# UGI Utilities Natural Gas-Fueled Explosion **Citation:** DCA17FP006 **Type / status:** incident / current **Agency:** National Transportation Safety Board **Effective:** 2017-07-02 **Published:** 2019-02-25 Accident. in Millersville, PA, USA. on 2017-07-02. UGI Utilities. Leak/explosion/fire ## Document text NTSB investigation DCA17FP006. Event Type: Accident Event Date: 2017-07-02 Event City: Millersville Event State Or Region: PA Event Country: USA Pipeline Operator: UGI Utilities Accident Type: Leak/explosion/fire Completion Status: Completed Report Number: PAB1901 Report Date: 2019-02-25 Probable cause: The National Transportation Safety Board determines that the probable cause of the natural gas explosion at 206 Springdale Lane was an improperly installed mechanical tapping tee that leaked and allowed gas to migrate into the house where it ignited. Tier1Name: Post-release Tier2Name: Explosion Tier1Name: Post-release Tier2Name: Fire (post-release) Tier1Name: Emergency response Tier2Name: Emergency response Tier1Name: Post-release Tier2Name: Evacuation Tier1Name: System operating Tier2Name: Product leak/release Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline handling/service Finding Tier3Name: Operating information Finding Modifier Name: Related operating info Finding Report Text: Pipeline - Pipeline handling/service - Operating information - Related operating info Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline handling/service Finding Tier3Name: Operating information Finding Modifier Name: Capability exceeded Finding Report Text: Pipeline - Pipeline handling/service - Operating information - Capability exceeded Finding Tier1Name: Personnel Finding Tier2Name: Experience/knowledge Finding Tier3Name: Knowledge Finding Modifier Name: Not specified Finding Report Text: Personnel - Experience/knowledge - Knowledge - Not specified Finding Tier1Name: Pipeline Finding Tier2Name: Pipline systems/equipment Finding Tier3Name: Fittings Finding Modifier Name: Malfunction Finding Report Text: Pipeline - Pipline systems/equipment - Fittings - 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 July 2, 2017, at 12:32 p.m. eastern daylight time, a natural gas-fueled explosion occurred at a single-family residence at 206 Springdale Lane, Millersville, Pennsylvania. The explosion killed one person and injured three others, destroyed the residence and significantly damaged six neighboring homes, one of which was subsequently condemned. What We Found We determined that the probable cause of the natural gas explosion at 206 Springdale Lane was an improperly installed mechanical tapping tee that leaked and allowed gas to migrate into the house where it ignited. PAB-19-01 <<>> National Transportation Safety Board Pipeline Accident Brief UGI Utilities Natural Gas-Fueled Explosion Millersville, Pennsylvania July 2, 2017 The Accident On July 2, 2017, at 12:32 p.m. eastern daylight time, a natural gas-fueled explosion occurred at a single-family residence at 206 Springdale Lane, Millersville, Pennsylvania.1 The explosion killed one person and injured three others, destroyed the residence and significantly damaged six neighboring homes, one of which was subsequently condemned. 2 (See figure 1.) Figure 1. Debris field at cul-de-sac area where explosion occurred. At 10:26 a.m., a neighborhood resident walking in the area smelled natural gas and reported it to the local gas utility, UGI Utilities, Inc. (UGI). 3 About half an hour later, a UGI service 1 All times in this accident brief are eastern daylight time. 2 Local authorities condemned the residence at 202 Springdale Lane. 3 On June 1, 1882, UGI is incorporated in Philadelphia as the United Gas Improvement Company, becoming the first public utility holding company in the United States. In February 1968, the company changed its name to UGI Corporation. NOTE: This report was reissued on October 26, 2023, with corrections to the timeline on pages 2 through 4 and corrections to the gas reading on page 4. NTSB/PAB-19/01 <<>> UGI Utilities Natural Gas-Fueled Explosion technician arrived to investigate and confirm the gas leak. 4 A Lancaster Area Sewer Authority (LASA) employee arrived on scene in response to UGI’s Emergency PA (Pennsylvania) One Call to mark the location of the sewer lines.5 About 15 minutes prior to the explosion, UGI personnel directed the resident of 206 Springdale Lane to evacuate the residence who left in their personal automobile. About 12:15 p.m., UGI requested fire department support. The first fire truck arrived at 12:28 p.m. and assumed a stand-by position. The house exploded about 4 minutes later. Following the accident, the natural gas main and service pipelines for the cul-de-sac homes were leak tested. All segments held pressure, except for the service line at the 206 Springdale Lane connection at the main; this segment had a Permalock mechanical tapping tee.6 Subsequent inspection of the Permalock mechanical tee revealed gas was leaking at the