The Case of the 2,600°F Molten Sparks

Oxy-Acetylene Cutting SparksThe Problem:

There were no as-built drawings for the plenum, absorber or vanes when the duct repairs started.  The location of the designated work place for patching the holes in the duct was 16’ long.   The trajectory of molten metal and sparks from the oxy-acetylene cutting of the corroded duct presented several possibilities to ignite the demister fire:

  1. The molten metal and sparks went directly onto the duct floor, bounced onto the polypropylene demister to ignite the fire. 
  2. There were corrosion holes in the vanes, similar to the holes in the top of the duct that went directly onto the polypropylene demister to ignite the fire. 
  3. The molten metal and sparks went directly down onto the polypropylene demister without contacting the floor of the duct to ignite the fire,
  4. The molten metal and sparks fell onto a vane inside the duct, slid along the vane and rolled inside the shaft onto the polypropylene demister to ignite the fire.

My Assessment: 

Polypropylene Demister Inside DuctOxy-acetylene cutting was performed on the corroded duct without checking and confidently knowing about flammable materials that may catch fire on the other side of the duct wall, i.e. inside the duct. This error in judgement violated common industry safety practices and standards of care for oxy-acetylene cutting by boilermakers as listed in USDOT CFR, OSHA, AWS Z49.1 and NFPA 51B.

Co-Gen Power Plant Duct & StackThe Consequences: 

The ignition temperature, at the oxyacetylene torch tip, when cutting thin stainless steel plate was 6,296°F.  The temperature of molten steel droplets was 2,600°F.  The time for the molten steel droplets to fall, due to gravity, from the top of the duct to the demister was 2½ seconds.  The heat loss of molten steel droplets in air was 100°F per second.  The beginning temperature at the oxy-acetylene torch tip lead to a final temperature at the end of the droplet’s fall onto the polypropylene demister that exceeded 2,000°F, which was double the ignition temperature of the polypropylene demister that caught fire.

The Lessons Learned:

The workplace safety policies and practices for “hot work” were not enforced.  The contractor had an obligation to determine all flammable items inside of the duct before cutting into the duct.  Contractors and Managers have mutual responsibilities for safety practices when oxy-acetylene cutting steel.