A winter freeze caused a fire protection sprinkler system failure at an elementary school in northern Texas causing the fire protection sprinkler system to operate and distribute water into the attic area of the school. U.S. Forensic was asked to determine the cause of the fire protection sprinkler system failure. The mechanical engineer assigned to the case was Jeffrey P. Stark, P.E.
The elementary school houses a wet and a dry pipe sprinkler system for fire protection. The wet pipe sprinkler system is intended for systems that will be protected by adequate heating or insulation and will not be exposed to below 40°F temperatures. The dry systems are intended where the temperature may reach 40°F and below. In a dry-pipe firewater system, the sprinkler piping is charged with compressed air rather than water. The compressed air holds the dry valve closed and prevents water from entering the system. When a sprinkler head is activated by the heat of a fire, the air exits the system and reduces the pressure on the outlet side of the dry valve. This allows the dry valve to open and water to flow to the activated sprinkler head.
An interview with the staff at the school revealed that they performed the routine inspections and tasks on the fire sprinkler system as part of the school’s monthly fire drill. One of the staff members was trained on performing these inspections and tasks by the local fire department. They performed the draining of the dry pipe sprinkler system at least twice at every location depending on the amount of water that is drained. When they performed the draining of the west end of the dry pipe sprinkler system, very little water came out as compared to the east end dry pipe sprinkler system.
Examination of the sprinkler system revealed that a tee pipe fitting for the dry pipe system in the west side of the attic had fractured.
Analyses & Findings – Sprinkler System Failure
According to the National Fire Protection Association (NFPA), the branch piping in a dry pipe sprinkler system shall be pitched at least ½ inch per 10 feet for drainage. The piping (north of the support) on the south run of the dry pipe sprinkler system was sloped towards the failed tee pipe fitting instead of being sloped towards the drain location. This proved that all of the water would not drain from the dry pipe sprinkler system. The tee pipe fitting that failed was midway between supports on the run of the tee. It supported the inlet of the branch piping which came from a higher elevation. There were no supports near the end of the branch piping. This induced an additional load on the tee pipe fitting and allowed the tee pipe fitting to sag between the run piping supports. This induced sag between the piping supports allowed water to collect in the tee pipe fitting that could not be drained.
The temperature remained in the low 20’s until 8:00 am and remained below freezing until 10:00 am. The rupture of the tee pipe fitting of the dry pipe sprinkler system was caused by water that remained in the pipe fitting even after being drained, freezing and turning to ice. The expansion of water into ice caused the rupture of the tee pipe fitting.
Based on information obtained and considered to date, U.S. Forensic has determined that the piping for the dry pipe sprinkler system where the failure occurred was not built to code as per the National Fire Protection Association 13: Standard for the Installation of Sprinkler Systems, which states that the branch piping in a dry pipe system shall be pitched at least ½ inch per 10 feet.