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Millard Refrigerated Services Ammonia Release

Overview

On August 23, 2010, at the Millard Refrigerated Services facility in Theodore, Alabama, a hydraulic shock event occurred during restart of the ammonia refrigeration system after a 7-hour power outage. The event caused catastrophic failure of roof-mounted piping and release of more than 32,000 pounds of anhydrous ammonia. Workers and members of the public were exposed. Hospital admissions and intensive care cases were reported. Product and equipment were contaminated.

Incident Snapshot

Field Value
Facility / Company Millard Refrigerated Services, Inc.
Location Theodore, Alabama
Incident Date 08/23/2010
Investigation Status The CSB issued its final report at a public meeting in East Rutherford, NJ on January 15, 2015.
Accident Type Anhydrous ammonia release
Final Report Release Date 01/15/2015

What Happened

A 7-hour power outage occurred before the incident. During restart of the plant’s ammonia refrigeration system, a hydraulic shock event occurred. The hydraulic shock caused a roof-mounted 12-inch suction pipe to catastrophically fail. The pressure developed by the hydraulic shock also caused an evaporator coil inside the facility to rupture.

Shortly before 9:00 am on August 23, 2010, the facility was loading two international ships with frozen poultry when the refrigeration system experienced the event and released 32,100 pounds of anhydrous ammonia. After the Operations Manager announced the release on the Millard facility intercom, three Millard employees went onto the roof in an attempt to mitigate the leak by closing manual valves that supplied ammonia to the blast freezer.

Approximately 15 minutes after being notified of the release, the Millard Plant Manager called 9-1-1. The Mobile Fire Department arrived 16 minutes later with hazardous materials technicians to assess and mitigate the release. The Theodore Volunteer Fire Department also responded on the south side of the canal. The U.S. Coast Guard halted water traffic on the industrial canal.

Millard employees on the roof closed valves and reported the release secured by 1:20 pm, about 4 hours after the initial release.

Facility and Process Context

Millard Refrigerated Services operated a 143,000-pound ammonia refrigeration system that supplied five product storage freezers and three blast freezers. The facility operated as a marine export facility that sent frozen meat abroad.

The refrigeration system was designed to handle liquid ammonia at a minimum temperature of -40°F to a maximum temperature of 110°F, and the normal design system operating pressure ranged from 8.8 inches of mercury vacuum to 210 psig.

The facility’s blast freezer was a 24,000 square foot space that operated at extremely low temperatures to rapidly chill or freeze product.

Consequences

  • Fatalities: 0
  • Injuries:
  • One Millard employee sustained injuries after briefly losing consciousness from ammonia inhalation.
  • One Millard employee injured his leg when he fell to the ship’s deck.
  • A second Millard employee sought medical treatment for heat exhaustion.
  • Nine ship crew members sought medical attention for ammonia exposure.
  • 143 offsite contractors reported exposure.
  • 32 workers were admitted to the hospital.
  • 4 were placed in intensive care.
  • Environmental release:
  • More than 32,000 pounds of anhydrous ammonia were released.
  • The release was also described as 32,100 pounds.
  • The ammonia cloud traveled 0.25 miles across the river adjacent to the plant.
  • Ammonia concentrations as high as 7,275 ppm were reported in the contaminated blast freezer.
  • Colorimetric gas detection tubes on the dock detected 250 to 450 ppm.
  • The U.S. Coast Guard reported concentrations of 500 to 600 ppm inside the Millard facility.
  • Facility damage:
  • A roof-mounted 12-inch suction pipe catastrophically failed.
  • An evaporator coil inside the facility ruptured.
  • A portion of the system’s blast freezer evaporator header ruptured.
  • The evaporator piping manifold and rooftop piping ruptured.
  • The failed evaporator coil contaminated 8 million pounds of poultry and packaging material.
  • Operational impact:
  • The refrigeration system experienced a hydraulic shock event during restart after a 7-hour power outage.
  • All other Millard employees evacuated the facility.
  • The release was secured by 1:20 pm, about 4 hours after the initial release.
  • The blast freezer evaporator defrost timer was originally intended to run the pump-out cycle for 20 minutes but only lasted 15 minutes on the days leading up to the incident.
  • The release caused interruption of operations and contamination of product and packaging material.
  • The Coast Guard halted water traffic in the industrial canal.