connection of the tee to the 2-inch main, and two of the four nylon bolts had fractured. At the time of the accident, the operating pressure of the line was 54 pounds per square inch, gauge (psig), as measured at the closest monitoring point that was about 0.5 mile away from 206 Springdale Lane. The Pipeline System The natural gas main on Springdale Lane was made from a polyethylene material that was installed on August 7, 1995. The maximum allowable operating pressure (MAOP) of the system main serving Springdale Lane was 60 psig. The service line to the 206 Springdale Lane residence was installed in June 1998, and was constructed of 1/2-inch polyethylene pipe. The mechanical tapping tee was installed at the same time as the service line. The mechanical tapping tee and service pipe had been in service for 19 years when the accident occurred.7 The pipe between the main and the meter at the residence was 50 feet long. The service line to the residence was inserted inside a 1-inch polyethylene pipe that served as a protective jacket. The jacket ran from the outlet of the service tee to the meter. UGI Actions Following the Gas Leak Report Shortly after the gas leak report, UGI dispatched a UGI technician to respond. At 11:00 a.m., the technician arrived on scene and began his assessment to determine the presence of a leak. Consistent with UGI policy and training, the technician began implementing UGI’s leak survey procedure. His investigation determined that the leak source was at an adjacent residence, which was 206 Springdale Lane. At 11:18 a.m., the technician called the on-call UGI senior supervisor of Operations Construction and Maintenance (senior supervisor). The technician requested a response crew because he measured high-gas-concentration readings and confirmed a leak. The technician told the senior supervisor that he suspected that a mechanical tee was leaking, and that he measured a 4 UGI refers to its service technicians as mechanics. Each mechanic has a “grade” designation of I, II, or III, which is determined by the degree of qualifications and experience. 5 For additonal information, see http://www.pa1call.org/PA811/Public/. 6 The Honeywell Elster Perfection PermaLock Mechanical Tapping Tee is a tee that can be installed on a main to supply the individual gas service to a gas customer without the need to shut down the natural gas main. 7 UGI Gas Operations Manual, section 70.20. 2 CORRECTED COPY <<>> UGI Utilities Natural Gas-Fueled Explosion reading of 80 percent gas in the sewer and 98 percent gas over the tee. Based on the gas readings, the technician classified the leak as an emergency leak. The senior supervisor contacted UGI Dispatch at 11:20 a.m. and requested that they call Emergency PA One Call to mark underground utilities because an excavation would be necessary. The Emergency PA One Call ticket was issued at 11:27 a.m. About 11:20 a.m., the senior supervisor began looking for a crew to respond to the leak. Using the UGI duty roster of employees available to respond to emergencies, the senior supervisor made several calls between 11:25 a.m. and 11:44 a.m. to the on-call duty operator, who is required to answer all calls for response during off hours, including weekends; however, that duty operator did not respond. The senior supervisor contacted the next duty operator appearing on the on-call list, who answered and agreed to respond to the scene. By 11:50 a.m., the senior supervisor had identified a crew to respond at the site. After identifying the response crew, the senior supervisor traveled to the scene. The senior supervisor told National Transportation Safety Board (NTSB) investigators that prior to arriving on-scene he reviewed the location of the closest shutoff valve to the site. On his way to the shutoff valve, he contacted the UGI engineer and informed her that he expected to shut down a main line. As the duty supervisor, the senior supervisor could exercise judgement to determine whether it was appropriate to either squeeze off the line or close a gas valve.8 After the discussion with other responding UGI employees, the senior supervisor decided to squeeze off the main line rather than closing the mainline valve, because he thought this action was the fastest way of stopping the flow of gas. Figure 2 shows the location of the valve. Figure 2. Location of gas shut-off valve in relation to accident site. The senior supervisor told NTSB investigators that the following factors influenced his decision to squeeze off the main: he considered that given the circumstances surrounding the leak, 8 To squeeze off a line is a method of controlling either the pressure or the flow through a plastic pipe by reducing its cross-sectional area by applying an external force. 