# Building Explosion and Fire - **operation:** document - **citation:** DCA16FP003 - **title:** Building Explosion and Fire - **source type:** incident - **agency:** National Transportation Safety Board - **status:** current - **official:** true - **published on:** 2019-04-29 - **effective on:** 2016-08-10 - **summary:** Accident. in Silver Spring, MD, USA. on 2016-08-10. Washington Gas Light Company. Leak/explosion/fire - **machine formats:** - **json:** https://regulus.evalyn.ai/document/ntsb-case-dca16fp003.json - **markdown:** https://regulus.evalyn.ai/document/ntsb-case-dca16fp003.md - **app url:** https://regulus.evalyn.ai/document/ntsb-case-dca16fp003 - **source url:** https://www.ntsb.gov/investigations/Pages/DCA16FP003.aspx **body:** NTSB investigation DCA16FP003. Event Type: Accident Event Date: 2016-08-10 Event City: Silver Spring Event State Or Region: MD Event Country: USA Pipeline Operator: Washington Gas Light Company Pipeline Type: Distribution Accident Type: Leak/explosion/fire Completion Status: Completed Report Number: PAR1901 Report Date: 2019-04-29 Probable cause: The National Transportation Safety Board determines that the probable cause of the explosion in building 8701 of the Flower Branch apartment complex was the failure of an indoor mercury service regulator with an unconnected vent line that allowed natural gas into the meter room where it accumulated and ignited from an unknown ignition source. Contributing to the accident was the location of the mercury service regulators where leak detection by odor was not readily available. Tier1Name: System operating Tier2Name: Explosion Tier1Name: Post-release Tier2Name: Fire/explosion (post-release) Tier1Name: Emergency response Tier2Name: Emergency shutoff Tier1Name: System operating Tier2Name: Product leak/release Tier1Name: System operating Tier2Name: System/equipment malf/failure Tier1Name: Emergency response Tier2Name: Inspection event Finding Tier1Name: Organizational Finding Tier2Name: Management Finding Tier3Name: Policy/procedure Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Management - Policy/procedure - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Documentation/record keeping Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Support/oversight/monitoring - Documentation/record keeping - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Enforcement Finding Modifier Name: Federal agency Finding Report Text: Organizational - Support/oversight/monitoring - Enforcement - Federal agency Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Documentation/record keeping Finding Modifier Name: Not specified Finding Report Text: Organizational - Support/oversight/monitoring - Documentation/record keeping - Not specified Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Enforcement Finding Modifier Name: State/Local agency Finding Report Text: Organizational - Support/oversight/monitoring - Enforcement - State/Local agency Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Documentation/record keeping Finding Modifier Name: State/Local agency Finding Report Text: Organizational - Support/oversight/monitoring - Documentation/record keeping - State/Local agency Finding Tier1Name: Organizational Finding Tier2Name: Development Finding Tier3Name: Design Finding Modifier Name: Inspection organization Finding Report Text: Organizational - Development - Design - Inspection organization Finding Tier1Name: Pipeline Finding Tier2Name: Pipline systems/equipment Finding Tier3Name: Regulator Finding Modifier Name: Failure Finding Report Text: Pipeline - Pipline systems/equipment - Regulator - Failure Finding Tier1Name: Pipeline Finding Tier2Name: Pipline systems/equipment Finding Tier3Name: Venting Finding Modifier Name: Malfunction Finding Report Text: Pipeline - Pipline systems/equipment - Venting - Malfunction Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline structure Finding Tier3Name: Pipe Finding Modifier Name: Design Finding Report Text: Pipeline - Pipeline structure - Pipe - Design Finding Tier1Name: Environment/Infrastructure Finding Tier2Name: Operating environment/control system Finding Tier3Name: Sensor/detector coverage/availability Finding Modifier Name: Awareness of condition Finding Report Text: Environment/Infrastructure - Operating environment/control system - Sensor/detector coverage/availability - Awareness of condition Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline handling/service Finding Tier3Name: Operating information Finding Modifier Name: Not inspected Finding Report Text: Pipeline - Pipeline handling/service - Operating information - Not inspected Finding Tier1Name: Personnel Finding Tier2Name: Action/decision Finding Tier3Name: Information processing/decision making Finding Modifier Name: Pipeline maintenance personnel Finding Report Text: Personnel - Action/decision - Information processing/decision making - Pipeline maintenance personnel Finding Tier1Name: Personnel Finding Tier2Name: Action/decision Finding Tier3Name: Information processing/decision making Finding Modifier Name: Emergency services personnel Finding Report Text: Personnel - Action/decision - Information processing/decision making - Emergency services personnel Finding Tier1Name: Pipeline Finding Tier2Name: Pipline systems/equipment Finding Tier3Name: Regulator Finding Modifier Name: Design Finding Report Text: Pipeline - Pipline systems/equipment - Regulator - Design Finding Tier1Name: Pipeline Finding Tier2Name: Pipline systems/equipment Finding Tier3Name: Regulator Finding Modifier Name: Malfunction Finding Report Text: Pipeline - Pipline systems/equipment - Regulator - Malfunction Official NTSB investigation data. NTSB findings determine probable cause and make safety recommendations; they do not adjudicate civil liability or regulatory violations. What Happened On August 10, 2016, at 11:51 p.m., eastern daylight time, a 14-unit apartment building, located at 8701 Arliss Street, in the unincorporated community of Silver Spring, in Montgomery County, Maryland, partially collapsed due to a natural gas-fueled explosion and fire. The explosion and fire also heavily damaged an adjacent apartment building, 8703 Arliss Street, which shared a common wall with building 8701. As a result of this accident, 7 residents died, 65 residents were transported to the hospital, and 3 firefighters were treated and released from the hospital. The damage from the accident exceeded $1 million. What We Found The probable cause of the explosion in building 8701 of the Flower Branch apartment complex was the failure of an indoor mercury service regulator with an unconnected vent line that allowed natural gas into the meter room where it accumulated and ignited from an unknown ignition source. Contributing to the accident was the location of the mercury service regulators where leak detection by odor was not readily available. What We Recommended As a result of this investigation, we made the following new safety recommendations. To the Pipeline and Hazardous Materials Safety Administration: Require that all new service regulators be installed outside occupied structures. (P-19-001) Require existing interior service regulators be relocated outside occupied structures whenever the gas service line, meter, or regulator is replaced. In addition, multifamily structures should be prioritized over single-family dwellings. (P-19-002) To the Public Service Commission of Maryland, the Commonwealth of Virginia State Corporation Commission Division of Public Utility Regulation, and the Public Service Commission of the District of Columbia: Following Washington Gas’s successful completion of Safety Recommendation P-19-009, audit and verify the performance of Washington Gas’s mercury service regulator replacement program, including its recordkeeping. (P-19-003) Oversee the replacement process for the mercury service regulators that Washington Gas has in service. (P-19-004) To the International Academies of Emergency Dispatch: Revise Protocol-60 Gas Leak/Gas Odor (Natural and Liquified Petroleum Gases) to direct dispatchers to notify the gas company when any odor call is received. (P-19-005) To the International Code Council: In coordination with the Gas Technology Institute and the National Fire Protection Association, incorporate provisions in the International Fuel Gas Code that requires methane detection systems for all types of residential occupancies with gas service. At a minimum, the provisions should cover the installation, maintenance, placement of the detectors, and testing requirements. (P-19-006) To the National Fire Protection Association: In coordination with the Gas Technology Institute and the International Code Council, revise the National Fuel Gas Code, National Fire Protection Association 54 to require methane detection systems for all types of residential occupancies with gas service. At a minimum, the provisions should cover the installation, maintenance, placement of the detectors, and testing requirements. (P-19-007) To the Gas Technology Institute: In coordination with the National Fire Protection Association and the International Code Council, work to develop standards for methane detection systems for all types of residential occupancies in both the International Fuel Gas Code and the National Fuel Gas Code, National Fire Protection Association 54. At a minimum, the provisions should cover the installation, maintenance, placement of the detectors, and testing requirements. (P-19-008) To Washington Gas Light Company: Throughout the Washington Gas network, implement an audit program to verify the data on the service forms used to determine the location and condition of mercury service regulators to ensure the accuracy of this safety-critical data. (P-19-009) Revise your procedures and field forms to require technicians to verify the integrity of vent lines following the testing of