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NOTE: This is an unformatted excerpt from our online fire protection training library.

Hydraulic Calculations

Density/Area Method

Choice Appropriate to the Occupancy Hazard

It is not necessary to meet all of the possible points on the design curve that is chosen and applied.  The designer has to pick a point on the curve that corresponds to the occupancy hazard deemed appropriate.

When estimating water supply requirements by using the simple process of multiplying the density times the area, you will notice that a higher density and smaller area of application will yield a lower flow.  For example, if the ordinary group 2 curve is used, the densities/areas range from 0.20 gpm/sq. ft for an area of 1,500 sq. ft. (0.20/1500) to 0.15/4000. 

The higher density requires an estimated flow of 300 gpm (0.2 x 1500), and the lower density requires a flow of 600 gpm (0.15 x 4000).  The lower density means that the fire area may be larger than if more water is applied to a smaller area.  Some authorities, mainly highly protected risk insurance companies, will opt for the conservative side of the curve, which is the larger area of application.  This is not only because the larger area requires more water, but also because if the smaller area is applied, and a branch line or a pipe supplying a number of sprinklers is obstructed, there will be a larger number of sprinklersFor example, if the area of application selected is 1,500 sq. ft. and each sprinkler has a spacing of 130 sq, ft. per sprinkler, a total of twelve sprinklers will be calculated. Assume that the area of application is three branch lines, flowing four sprinklers - if one branch line is blocked, the flow to 1/3 of the sprinklers is not happening. However, if a 3,000 sq. ft. area is chosen, and the area of application is using six branch lines with four sprinklers each, and one branch line is blocked, only 1/6 of the sprinklers will not flow - thus, there is a better chance of control, because more sprinklers are flowing. that can still be flowing.  Thus, the actual discharge density coming from the system should be higher, even with a couple of sprinklers obstructed.

NOTE: This is an unformatted excerpt from our online fire protection training library.

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