MECHANICAL CONTRACTING e Continued from p 50
safety factor compared to Supply Curve B (280 gpm) but an almost insignificant safety factor compared to Supply curve A (40 gpm). Some question whether a safety
engineering, and inspection/testing/maintenance, sprinklers systems have been successful due to “built in” safety factors including the following3:
and 602) and so on until all 17 sprinklers were flowing. The results are presented in Table 1. This analysis indicates that the first
sprinkler to operate provides 260 percent of the minimum design density. As more and more sprinklers operate, the density provided by each sprinkler naturally decreases, but, even with all 17 sprinklers operating, the most remote sprinkler still provides 110 percent of the design density. Not until the fourth sprinkler operates does any sprinkler deliver less than 0.2 gpm/sq.ft. With the entire density area flowing, the average density is 0.21 gpm/sq.ft., with the maximum density at Node 604 of 0.32 gpm/sq.ft.
Accounting for hose stream allowances
Figure 1: Impact of the variation in curve slope on safety factors 52
factor should be an amount of pressure or an amount of flow between the sprinkler system demand and the water supply curve. Since system flow and pressure are interrelated, the authors suggest that the safety factor should be the length of the line between the sprinkler system demand point and the point where the demand curve intersects the supply curve (see Figure 2). Equation 1 can be used to calculate the intersection of these two lines on a N1.85 logarithmic graph. Using this approach, a safety factor can be
Equation 1 Where: PJ =Pressure at junction point (psi) PS=Static pressure from flow test (psi) PR=Residual pressure from flow test (psi) QF=Flow from flow test (gpm) QJ=Flow at junction point
specified as a pressure or flow and will have meaning because it is measured as the pressure or flow component of the sloped line between demand and supply curves.
Inherent sprinkler system safety factors
Fire sprinkler systems have
enjoyed an enviable track record since the first sprinkler was invented in 1874. Beyond regulation, good
1. Initial densities are higher due
to the descending supply curve. 2. Calculations are started with the
design density requirement at the end sprinkler and inherently the average density in the system will be higher. 3. The hydraulically most remote
areas are calculated; any other configuration of sprinkler operation will produce higher delivered densities. 4. Calculations are developed on a
rectangular pattern, which is the most severe condition. 5. The friction coefficient for wet-
pip systems will probably average higher than the calculated C=120 resulting in higher delivered densities. 6. The hose stream demand
included in the total water supply is available to sprinklers in the early stages of a fire, further increasing the delivered density of the sprinklers. Safety factor No. 4 is
Figure 2: Safety factor measurements
illustrated in Figure 3. Safety factor No. 2 is best illustrated by the following analysis conducted by the authors on a light hazard wet-pipe sprinkler system in an academic building designed to deliver 0.1 gpm/sq.ft. over the most remote 1,500 sq.ft. The hydraulically most remote area contained 17 sprinklers. Successive sprinkler calculations were prepared that modeled the most remote sprinkler operating (Node 601), followed by the two most remote sprinklers operating (Nodes 601
Figure 3: Inherent safety factor due to sprinkler spray pattern
degraded water supply curve. This concept has been developed and promoted by J. Michael (Mike) Thompson, P.E.; a founding partner of the Protection Engineering Group. Mike has developed software providing a number of hydraulic tools, one of which plots supply vs demand curves in this fashion.
Density Concerning sprinkler system
design, fire protection professionals speak in terms of density; the rate of water application per unit area at the
3. “Rack Storage of Materials, NFPA 231C-1986.” IRInformation, Industrial Risk Insurers, February 1, 1990,
9.IN.10.1.2C. Sprinkler hydraulic calculations
are required to account for the water used by the fire department to manually suppress a fire. This is referred to as a hose stream allowance. Typically this is shown on a hydraulic graph as a line of a length equal to the allowance (in gpm) and extending horizontally from the maximum sprinkler demand. The problem with this depiction of the total system demand versus the supply is that the hose streams are not flowing at the maximum pressure demand of the sprinklers and not just at flows higher than the maximum sprinkler demand. In reality, the fire department is
“taking this amount of water away” from the available supply and the sprinkler system is “left” with a
phc june 2011
www.phcnews.com
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