3 CORRECTED COPY <<>> UGI Utilities Natural Gas-Fueled Explosion even if the valve were turned off, the gas would continue to flow to the leak until the system lost pressure; and the line could be quickly and successfully excavated and squeezed, as compared to the time necessary to reach the valve. The senior supervisor stated that he received a phone call from the technician, who said that he had measured a lower explosive limit (LEL) of 11 percent in the basement at 202 Springdale Lane and that he could not gain access to the house at 206 Springdale Lane.9 The senior supervisor informed the technician that he should do whatever was necessary, including a forced entry, to clear the house at 206 Springdale Lane. Shortly after noon, the senior supervisor arrived at the scene and continued making calls from his vehicle to his management and others to keep them abreast of the situation. Within 10 minutes of arriving, the senior supervisor made an assessment that the fire department was needed. At 12:14 p.m., he called UGI Dispatch to request that they call 9-1-1, which was accomplished 1 minute later. A second UGI technician arrived on the scene at 12:19 p.m. The senior supervisor and the duty operator began excavating the gas main pipeline. The first UGI technician on the scene had twice visited the residence (202 Springdale Lane) to the left of the accident house and had talked with the residents. The LEL measurement on the first visit was 10 percent. Returning 30 minutes later, he advised the residents of 202 Springdale Lane that there was no need to evacuate as the LEL measurement was at 11 percent, which is less than UGI’s 20 percent LEL threshold for resident evacuation. While the main line was being excavated, the occupant of 206 Springdale Lane responded to the first UGI technician’s knocks on the door. Based on the gas concentration readings of 10 to 11 percent gas in air in the house, which would represent about 200 percent LEL concentration (well above UGI’s 20 percent LEL threshold for resident evacuation), the technician decided to evacuate the house. 10 The occupant requested permission to leave in her personal vehicle that was stored in the garage. Although the technician did not allow the occupant to use her powered garage door opener, he allowed the occupant to start her vehicle in the garage and depart the area. The electrical power to the neighborhood remained energized at the time of the accident.11 The UGI technician, who had first arrived on the scene, was near the gas meter for 206 Springdale Lane when the explosion occurred. The two gas employees, who were digging at the main in front of the house, had fully excavated the plastic main and were ready to squeeze-off the plastic main line when the explosion occurred. The representative from LASA was walking in the cul-de-sac when the explosion occurred. The technician located near the gas meter was killed and three others were injured. 9 Lower explosive limit (LEL) is the minimum concentration of vapor (percentage by volume) in air below which propagation of a flame does not occur in the presence of an ignition source. 10 Atmospheres containing gas concentrations between 5 to 17 percent will support ignition and are considered highly flammable. 11 At 12:59 p.m., the UGI Dispatch contacted the local electric power provider, PPL Electric Utilities (PPL), and requested that they shut off the power to the area. At 1:08 p.m., PPL confirmed that the electric power was shut off and that its personnel were on site. 4 CORRECTED COPY <<>> UGI Utilities Natural Gas-Fueled Explosion Technician Experience and Qualifications The technician, who was killed in the explosion, had over 16 years of utility experience. In 2001, he started working at UGI. He was a technician in the Construction and Maintenance department. Since April 2003, he held an Investigating Leak and Odor Complaints qualification. He held 73 operator qualification tasks that covered all aspects of gas utility work, including emergency response, pipe joining, repair and maintenance, and inspection activities. Since 2013, he had responded to and resolved 446 emergency calls. Of those 446 calls, 238 were gas leaks. Of those 238 leaks, 27 were hazardous underground gas leaks. Mechanical Tapping Tees NTSB’s examination of the tee assembly involved in the accident revealed the assembly was incorrectly installed because a locking sleeve, an important piece of hardware that served as an attachment between the tee and the main, was not attached to the main. The tee is also secured to the main by four nylon bolts. Although four nylon bolts were installed, two of them were fractured. Since the locking sleeve was not attached to the main, additional tensile stress was placed on the four nylon bolts, which then become the fail-safe feature holding the tee assembly in place. The NTSB determined two of the four nylon bolts fractured when in service in a manner consistent with slow crack growth. The incorrect installation of the tee, combined with the in-service fracture of the two nylon bolts, allowed gas to escape from the tee assembly. See Safety Recommendation Report PSR-18/01 Installation of PermaLock Mechanical Tapping Tee Assemblies, including Safety