressures available to early suppression fast response sprinklers and other storage sprinklers.