Sight glass rupture incident

On April 9, 1986, Employees #1 and #2 were tightening a manhole cover plate on a 1,000 gallon capacity steam jacketed pressure vessel. The vessel was in service, agitating and heating a wax emulsion product by a pressure emulsion process when a 4 in. diameter, 3/4 in. thick sight glass ruptured. It threw out an explosion-proof light that was mounted over the sight glass and broke a 1 in. diameter steam line. Employee #1 was killed and Employee #2 was burned by steam and the hot wax emulsion product. The pressure relief valve on the 75 psi vessel was rated at 150 psi and was not operational. No maintenance or vessel inspection documentation could be located.
Source:osha.gov