Respondent owns and operates a natural gas processing well site and facility, which includes a separator used for separating well fluids produced from the natural gas well into gaseous and liquid components and a tank battery which consists of interconnected storage tanks situated to receive output from the natural gas well and separator. Natural gas is a highly flammable gaseous hydrocarbon mixture containing methane, ethane, propane and butane, which are extremely hazardous substances listed in Table 3 of 40 CFR ยง 68.130 that may ignite, flashback or explode when exposed to a source of ignition. On Monday, February 3, 2014, multiple maintenance projects were underway at the facility. A coil tubing job was underway on one part of the facility. Cuttings and crude oil from the coil tubing job were placed in the “water storage tank” for storage. A welder and assistant were tasked with connecting a two inch water line running from the “water tank”. Hydrocrarbon vapors collected in the two inch line due to the offloading of cuttings and crude oil that had been placed in the “water tank” from the coil tubing job. When the welder struck an arc on the two inch line, the flammable vapors ignited and traveled down the two inch line until they reached the water tank, which then exploded.