Freezing in Fire Sprinkler Systems During Activation at Low Temperatures

Abstract: The subject of this thesis is theoretical examination of freezing phenomena inside fire sprinkler systems during activation at low temperatures in the surroundings. Many sprinkler systems are installed in environments such that they must be operable at subfreezing temperature conditions. During activation of such a system, water is assumed to penetrate a significant distance and without interruption through cooled piping to the sprinkler. The motivation for this study is to investigate whether this is a reasonable assumption. The objective of this thesis is therefore to present calculation methods that can be used to predict whether system failure due to flow stoppage caused by ice growth at activation is to be feared and to make general observations, whenever possible. Our attention is directed mainly towards high pressure water mist sprinkler systems of dry pipe type. In order to find as far as possible analytical calculation procedures that can be customized to our problem, the approach was literature studies in the field. The result of this work shows that a sprinkler system during activation may experience complete blockage in two distinctive ways; either due to dendritic or (more unlikely) annular ice growth. The dendritic ice formation mode is characteristic for rather moderate subfreezing temperatures in proximity of 0 °C and is associated with the phenomenon of supercooling of the flowing volume of water which leads to sudden slush ice growth when nucleation starts. On the other hand, at the annular ice growth mode characteristic for lower temperatures, an ice shell at the pipe wall is created immediately in contact with water but is continuously melted away on its upstream-side by gradually warmer water (due to heat up of the system by incoming water). At the same time, the existing models have been found unsatisfactory to provide complete quantitative description of these phenomena to an extent sufficient to solve our problem. This means that complete blockage of a sprinkler system at activation still cannot be predicted with certainty. Nevertheless, we believe that we have sufficient grounds to claim that in a high pressure water mist system, flow stoppage in the piping (and piping only) due to ice formation (dendritic or annular) should be considered as unlikely. However, this cannot be generalized to high pressure nozzles in these systems. For conventional sprinkler systems, annular ice growth is not believed to cause complete flow blockage until the surroundings temperatures are considerably lower than –30 °C but at the same time, these systems are probably vulnerable for complete blockage due to dendritic ice growth. Hence, we propose experimental activities in order to provide a base for future development of the existing models towards our application.