Skip to content

DPC Enterprises Glendale Chlorine Release

Overview

On November 17, 2003, chlorine gas was released from the DPC Enterprises chlorine repackaging facility in Glendale, Arizona, near Phoenix. The release occurred when chlorine vapors from a rail car unloading operation escaped from a scrubber system designed to recapture the material. Excess chlorine vented to the scrubber depleted the active caustic scrubbing material, over-chlorinated the scrubber, and triggered a bleach decomposition reaction that released toxic gases into the surrounding community. Fourteen people, including police officers and residents, required medical treatment.

Incident Snapshot

Field Value
Facility / Company DPC Enterprises, L.P.
Location Glendale, AZ
Incident Date 11/17/2003
Investigation Status The CSB's final report was issued at a news conference in Glendale on February 28, 2007.
Accident Type Chlorine gas release from a chlorine repackaging facility
Final Report Release Date 02/28/2007

What Happened

  • At about 11:30 a.m. on November 17, 2003, an uncontrolled decomposition reaction in a batch scrubber released chlorine gas into the air at the DPC Enterprises, L.P. chlorine repackaging facility in Glendale, Arizona.
  • At approximately 7:00 that morning, operators recorded the ORP meter reading of 490 mV and tested the solution in the scrubber; they measured a caustic concentration of 1.60 percent caustic (by weight).
  • Shortly after 9:00 a.m., the operators began transferring chlorine to the bulk road trailer.
  • At 10:00 a.m., operators recorded an ORP reading of 510 mV, again tested the scrubber’s contents, and recorded the caustic concentration at 1.18 percent.
  • According to the operators, the first safety alarm on the caustic scrubber, set at an ORP reading of 530 mV, activated at approximately 10:15 a.m.; an operator pressed the acknowledge button to silence the alarm, checked the ORP value, and returned to other tasks.
  • Shortly after 11:00 a.m., the second safety alarm, set at an ORP reading of 545 mV, activated; an operator pressed the alarm acknowledge button to silence the alarm and went to get a container for a scrubber solution sample.
  • Upon returning to the scrubber area, the operator heard rumbling and saw liquid splashing from, and a green cloud forming around, the scrubber.
  • The operator instructed nearby personnel to evacuate and pushed an emergency shutdown button, which closed automatic valves on the loading line and the scrubber vent line connected to the bulk road trailer.
  • He activated the plant’s emergency alarm, and evacuated with other DPC employees to the designated assembly area.
  • DPC’s plant manager called 911 and then telephoned neighboring businesses to inform them of the release.
  • The Phoenix Fire Department was first to arrive on the scene, and were joined by the Glendale and Tempe fire departments and the Glendale and Phoenix police departments.
  • Responders established initial boundaries for the potentially hazardous area, and later expanded the boundaries when plume modeling by the Tempe Fire Department indicated that the potentially hazardous area could be larger.
  • Authorities used an automated telephone call-down system and media announcements to notify the community in the potentially hazardous area to evacuate.
  • Police officers also drove through the evacuation area and used their public address systems to notify residents.
  • Responders set up water sprays to absorb chlorine gas, and entered the site at approximately 1:30 p.m. to close manual valves associated with the railcar, bulk road trailer, and scrubber.
  • Phoenix Fire Department responders measured chlorine concentrations of 20-35 parts per million (ppm) close to the scrubber, with higher spikes when gases periodically vented.
  • The rate of venting eventually decreased, and all evacuees were allowed to return to their homes about four and one half hours after the over-chlorination of the scrubber.
  • Minor venting of chlorine from the scrubber continued until DPC personnel added caustic to the scrubber to stabilize the contents and absorb any remaining chlorine.
  • No further emissions were detected.
  • As a result of the incident, 11 police officers and five citizens were evaluated for symptoms consistent with chlorine exposure.

Facility and Process Context

  • DPC Enterprises, L.P., is privately held and owns and operates six chlorine repackaging facilities.
  • The company employs 50, including nine at the Glendale site.
  • A centralized group in Houston provides management, engineering, health, safety, environmental, and security services to both DPC Enterprises and DPC Industries.
  • At the site, DPC receives liquid chlorine in railcars and repackages it into smaller containers to distribute to local customers, and also manufactures sodium hypochlorite (or bleach).
  • The caustic scrubbers used to control chlorine emissions are located in the southwest section, adjacent to the chlorine railcar unloading and bulk road trailer loading area.
  • The chlorine building contains cylinder loading and bleach manufacturing facilities.
  • The Glendale scrubbers have two purposes: to capture chlorine vented from repackaging operations and to produce saleable bleach (sodium hypochlorite solution) for distribution to local industrial and commercial customers.
  • The two Glendale scrubbers are 4,000 gallon, fiberglass reinforced plastic tanks.
  • They operate as batch chemical reactors, with one unit receiving chlorine (the online scrubber), and the other operating as a backup (the standby scrubber).
  • The Glendale site is covered by OSHA’s Process Safety Management (PSM) regulation and EPA’s Risk Management Program (RMP) regulation.
  • The caustic scrubber is also permitted by Maricopa County as an air pollution control device.