Recommendations P-18-001 through -004, for details pertaining to examination and testing of tee assemblies. Postaccident Actions UGI took the events of July 2, 2017, as an opportunity to reinforce and enhance the procedures and programs specifically revolving around emergency response to ensure employee preparedness, competence, and confidence when responding to emergency situations. The following summarizes the emergency response improvements UGI implemented across departments and job functions: 1. System Improvement Initiatives Immediately following the accident, UGI instituted the following initiatives to manage the integrity of the system on Springdale Lane from the intersection of Burr Oak Drive west to the end of the cul-de-sac. UGI implemented the following improvements: • Conducted a daily driving leak survey of the main in the neighborhood • Remediated or replaced all known mechanical tees • Replaced the entire gas main 5 <<>> UGI Utilities Natural Gas-Fueled Explosion • Took actions to manage the integrity of the surrounding neighborhood defined by the boundaries of North Duke Street, Blue Rock Road, Letort Road, and Little Conestoga Creek 2. Pipeline and Public Safety Improvements The UGI Pipeline and Public Safety department undertook several training initiatives to emphasize the incident command system. Outreach efforts with external emergency response departments was increased to strengthen the awareness of natural gas pipeline safety. 3. Training and Field Compliance Improvements UGI updated its training to improve the skill set of its employees when responding to an emergency. 4. Pipeline Facility Improvements UGI implemented pipeline facility enhancement to ensure the safe and reliable delivery of natural gas to its customers. This enhancement included the increased remediation activity of mechanical tees throughout its distribution system, by either remediating or replacing 2,577 total mechanical tees since the accident [through October 4, 2018]. UGI continues to examine the main and service facilities to determine the mechanical tees that must be replaced. 5. Dispatch Improvements UGI modified its dispatch procedures and protocols. UGI streamlined dispatch processes to proactively identify the need for electrical shutdown and additional help. 6. Standard and Procedure Improvements UGI created an emergency plan that is separate from its Gas Operations Manual (GOM). UGI’s emergency plan contains similar content as previously included in GOM 60.50– Emergency Plan; however, the procedures have been revised to add further clarity to the emergency response situations and the actions undertaken. 7. 9-1-1-Related UGI Initiatives To enhance safety and to ensure 9-1-1 emergency responders are timely notified, UGI: • Developed a first-hour checklist for first responders that provides guidance on when to (1) contact the local emergency response agencies: 9-1-1 and Emergency Medical Services, (2) shut off electrical power in the area, and (3) conduct an evacuation • Updated its response procedures to include “safety perimeter" criteria • Reinforced the discretion that UGI’s first responders must shut down a pipeline 6 <<>> UGI Utilities Natural Gas-Fueled Explosion • Created checklists for leak investigations • Identified in its emergency response procedures specific emergency situations where immediate 9-1-1 notification is required by UGI personnel • Reviewed and updated its dispatch procedures to include a decision matrix • Identified situations and keywords where immediate 9-1-1 notification is required 8. Safety Culture Assessment Project In June 2018, UGI contracted DuPont Sustainable Solutions (DSS) to assess the UGI safety culture and the efficacy of UGI safety-related initiatives and incident investigation process to create a unified safety brand. The assessment included a comprehensive evaluation that provided a baseline to understand the current state and the development of an improvement strategy to advance the culture, systems, and processes as part of a world-class safety program. Pennsylvania Public Utility Commission Enforcement Action On October 4, 2018, the Pennsylvania Public Utility Commission Bureau of Investigation and Enforcement, issued a formal complaint against UGI that proposed a cumulative civil penalty of $2,090,022. This complaint, which was an enforcement action of that regulatory agency, alleged several safety violations that occurred during the gas leak response on Springdale Lane. Previously Issued Safety Recommendations As a result of this natural gas-fueled explosion on June 25, 2018, the NTSB issued the following four safety recommendations: To the Pipeline and Hazardous Materials Safety Administration: Work with state pipeline regulators to incorporate into inspection programs, a review to ensure that gas distribution pipeline operators are using best practices recommended by the manufacturer in its distribution integrity management programs, including using the specified tools and methods, to correctly install Permalock mechanical tapping tee assemblies. (P-18-001) Safety Recommendation P-18-001 is classified Open—Initial Response Received. Reference the use of external sources of information for threat identification in your frequently asked questions for preparation of distribution integrity management programs. (P-18-002) Safety Recommendation P-18-002 is classified Closed-Acceptable Action. 