I worked for two research universities, building custom research equipment. All of the equipment has to be extremely clean for use, much of it was used in very high vacuum experiments, some of the experiments required very large vacuum pumps, that had to be set up in the basement, requiring piping to reach the labs. A new building was built and 3, 6" vacuum lines with flanges welded on needed to be shortened, when I looked into the vacuum lines the inside surface was covered with a scum of grease and dirt, I squirted some acetone on a paper towel and cleaned the mess off the area I had to weld, I was tiging the flanges back on the lines, lines one and two went fine, when I struck an arc on the third line there came a rumbling sound and the reinforced concrete floor started shaking madly! in an instant a red flame 2 feet long shot out of the pipe, I jumped backward out of the equipment room I was working in, just as a student came through another door to tell me that the floor was shaking in his lab and knocked all of his stuff on to the floor.