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It was determined that a five (5) gallon electric hot water heater located in a storage room immediately adjacent to the high school cafeteria catastrophically failed causing severe damage to the surrounding area.
The blast occurred at 6:05 p.m. Thursday, May 11, 2000. Two custodians, Sharon Brennon, and John Morrata, were working in the cafeteria minutes prior to the failure. They indicated that they were working immediately beside the wall that collapsed just before they went for a coffee break. Just as they took their break, Ms. Brennon indicated that they heard a “big boom” and immediately ran for the door fearing a secondary gas explosion. Ms. Brennon and Mr. Morrata did not previously respond to any problems with hot water that day, but they worked the afternoon shift that did not require any demand for hot water on that system.
Earlier in the day prior to the failure, the day shift custodian Aubrey Williams indicated that they he had responded to several calls for cold water in the cafeteria kitchen. He indicated that he had to reset the tripped hot water heater circuit breaker 2-3 times between 10:00 a.m. and 11:30 a.m. Because his efforts did not correct the cold water problem, he thought there may have been some other breakers tripped in an area that he could not access due to asbestos removal.
The vessel that failed was a 1981 Hatco Electric Hot Water Heater, model # HCI-2464, serial #27492-TL110. The vessel was UL listed with a 150 PSI rated working pressure. The heater was rated for 1 amp, 3 phase with a total kilowatt rating of 17.2.
The hot water heater failed at a weakened area near the welded longitudinal lap joint. It appears that this thinned area may have been leaking slightly during the day resulting in abnormal conditions in the heater. These abnormal conditions may have contributed to the temperature control and breaker problems indicated earlier in the day. As the thin area failed the longitudinal seam also failed along the heat effected zone of the weld.
In order for the vessel to fail catastrophically with force sufficient to cause the building structural damage, the temperature of the water in the vessel had to be over 212 F in order for the water to flash into steam. The vessel was equipped with a pressure-temperature safety valve that should have been designed to prevent the vessel from achieving excessive pressures and temperatures. The stamping on the valve was not be found in the debris, however the valve was sent to the National Board lab for testing.
The report back from the National Board testing lab conclusively determined that the pressure temperature valve failed to operate and did not prevent the temperature of the vessel from reaching 212 F. The water heater had a maximum allowable working pressure of 150 PSI, but pressure temperature valve tested after the blast held tightly closed up to a pressure of 184 PSI before the test was stopped. Rust deposits were on the valve bonnet surface in the path suspected to have originated from the bonnet vent on the outlet side and leading to the interface between the bonnet and the body. Additionally several elements of the valve were missing either by the force of the blast or they were removed prior to the blast by untrained personnel at the school.
The valve lifting lever was missing probably as a result of the blast. The valve spring and the thermal element that traditionally exists on such a valve to protect the heater from excessive temperature was not present. Since the thermal element could not be located in the debris it is suspected that the element was either removed from the valve to prevent valve leakage or the valve was installed without the thermal element. Please see attached National Board Lab Report for more details.
Failure of the pressure-temperature safety valve along with the corroded and weakened condition of the vessel resulted in the ultimate failure of the vessel. Other schools in the Avon school system were visited to ensure there were not similar vessels in operation at those facilities.It is recommended that a release be issued to cover the recommended preventative programs schools, homes and businesses should follow to ensure a similar occurance does not happen with more tragic results.
This article was reproduced with permission from www.Safteng.net