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Marcus Oil and Chemical Tank Explosion

Overview

Case study of a polyethylene wax processing facility explosion and fire at Marcus Oil and Chemical in Houston, Texas, on December 3, 2004. The CSB identified pressure vessel repairs and alterations, and nitrogen inerting system design and operation, as key issues.

Incident Snapshot

Field Value
Facility / Company Marcus Oil and Chemical
Location Houston, TX
Incident Date 12/03/2004
Investigation Status The CSB issued a case study report on this incident at a news conference in Houston on June 6, 2006.
Accident Type Chemical Manufacturing - Fire and Explosion Investigation
Final Report Release Date 06/06/2006

What Happened

  • On Friday, December 3, 2004, at about 5:50 pm, employees heard a loud "pop" then saw light from a fire reflecting off a shiny tanker truck parked near the process equipment.
  • About 45 seconds later, a violent explosion occurred and a fire fueled by molten wax erupted near the main warehouse.
  • The warehouse and nearby equipment were quickly involved in the fire.
  • The Houston Fire Department arrived approximately five minutes after the explosion.
  • Firefighters extinguished the three-alarm blaze by midnight, approximately seven hours after the explosion.

Facility and Process Context

  • Marcus Oil and Chemical operated a polyethylene wax processing facility on the southwest side of Houston, Texas.
  • Established in 1987, Marcus Oil refines high-density polyethylene waxes used in coatings, adhesives, polishes, rubber processing, and textiles.
  • The annual capacity was in excess of 250 million pounds.
  • The normal process started with pumping feedstock from tanker trucks into a wash tank where impurities settled to the bottom.
  • The wax then was processed to extract hexane and other hydrocarbons, solidified into small pellets, packaged, and shipped.
  • The extracted hydrocarbons were stored in aboveground storage tanks for later sale.
  • Marcus Oil management reported that nitrogen was used throughout the process equipment to protect the molten wax from contact with oxygen in the air.
  • Operators periodically removed the residual wax with concentrated impurities (called "rag") from the wash tank.
  • The removed rag solidified and was stored in the warehouse for later processing.
  • Employees hand-loaded solidified rag into Tank 4 where it was remelted using high-temperature steam coils.
  • A pump transferred the molten rag into Tanks 6 and 7 where it awaited transfer to the process unit.
  • Steam piping in the tanks maintained the molten wax at approximately 300oF.
  • Operators used the nitrogen system to pressurize Tanks 6 and 7 to force the liquid rag through an elevated section of pipe connected to the process unit feed pump.
  • The rag was pumped through the process unit to remove residual hydrocarbons.
  • The refined rag was solidified into small pellets, packaged, and shipped.
  • The nitrogen was produced onsite using a small nitrogen generator, and was stored in two large pressure vessels.
  • The maximum pressure in the nitrogen system was 120 psig.
  • A pressure regulator reduced the nitrogen pressure to between 40 and 70 psig in Tanks 6 and 7.
  • A temporary hose was connected between the compressed air system and the nitrogen distribution system downstream of the nitrogen generator to resolve the production delay.
  • They subsequently replaced the temporary hose with a permanent pipe and valve connection.
  • This system modification eliminated the extra work required to route and connect the temporary hose each time the nitrogen system pressure fell too low.

Consequences

  • Fatalities: None reported.
  • Injuries: Three firefighters were slightly injured while fighting the fire, and local residents sustained minor injuries from flying glass.
  • Environmental Release: Not reported.
  • Facility Damage: The explosion shattered windows in buildings and vehicles and caused structural damage as far as one-quarter mile away. Significant interior damage resulted when suspended ceilings and light fixtures were blown down in the onsite buildings, nearby businesses, and a church. Tank 7 was propelled 150 feet where it impacted a warehouse belonging to another business. The 50,000 lb vessel was propelled into surplus equipment stored nearby, and came to rest against a warehouse on an adjacent property more than 150 feet away.
  • Operational Impact: The warehouse and nearby equipment were quickly involved in the fire. Firefighters extinguished the three-alarm blaze by midnight, approximately seven hours after the explosion. The resulting fire burned for nearly seven hours.

Key Findings

Immediate Causes

  • The weld used to reclose the temporary opening on Tank 7 failed during the incident because the repair weld did not meet generally accepted industry quality standards for pressure vessel fabrication.
  • The internal pressure of Tank 7 (67 psig) likely exceeded the strength of the defective weld on the patch plate.
  • The oxygen concentration in Tank 7 permitted the flame to flash back into the vessel.
  • An internal deflagration blew the vessel head into multiple fragments.

Contributing Factors

  • Operators pressurized Tanks 6 and 7 with nitrogen gas containing 18 percent oxygen instead of the intended concentration of not more than 8 percent oxygen.
  • The tank rapidly depressurized through the 24-inch hole.
  • Hydrocarbon vapor, compressed air, and hot liquid wax were ejected through the hole.
  • Sparks were generated when the patch plate struck the concrete pad, and likely ignited the wax and hydrocarbon vapors.
  • The 50,000 lb vessel was propelled into surplus equipment stored nearby.
  • Burning polyethylene wax was blown against the warehouse and other equipment and ignited combustible materials.
  • Marcus Oil installed a connection between the nitrogen and compressed air systems to provide rapid repressurization of the nitrogen system when the nitrogen pressure was too low to move the molten wax from the tanks to the process unit.
  • The company assumed that compressed air was an acceptable substitute for nitrogen during rag wax processing.
  • During rag processing, product discoloration from air exposure was not a concern.
  • Management did not evaluate the hazards that resulted from this process change.
  • Pressurizing the nitrogen system with compressed air rather than waiting for the nitrogen generator to repressurize the system contaminated the nitrogen gas with as much as 18 percent oxygen.
  • This oxygen level was sufficient to support combustion of the hydrocarbon vapor and wax inside the tanks.
  • The pressure vessel installation and alterations made by Marcus Oil did not follow the ASME Boiler and Pressure Vessel Code or the National Board Inspection Code (NB-23).
  • Poor welding severely weakened Tank 7 and led to its catastrophic failure.
  • Some had operating pressures in excess of 100 psig, yet none was equipped with a pressure relief device.