indoor service regulators throughout the Washington Gas network. (P-19-010) Establish a time frame with specific dates and milestones for the replacement of mercury service regulators throughout the Washington Gas network that recognizes the need to expedite this program and that prioritizes multifamily dwellings where mercury service regulators are located inside the property. (P-19-011) Install all new service regulators outside occupied structures. (P-19-012) Relocate existing interior service regulators outside occupied structures whenever the gas service line, meter, or regulator is replaced. In addition, multifamily structures should be prioritized over single-family dwellings. (P-19-013) PAR-19-01 <<>> Building Explosion and Fire Silver Spring, Maryland August 10, 2016 Accident Report NTSB/PAR-19/01 PB2019-100722 National Transportation Safety Board <<>> NTSB/PAR-19/01 PB2019-100722 Notation 58957 Adopted April 24, 2019 Pipeline Accident Report Building Explosion and Fire Silver Spring, Maryland August 10, 2016 National Transportation Safety Board 490 L’Enfant Plaza, S.W. Washington, D.C. 20594 <<>> National Transportation Safety Board. 2019. Building Explosion and Fire, Silver Spring, Maryland, August 10, 2016. NTSB/PAR-19/01. Washington, DC. Abstract: On August 10, 2016, at 11:51 p.m. eastern daylight time, a 14-unit apartment building, located at 8701 Arliss Street, in Silver Spring, Montgomery County, Maryland, partially collapsed due to a natural gas-fueled explosion and fire. The explosion and fire also heavily damaged an adjacent apartment building which shared a common wall with building 8701. As a result of this accident, 7 residents died, 65 residents were transported to the hospital, and 3 firefighters were treated and released from the hospital. The investigation focused on the following safety issues: the location and inspection of service regulators within a structure; the inspection of the gas meter assembly; the notification of the natural gas odor to Washington Gas Light Company; and the detection of natural gas through odorants and methane. As a result of this investigation, the National Transportation Safety Board (NTSB) makes new safety recommendations to the Pipeline and Hazardous Materials Safety Administration; the Public Service Commission of Maryland, the Commonwealth of Virginia State Corporation Commission Division of Public Utility Regulation, and the Public Service Commission of the District of Columbia; the International Academies of Emergency Dispatch; the International Code Council; the National Fire Protection Association; the Gas Technology Institute; and Washington Gas Light Company. The National Transportation Safety Board (NTSB) is an independent federal agency dedicated to promoting aviation, railroad, highway, marine, and pipeline safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The NTSB makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and statistical reviews. The NTSB does not assign fault or blame for an accident or incident; rather, as specified by NTSB regulation, “accident/incident investigations are fact-finding proceedings with no formal issues and no adverse parties … and are not conducted for the purpose of determining the rights or liabilities of any person.” 49 C.F.R. § 831.4. Assignment of fault or legal liability is not relevant to the NTSB’s statutory mission to improve transportation safety by investigating accidents and incidents and issuing safety recommendations. In addition, statutory language prohibits the admission into evidence or use of any part of an NTSB report related to an accident in a civil action for damages resulting from a matter mentioned in the report. 49 U.S.C. § 1154(b). For more detailed background information on this report, visit http://www.ntsb.gov/investigations/dms.html and search for NTSB accident ID DCA16FP003. Recent publications are available in their entirety on the Internet at http://www.ntsb.gov. Other information about available publications also may be obtained from the website or by contacting: National Transportation Safety Board Records Management Division, CIO-40 490 L’Enfant Plaza, SW Washington, DC 20594 (800) 877-6799 or (202) 314-6551 NTSB publications may be purchased from the National Technical Information Service. To purchase this publication, order product number PB2019-100722 from: National Technical Information Service 5301 Shawnee Rd. Alexandria, VA 22312 (800) 553-6847 or (703) 605-6000 http://www.ntis.gov/ <<>> NTSB Pipeline Accident Report Contents Figures ........................................................................................................................................... iii Tables ............................................................................................................................................ iv Abbreviations and Acronyms .......................................................................................................v