Comparison of sprinkler activation times under flat and corrugated metal deck ceiling

The distance of sprinklers below ceiling is of fundamental importance since it has a great influence on activation time. Accepted distances are specified in different sprinkler standards such as NFPA 13, FM 2-0 and EN 12845 for different configurations. This distance is unambiguously defined for flat smooth ceilings, but for corrugated ceilings, such as corrugated metal deck ceilings found in many warehouses, the distance can be measured from either the top or the bottom of the deck. NFPA 13 states that it should be measured from the bottom if the thickness of the deck is below 75 mm and from the top if it is higher. The requirement in NFPA 13 has been adopted from the FM datasheets, but the authors have been unable to identify any background to this requirement. In this paper, the activation time of sprinkler heads at different distances below a corrugated ceiling with thickness of 125 mm was simulated using FDS 6.6.0 and compared to distances below a flat smooth ceiling. The case was based on a warehouse with 6-tier storage of Uncartoned Expanded Plastics (UEP) with a ceiling height of 11.2 m and clearance to storage of 2.6 meters. Only the activation of the first sprinkler was simulated since it was expected that the drag from droplets of previously activated sprinkler heads would thicken the ceiling jet, making the activation time of additional sprinkler less sensitive to the distance from ceiling. The results show that the corrugated ceiling has a large influence on the ceiling jet allowing sprinklers further from the ceiling to activate. In the case studied, the activation time of sprinklers below the corrugated ceiling more closely resembled those under a flat smooth ceiling when the distance below the corrugated ceiling was measured from the bottom of the deck. More simulations and experiments are however needed before any guidance on changes in sprinkler standards can be given.