{"operation":"document","citation":"PLD21FR002","title":"Atmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance","source_type":"incident","agency":"National Transportation Safety Board","status":"current","official":true,"published_on":"2022-11-02","effective_on":"2021-06-28","summary":"Accident. in Farmersville, TX, USA. on 2021-06-28. Atmos Energy. Rupture/fire","machine_formats":{"json":"https://regulus.evalyn.ai/document/ntsb-case-pld21fr002.json","markdown":"https://regulus.evalyn.ai/document/ntsb-case-pld21fr002.md"},"app_url":"https://regulus.evalyn.ai/document/ntsb-case-pld21fr002","source_url":"https://www.ntsb.gov/investigations/Pages/PLD21FR002.aspx","body":"NTSB investigation PLD21FR002.\n\nEvent Type: Accident\n\nEvent Date: 2021-06-28\n\nEvent City: Farmersville\n\nEvent State Or Region: TX\n\nEvent Country: USA\n\nPipeline Operator: Atmos Energy\n\nPipeline Type: Gas Transmission - Regulated\n\nAccident Type: Rupture/fire\n\nCompletion Status: Completed\n\nReport Number: Unknown\n\nReport Date: 2022-10-19\n\nProbable cause: The National Transportation Safety Board determines that the probable cause of the June 28, 2021, explosion was a leaking mainline valve that allowed natural gas to enter the launcher where it mixed with air, creating a flammable gas–air mixture that was ignited by an undetermined source. Contributing to the explosion and its severity were Atmos Energy Corporation’s procedures and training practices that did not prepare workers to recognize and safely respond to abnormal operating conditions.\n\nTier1Name: Pigging\n\nTier2Name: Explosion\n\nTier1Name: Pigging\n\nTier2Name: System/equipment malf/failure\n\nTier1Name: Pigging\n\nTier2Name: Maintenance event\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Management\n\nFinding Tier3Name: Policy/procedure\n\nFinding Modifier Name: Pipeline operator\n\nFinding Report Text: Organizational - Management - Policy/procedure - Pipeline operator\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Support/oversight/monitoring\n\nFinding Tier3Name: Training\n\nFinding Modifier Name: Pipeline operator\n\nFinding Report Text: Organizational - Support/oversight/monitoring - Training - Pipeline operator\n\nFinding Tier1Name: Pipeline\n\nFinding Tier2Name: Pipline systems/equipment\n\nFinding Tier3Name: Valves\n\nFinding Modifier Name: Damaged/degraded\n\nFinding Report Text: Pipeline - Pipline systems/equipment - Valves - Damaged/degraded\n\nOfficial NTSB investigation data. NTSB findings determine probable cause and make safety recommendations; they do not adjudicate civil liability or regulatory violations.\n\nWhat Happened\nOn June 28, 2021, about 3:35 p.m. local time, a natural gas explosion occurred during routine maintenance activities at an Atmos Energy Corporation (Atmos) facility near Farmersville, Texas. [1] The natural gas ignited and exploded after workers inserted an in-line tool (or pig) into a launcher. [2] Seven workers from Atmos, FESCO, Ltd. (FESCO), and Bobcat Contracting, L.L.C. (Bobcat) were on-site at the time of the accident and performing work for Atmos.[3] The explosion occurred while the workers were removing the pig insertion tool during the sixth of a series of in-line inspections. [4] The explosion was directed through the open launcher door, ejecting the pig from the launcher, injuring two of the workers and killing two more. At the time of the accident, the weather was 75°F with light to heavy rain showers. No cloud-to-ground lightning strikes were reported within 15 miles of the accident site. Atmos estimated property damage at the site to be about $580,000.\nThe five surviving workers described the events leading up to the explosion in interviews with National Transportation Safety Board (NTSB) investigators. Shortly before the explosion, workers used a portable flaring system to vent natural gas from the launcher. [5] A worker opened the 1-inch valve that connected the flare flow line to the launcher and allowed natural gas to vent from the launcher to the flare tip, where it was successfully ignited. As natural gas pressure in the launcher decreased and less natural gas flowed to the flare tip, the work crew observed the flame die down and extinguish. The Atmos workers, who were overseeing the contractor personnel, did not expect gas to be leaking from the mainline valve, but they did note in interviews that they thought the flare system would provide a safe path for gas leaking past the mainline valve to vent to atmosphere if any leakage happened to occur. [6] The workers were not using any form of gas monitoring other than observing the flare tip. [7]\n\nWorkers began performing the remaining steps needed to load the pig about 3:28 p.m.: photographing the pig, opening the launcher door, lifting the pig into position with an excavator, attaching a grounding cable to the launcher and insertion tool, manually inserting the pig into the launcher, sliding the pig further into the launcher with the excavator, and removing the insertion tool. Before the insertion tool was completely removed, workers at the site heard a loud sound, and one worker observed a flash near the open launcher door.