Updated: 10:53 a.m.
The National Transportation Safety Board says an August 2017 natural gas explosion at Minnehaha Academy happened after a pipefitting crew mistakenly thought a key valve was closed before they started disassembling a gas installation at the school.
A report issued Tuesday by the NTSB found that a pipefitter and his son, working for Master Mechanical, were preparing to move a meter in the basement of the Minneapolis private school, believing that a “plug valve” in the line was shut off because the handle was perpendicular to the gas piping.
The explosion killed two school employees and leveled a section of the upper school campus.
A post-explosion examination of the valve found the internal shutoff in the valve was in the open position, and that investigators found the valve mechanism frozen in place.
Examination of the valve after the accident confirmed the valve handle had previously been in a different position.
“The investigators were unable to determine how the valve wrench became positioned in that perpendicular orientation because preaccident photographs, provided by CPEMG (CenterPoint Energy Minnesota Gas) and taken about 1.5 years prior to the accident, showed the wrench positioned parallel to the piping, thus indicating the valve was open,” the report noted.
The two employees, a Master Mechanical field foreman and his son, a construction helper, were working in the school’s basement. Both believed that the valve was closed, despite small indicators showing it was open, when the construction helper started taking apart piping downstream from the valve, the NTSB said.
The frozen valve would have made it impossible to shut off the gas quickly. Witnesses reported a loud “horrendous flow of air” coming from the basement and the smell of gas, and the building exploded a minute later.
The report says that CenterPoint suspended similar meter work after the blast and instituted new policies about what activities and equipment the company was responsible for and what contractors would be responsible for -- as well as pre-construction reviews of exactly how to shut off gas flow prior to work on gas piping.
Eagan-based Master Mechanical did not immediately respond to a request for comment on the NTSB findings.
The blast killed the school’s janitor, 82-year-old alumnus John Carlson, and receptionist Ruth Berg, 47. Nine others were hurt in the blast.
The school has since rebuilt on the same site, and welcomed students back to campus this fall, after holding classes at a temporary site in Mendota Heights.
CenterPoint Energy and Master Mechanical settled a wrongful death lawsuit with Berg’s family last year. Five new lawsuits were filed in the case in July.