Executive Summary .................................................................................................................... vii 1. Factual Information and Analysis ..........................................................................................1 1.1 The Accident ............................................................................................................................1 1.2 Accident Site and Property Description ...................................................................................2 1.2.1 Gas Meter/Utility Room ................................................................................................5 1.3 The Investigation ......................................................................................................................5 1.3.1 Gas Odor Leak Calls ......................................................................................................5 1.3.2 Previous Gas Odor Calls ................................................................................................6 1.3.3 Emergency Response .....................................................................................................6 1.3.4 Postaccident Gas Leak Surveys and Odorant Testing ...................................................7 1.3.5 Fire and Arson Investigations ........................................................................................8 1.3.6 Explosion Damage and Evidence ..................................................................................8 1.3.7 Gas Service to the Flower Branch Apartment Complex ................................................9 2 Safety Issues ............................................................................................................................10 2.1 Introduction ............................................................................................................................10 2.2 Gas Meter Assembly Inspection .............................................................................................10 2.3 Service Regulators ..................................................................................................................12 2.3.1 Service Regulator Overview ........................................................................................12 2.3.2 NTSB Materials Laboratory Examination of the Mercury Service Regulators ...........17 2.3.3 Upper Regulator Union Assembly ...............................................................................19 2.3.4 Lower Regulator Union Assembly ..............................................................................21 2.3.5 Vent Line Inspection Practices ....................................................................................22 2.3.6 Examination and Exclusion of Gas Appliances in the Meter Room ...........................22 2.3.7 Mercury Service Regulator Testing .............................................................................24 2.3.8 Natural Gas Accumulation in the Meter Room of Building 8701 ...............................26 2.3.9 Mercury Service Regulator Replacement ....................................................................28 2.3.10 Location of Natural Gas Service Regulators ...............................................................29 2.4 Natural Gas Odor Notification ...............................................................................................30 2.4.1 Kay Management .........................................................................................................30 2.4.2 Washington Gas ...........................................................................................................31 2.4.3 Montgomery County Fire and Rescue Services ...........................................................31 2.5 Odorant and Methane Detection of Natural Gas ....................................................................32 2.5.1 Odorant Detection ........................................................................................................32 2.5.2 Methane Detection .......................................................................................................34 i <<>> NTSB Pipeline Accident Report 3 Postaccident Actions ..............................................................................................................39 4 Conclusions .............................................................................................................................40 4.1 Findings ..................................................................................................................................40 4.2 Probable Cause .......................................................................................................................41 5 Recommendations ..................................................................................................................42 