\n\nAn Atmos worker called 9-1-1 at 3:35 p.m. The Collin County Assistant Fire Marshal was the first emergency responder on the scene; he arrived at 3:41 p.m., determined that two of the four injured workers were deceased, and began to assist the two injured survivors. He observed no fire. Other responding agencies included the Farmersville Fire Department, the Texas Department of Public Safety, and the Collin County Sheriff’s Office. The Collin County Sheriff’s Office took control of the site and documented the accident scene. The Collin County Sheriff’s Office also called the Federal Bureau of Investigation to investigate for criminal activity. No criminal activity was found.\n\n[1] (a) For more detailed information about this investigation, see the public docket at https://data.ntsb.gov/Docket/Forms/searchdocket and search for number PLD21FR002. Use the CAROL Query to search safety recommendations and investigations. (b) All times in this report are local time unless otherwise noted.\n\n[2] A launcher is the portion of the pipeline facility used to insert in-line tools, commonly referred to as pigs, into a pipeline. Pig is a generic name for a device that is inserted into a pipeline and travels through the pipeline. There are numerous types of pigs used for inspection, cleaning, and separating products in pipelines. In this accident, the pig was a gauging pig for detecting dents or internal obstructions that could cause a pig to become stuck in a pipeline.\n\n[3] Bobcat workers were providing pig loading services and FESCO workers were providing flaring services; Atmos personnel were overseeing the overall in-line inspections and approving each step in the process.\n[4] An insertion tool is a device used to push a pig into a launcher. In this case, the insertion tool was a 16-foot-long metal pole with a cup welded to the end. (b) Pigs were successfully loaded on June 21, 22, 23, 24, and 27, 2021.\n\n[5] A portable flaring system is used to direct natural gas from the launcher through a detachable flare flow line, up a portable stack, and to a flare tip, where it is ignited and burned.\n\n[6] The mainline valve is one of two valves between the launcher and the transmission pipeline; the pig passes through the mainline valve to enter the transmission pipeline.\n\n[7] The behavior of the flare tip would not indicate the presence or absence of natural gas, but it would provide information about the pressure of gas in the launcher. As such, it provided useful information but was not a reliable indicator of a safe or hazardous atmosphere at the site.\n\nWhat We Found\nWe determined that the probable cause of the June 28, 2021, explosion was a leaking mainline valve that allowed natural gas to enter the launcher where it mixed with air, creating a flammable gas–air mixture that was ignited by an undetermined source. Contributing to the explosion and its severity were Atmos Energy Corporation’s procedures and training practices that did not prepare workers to recognize and safely respond to abnormal operating conditions.\n\nPIR-22-03\n<<<PAGE 1>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nIssued: October 19, 2022 Pipeline Investigation Report: PIR-22/03\nAtmos Energy Corporation\nNatural Gas–Fueled Explosion During\nRoutine Maintenance\nFarmersville, Texas\nJune 28, 2021\n1. Factual Information\n1.1 Accident Summary\nOn June 28, 2021, about 3:35 p.m. local\ntime, a natural gas explosion occurred during\nroutine maintenance activities at an Atmos Energy\nCorporation (Atmos) facility near Farmersville,\nTexas.\n1 The natural gas ignited and exploded after\nworkers inserted an in-line tool (or pig) into a\nlauncher.2 (See figure 1.) Seven workers from\nAtmos, FESCO, Ltd. (FESCO), and Bobcat\nContracting, L.L.C. (Bobcat) were on-site at the\ntime of the accident and performing work for\nFigure 1. Pig before the\nexplosion. (Courtesy of Bobcat\nContracting.)