Abstract: At 10:00 a.m. on June 8, 2020, Employee #1 was working for a pipe fitting contractor. Employee #1 was removing steel bolts from flange fitting to open the process piping. The jobsite was a roofing materials production facility, and the nature of work was adding process piping and equipment to enlarge an existing chemical process. The scope of this work included tying in new relief valve piping to the existing line process piping. This line piping is normally empty. The additive is a corrosive chemical with a pH of 10.8. When deliveries are made the piping is used to convey the additive from the delivery truck to a fixed storage tank located outdoors. It is only during deliveries that the line contains the flowing additive. However, approximately 8 gallons of residual liquid remains trapped in a 2 inch vertical pipe riser. This liquid is trapped because of a one-way check valve located downstream of the riser. Employee #1 was opening the pipe flange for the purpose of draining this residual liquid prior to adding the relief valve. Employee #1 began to open the four-bolt pipe flange located between the pipe riser and check valve. Employee #1 removed the two bottom bolts from the flange to allow the residual additive to safely drain from this line. Employee #1 observed that liquid had stopped flowing from the pipe flange and began to remove one of the two remaining top flange bolts. Unknown to employee #1, the gasket inside the pipe flange became displaced during drainage. The stoppage was due to the dislodged gasket blocking this flow. It was not possible to detect the displaced gasket by visual means, thus Employee #1 removed the third bolt allowing the residual additive to drain onto Employee #1. The clothing of Employee #1 became saturated in the torso and waist areas. Employee #1 was hospitalized to treat 2nd degree chemical burns.