Consequences

  • Fatalities: 0
  • Injuries: Five residents and 11 police officers sought medical attention for symptoms of chlorine exposure and were treated and released.
  • Environmental release: A chlorine gas release occurred; the CSB estimated the scrubber could have released up to 1,920 pounds of chlorine. Hazardous emissions continued for about six hours.
  • Facility damage: No facility damage was reported in the provided record.
  • Operational impact: Residents and workers in a 1.5 square mile zone were told to evacuate. All evacuees were allowed to return to their homes about four and one half hours after the over-chlorination of the scrubber.

Key Findings

Immediate Causes

  • Excess chlorine vented to the scrubber, where it completely depleted the active scrubbing material (caustic) and over-chlorinated the scrubber.
  • The resulting bleach decomposition reaction released a cloud of toxic gases into the surrounding community.
  • An uncontrolled decomposition reaction in a batch scrubber released chlorine gas into the air at the DPC Enterprises, L.P. chlorine repackaging facility in Glendale, Arizona.

Contributing Factors

  • Contrary to procedure, practice at the DPC site was to continue chlorine flow to the scrubber during quality control testing.
  • Management did not detect this deviation.
  • The bleach production (scrubber) SOP did not reflect the sensitivity of the process to over-chlorination.
  • The SOP specified no actions to be taken upon receipt of the fourth (final) alarm.
  • The SOP did not document which operations produced high rates of chlorine venting.
  • The SOP was available for employee review, but was not routinely used in daily operation.
  • Operator training, based on the operating procedure, did not address the sensitivity of the scrubber to over-chlorination or the safety and environmental consequences of over-chlorination.
  • The ORP meter readings were susceptible to errors due to temperature swings, changes in the initial caustic concentration, variation in the chemistry of the water used to prepare the caustic solution, sensor fouling, and installation-specific factors.
  • The Glendale Police Department officers were not wearing their APRs when they were exposed because the incident command system did not deliver timely information, police dispatchers sent officers directly into the isolation zone without first directing them to a staging area, and some officers carrying APRs failed to use them.
  • The exposed officers had also not received their annual hazardous materials refresher training.
  • Published guidance on scrubber over-chlorination provided no specific information on the composition, quantity, or duration of emissions expected during over-chlorination incidents, delaying stabilization of the scrubber and extending the duration of the incident.

Organizational and Systemic Factors

  • DPC’s corporate standards relied solely on procedural safeguards against scrubber over-chlorination.
  • DPC’s corporate hazard assessment process did not identify or address the consequences of failure to follow the bleach manufacturing SOP, including potential off-site consequences.
  • DPC’s internal PSM/RMP audit program did not detect deficiencies in operating procedures, training, operating practice, process safety information, and hazard assessment.
  • DPC management failed to recognize that practice deviated from the written bleach production SOP.
  • The same corporate group performing the audit had also developed the site PSM program, written the site operating procedures, and participated in or led the site PHAs.
  • DPC Enterprises’ sites have not yet been verified and certified under a voluntary safety system.
  • Inadequate integration of the Glendale Police Department into the incident command structure prevented the timely transmission of critical safety information to responding officers.
  • Deployment of Glendale Police Department officers into chlorine-impacted area without briefing or safety equipment checks allowed them to enter hazardous locations without APRs.
  • Inadequate hazardous material training led to Glendale Police Department officers not wearing their APRs.

Failed Safeguards or Barrier Breakdowns

  • DPC relied on a single administrative safeguard to prevent scrubber over-chlorination: an SOP.
  • No automated control actions occur based on the ORP meters’ outputs.
  • The first ORP alarm required acknowledgment, remaining in area, and sampling at 15-minute intervals.
  • The second ORP alarm had no action specified.
  • The third ORP alarm required acknowledgment and, if venting at a high rate, stopping chlorine flow to scrubber and sampling at 5-minute intervals.
  • The fourth ORP alarm had no action specified.
  • The SOP specified no actions to be taken upon receipt of the fourth (final) alarm.
  • The SOP did not provide clear warning that the time between the final alarm and over-chlorination could be brief.
  • The PHAs did not review the scrubber operating procedure and did not directly address failure to turn off the chlorine flow to the scrubber at the end of a batch.
  • The Glendale PHAs did not identify and address the known scrubber failure mode of over-chlorination.
  • The PSM-required audit failed to detect the missing process safety information and scrubber operating procedure problems uncovered during the CSB’s investigation.
  • The audit did not rigorously examine the underlying PSM program elements.
  • The Glendale system’s call-down system had technical and coordination issues that caused confusion.
  • The Glendale Police Department officers did not wear their APRs when they were exposed.
  • The published guidance available at the time of the incident did not recommend specific safeguards to prevent, control, or mitigate the consequences of scrubber over-chlorination.