\n1 (a) For more detailed information about this investigation, see the public docket at\nhttps://data.ntsb.gov/Docket/Forms/searchdocket and search for number PLD21FR002. Use the CAROL\nQuery to search safety recommendations and investigations. (b) All times in this report are local time unless\notherwise noted.\n2 A launcher is the portion of the pipeline facility used to insert in-line tools, commonly referred to as\npigs, into a pipeline. Pig is a generic name for a device that is inserted into a pipeline and travels through\nthe pipeline. There are numerous types of pigs used for inspection, cleaning, and separating products in\npipelines. In this accident, the pig was a gauging pig for detecting dents or internal obstructions that could\ncause a pig to become stuck in a pipeline.\n1\n\n<<<PAGE 2>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nAtmos.3 The explosion occurred while the workers were removing the pig insertion tool\nduring the sixth of a series of in-line inspections.\n4 The explosion was directed through\nthe open launcher door, ejecting the pig from the launcher, injuring two of the workers\nand killing two more. At the time of the accident, the weather was 75°F with light to\nheavy rain showers. No cloud-to-ground lightning strikes were reported within 15 miles\nof the accident site. Atmos estimated property damage at the site to be about $580,000.\nThe five surviving workers described the events leading up to the explosion in\ninterviews with National Transportation Safety Board (NTSB) investigators. Shortly before\nthe explosion, workers used a portable flaring system to vent natural gas from the\nlauncher.5 A worker opened the 1-inch valve that connected the flare flow line to the\nlauncher and allowed natural gas to vent from the launcher to the flare tip, where it was\nsuccessfully ignited. (See figure 2.) As natural gas pressure in the launcher decreased\nand less natural gas flowed to the flare tip, the work crew observed the flame die down\nand extinguish. The Atmos workers, who were overseeing the contractor personnel, did\nnot expect gas to be leaking from the mainline valve, but they did note in interviews that\nthey thought the flare system would provide a safe path for gas leaking past the mainline\nvalve to vent to atmosphere if any leakage happened to occur.\n6 The workers were not\nusing any form of gas monitoring other than observing the flare tip.\n7\n3 Bobcat workers were providing pig loading services and FESCO workers were providing flaring\nservices; Atmos personnel were overseeing the overall in-line inspections and approving each step in the\nprocess.\n4 (a) An insertion tool is a device used to push a pig into a launcher. In this case, the insertion tool was a\n16-foot-long metal pole with a cup welded to the end. (b) Pigs were successfully loaded on June 21, 22, 23,\n24, and 27, 2021.\n5 A portable flaring system is used to direct natural gas from the launcher through a detachable flare\nflow line, up a portable stack, and to a flare tip, where it is ignited and burned.\n6 The mainline valve is one of two valves between the launcher and the transmission pipeline; the pig\npasses through the mainline valve to enter the transmission pipeline.\n7 The behavior of the flare tip would not indicate the presence or absence of natural gas, but it would\nprovide information about the pressure of gas in the launcher. As such, it provided useful information but\nwas not a reliable indicator of a safe or hazardous atmosphere at the site.\n2\n\n<<<PAGE 3>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nFigure 2. Accident site. (Courtesy of Wylie Fire Department.)\nWorkers began performing the remaining steps needed to load the pig about\n3:28 p.m.: photographing the pig, opening the launcher door, lifting the pig into\nposition with an excavator, attaching a grounding cable to the launcher and insertion\ntool, manually inserting the pig into the launcher, sliding the pig further into the launcher\nwith the excavator, and removing the insertion tool.\n8 Before the insertion tool was\ncompletely removed, workers at the site heard a loud sound, and one worker observed a\nflash near the open launcher door.\nAn Atmos worker called 9-1-1 at 3:35 p.m. The Collin County Assistant Fire\nMarshal was the first emergency responder on the scene; he arrived at 3:41 p.m.,\ndetermined that two of the four injured workers were deceased, and began to assist the\ntwo injured survivors. He observed no fire. Other responding agencies included the\nFarmersville Fire Department, the Texas Department of Public Safety, and the Collin\nCounty Sheriff’s Office. The Collin County Sheriff’s Office took control of the site and\ndocumented the accident scene. The Collin County Sheriff’s Office also called the\nFederal Bureau of Investigation to investigate for criminal activity. No criminal activity was\nfound.