6 Appendix .................................................................................................................................47 6.1 Appendix A: The Investigation ..............................................................................................47 6.2 Appendix B: Sample of 20 NTSB Investigations of Residential and Commercial Building Pipeline Accidents Between 1971 and 2018 ..................................................................................47 References .....................................................................................................................................52 ii <<>> NTSB Pipeline Accident Report Figures Figure 1. The accident location, 8701 Arliss Street, before the August 10, 2016, explosion. ....... 1 Figure 2. Buildings 8701 and 8703 Arliss Street, Silver Spring, Maryland, after the explosion and fire. ........................................................................................................................................... 2 Figure 3. Layout of the Flower Branch apartment complex. ......................................................... 3 Figure 4. A plan for the basement/terrace floor for 8701 Arliss Street and the shared-wall portion of 8703 Arliss Street. .......................................................................................................... 4 Figure 5. Portions of mercury service regulators and gas meters in building 8701 showing heat/fire/explosion damage and the unconnected lower regulator vent line. ................. 11 Figure 6. Building 8701 after debris removal and ATF reconstruction. ...................................... 12 Figure 7. Exemplar configuration of a mercury service regulator assembly in a similar Flower Branch apartment building. .............................................................................................. 14 Figure 8. Exemplar mercury service regulator taken from a nearby Flower Branch apartment building. ....................................................................................................................... 15 Figure 9. Cross sectional image of a mercury service regulator. ................................................. 16 Figure 10. Mercury service regulators, service line, jurisdictional pipeline, and vent line after the accident. .................................................................................................................................. 18 Figure 11. Photograph of the lower regulator union assembly parts. .......................................... 19 Figure 12. A photograph of the exposed external threaded nipple from the upper regulator union assembly.............................................................................................................................. 20 Figure 13. Photograph of the disassembled union assembly for the accident lower gas regulator after cleaning. ................................................................................................................................ 21 Figure 14. Hot water heater at the accident scene after the explosion. ........................................ 23 Figure 15. The main burner assembly, including the separated pipe and the gas control valve assembly. ............................................................................................................................. 24 Figure 16. Three-dimensional visualization of the meter room. .................................................. 27 iii <<>> NTSB Pipeline Accident Report Tables Table 1. Valve seat obstruction failure test data. ......................................................................... 25 Table 2. Diaphragm tear failure test data for a torn upper regulator. ........................................... 25 Table 3. Loss of mercury failure test data. ................................................................................... 26 iv <<>> NTSB Pipeline Accident Report Abbreviations and Acronyms ANSI American National Standards Institute ASME American Society of Mechanical Engineers ATF US Bureau of Alcohol, Tobacco, Firearms and Explosives BSI British Standards Institution CFD computational fluid dynamics CFH cubic feet per hour CFR Code of Federal Regulations COMAR Code of Maryland Regulations DOT US Department of Transportation ECC Montgomery County Emergency Communication Center EPA US Environmental Protection Agency GAO US Government Accountability Office GTI Gas Technology Institute IAED International Academies of Emergency Dispatch ICC International Code Council IFGC International Fuel Gas Code LEL lower explosive limit LP liquified petroleum MAOP maximum allowable operating pressure MCFRS Montgomery County Fire and Rescue Service MCPD Montgomery County Police Department NCSL National Conference of State Legislatures NFPA National Fire Protection Association v <<>> NTSB Pipeline Accident Report NFPA 54 National Fuel Gas Code NPS