Key Findings

Immediate Causes

  • hydraulic shock
  • condensation-induced shock
  • vapor-propelled liquid
  • the manual clearing of an ammonia alarm in the control system interrupted the defrost cycle
  • the evaporator switched directly from defrost mode into refrigeration mode without bleeding hot gas from the evaporator coil
  • the suction stop valve was prematurely opened
  • the low-temperature liquid and hot gas mixed in the same pipe, causing the hot gas void to collapse as it rapidly condensed to a liquid

Contributing Factors

  • a 7-hour power outage
  • manual bypass of the programmed defrost sequence
  • lack of restricted access to control system modifications
  • short pump-out time
  • the blast freezer evaporators were grouped to one set of control valves
  • four evaporator coils were grouped to one set of control valves
  • large internal coil volume
  • low-load period in the middle of the night
  • the control system contained a programming error that permitted the system to go from soft gas directly to refrigeration mode without bleeding the high pressure from the coil or preventing the low-temperature suction valve from opening
  • other blast cell evaporators kept operating and ammonia fed to the ruptured suction line
  • the employees attempted to isolate the source of the leak instead of shutting down the ammonia system

Organizational and Systemic Factors

  • Millard documentation stated that the control system contained a programming error
  • Millard’s investigation report found that the manual clearing of an ammonia alarm in the control system interrupted the defrost cycle
  • the manual clearing was enabled by a lack of restricted access to control system modifications
  • the Millard emergency procedure instructed personnel to locate the leak and attempt to isolate the leak
  • the procedure stated that control of an ammonia leak will require that the leak be stopped by valving off a pipe or piece of equipment
  • the procedure defined criteria for when the e-stop button could be activated including natural disasters or any other emergency when shutdown is deemed necessary by authorized personnel
  • the plan engineer made the decision to locate and isolate the release instead of shutting down the ammonia system

Failed Safeguards or Barrier Breakdowns

  • the control system did not recognize that the blast freezer evaporator unit contained high-pressure hot gas when it allowed the suction stop valve to open during the system restart
  • password-protected controls were not in place
  • the bleed phase was omitted
  • the defrost control sequence did not automatically bleed any coil that was in defrost prior to the power outage upon restart
  • the pump-out cycle lasted 15 minutes instead of the originally intended 20 minutes
  • the emergency stop button was not activated earlier
  • the employees attempted to isolate the release while the refrigeration system was running

Recommendations

  1. Recommendation 1
    Recipient: Not specified
    Status: Not specified
    Summary: For the design of ammonia refrigeration systems, avoid grouping multiple evaporators to a single set of control valves. This is especially important for large capacity evaporators in excess of 20 tons. Evaporators with hot gas defrost systems should be controlled by individual valve control groups dedicated to each evaporator coil.

  2. Recommendation 2
    Recipient: Not specified
    Status: Not specified
    Summary: Program or configure defrost control systems with interlocks to ensure the low-temperature liquid feed and hot gas remain isolated during the initiation and termination of the hot gas defrost cycle in the event of a power outage, cycle interruption, or other abnormal situation. Program the defrost control sequence to automatically depressurize or bleed the coils in defrost upon restart after an outage or interruption, prior to opening the suction stop valve to set the evaporator into cooling mode.

  3. Recommendation 3
    Recipient: Not specified
    Status: Not specified
    Summary: Avoid the manual interruption of evaporators in defrost and equip control systems with password protected controls to ensure only trained and authorized personnel have the authority to manually override system processes.

  4. Recommendation 4
    Recipient: Not specified
    Status: Not specified
    Summary: For time-initiated hot gas defrost systems, ensure pump-out times are long enough to remove a sufficient amount of residual liquid refrigerant in the evaporator coils prior to introducing hot gas, especially after low-load periods or power outages.

  5. Recommendation 5
    Recipient: Not specified
    Status: Not specified
    Summary: In the event of an ammonia release that cannot be promptly isolated, activate the emergency shut-down switch to de-energize pumps, compressors and valves instead of attempting to isolate leaking equipment while the refrigeration system is running. Shutting down the equipment will stop the circulation of ammonia and limit the release of additional ammonia from components running upstream of failed equipment or piping.

Key Engineering Lessons

  • Hot gas defrost systems should be designed so that low-temperature liquid feed and hot gas remain isolated during initiation and termination, including after power outages or other interruptions.
  • Defrost control sequences should automatically depressurize or bleed coils in defrost before opening the suction stop valve to return the evaporator to cooling mode.
  • Multiple evaporators grouped to a single set of control valves can increase the potential for a hydraulic shock event and should be avoided for large capacity evaporators.
  • Manual interruption of evaporator defrost cycles should be prevented or tightly controlled through password-protected access for trained and authorized personnel only.
  • Pump-out times must be long enough to remove residual liquid refrigerant from evaporator coils before introducing hot gas, especially after low-load periods or power outages.
  • If an ammonia release cannot be promptly isolated, the emergency shut-down switch should be used to de-energize pumps, compressors, and valves rather than attempting to isolate the leak while the refrigeration system is running.

Source Notes

  • Priority 1 final report facts were used to resolve conflicts and establish the authoritative incident description.
  • The supporting news release confirms the incident location and public impact but provides limited technical detail.
  • The transcript contains additional discussion of the Millard incident and also includes unrelated US Ink material; only Millard facts were used here where explicitly stated.
  • Where the source documents used slightly different wording or quantities, the final report wording was preferred.

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