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Delaware City Refining Company

Overview

On November 29, 2015, an operator at the Delaware City Refining Company’s sulfuric acid alkylation unit suffered second-degree burns to the face and neck and third-degree burns to the wrist from a flash fire while operations personnel were preparing equipment for maintenance by de-inventorying and draining vessels. The event occurred when hydrocarbons backflowed from a pressurized depropanizer column through a leaking closed isolation valve into vessels being drained to the oil water sewer, where the material ignited.

Incident Snapshot

Field Value
Facility / Company Delaware City Refining Company (DCRC)
Location Delaware City, DE
Incident Date 11/29/2015
Investigation Status The CSB's final report was released at a news conference in Wilmington, DE on May 18, 2017.
Accident Type Oil and Refining - Fire and Explosion
Final Report Release Date 05/18/2017

What Happened

  • On Sunday, November 29, 2015, an operator at the Delaware City Refining Company’s sulfuric acid alkylation unit suffered second-degree burns to the face and neck and third-degree burns to the wrist from a flash fire.
  • The incident occurred when operations personnel were preparing equipment for maintenance work by de-inventorying and draining vessels located between two isolation points.
  • A single block valve isolated the vessels being decontaminated from a pressurized and inventoried depropanizer column containing hydrocarbons; unknown to operations personnel, the valve leaked in the closed position, resulting in backflow of flammable material from the depropanizer.
  • When an operator opened the vessel drain valve to empty what he assumed was condensate water from the vessel to the oil water sewer, the hydrocarbons from the depropanizer also released to the sewer and ignited, resulting in a flash fire.
  • Earlier in the week, operators planned to isolate the caustic settler from the pumps by closing the caustic drain valve, but four days prior to the incident operators learned that the caustic drain valve was leaking and therefore would not seal properly.
  • Because the valve would not seal, operations expanded the isolation to another block valve downstream of the coalescer and settler.
  • The dayshift operator shut down the propane flow, filled the caustic settler with process condensate water, connected a nitrogen hose, and then removed the depropanizer from service by closing the column’s inlet and outlet valves.
  • The dayshift operator drained some condensate water to a quench drum and then drained the rest to the oil water sewer and began the second and final condensate water filling until shift change.
  • The nightshift operator reviewed the dayshift operator’s handwritten list of steps and proceeded with draining the vessels from the second condensate wash.
  • At about 6:00 pm the nightshift operator opened the drain valve to the OWS to drain the carry-over condensate water into the OWS system.
  • Shortly after opening the 3-inch drain valve to the OWS, he heard a pop and saw a wall of fire advancing toward him.
  • He left the area and turned on stationary fire monitors.
  • The fire burned for approximately one hour before onsite refinery emergency response teams extinguished the fire by isolating the fuel source.

Facility and Process Context

  • The incident occurred in the caustic wash section of the alkylation unit.
  • Caustic sodium hydroxide is used to scrub trace amounts of sulfur dioxide from the purge streams in a caustic settler.
  • The solution is then fed to a coalescer or “carry over” vessel to ensure the caustic and water mixture is removed from the hydrocarbon stream prior to entering the depropanizer column.
  • The alkylation unit at DCRC receives butylenes from the fluid coker unit and the fluid catalytic cracker (FCC) unit.
  • The alkylation unit contains two alkylation reactor sections, each comprised of two reactors that run in parallel.
  • The depropanizer column maintains optimal propane concentration in the unit by removing excess propane.
  • Prior to entering the depropanizer column, the alkylation reactor purge streams travel through a caustic wash system to mitigate the risk of corrosion by removing trace amounts of sulfur dioxide.
  • The facility has two sewer systems: an OWS system and a storm water sewer.
  • The OWS runs through a monitor box to the wastewater treatment plant.

Consequences

  • Fatalities: 0
  • Injuries: 1
  • Environmental release: hydrocarbons from the depropanizer also released to the sewer
  • Facility damage: Not specified in the source extract
  • Operational impact: The fire burned for approximately one hour before onsite refinery emergency response teams extinguished the fire by isolating the fuel source.

Key Findings

Immediate Causes

  • The four-inch coalescer outlet gate valve was later determined to have a slow leak through the valve seat, and likely allowed hydrocarbons in the depropanizer column to backflow into the caustic settler and coalescer during the equipment preparation activity.
  • A furnace, located about 120 feet east of the caustic settler, most likely ignited the hydrocarbons when the nightshift operator began draining the contents of the vessels into the OWS.