nominal pipe size NTSB National Transportation Safety Board OTD Operations Technology Development PHMSA Pipeline and Hazardous Materials Safety Administration PSC Public Service Commission of Maryland psig pounds per square inch, gauge RMD residential methane detectors RSPA Research and Special Programs Administration SCFH standard cubic feet per hour UEL upper explosive limit UL Underwriter’s Laboratories WC water column WG Washington Gas Light Company WSSC Washington Suburban Sanitary Commission vi <<>> NTSB Pipeline Accident Report Executive Summary On August 10, 2016, at 11:51 p.m., eastern daylight time, a 14-unit apartment building, located at 8701 Arliss Street, in the unincorporated community of Silver Spring, in Montgomery County, Maryland, partially collapsed due to a natural gas-fueled explosion and fire. The explosion and fire also heavily damaged an adjacent apartment building, 8703 Arliss Street, which shared a common wall with building 8701. As a result of this accident, 7 residents died, 65 residents were transported to the hospital, and 3 firefighters were treated and released from the hospital. The damage from the accident exceeded $1 million. The following are safety issues in this accident: • the location and inspection of service regulators within a structure • the inspection of the gas meter assembly • the notification of the natural gas odor to Washington Gas Light Company • the detection of natural gas through odorants and methane The National Transportation Safety Board determines that the probable cause of the explosion in building 8701 of the Flower Branch apartment complex was the failure of an indoor mercury service regulator with an unconnected vent line that allowed natural gas into the meter room where it accumulated and ignited from an unknown ignition source. Contributing to the accident was the location of the mercury service regulators where leak detection by odor was not readily available. vii <<>> NTSB Pipeline Accident Report 1. Factual Information and Analysis 1.1 The Accident On August 10, 2016, at 11:51 p.m., eastern daylight time, a 14-unit apartment building, located at 8701 Arliss Street, in the unincorporated community of Silver Spring, in Montgomery County, Maryland, partially collapsed due to a natural gas-fueled explosion and fire.1 (See figure 1.) Figure 1. The accident location, 8701 Arliss Street, before the August 10, 2016, explosion. (Photograph from Google Earth, dated November 2015.) The explosion and fire also heavily damaged an adjacent apartment building, 8703 Arliss Street, which shared a common wall with building 8701. (See figure 2.) 1 All times mentioned in this report are eastern daylight time, unless otherwise specified. 1 <<>> NTSB Pipeline Accident Report Figure 2. Buildings 8701 (right-center) and 8703 (left) Arliss Street, Silver Spring, Maryland, after the explosion and fire. (Photograph provided by the Public Service Commission of Maryland.) As a result of this accident, 7 residents died, 65 residents were transported to the hospital, and 3 firefighters were treated and released from the hospital.2 The damage from the accident exceeded $1 million. 1.2 Accident Site and Property Description Buildings 8701 and 8703 Arliss Street were part of the Flower Branch Apartments, a 26-building complex that was constructed in 1955. (See figure 3.) The apartment complex was managed by Kay Management, a real estate management and development company. 2 Some of the injuries reported included respiratory problems from smoke inhalation and injuries from falling or burns. 2 <<>> NTSB Pipeline Accident Report Figure 3. Layout of the Flower Branch apartment complex. Building 8701 is identified by a red star. (Graphic provided by Kay Management.) Building 8701 Arliss Street was a four-level structure with a basement and three floors. Four apartments were located on each of the upper three levels. The basement/terrace level consisted of one apartment, a meter room, a storage room, and a property management and rental office. (See figure 4.) 3 <<>> NTSB Pipeline Accident Report Figure 4. A plan for the basement/terrace floor for 8701 Arliss Street and the shared-wall portion of 8703 Arliss Street. (Graphic provided by Kay Management.) According to Washington Gas Light Company’s (WG) construction and service records for 8701 Arliss Street, between the years of 1955 and 1969 the natural gas main along Piney Branch Road that supplied this address was a medium-pressure main constructed of 6-inch inside diameter wrapped steel. The service line from the main that supplied 8701 Arliss Street was 2-inch nominal pipe size (NPS) wrapped steel, connected to a 1-inch NPS inlet pipe and a 1-inch outlet pipe after the service regulators. 