Recommendations

  1. 2004-2-I-AZ-R1 | Recipient: DPC Enterprises, L.P. | Status: Closed-Acceptable Action | Establish and implement DPC corporate engineering standards that include adequate layers of protection on chlorine scrubbers at DPC facilities, including: additional interlocks and shutdowns, such as automatically stopping chlorine flow to the scrubber upon oxidation-reduction potential alarm; mitigation measures, such as systems to automatically add caustic to over-chlorinated scrubbers, or back-up scrubbing capability to treat emissions from over-chlorinated scrubbers; increases in the final caustic concentration in the scrubbers to eight percent or higher to provide a substantial safety margin against over-chlorination; and use of the site's continuous bleach manufacturing system to convert scrubber solution to saleable bleach.
  2. 2004-2-I-AZ-R2 | Recipient: DPC Enterprises, L.P. | Status: Closed-Acceptable Action | Revise scrubber SOPs to include clearly described operating limits and warnings about the consequences of exceeding those limits, and the safety and environmental hazards associated with scrubber over-chlorination.
  3. 2004-2-I-AZ-R3 | Recipient: DPC Enterprises, L.P. | Status: Closed-Acceptable Action | Train employees on the revised SOPs and include a test to verify understanding. Periodically review operator understanding of and conformance to the scrubber SOPs.
  4. 2004-2-I-AZ-R4 | Recipient: DPC Enterprises, L.P. | Status: Closed-Acceptable Action | Include scrubber operation in facility PHAs. Ensure that they include lessons learned from this incident and other DPC scrubber incidents, as well as industry experience with over-chlorination, and consider off-site consequences when evaluating the adequacy of existing safeguards.
  5. 2004-2-I-AZ-R5 | Recipient: DPC Enterprises, L.P. | Status: Closed-Acceptable Action | Use a qualified, independent auditor to evaluate DPCs PSM and RMP programs against best practices. Implement audit recommendations in a timely manner at all DPC chlorine repackaging sites.
  6. 2004-2-I-AZ-R6 | Recipient: DPC Enterprises, L.P. | Status: Closed-Acceptable Action | Implement a recognized safety management system, including third party verification and certification, to achieve documented continuous improvement in safety performance at Glendale and the other DPC chlorine repackaging sites.
  7. 2004-2-I-AZ-R13 | Recipient: Maricopa County Air Quality Department | Status: R13: Closed-Acceptable Action | Revise DPC’s permitted operating conditions to specify a minimum scrubber caustic concentration of 8 percent or more, as determined by laboratory measurement, with measurements taken daily and upon completion of each scrubber batch.
  8. 2004-2-I-AZ-R14 | Recipient: Chlorine Institute (CI) | Status: Closed – Acceptable Action | Clarify the chemistry involved in over-chlorination incidents so that "Chlorine Scrubbing Systems, Pamphlet 89," and other pertinent publications ensure that the recommended practices and safeguards prevent, mitigate, and control hazardous releases due to bleach decomposition, and provide sufficient detail on the safety and environmental consequences of over-chlorination to enable companies to provide emergency responders with information on the potential characteristics of over-chlorination events, and on the best means of mitigating the bleach decomposition reaction following a release.

Key Engineering Lessons

  • Do not rely on a single administrative safeguard, such as an SOP, to prevent scrubber over-chlorination.
  • Provide automatic interlocks or shutdowns tied to ORP alarms so chlorine flow can be stopped before the scrubber is over-chlorinated.
  • Maintain a substantial caustic concentration margin in the scrubber; the CSB recommendations specify eight percent or higher.
  • Include scrubber operation in process hazard analyses and explicitly evaluate the known over-chlorination failure mode and off-site consequences.
  • Operating procedures and training must clearly address the sensitivity of the scrubber to over-chlorination and the consequences of exceeding operating limits.
  • Emergency response guidance for over-chlorination must describe the expected emissions and the means of mitigating bleach decomposition after a release.

Source Notes

  • Priority 1 final report content was not separately provided in the extracts; the consolidated incident record relies on the CSB final report-derived supporting document and recommendation status pages.
  • Where multiple documents differed, higher-priority source content was used for recommendation status and technical findings.
  • The community meeting transcript was used only for statements made during the meeting and not as a substitute for final investigative findings when those were available elsewhere in the provided extracts.

Similar Incidents

Incidents sharing the same equipment, root causes, or hazard types.

Same Equipment

Same Root Cause

Same Hazard

← View in Knowledge Graph