The U.S. Chemical Safety Board said an unqualified welder made poor quality welds in a pressure vessel, and the low-quality welds led to the violent explosion of a 50,000-pound steel pressure vessel December 3, 2004. The tank was at the Marcus Oil and Chemical facility in Houston.

In its final report on the chemical plant explosion, the Chemical Safety Board called on Houston to adopt new safety regulations governing the construction and modification of the industrial pressure vessels for gases or liquids. The board issued its final report on the disaster June 6.

The vessel that exploded was a horizontal tank 12 feet in diameter, 50 feet long. It operated at a pressure of approximately 67 pounds per square inch. The explosion reportedly was felt 25 miles away.

Chemical Safety Board investigators said the failed vessel was modified by Marcus Oil, as were several other pressure vessels at the facility. Following the modifications, the vessels were resealed with a steel plate that was welded over the two-foot diameter temporary openings in each vessel.

The board said those repair welds did not meet accepted industry quality standards for pressure vessels, and that Marcus Oil did not use a qualified welder or proper welding procedures to reseal the vessels. Additionally, the board said Marcus Oil did not pressure-test the vessels after the welding was completed.

In a press release on the investigation, the Chemical Safety Board said the repair weld on Tank No. 7 failed under pressure on Dec. 3, 2004. The rupture ejected molten wax and flammable hydrocarbons and, because Marcus Oil used air instead of nitrogen to pressurize the vessel, oxygen inside the tank allowed the ignition of the flammable material. The board said sparks from the rupturing metal were the likely cause of ignition that followed the rupture.

The ensuing fire spread back into the damaged tank causing a violent explosion, propelling the 25-ton vessel more than 150 feet, where it came to rest against a warehouse on an adjacent property. Investigators later found a variety of large metal fragments in the surrounding community, including a 120-pound steel plate located in a field 900 feet away.

Defective welds cut vessel strength 75 percent
“Marcus Oil could not provide our team with any documentation concerning the design, construction, or safe operating pressure of the vessels. The Chemical Safety Board estimated that the defective welds had decreased the strength of the vessels by more than 75 percent. It is likely that the welds were further weakened by metal fatigue from hundreds of operating cycles over many years. The weld on Tank No. 7 finally failed catastrophically during a routine production run,” said John Vorderbrueggen, the lead investigator for the board. Vorderbrueggen was quoted in the board’s press release.

Vorderbrueggen noted that documentary and physical evidence were lost after the incident when the city demolished the fire-damaged building where Marcus Oil records were stored. Investigators from Washington, D.C., arrived at the scene later.

The Chemical Safety Board’s report noted that Texas is one of 11 states that have not adopted national safety standards for pressure vessels. The national standards that were first developed in 1915 by the American Society of Mechanical Engineers (ASME) as the Boiler and Pressure Vessel Code, provides rules for pressure vessel design, fabrication, weld procedures, welder qualifications, and pressure testing. In addition, the National Board of Boiler and Pressure Vessel Inspectors has established rules for pressure vessel repairs and alterations, the National Board Inspection Code. However, Texas is one of 17 states that do not require adherence to the National Board Inspection Code. The code requires alterations to pressure vessels to be inspected, tested, certified, and stamped.

Code enforcement could have prevented explosion
“If the provisions of internationally recognized pressure vessel safety codes had been required and enforced, this accident would almost certainly not have occurred,” Chemical Safety Board Member John S. Bresland said.

The Board called on the City of Houston to expand the current building ordinance to require mandatory compliance with both the ASME Code for all new pressure vessels and the National Board Inspection Code for all pressure vessel repairs and alterations. The Board separately 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 explosion at Marcus Oil was felt over a wide area in Houston and ignited a fire that burned for seven hours, injuring three Houston firefighters and several residents. The explosion was so violent that glass and steel fragments from the pressure vessel were thrown up to a quarter-mile away, building and car windows were shattered, and nearby buildings experienced significant structural and interior damage.

The Marcus Oil facility refines polyethylene waxes, a byproduct from the petrochemical industry, for industrial use.

The case study report and accompanying safety recommendations have been posted to the agency’s website, www.csb.gov.

The Chemical Safety Board is an independent federal agency charged with investigating industrial chemical accidents.

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