7 <<>> UGI Utilities Natural Gas-Fueled Explosion To Honeywell International, Incorporated: Update your Permalock mechanical tapping tee assembly installation instructions to specify the exact tools that should be used during installation and explain what an installer should sense while using those tools throughout the installation process. (P-18-003) Safety Recommendation P-18-003 is classified Open—Initial Response Received. Specify in your Permalock mechanical tapping tee assembly installation instructions a not-to-exceed torque limit for Nylon bolts and have that value checked and adjusted with a torque wrench immediately after installation. (P-18-004) Safety recommendation P-18-004 is classified Open—Initial Response Received. Probable Cause The National Transportation Safety Board determines that the probable cause of the natural gas explosion at 206 Springdale Lane was an improperly installed mechanical tapping tee that leaked and allowed gas to migrate into the house where it ignited. For more details about this accident, visit www.ntsb.gov/investigations/dms.html and search for NTSB accident identification number DCA17FP006. Issued: February 25, 2019 Revised: October 26, 2023 The NTSB has authority to investigate and establish the facts, circumstances, and cause or probable cause of a pipeline accident in which there is a fatality or substantial property damage, or significant injury to the environment. (49 U.S. Code, Section 1131 - General authority) 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.” Title 49 Code of Federal Regulations, Section 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. Code, Section 1154(b). 8 PSR-18-01 <<>> National Transportation Safety Board Washington, DC 20594 Safety Recommendation Report Installation of PermaLock Mechanical Tapping Tee Assemblies Accident/Incident Number: DCA17FP006 Operator: UGI Utilities, Inc. Accident: Residential Natural Gas Explosion Location: Millersville, Pennsylvania Date: July 2, 2017 Recommendation Number: P-18-1 through -4 Adopted: June 18, 2018 The National Transportation Safety Board (NTSB) is providing the following information to urge the Pipeline and Hazardous Materials Safety Administration (PHMSA) and Honeywell to take action on safety recommendations intended to prevent the incorrect installation of PermaLock mechanical tapping tee assemblies in gas distribution systems. These recommendations are derived from our ongoing investigation of a fatal accident involving a natural gas explosion and fire in Millersville, Pennsylvania. The NTSB is issuing two recommendations to PHMSA and two recommendations to Honeywell. Ongoing Investigation On July 2, 2017, at 12:32 p.m. eastern standard time, a natural gas explosion and fire destroyed a single-family residence at 206 Springdale Lane, in Millersville, Pennsylvania. One person died, and three people were injured. Two nearby residences were severely damaged and condemned for demolition. The NTSB investigation is ongoing, and the cause of the accident has not been determined. UGI Utilities, Inc. (UGI) supplied natural gas to the Millersville residences through a plastic natural gas pipeline (main) and service lines, which UGI operated at a pressure of 54 pounds per square inch, gauge. The main was 2 inches in diameter and made of Aldyl polyethylene; the 0.50-inch diameter service lines were made of polyethylene. Postaccident, the main and service lines at the accident site were pressure tested, which indicated that a PermaLock mechanical tapping tee assembly was leaking gas at the connection of the tee assembly to the main in front of the destroyed residence. UGI installed the tee assembly in June of 1998; it had been in service for 19 years when the accident occurred. (See figure.) 58092 PSR-18/01 <<>> Stop (for cap) To service line Stop (for the outer lip portion of the locking sleeve) Figure. Cross-section diagram of an exemplar PermaLock mechanical tapping tee assembly, provided by Honeywell. PermaLock Mechanical Tapping Tee Assembly Perfection Corporation, later known as Elster Perfection Corporation, which is currently a division of Honeywell, manufactured the PermaLock mechanical tapping tee assembly involved in the accident. 1 Since 1987, three versions of the PermaLock tee assembly have been manufactured, and millions of the tee assemblies have been sold worldwide.2 Each of the three versions has a different cutter tool design and method of attaching the tower to the main. The tee assembly in this accident was manufactured in 1998; it was the third version. The tee assembly was made of medium density polyethylene and consisted of a tower (upper half) and a base (lower 1 For simplicity, in this report, the NTSB will use Honeywell as the umbrella company name encompassing the Elster Perfection Corporation and the Perfection Corporation. 