CSB Report: Defective Welds Contributed to Houston Refinery Explosion

The Chemical Safety and Hazard Investigation Board (CSB) determined that a number of errors including defective welds were the cause of a December 2004 chemical plant explosion that ignited a fire that burned for 7 hours in southwest Houston.

The 50,000-pound steel pressure vessel that exploded at the Marcus Oil and Chemical facility on the evening of Dec. 3, 2004, slightly injured three firefighters, while several residents sustained cuts from flying glass. Steel fragments from the explosion were thrown up to a quarter-mile from the plant, which refines polyethylene waxes for industrial use.

CSB investigators determined that Marcus Oil did not use a qualified welder or proper welding procedure to reseal the vessels and did not pressure-test the vessels after repairs were made on the failed vessel, known as Tank No. 7.

According to CSB officials, the repair weld on Tank No. 7 failed under pressure, ejecting molten wax and flammable hydrocarbons. Marcus Oil used air instead of nitrogen to boost the pressure of the vessel, and the oxygen inside the tank allowed the ignition of the flammable material, most likely by sparks from the metal fragments.

The fire spread back into the damaged tank, causing a violent explosion, which propelled the 25-ton vessel more than 150 feet. The tank came to rest against a warehouse on an adjacent property.

The incident's lead investigator, John Vorderbrueggen, said that although substantial amounts of documentary and physical evidence were lost when the city demolished the fire-damaged building where Marcus Oil records were stored, the agency was able to estimate that the defective welds had decreased the strength of the vessels by more than 75 percent and that the weld on Tank No. 7 finally failed catastrophically during a routine production run.

CSB Wants Houston to Adopt New Regs

As a result of the incident, CSB called on the city of Houston to adopt new safety regulations instituted by the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code that govern the construction and modification of pressure vessels industrial process and storage containers that hold pressurized gases or liquids.

According to CSB, Texas is one of 11 states that have not adopted national safety standards for pressure vessels and one of 17 states that do not require adherence to the National Board Inspection Code, which requires alterations to pressure vessels to be inspected, tested, certified and stamped.

"If the provisions of internationally recognized pressure vessel safety codes had been required and enforced, this accident would almost certainly not have occurred," CSB Board Member John Bresland said. "The presence of unregulated, uninspected and improperly maintained pressure vessels within an urban area like Houston is a serious concern."

CSB also recommended that Marcus Oil repair all modified pressure vessels to conform to the National Board Inspection Code requirements, install relief devices on all pressure vessels, and avoid the contamination of its nitrogen supply with air to prevent fires.

The case study report and accompanying safety recommendations have been posted to the agency's Web site.

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