There are very few instances where OSHA uses such clear language, but in 1910.252(a) they use the phrase “absolutely certain” and for good reason! This explosion occurred at a wastewater treatment plant in the city of Gumi, South Korea, on October 19, 2016.
1910.252(a)(3)(i) Used containers.
No welding, cutting, or other hot work shall be performed on used drums, barrels, tanks or other containers until they have been cleaned so thoroughly as to make absolutely certain that there are no flammable materials present or any substances such as greases, tars, acids, or other materials which when subjected to heat, might produce flammable or toxic vapors. Any pipe lines or connections to the drum or vessel shall be disconnected or blanked.