3 The main had a maximum allowable operating pressure (MAOP) of 25 pounds per square inch, gauge (psig) and the operating pressure at the time of the explosion was between 20 and 22 psig.4 This line had been servicing the address for over 60 years until the explosion on August 10, which severed a portion of the service line. The service line construction was completed on May 31, 1955, and the installation of the natural gas meters and regulators was completed on June 15, 1955. The gas main and service line were buried about 3 feet below the surface. The 2-inch service line ran along the front of the apartment buildings, including 8701 Arliss Street. On September 25, 1969, the service line was relocated about 80 feet and disconnected 3 NPS is nominal pipe size as defined the American Society of Mechanical Engineers (ASME) B36-10M-2018 standards for wrought steel pipe (ASME 2018). 4 MAOP refers to how much pressure the walls of the pipeline may safely hold in normal operations. 4 <<>> NTSB Pipeline Accident Report from the 6-inch medium-pressure wrapped steel main on Piney Branch Road. The service line was then connected to a 4-inch medium-pressure wrapped steel main that ran along Arliss Street. 1.2.1 Gas Meter/Utility Room The gas meters were located in the meter room in the basement of 8701 Arliss Street on the north facing wall. The gas meter assembly included 15 meters that served 13 apartments, and maintenance and management offices. 5 The electric meter panel and a commercial-grade gas water heater were positioned about 6 feet away from the gas meter assembly. The room was also used for equipment and supply storage. At the time of the accident, it contained supplies and equipment needed for maintenance. 6 The meter room had restricted access and only two windows: one enclosed with glass and the other covered with a piece of wood with two small holes for ventilation. The room was secured by a knob lock, a vertical deadbolt lock, and was armed with an alarm system.7 Only Kay Management staff and Montgomery County Fire and Rescue Service (MCFRS) responders were authorized access to the meter room. In February 2016, the lock to the meter room was changed; however, Kay Management had not placed a copy of the new key in the MCFRS Knox Box, located on the premises.8 1.3 The Investigation 1.3.1 Gas Odor Leak Calls Prior to the accident, several residents of buildings 8701 and 8703 reported to investigators that they smelled gas in the weeks and months leading up to the explosion. One resident reported that he smelled natural gas after returning from work around 11:30 p.m. on August 10. While he was taking out the trash, around 11:50 p.m., he smelled a gas odor in the stairwell and walked down to the basement to locate the source. He told National Transportation Safety Board (NTSB) investigators that he found the odor became stronger as he walked into the basement and he could hear a hissing noise from behind the locked doors of the meter room. He said that he planned to call 9-1-1 to report the gas odor after he finished taking out the trash. However, before he could report the leak, as he walked back from the community trash containers about 120 feet away, the explosion occurred.9 5 The management office was an apartment converted for business purposes. 6 Kay Management provided an inventory list for the meter room in 8701 Arliss Street. 7 According to the US Bureau of Alcohol, Tobacco, Firearms and Explosions (ATF) Origin and Cause report, the knob lock was locked, and the vertical dead bolt was not locked, prior to the explosion. For more information, see “ATF Report” in NTSB Docket DCA16FP003. 8 A Knox Box system is an access box for fire departments to use in the event of an emergency to enter structures or areas that are deemed difficult. This is required under National Fire Protection Association (NFPA) 1, which is a comprehensive fire code that the State of Maryland has adopted by reference in its Fire Prevention Code. 9 For more information, see “Interview transcript with Resident A” in NTSB Docket DCA16FP003. 5 <<>> NTSB Pipeline Accident Report 1.3.2 Previous Gas Odor Calls Between September 2015 and June 2016, Kay Management received six gas odor calls from residents of 8701 Arliss Street. 10 Members of its maintenance staff said that when they investigated, they either did not detect gas through their gas-detection equipment or would report that the painting of apartments was causing the smell. MCFRS fire personnel responded to an odor call at building 8701 on July 25, 2016. According to interviews with Flower Branch residents and employees from Kay Management, residents in the building typically called either the management company or 9-1-1 to report gas odors. WG has no record of responding to or receiving any notifications of gas odor calls from 8701 Arliss Street in the 5 years prior to the accident. The same resident who detected the natural gas odor on the night of the explosion had called 9-1-1 on July 25, 2016, to report a strong smell of natural gas both outside and inside of building 8701. The resident told the 9-1-1 operator that the strongest smell of natural gas was