Design of Dry Pipe Sprinkler Systems to Meet the Water Delivery Time Restriction in Industrial Freezers

Abstract

Dry-pipe sprinkler systems are commonly used when the ambient temperature of a protected space is below freezing temperature of water. Because the degree of fire spread prior to sprinkler actuation is greater than that of the wet system in comparable circumstances, a greater coverage area is often required in a drysystem than that of a wet system. In addition, the dry system also requires the water delivery time to the most remote sprinkler be within a certain time limit. Dry-pipe ceiling-sprinkler restriction imposed by NFPA 13 for the protection of Class 2 materials in rack-storage higher than 7.6-m under 12-m ceiling is a maximum 30-seconds water delivery time with a 4-heads-open assumption. An in-house computer program developed to estimate water delay times in dry-pipe systems has been extensively used at FM Global for the last 15 years to address the water delivery time requirement mentioned above for industrial freezers. This paper describes how the program was developed and how the 30-seconds water-delivery time requirement could be met by choosing proper operating conditions through numerous real design cases. The examples showed that by adjusting many operating control parameters, most of the system could meet the 30-seconds water delivery time restriction without making substantial alteration in system configurations. The case study also shows that the air trip time linearly increases with the system volumes as anticipated. The water transit time also increases with the system volume in general, however, the dependence was more complex, which indicates that the system configuration plays an important role in the transit time in dry-pipe systems. Overall, the case study shows that the program had enabled the system design to be much more flexible and cost effective.