Final Report from U.S. Chemical Safety Board on 2004 Explosion Calls on Houston to Enact Stricter Pressure Vessel Regulations

June 6, 2006

Houston, Texas, June 6, 2006 - In its final investigation report on a December 2004 chemical plant explosion in southwest Houston, the U.S. Chemical Safety Board (CSB) today called on the city to adopt new safety regulations governing the construction and modification of pressure vessels - industrial process and storage containers that hold pressurized gases or liquids. The case study report issued at a news conference this morning describes the violent explosion of a 50,000-pound steel pressure vessel on the evening of December 3, 2004, at the Marcus Oil and Chemical facility on Minetta Street in southwest Houston. The explosion was felt over a wide area in Houston and ignited a fire that burned for seven hours. Three Houston firefighters were slightly injured during the response to the blaze. Several residents sustained cuts from flying glass, and steel fragments from the explosion were thrown up to a quarter-mile from the plant. Building and car windows were shattered, and nearby buildings experienced significant structural and interior damage.

The Marcus Oil facility, which was established in 1987, refines polyethylene waxes for industrial use. The crude waxes, which are obtained as a byproduct from the petrochemical industry, contain flammable hydrocarbons such as hexane. At Marcus Oil, the waxes are processed and purified inside a variety of steel process vessels. The vessel that exploded was a horizontal tank 12 feet in diameter, 50 feet long, and operated at a pressure of approximately 67 pounds per square inch.

CSB investigators determined that the failed vessel, known as Tank No. 7, had been modified by Marcus Oil to install internal heating coils, as were several other pressure vessels at the facility. Following installation of the coils, each vessel was resealed by welding a steel plate over the two-foot diameter temporary opening. The repair welds did not meet accepted industry quality standards for pressure vessels. Marcus Oil did not use a qualified welder or proper welding procedure to reseal the vessels and did not pressure-test the vessels after the welding was completed.

On the evening of December 3, 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. The material was likely ignited 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, where it came to rest against a warehouse on an adjacent property. CSB 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.

Lead Investigator John Vorderbrueggen said, "Marcus Oil could not provide our team with any documentation concerning the design, construction, or safe operating pressure of the vessels. The CSB estimated that the defective welds had decreased the strength of the vessels by more than 75%. It is likely that the welds were further weakened by metal fatigue from hundreds of operating cycles over many years. The weld on Tank #7 finally failed catastrophically during a routine production run."

Mr. Vorderbrueggen noted that substantial amounts of documentary and physical evidence were lost the Saturday morning immediately following the incident when the city demolished the fire-damaged building where Marcus Oil records were stored. CSB investigators from Washington, DC, arrived at the scene later that day.

The CSB report pointed out that Texas is one of 11 states that have not adopted national safety standards for pressure vessels. First developed in 1915, the American Society of Mechanical Engineers (ASME) 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.

"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 S. Bresland said. "Pressure vessels potentially contain huge amounts of stored energy, and if they fail they can pose a grave danger to lives and property, as clearly demonstrated by the accident at Marcus Oil. The presence of unregulated, uninspected, and improperly maintained pressure vessels within an urban area like Houston is a serious concern."

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 case study report and accompanying safety recommendations have been posted to the agency's website,

The CSB is an independent federal agency charged with investigating industrial chemical accidents. The agency's Board members are appointed by the president and confirmed by the Senate. CSB investigations look into all aspects of chemical accidents, including physical causes such as equipment failure as well as inadequacies in safety management systems, regulations, and industry standards. The Board does not issue citations or fines but does make safety recommendations to plants, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA. Visit our website,

For more information, contact Director of Public Affairs Dr. Daniel Horowitz at (202) 441-6074 (cell) in Houston or Public Affairs Specialist Kara Wenzel in Washington, DC, at (202) 261-7642 or (202) 577-8448 (cell).


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