This school's gas valve looked to be closed, then it exploded
David Wagman | December 03, 2019Piping that was being disassembled upstream of a gas service meter resulted in a gas leak that touched off an explosion at a school in Minneapolis in August 2017, destroying a building, injuring nine people and killing two.
The National Transportation Safety Board (NTSB) in early December released its findings into the accident. It said that a contributing factor was the lack of detailed documentation that clearly established the scope of work to be performed.
At the time of the explosion, two workers were installing new piping as part of work to relocate gas meters from the basement of the building to the outside. Two new meters mounted on a wall were ready for the new piping to be connected. As workers were removing the existing piping, a full-flow natural gas line at pressure was opened. The workers were unable to stem the gas release, so they evacuated the area.
A school maintenance worker heard and smelled the natural gas release and went to its source in the basement meter room where the workers had been. As he exited the basement, he made an announcement over his hand-held radio that there was gas in the building and to evacuate immediately. As he made his radio call, he ran up the stairs and searched for occupants. Less than one minute later, the building exploded.
Relocating service meters
The school, Minnehaha Academy, was in recess for summer vacation, but 36 staff were on hand to work on administrative tasks. The basement of the central building structure contained a utilities/storage room, a boiler room and a “utility bunker.” The bunker was an extension of the basement spaces built beneath a ground-level concrete slab that extended out from the west basement wall of the building. The utility bunker, which was outside the foundation footprint of the building, contained the gas service meter equipment and was accessed by a basement door from the boiler room.
The school was in recess for summer vacation, but 36 staff were on hand to work on administrative tasks. Source: KDLT NewsThe contract work involved relocating the gas service meters from the inside of the building to the outside of that building. The construction print ticket for the specific phase of the project included connecting the local gas utility's relocated meters using 2 in black threaded gas piping, welding 4 in gas piping, welding underground pipes, conducting core-drilling as necessary, performing the work during normal business hours, and securing a Minneapolis city permit for the work.
About a month before the accident, the utility had installed two new sets of gas service meter equipment on the outside of the building.
Stuck valve?
The NTSB report said that interview statements by two contract employees indicated that, prior to the explosion, they had encountered a plug valve that was connected to and located immediately prior to one meter in which the wrench was “stuck” in the closed position (that is, the wrench could not be turned).
A construction helper stated that he knew the valve was closed because the wrench was positioned perpendicular to the valve piping (the inlet/outlet connection ports of the valve).
The field foreman also stated that he had determined that the plug valve was closed and that it was safe for the construction helper to begin disassembling piping downstream of the valve.
Based on the NTSB interviews, upon instruction from the field foreman, the construction helper was then assigned to disconnect the piping from the gas service meter.
An on-scene examination by the NTSB of the plug valve, showed that the valve wrench was, indeed, positioned perpendicular to the piping. Disassembly of the valve components, however, showed that the internal “flow control” component of the valve (the valve plug) was in the open position.
Investigators were unable to determine how the valve wrench became positioned in that perpendicular orientation. Pre-accident photographs taken about 18 months before the accident showed the wrench positioned parallel to the piping, thus indicating the valve was open.
Confirmation test
The NTSB said that it is customary to align the handle so that it is parallel with the piping when the valve is open. Similarly, the handle is positioned perpendicular to the piping when the valve is closed. It said that confirmation of this "is the responsibility of the crew working on the piping." Confirmation would entail removing the handle and noting the position of the plug assembly and viewing the indicator or "witness mark" on the body of the valve.
After the accident, the local utility, Minnesota Gas, implemented procedural changes for similar types of work. The changes included a mandatory meeting prior to the start of work to define the demarcation point between utility facilities and customer piping and to discuss when, where and how gas will be turned off.
In addition, state utility regulators issued new rules mandating the installation of excess flow valves and manual service line shutoff valves for all Minnesota natural gas distribution systems.
It was probably unwise for the maintenance worker to use a transmitter when gas was smelled. They don't allow transmitters around dynamite. Who in the school would be listening at his frequency?
1. There is no mention if the radio was intrinsically safe.
2. Transmission via radio was the most expeditious means of communication whether or not there were any listeners.
In reply to #2
There was no mention of danger before most of the mass shootings either.
So your attitude is, I am going to transmit even if it kills me?
I seem to read you a lot Mr. Wagman.
My question would be what was determined to have been the ignition of the blast?
Was the radio a factor? The blast sites where they restrict the use of shortwave radios is done so because that is what they use to detonate the explosive, I think - please correct if I'm wrong.
On every large gas valve I have seen (and remember) there is this "witness mark" - a line indicating the position of the flow direction either through the valve or if perpendicular to the pipe indicating it's closed. Any Plumber of Pipe-fitter is aware of this rather important fact. On smaller valves they have a "fixed" handle which is known to indicate the flow (or not)of the fluid inside the pipe. Also I have a problem with the disassembly of the piping. If a pipe or fitting was loosened by unscrewing it would certainly begin to spew gas which should/would be an indication of the problem. then of coarse you would re-tighten it and proceed to get the gas turned off. Certain individuals should not be allowed to be in the position to cause this type of disaster. However I have seen this happen, but without the explosion part, on one of my last jobs as superintendent. Same "fitter" apparently hired from the "head down", caused 2 evacuations of same project by just being stupid.
But those valves large enough to have the "wrench" you mention as stated should be removed and attached nearby so as not to give the false impression of being closed. The "nut" on top of those valves is usually square so if the wrench is left in place it could either be closed or OPEN.
I was posting our insurance company's "required safety posters" in my construction office one day and our concrete contractor's superintendent offered this observation: You only need one poster - "NO DUMMIES".
I agree, Jim
The contractor was not competent to perform this work. The handle can of course be in any position on a plug cock, and is not determinate of the actual valve position. That is the bottom line.