\n1.2 Before the Explosion\nAtmos communicated the work requirements for the in-line inspections to\nrepresentatives from FESCO and Bobcat during a stakeholder meeting on June 15,\n2021, and Atmos workers communicated requirements verbally on-site to Bobcat and\n8 A grounding cable prevents the buildup of static charge, which can result in sparks.\n3\n\n<<<PAGE 4>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nFESCO workers assigned to this task. Workers were to use Atmos’ procedure for loading\npigs, and the Bobcat foreman was aware of this procedure. Although each Atmos worker\nhad performed pigging work for no fewer than 8 years, they were not aware of Atmos’\npigging-specific procedures, which were formally documented in Appendix R of its\nPipeline Integrity Management Plan in 2019. (See section 1.6.3 for details on Atmos’\npigging procedures.)\nOn June 21, 2021, Atmos, Bobcat, and FESCO workers began a series of in-line\ninspections, and FESCO installed a portable flaring system.9 During the first pig loading\nin the series, workers suspected that the mainline valve was leaking when the flare did\nnot extinguish as expected.10 The workers adjusted the position of the mainline valve\nand found a position where the leak stopped or reduced to the point that the flare\nextinguished. They marked that position on the valve’s exterior and did not have issues\nwith the flare extinguishing during subsequent runs.\n1.3 Worker Information\nThe seven workers from Atmos, FESCO, and Bobcat on-site at the time of the\naccident had more than 50 years of cumulative pipeline experience. The NTSB’s review\nof company personnel records showed that none of the workers had a history of\ndisciplinary actions related to pipeline safety.\nAll seven workers had stop work authority, which allowed them to stop work if\nthey had a safety concern. None of the workers expressed a safety concern before the\nexplosion.\n1.4 Toxicological Testing\nRequired post-accident drug and alcohol testing of the three uninjured workers\ndid not detect any alcohol or other tested-for substances.11 Toxicological testing\nperformed by the Federal Aviation Administration Forensic Sciences Laboratory on\n9 Atmos began using flaring systems to support pig loading and launching activities in February 2021,\nabout 4 months before the explosion, to reduce methane emissions.\n10 Atmos stated that it expected its employees to rely on their experience, training, qualifications, and\njudgment to determine when reporting potential issues with valves was necessary. Atmos was not aware of\nany reports of potential issues with the mainline valve in the 10 years before the explosion.\n11 In accordance with Title 49 Code of Federal Regulations (CFR) 40.85, and as detailed in 49 CFR\n40.87, each of these three workers underwent a post-accident alcohol breath test and a urine drug test for\nmarijuana metabolites, cocaine metabolites, amphetamines, opioids, and phencyclidine (PCP).\n4\n\n<<<PAGE 5>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nspecimens from the two injured and two deceased workers did not detect any tested-for\nsubstances.12\n1.5 Postaccident Testing, Examinations, and Modeling\n1.5.1 Valve Testing and Examinations\nInvestigators tested and examined the mainline valve, which was a 24-inch-\ndiameter side-entry ball valve. Pressure testing revealed that the mainline valve was\nleaking, and examinations found that its sealing surfaces were scratched and gouged.\n(See figure 3.) This damage created leak paths along the sealing surfaces. The damage\nwas consistent with foreign debris (such as metallic particles) entering the valve from an\nexternal source, and the scratches’ and gouges’ orientations were consistent with wear\nincurred during normal valve operation. The mainline valve seals were adequately\nlubricated at the time of the explosion and no abnormal wear was reported. No issues\nwere found with the materials or the process for manufacturing the valve.\n12 The Federal Aviation Administration Forensic Sciences Laboratory tests specimens for a wide variety\nof substances including toxins, prescription and over-the-counter medications, and illicit drugs; information\nabout these substances can be found at the Civil Aerospace Medical Institute WebDrugs website\n(https://jag.cami.jccbi.gov/toxicology).\n5\n\n<<<PAGE 6>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nFigure 3. Damaged mainline valve sealing surfaces visible after disassembly into ring (left) and\nball (right) components.