Employees were working to drain condensate in a dead-leg tee on the compressed air header in preparation for hooking up a rental compressor to the header. Employee 1 opened the isolation valve to drain the tee and the butterfly valve opened more than expected. The disconnected 3-inch hose at the end of the line began whipping around and the fitting on the end of the hose struck Employee 2 in the lower right leg resulting in a compound fracture.