Fire Sprinklers Continued from page 36

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 there- of. Although well intended, an arbitrary safety factor irre- spective of the slope of the water supply curve may not actu- ally provide much “safety.” This is best illustrated in Figure 1, where a 5 psi difference provides a good 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 factor should be an

amount of pressure or an amount of flow between the sprin- kler system demand and the water supply curve. Since sys- tem 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 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.

Figure 3: Inherent safety factor due to sprinkler spray pattern

in” safety factors including the following : 1. Initial densities are higher due to the descending supply

curve. 2. Calculations are started with the design density require-

ment 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 high- er 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 proba-

bly average higher than the calculated C=120 resulting in higher delivered densities.

Table 1: Inherent safety factors due to number of sprinklers operating

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 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 calcula- tions were prepared that modeled the most remote sprinkler operating (Node 601), followed by the two most remote sprinklers operating (Nodes 601 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

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

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 engineering, and inspection/testing/mainte- nance, sprinklers systems have been successful due to “built

Page 38/Plumbing Engineer

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 sprin- kler 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 Sprinkler hydraulic calculations are required to account

for the water used by the fire department to manually sup- press 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 hori- zontally 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 maxi- mum pressure demand of the sprinklers and not just at flows Continued on page 40

May 2011

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