\nInvestigators also tested the kicker valve (the only other valve that connected the\nlauncher to Atmos’ gas transmission system) and determined that it was not leaking.13\n1.5.2 Flammability Conditions\nThe NTSB studied flammability conditions within the launcher by modeling the\nmigration of natural gas through the launcher body, piping, and valves.14 The results\nindicated that the concentration of natural gas within the launcher barrel would have\nremained at or near 100 percent after the flare was extinguished and before opening the\nlauncher door. Once the launcher door was opened, natural gas would have flowed out\nnear the top of the open door and air would have entered near the bottom. As the\nnatural gas was displaced by air, an explosive gas–air mixture would have developed\n13 The kicker valve is used to pressurize the launcher after the door is closed, launching the pig into the\ntransmission pipeline.\n14 (a) This modeling was completed using a computational fluid dynamics software package developed\nby the National Institute for Standards and Technology called Fire Dynamics Simulator. (b) Natural gas is\nless dense than air when at the same temperature and pressure, so it tends to migrate upward when mixed\nwith a normal atmosphere.\n6\n\n<<<PAGE 7>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nwithin the launcher, but as long as further natural gas was not introduced, continued\nventing through the open launcher door would have caused the natural gas\nconcentration to fall below the lower explosive limit within a few minutes.15\nInsertion of the pig was modeled as a complete blockage within the launcher\nbarrel.16 When the pig fully obstructs the launcher barrel, the equalizer piping (a valved\npipe that allows equalization of pressure within the launcher on both sides of the pig) is\nthe only way for gas to move between the launcher barrel sections on either side of the\npig. (See figure 4.) Modeling that included the leak from the mainline valve showed that\nonce the blockage was introduced, the concentration of natural gas in the launcher\nbetween the pig and open launcher door would have decreased while the concentration\nbetween the pig and mainline valve would have increased.\nFigure 4. Diagram of pig launcher.\n1.5.3 Ignition Source\nNTSB investigators considered the as-found launcher and connected equipment,\ntools used, the mainline valve leak, flammability conditions, and the sequence of events\n15 (a) Natural gas forms an explosive mixture in air at concentrations between the lower explosive limit\n(5 percent) and upper explosive limit (15 percent). (b) When the model was adjusted to include the leak\nfrom the mainline, a thin layer of flammable gas–air mixture persisted in the top of launcher.\n16 In reality, a pig may allow a small amount of gas to seep past if it does not create a gas-tight seal.\n7\n\n<<<PAGE 8>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nbefore the explosion to evaluate possible ignition scenarios. The NTSB did not\ndetermine the ignition source.\n1.6 Regulations, Rules, and Procedures\n1.6.1 Federal Regulations on Covered Tasks and Worker Qualifications\nThe Pipeline and Hazardous Materials Safety Administration considers pig loading\nand launching to be a covered task under Title 49 Code of Federal Regulations (CFR)\nPart 192.801, a position documented in Interpretation Response #PI-22-0008. A covered\ntask is an activity identified by an operator that is performed on a pipeline facility, is an\noperations or maintenance task, is performed as a requirement of 49 CFR Part 192, and\naffects the operation or integrity of the pipeline. Employees must be qualified for\ncovered tasks and are considered qualified if they have been evaluated, can perform\nassigned covered tasks, and can recognize and react to abnormal operating conditions\n(see 49 CFR 192.803).17\n1.6.2 Atmos Worker Qualifications\nAtmos did not designate launching pigs as a covered task as defined by federal\nregulations. Because Atmos did not designate launching pigs as a covered task, it did\nnot require its employees or contractors to be qualified under the regulatory definition\nto launch pigs. Instead, Atmos relied on on-the-job training for tasks specifically related\nto pigging.