Investigators say the explosion at the Marcus Oil and Chemical plant near Main and Fondren occured when a repair weld on a 25-ton, 50-foot long metal pressure vessel failed, igniting flammable material inside. The blast sent the tank 150-feet through the air and shards of metal into nearby neighborhoods and businesses. The CSB's lead investigator John Vorderbrueggen says sloppy welding that wasn't tested led to the explosion.
"Everytime you weld on a pressure vessel, you're required to perform a hystrostatic test. On a vessel that big that takes a lot of water, but that's the only way to prove that it's safe. We also concluded that had they tested this vessel, they would have tested it to one-and-a-half times its pressure rating, so it would have been tested to about 120 psi. That plate probably would have failed when it was originally tested, but it was never challenged."
The CSB also found that Marcus used air instead on nitrogen to boost pressure in the tank and the oxygen allowed the ignition of the flammable material inside, a polyethelene wax used in industry. Although most of the pressure vessels at the plant were regulated under city codes, the one that blew-up wasn't, which led the CSB to recommend that the city adopt new safety regulations for all pressurized tanks.
"The city of Houston, through their fire ordinances, does require most of the pressure vessels that were at Marcus Oil, to comply with the pressure vessel code. But it's the rest of the vessels that haven't been picked-up by the city ordinance and this is really an opportunity, and it's not a broad leap for the city, to add this recommendation and to fully implement this recommendation."
The Mayor's office says it hasn't seen the official recommendations yet. An official there says current codes are enforced by the fire department, but new pressure vessel regulations could fall under the city's plumbing code and would likely require city council approval. This is the CSB's John Bresland.
"If the provisions of internationally recognized pressure vessel safety codes had been required and enforced, this accident would almost certainly have not occurred. Pressure vessels contain potentially huge amounts of stored energy and if they fail, they can pose a grave danger to lives and property as cleary demonstrated by the accident at Marcus Oil."
Since the explosion, Marcus Oil and Chemical has been fined by the Occupational Safety and Health Administration and has yet to resume regular operations.