Organizational and Systemic Factors

  • Marcus Oil was unable to provide any documentation of the design, construction, and safe operating pressure for Tank 7, or three other identical process tanks (Tanks 5, 6, and 8).
  • Marcus Oil had altered Tanks 5 - 8: each had a 24-inch diameter temporary opening cut into one end to install a steam pipe that heated the wax above the melting temperature.
  • Marcus Oil did not use a qualified welder or proper welding procedure to reweld the plate on the vessel heads and install the steam pipe nozzles in the shells.
  • Marcus Oil personnel also acknowledged that they did not hydrostatically pressure test the vessels after the welding was completed.
  • The CSB also found that Tanks 5, 6, 7, and 8, the nitrogen storage vessels, and the compressed air storage vessel were not equipped with pressure relief devices as required by the ASME Boiler and Pressure Vessel Code.
  • Texas did not require Marcus Oil to comply with the ASME Code for new pressure vessels.
  • Texas does not require use of the National Board Inspection Code (NB-23) for pressure vessel repairs and alterations.
  • The City buildings department did not verify that Marcus Oil had correctly implemented the city ordinance on these pressure vessels or the four wax storage tanks before Marcus Oil began operating the process equipment.
  • The connection installed by Marcus Oil between the nitrogen and the compressed air systems increased the oxygen concentration in the inerting gas to an unsafe level.

Failed Safeguards or Barrier Breakdowns

  • The weld used to reclose the temporary opening on Tank 7 failed.
  • The repair weld did not meet generally accepted industry quality standards for pressure vessel fabrication.
  • The original, flame-cut surface was not ground off the plate edges before rewelding the joint.
  • The weld did not penetrate the full thickness of the vessel head.
  • The welds contained excessive porosity (holes from gas bubbles in the weld).
  • Marcus Oil did not use a qualified welder or proper welding procedure.
  • Marcus Oil did not hydrostatically pressure test the vessels after the welding was completed.
  • The nitrogen system pressure occasionally dropped below the minimum pressure required to transfer the material from the tanks because the generator could not keep up with the process demand.
  • Personnel did not realize that adding air directly into the nitrogen system contaminated the gas in the nitrogen storage vessels with oxygen.
  • The nitrogen storage vessels were not equipped with pressure relief devices as required by the ASME Boiler and Pressure Vessel Code.
  • Tanks 5, 6, 7, and 8, the nitrogen storage vessels, and the compressed air storage vessel were not equipped with pressure relief devices as required by the ASME Boiler and Pressure Vessel Code.
  • The City buildings department did not verify that Marcus Oil had correctly implemented the city ordinance on these pressure vessels or the four wax storage tanks before Marcus Oil began operating the process equipment.

Recommendations

  1. 2005-02-I-TX-R1 | Recipient: Marcus Oil | Status: Not provided | Summary: Implement the requirements of the National Board Inspection Code (NB-23) to repair all pressure vessels that have been altered at the facility. Require the application of NB-23 to all future pressure vessel repairs and alterations.
  2. 2005-02-I-TX-R2 | Recipient: Marcus Oil | Status: Not provided | Summary: Install pressure relief devices on all pressure vessels as required by the ASME Boiler and Pressure Vessel Code, Section VIII.
  3. 2005-02-I-TX-R3 | Recipient: Marcus Oil | Status: Not provided | Summary: Require all newly installed pressure vessels to conform to the requirements of the ASME Boiler and Pressure Vessel Code, Section VIII.
  4. 2005-02-I-TX-R4 | Recipient: Marcus Oil | Status: Not provided | Summary: Train personnel on the safe operation and maintenance of the nitrogen system and the importance of controlling oxygen contamination in the inerting gas.
  5. 2005-02-I-TX-R5 | Recipient: City of Houston | Status: Not provided | Summary: Amend the city building ordinances to require all newly installed pressure vessels to comply with the ASME Boiler and Pressure Vessel Code, Section VIII.
  6. 2005-02-I-TX-R6 | Recipient: City of Houston | Status: Not provided | Summary: Amend the city building ordinances to require pressure vessel repairs and alterations to comply with the National Board Inspection Code (NB-23).

Key Engineering Lessons

  • An inerting system must provide an oxygen-deficient gas that is unable to support combustion; never connect inerting systems to any system that can contaminate the inerting gas with oxygen.
  • Pressure vessel repairs and alterations must follow recognized codes and standards, including the National Board Inspection Code (NB-23), and newly installed pressure vessels must conform to the ASME Boiler and Pressure Vessel Code, Section VIII.
  • Pressure vessels operated above 15 psig should be equipped with a correctly sized and certified pressure relief device as required by the ASME Boiler and Pressure Vessel Code.
  • Weld quality and post-repair testing are critical for altered pressure vessels; qualified welders, proper welding procedures, and hydrostatic pressure testing are necessary safeguards.

Source Notes

  • All fields were consolidated from the provided final report extract, which has source_priority 1 and therefore overrides lower-priority sources.
  • No external facts were added; empty strings were preserved where the source text did not provide a value.

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