Two employees working from separate stepladders were replacing a leaky pressure-reducing valve in an overhead low pressure (12 psi upstream from the pressure-reducing valve), 2-inch diameter, steel steam line. There was a shutoff ball valve approximately 8 inches upstream from the pressure-reducing valve. In the process of replacing the pressure-reducing valve that was threaded into the line, the nearby shutoff ball valve (which was also threaded into the line) became disassembled, releasing steam that caused burns to one of the employees. The shutoff valve was closed before beginning the replacement, but it was not locked/tagged out. The main steam valve for the line, which was approximately 15 feet away, was also not closed or locked/tagged out at the time.