Abstract: On February 21, 2004, Employees #1, #2, #3, and #4 were cleaning a tank, which was part of a compressor used to recover gas mixed with methylacetylene-propadiene and liquid propane from rework cylinders in a production process. The cleaning was being done in a separate area of the plant from the production operation. The gas mixed with methylacetylene-propadiene was bottled prior to the cleaning, and a tarry sludge accumulated in the tank's bottom from the methylacetylene-propadiene operation. When the tank was opened and tank cleaning began, the sludge was released. A nearby located propane heater for the steam heated a pressure washing unit, which apparently ignited the vapors emanating from the tank and the sludge into the work room. Employees #1, #2, and #4 were hospitalized with 1st and 2nd degree burns. Employee #3 was treated for injuries, however he was not hospitalized.