Contributing Factors

  • The refinery lacked specific procedures for removing the caustic settler and coalescer from service.
  • DCRC also lacked any general procedures to ensure that preparing equipment for maintenance, prior to lockout-tagout (LOTO), was done safely.
  • DCRC relied on an isolation method that used a single closed isolation valve, which was later determined to have a slow leak, to act as a barrier between the hydrocarbons in the depropanizer and the vessels they were draining.
  • The single closed isolation valve method did not adequately isolate the depropanizer column from the coalescer and caustic settler to prevent hydrocarbons in the column from backflowing into the vessels and providing a flammable atmosphere when the nightshift operator began draining the vessels into the OWS system.
  • When alkylation unit operators expanded the isolation to include the coalescer and caustic settler, they closed only one valve between the depropanizer column and the equipment they were attempting to isolate.
  • The expanded isolation plan was much more complex than the original, as it involved isolating more equipment and de-inventorying vessels.
  • DCRC had no procedure or process to ensure that isolation points were holding prior to starting equipment preparation activities.
  • Nor was there a process or procedure to require the use of more protective forms of isolation for equipment preparation activities when possible.
  • The pathway for the OWS is located in an orientation that brings it in close proximity to the operating furnace, with the closest drain about 25 feet from a furnace.

Organizational and Systemic Factors

  • An operator looked for a procedure or job aid for the task of taking those vessels out of service; however, no job aid or procedure existed, so the dayshift operator and board operator developed informal instructions for isolating and de-inventorying that equipment.
  • DCRC stated that an MOC did not apply to the maintenance preparation activity or the expansion of the isolation scheme because there was no standing procedure, job aid, or instruction from which to deviate.
  • The CSB concludes that the new plan should have been considered a change.
  • DCRC created a handwritten list of steps for performing the maintenance preparation activity and did not deviate from an existing procedure when expanding the isolation.
  • In the absence of a robust MOC program, operating decisions are based solely on experience without the benefit of a hazard analysis.

Failed Safeguards or Barrier Breakdowns

  • A single block valve isolated the vessels being decontaminated from a pressurized and inventoried depropanizer column containing hydrocarbons.
  • The valve leaked in the closed position.
  • No job aid or procedure existed for taking the coalescer and caustic settler out of service.
  • DCRC lacked general procedures for preparing equipment for maintenance and processes for identifying and addressing potential hazards associated with those tasks.
  • DCRC had no formalized process, general procedure, or job aid for ensuring equipment was properly isolated, de-inventoried, or decontaminated prior to maintenance work.
  • DCRC relied on an isolation method that used a single closed isolation valve.
  • DCRC had no procedure or process to ensure that isolation points were holding prior to starting equipment preparation activities.
  • There was no process or procedure to require the use of more protective forms of isolation for equipment preparation activities when possible.

Recommendations

  1. Recommendation ID: Not provided. Recipient: Not provided. Status: Not provided. Summary: Not provided in the source extract.
  2. Recommendation ID: Not provided. Recipient: Not provided. Status: Not provided. Summary: Not provided in the source extract.

Key Engineering Lessons

  • Operational tasks that involve preparing equipment for maintenance can be uncommon and non-routine; procedures should explicitly cover preparation of process equipment for maintenance.
  • Pre-planning and hazard identification should occur before equipment preparation work begins, including assessment of hazards introduced by the work and steps to mitigate them.
  • Single block valves should not be relied upon to isolate hazardous energy sources for de-inventorying or decontaminating activities prior to maintenance work, even when newly installed.
  • If an equipment preparation task or isolation plan must be modified or expanded because of leaking valves or changing conditions, the change should be evaluated for new hazards.
  • Materials containing, or potentially containing, hydrocarbons or flammables should not be drained into the sewer or other systems not specifically designed for such service, especially where ignition sources are present.
  • Effective preplanning and hazard identification are necessary for proper equipment isolation and decontamination.
  • MOC systems should include non-traditional changes that do not fit into common MOC categories.

Source Notes

  • All facts were taken from the provided final report extract, which has source_priority 1 and therefore overrides lower-priority information.
  • The recommendations section in the source extract did not provide recommendation IDs, recipients, statuses, or summaries.
  • The incident is described in the source as a flash fire at the Delaware City Refinery in the caustic wash section of the alkylation unit.

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