\n1.6.3 Atmos Pigging Procedures\nAtmos formally documented its pigging procedure in Appendix R of its Pipeline\nIntegrity Management Plan in 2019. The procedure was still in place at the time of the\naccident. The procedure warned that pigging can be dangerous and instructed\npersonnel to monitor for hydrocarbons (such as natural gas), to verify a pressure gauge\nhad been installed to monitor and relieve pressure inside the barrel, and never to stand\ndirectly in front of the launcher door. The procedure was intended to serve as a guide for\nloading a pig under ideal situations and configurations. However, the configuration\n17 An abnormal operating condition is a condition identified by the operator that may indicate a\nmalfunction of a component or deviation from normal operations that may (a) indicate a condition\nexceeding design limits or (b) result in a hazard(s) to persons, property, or the environment (49 CFR\n192.803).\n8\n\n<<<PAGE 9>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nshown in the procedure did not match the launcher involved in this accident, and the\nprocedure did not mention the use of a flaring system.\nThe procedure also required workers to make adjustments as needed to ensure\nthat pig loading was performed in a safe and controlled manner, but abnormal\noperating conditions specific to pig loading were not indicated in this or any other\nAtmos procedure.\n1.7 Postaccident Actions\nAtmos suspended all pigging operations on in-service pipelines immediately\nfollowing the accident and initiated a review of its related processes and procedures.\nAtmos revised their pigging procedure, established a new covered task for pigging\noperations, and developed associated training. The revised procedure standardizes\nlauncher designs, requiring additional ports for venting, pressure gauge measurements,\nand gas monitoring; a grounding lug; and double block and bleed valving for valves that\nconnect launchers to their natural gas transmission system.18 The revised procedure also\nrequires a site-specific valve sequencing plan, job safety analyses, oversight by a third-\nparty inspector, purging natural gas from the launcher with nitrogen, mandatory\nstabilization periods, criteria for stopping work if pressure and gas concentration levels\nrise beyond a defined threshold, and delineation of hazard zones in proximity to\nlaunchers and receivers. Additional specifications for on-site tools and grounding\nrequirements were also included to further reduce the risk of inadvertent ignition.\n2. Analysis\nWorkers from Atmos, FESCO and Bobcat were performing routine pig loading on\na natural gas pipeline when an explosion occurred in the launcher. The explosion was\ndirected through the open launcher door toward four workers, injuring two and killing\ntwo.\nThe week before the explosion, workers suspected the mainline valve was leaking\nand adjusted the valve’s position until they found a position where the leak stopped or\nreduced to the point that the flare extinguished. They marked this position on the\nexterior of the mainline valve and did not have issues with the flare extinguishing on\nsubsequent runs. The Atmos workers overseeing the operation did not expect the valve\nto leak but believed that if any natural gas did leak into the launcher from the mainline\nvalve, it would vent safely through the flare system to the atmosphere.\n18 (a) A grounding lug helps to prevent the buildup of static charge. (b) Double block and bleed valving\nuses two in-line valves that can be closed to vent the line between them.\n9\n\n<<<PAGE 10>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nOn the day of the explosion, workers began the routine task of loading a pig for\nthe sixth time in about a week. Workers were equipped with a straight metal rod as their\npig insertion tool, necessitating they stand in front of the open launcher.\nThe flare tip extinguished as expected, and based on modeling, the natural gas\nconcentration remained at or near 100 percent even after workers depressurized the\nlauncher. If the mainline valve had sealed properly, after the launcher door was opened,\nthis remaining natural gas would have vented from the launcher to the atmosphere while\nair entered, creating a gas–air mixture whose concentration would have fallen below the\nlower explosive limit within a few minutes.\nOpening the launcher door did allow natural gas to vent to atmosphere,\ndecreasing the concentration within the launcher. However, the mainline valve was\nleaking, which allowed additional natural gas to enter the launcher while workers were\nloading the pig. After the pig was loaded into the launcher, the pig obstructed the pipe\nbetween the mainline valve and launcher door. This obstruction and the leaking mainline\nvalve caused natural gas concentrations between the pig and mainline valve to rise to an\nexplosive concentration. The workers were not using any means of detecting or\nmeasuring gas that could have alerted them to the hazardous condition. The gas ignited,\ncausing an explosion.