detected on the first level of the apartment building. The 9-1-1 operator told the resident not to turn on anything that could create an ignition source, and to evacuate himself and others out of the building. Neither the resident nor the 9-1-1 operator contacted the gas company. On July 25, 2016, MCFRS fire personnel arrived at building 8701 around 10:20 p.m. They approached the building using a calibrated four-gas multimeter.11 Residents directed the fire personnel to the main hallway of the first floor, where no combustible gas readings or natural gas odors were detected. Although one responder reported smelling “something,” it did not register on his detection equipment. The fire personnel proceeded through the apartment building, checking inside occupied apartments and underneath doors of unoccupied apartments. The fire personnel moved to the basement to test the storage and meter rooms but were unable to gain access to the meter room because the locks had recently been changed and the new key was unavailable. The fire personnel wedged the top of the doors open but detected no gas. MCFRS departed at 10:33 p.m. because they did not detect any gas readings throughout the building.12 The Kay Management property manager was not notified of the odor leak call and only learned of it after a conversation with a resident days later. 13 There are no records of additional conversation between Kay Management and MCFRS regarding the July 25 odor complaint. 1.3.3 Emergency Response On August 10, 2016, at 11:52 p.m., the MCFRS Emergency Communication Center received a 9-1-1 call reporting the explosion and fire. Buildings 8701 and 8703 Arliss Street were evacuated by two Montgomery County Police Officers who were within 1,000 feet from them 10 For more information, see “Operations – Kay Management (All Work Orders 08-01-15 thru 08-25-16)” in NTSB Docket DCA16FP003. 11 Four-gas multimeters are devices that measure the atmosphere for the following four parameters: oxygen concentration (in percentage O2), flammability (in percentage of lower explosive limit [LEL]), carbon monoxide concentration (in parts per million), and hydrogen sulfide concentration (in parts per million). 12 For more information, see “Reg. Oversight – MCFRS Statements on July 25, 2016, incident” and “Interview of Montgomery County Fire and Rescue Lieutenant” in NTSB Docket DCA16FP003. 13 For more information, see “Interview of Manager of Flower Branch Apartments-Errata” in NTSB Docket DCA16FP003. 6 <<>> NTSB Pipeline Accident Report when the explosion occurred. MCFRS dispatched units to the accident scene at 11:54 p.m., and the first unit arrived at 11:58 p.m. to begin rescue and fire suppression efforts.14 Residents of nearby apartment buildings within the complex voluntarily evacuated themselves. The MCFRS Emergency Communication Center called WG at 12:08 a.m. on August 11, 2016. Within an hour of the initial notification, WG personnel arrived on scene and shut off the 4-inch diameter main supplying gas to the five apartment buildings along Arliss Street, and the MCFRS responders extinguished the fire shortly thereafter. The NTSB concludes that the postexplosion responses by both MCFRS and WG were prompt and adequate. 1.3.4 Postaccident Gas Leak Surveys and Odorant Testing NTSB investigators arrived on scene the morning of August 11, 2016. WG pressure tested the main, which held pressure, indicating there was no leak.15 Soil bar hole tests were conducted within 3 hours of the accident in areas adjacent to the main and the 2-inch service lines in the front and back of buildings 8701 and 8703 detected no gas readings.16 Building 8701’s gas regulator vent line, which is a pipe that directs the natural gas away from the building and into the atmosphere in case of an over-pressure scenario, was also tested and showed no obstruction in the vent line. Overseen by the Public Service Commission of Maryland (PSC), WG performed postaccident odorant testing at two different regulator stations in Silver Spring, Maryland, and odorant levels were found compliant with Pipeline and Hazardous Materials Safety Administration (PHMSA) regulations that require natural gas to be odorized and detected at 1/5th (or 20 percent) of the lower explosive limit (LEL). 17 This safety feature is required for all distribution lines. The State of Maryland has a more stringent requirement for odorant levels, which must be detectable at 1/10th (10 percent) of the LEL.18 The testing indicated that the levels also complied with the Code of Maryland Regulations (COMAR) requirement. At the Flower Branch complex, the natural gas service lines were also tested for odorant levels and met the regulatory limits.19 14 For more information, see “Survival Factors” in NTSB Docket DCA16FP003 for detailed event chronology and emergency response activities. 15 The operating pressure of the gas line that feeds into apartment - **truncated:** true - **body characters:** 145728