Abstract: This incident was also covered in investigation summary Id 142282. This investigation covers the second contractor hired by the refinery's owner and operator. On or before December 21, 2021, a leak was found on a flange for a bypass line on the Hydro Desulfurization Unit (HDU-1) at refinery operated by an international oil company. The leak was detected because the leak had auto ignited. The refinery owner's fire team was called out to put the fire out. After the fire was put out; steam lances were continuously directed on the flange because of the potential of another flash fire from the leaking heated naphtha. The refinery's owner hired two contract companies to perform a repair of the leaking flange using a method called hot bolting, a method of removing and replacing one bolt at a time so workers could install a wire wrap and seal the leak. The replacement bolts were longer and had injection collars. Workers could do the wire wrap and inject the bolts with a sealant to stop the leak. One contracting firm would remove and replace the bolts, while the other contractor's employees assisted. At 1:00 a.m. on December 23, 2021, two employees were working for the contracting firm providing the assistance. As they were loosening flange bolts, the naphtha exploded. One employee suffered third-degree burns, and he was struck by flying debris. He was hospitalized. The other employee was not hospitalized. The employee's supervisors knew that the flange had auto ignited the day before they were called to come and fix the leak. The employees were using general PPE required clothing at the time of the incident (hard hat, flame-resistant clothing, steel toe boots, and safety glasses). They were not wearing a higher level of PPE.