MECHANICAL CONTRACTING | SPRINKLER SYSTEMS |
Random thoughts concerning fire sprinklers and the water that passes through them
BY THOMAS W. GARDNER P.E., FSFPE, LEED AP ALEX MUNGUIA P.E. APRIL M. MUSSER P.E.
B 50
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 formula. 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
Fire protection professionals should not only know what NFPA 13 requires for various hazards but they should also have a “feel” for the numbers if they are to truly understand how these systems can/will perform.
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 successively expanded hydraulic design criteria, such as area/density curves for different hazard severities. The advent of electronic calculators and personal computers made application 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 calculation 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 intermediate 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 professionals performing sprinkler system design is based on testing/measuring available water supplies (i.e., hydrant flow 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 performed such a test, it is obvious that the resulting data will have a significant margin of error.1 Unfortunately, many take that test
data as gospel (instead of an approximation) and prepare hydraulic calculations without regard to its accuracy. A hydrant water flow measurement of 890.2937 gpm implies a high level of measurement accuracy, which is usually not the case. 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 conservation 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 density 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.2 The good news is that the
successful performance of sprinkler systems designed with the Hazen- Williams formula demonstrates an acceptable degree of accuracy. The bad news is that fire protection professionals utilize calculators or computers and therefore report required flows and pressures of two (or more) decimal places. The calculations simply 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 pressures/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 system hydraulics, including the following: • Accuracy of the water supply
test data • Changes (degradation) in the
water supply over time • Corrosion of internal piping
surfaces over time • Building configuration changes
that may be detrimental to successful application of sprinkler spray • Human error To account for and protect from
these unknowns, many authorities having jurisdiction (AHJ) require a safety factor be applied to the hydraulic calculations. Many AHJs require safety factors that are a delta between the required pressure and the available pressure. Sometimes this is specified as a minimum fixed difference, as a percent of the total available pressure (at the demand flow) or as some combination thereof. Although well intended, an arbitrary safety factor irrespective of the slope of the water supply curve may not actually provide much “safety.” This is best illustrated in Figure 1, where a 5 psi difference provides a good
e Continued on p 52
1. Although use of hand held pitot tubes is still prevalent, today there are diffusers and digital flow meters with internally fixed pitot tubes which are more accurate and much easier to use. 2. NFPA 13 Automatic Sprinkler Systems Handbook, 2010 Edition, J. Lake, page 833.
phc june 2011
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