\nAlthough the NTSB did not determine the ignition source, two possible ignition\nscenarios were identified: (1) an ignition source was present between the pig and\nmainline valve, or (2) a flammable gas–air mixture and an ignition source were present\nbetween the pig and open launcher door which flashed back to the other side of the pig.\nAs a result of the explosion, Atmos revised their pigging procedure to include gas\nmonitoring for hazardous atmosphere and purging the launcher with nitrogen to expel\nnatural gas before opening the launcher door during pigging operations.\nMinimum federal safety standards in 49 CFR Part 192 require that operators\nensure through evaluation that individuals be “qualified” to perform covered tasks, such\nas pig loading. Qualification includes the ability to safely handle abnormal operating\nconditions. Atmos was therefore required to ensure its workers and contractors could\nrecognize and react to abnormal operating conditions that could be encountered during\npig loading, such as a leaking mainline valve.\nAt the time of the accident, Atmos did not require its employees or contractors to\nbe qualified to load pigs; instead, Atmos expected its employees to rely on their\nexperience and on-the-job training. Despite their many years of collective experience,\nthe workers were not qualified as required by federal regulations and were not using gas\nmonitors to monitor for hazardous atmosphere. Since the explosion, Atmos has\nestablished a new covered task for pigging procedures; workers performing pigging\noperations must now be qualified as required by 49 CFR Part 192, which includes the\n10\n\n<<<PAGE 11>>>\n\nAtmos Energy Corporation Natural Gas–Fueled Explosion During Routine Maintenance\nPIR-22/03\nability to safely handle abnormal operating conditions. The new procedure also includes\nguidance on hazard zones near launchers and receivers and criteria for stopping work.\n3. Probable Cause\nThe National Transportation Safety Board determines that the probable cause of\nthe June 28, 2021, explosion was a leaking mainline valve that allowed natural gas to\nenter the launcher where it mixed with air, creating a flammable gas–air mixture that was\nignited by an undetermined source. Contributing to the explosion and its severity were\nAtmos Energy Corporation’s procedures and training practices that did not prepare\nworkers to recognize and safely respond to abnormal operating conditions.\nThe National Transportation Safety Board (NTSB) is an independent federal\nagency dedicated to promoting aviation, railroad, highway, marine, and pipeline safety.\nEstablished in 1967, the agency is mandated by Congress through the Independent\nSafety Board Act of 1974, to investigate transportation accidents, determine the probable\ncauses of the accidents, issue safety recommendations, study transportation safety issues,\nand evaluate the safety effectiveness of government agencies involved in transportation.\nThe NTSB makes public its actions and decisions through accident reports, safety studies,\nspecial investigation reports, safety recommendations, and statistical reviews.\nThe NTSB does not assign fault or blame for an accident or incident; rather, as\nspecified by NTSB regulation, “accident/incident investigations are fact-finding\nproceedings with no formal issues and no adverse parties … and are not conducted for\nthe purpose of determining the rights or liabilities of any person” (Title 49 Code of\nFederal Regulations section 831.4). Assignment of fault or legal liability is not relevant to\nthe NTSB’s statutory mission to improve transportation safety by investigating accidents\nand incidents and issuing safety recommendations. In addition, statutory language\nprohibits the admission into evidence or use of any part of an NTSB report related to an\naccident in a civil action for damages resulting from a matter mentioned in the report\n(Title 49 United States Code section 1154(b)).\nFor more detailed background information on this report, visit the NTSB\ninvestigations website and search for NTSB accident ID PLD21FR002. Recent publications\nare available in their entirety on the NTSB website. Other information about available\npublications also may be obtained from the website or by contacting—\nNational Transportation Safety Board\nRecords Management Division, CIO-40\n490 L’Enfant Plaza, SW\nWashington, DC 20594\n(800) 877-6799 or (202) 314-6551\n11","truncated":false,"body_characters":31734}