Fire Sprinklers and the Water That Passes Through Them
By Thomas W. Gardner, P.E., FSFPE, LEED AP; Alex Munguia, P.E.; and April M. Musser, P.E. B
efore hydraulic calculations, fire sprinkler systems were designed by the pipe schedule method, which limits the number of sprinklers supplied by piping of
a specific diameter. In 1903, after studying recorded friction loss measurements produced by dozens of experimenters, Allen Hazen and Gardner Williams published an empirical formula now known as the Hazen-Williams friction loss for- mula. Until the early 1970s, using this friction loss formula was tedious, requiring the use of logarithms and slide rules. Hydraulic calculations were first introduced into NFPA
13 Standard for the Installation of Sprinkler Systems in the 1966 edition. In 1972, the concept of sizing system piping and water supplies based on density and area of expected sprinkler operation was introduced. Between 1966 and 1978, the standard was revised four times to include succes- sively expanded hydraulic design criteria, such as area/den- sity curves for different hazard severities. The advent of electronic calculators and personal computers made applica- tion of the Hazen-Williams formula routine and, as a result, hydraulically designed systems eventually became the norm. This article reviews and discusses selected water supply and hydraulic issues concerning fire sprinkler systems.
Significant digits The significant digits of a number are those digits that
carry meaning contributing to its precision. For example, 1.4136 carries more significant digits and precision than say 1.2. The number of significant digits in an answer to a cal-
Figure 1: Impact of the variation in curve slope on safety factors
Another area where fire protection professionals must be
cognizant of significant digits is in the calculation of the sprinkler piping network. As mentioned above, the Hazen- Williams formula is an empirical formula and therefore not an exact derivation of mathematical and physical conserva- tion equations. Therefore, the Hazen-Williams equation has certain limitations, such as not being applicable to turbulent water flow. More accurate fluid flow formulas account for turbulence and the variation of fluid densities and viscosities over a range of temperatures. NFPA 13 requires the Hazen- Williams formula for water-only systems because the densi- ty and viscosity of water do not significantly change over the range of temperature where water is used for fire protection and the effect of turbulence is extremely minor. The good news is that the successful performance of
culation will depend on the number of significant digits in the data used to derive the answer. Common significant digit mistakes in calculations include reporting more digits in an answer than justified by the number of digits in the data or rounding off to a smaller number of digits in an intermedi- ate calculation and then reporting even more digits in the final answer (e.g., rounding off to two digits in friction loss in each pipe, then adding up the friction loss of several pipes and stating a three-digit total). Much of the work required by fire protection profession-
als performing sprinkler system design is based on test- ing/measuring available water supplies (i.e., hydrant flow
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sprinkler systems designed with the Hazen-Williams formu- la demonstrates an acceptable degree of accuracy. The bad news is that fire protection professionals utilize calculators or computers and therefore report required flows and pres- sures of two (or more) decimal places. The calculations sim- ply don’t support the reported significant digits. In the book Sprinkler Hydraulics, Harold S. Wass makes this very point and suggests ignoring everything to the right of the decimal point. We suggest something similar: Round demand pres- sures/flows up to the next whole number and round supply pressures/flows down to the next whole number.
Calculation safety factors There are a number of unknowns concerning sprinkler Continued on page 38
May 2011
test and fire pump tests). Anyone who has ever held a pitot tube in a stream of water discharging from a fire hydrant knows the difficulty of trying to keep the pitot in the correct position and read the velocity pressure from the gauge as the needle “bounces” around, while wiping water off the face of the gauge and out of your eyes. Even if you have never per- formed such a test, it is obvious that the resulting data will have a significant margin of error. Unfortunately, many take that test data as gospel (instead of an approximation) and prepare hydraulic calculations without regard to its accura- cy. A hydrant water flow measurement of 890.2937 gpm implies a high level of measurement accuracy, which is usu- ally not the case.
Figure 2: Safety factor measurements
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