2 The first version of the PermaLock tee assembly had a flat cap and a one-piece cutter design that projected into the main; it was manufactured from 1987 to 1990. The second version had a domed cap and a cutter with a retractable center piece; it was manufactured from 1990 to 1996. The third version has a flat cap, a cutter tool, and a locking sleeve, and it is shipped with a depth tube; manufacturing of this version began in 1995 and continues today, in 2018. 2 <<>> half) joined together around the outer surface of a main by four Nylon 6/6 bolts.3 The tower contained an internal circular steel cutter tool and a steel locking sleeve.4 Honeywell ships the third version of the tee assembly with general written installation instructions and a plastic depth tube.5 The depth tube is provided so that the person installing the tee assembly can verify that the locking sleeve attaches to the main. According to the written instructions, the cutter tool pierces a hole in the main, and the locking sleeve attaches the tower to the main. The cutter tool is then decoupled (separated) from the locking sleeve and remains stored in the tower for the service life of the tee assembly. NTSB examination of the PermaLock tee assembly involved in the accident has revealed that the tee assembly was incorrectly installed. Although the cutter tool pierced a hole in the main, the locking sleeve did not progress down far enough into the tower to form threads in that hole. As a result, the locking sleeve was not attached to the main.6 The NTSB also found that two of the four Nylon bolts on the tee assembly were fractured in a manner consistent with slow crack growth. 7 These findings indicate the likelihood of varying tee assembly installation techniques. Installation Instructions Honeywell’s written installation instructions, which are shipped with the third version of the PermaLock tee assembly, describe the general requirements of the installation, but they do not specify all of the necessary details to properly accomplish the installation and ensure that the locking sleeve attaches to the main. 8 Honeywell also produced an instructional video on the installation of the third version of the PermaLock tee assembly. 9 In the video, the instructor explains that using shorter wrenches allows the person installing the tee assembly to better sense resistance on the wrench handle during the various installation phases. The instructor further explains that using longer wrenches reduces that ability to sense the various installation phases via the wrench handle. However, the instructor does not specify the appropriate size of the wrench that should be used. In reviewing the written instructions and the instructional video for the tee assembly involved in the accident, the NTSB has found that the different formats provided varying 3 (a) Starting in 2001, the bolt material on the third version was upgraded to Nylon 11. Beginning in 2005, the third version of the tee assembly was upgraded to and made of high density polyethylene. (b) Honeywell also offers accessories for the third version, such as stainless steel bolts and nuts for use in lieu of the Nylon bolts for tee assemblies installed on 1.25-inch diameter and 2-inch diameter mains. 4 When joined together, the cutter tool and the locking sleeve are referred to as the cutter assembly. 5 The specified length of the depth tube varies in accordance with the various tee assembly sizes. 6 NTSB x-ray computed tomography images and post disassembly inspection of the tee assembly confirmed this condition. 7 Slow crack growth, also known as stress rupture, occurs when sustained tensile stresses in parts made from polymeric materials enable a network of fine craze cracks that coalesce and propagate with time, ultimately resulting in fracture of the part. See K. G. Budinski and M. K. Budinski, Engineering Materials: Properties and Selection, 9th ed. (Upper Saddle River, New Jersey: Prentice Hall, 2010: 275-278). 8 A copy of these instructions is available in the docket for the Millersville accident investigation; see the NTSB Docket Management System and search for accident ID DCA17FP006. 9 The video was produced in 1996, about 1 year after Honeywell began manufacturing the third version of the tee assembly. A copy of the video is available in the docket for the Millersville accident investigation; see the NTSB Docket Management System and search for accident ID DCA17FP006. 3 <<>> amounts of critical information about the installation process, which likely affected the installation outcomes. UGI Training Program Title 49 Code of Federal Regulations (CFR), Part 192, Subpart N, effective October 2002, requires that personnel installing mechanical tapping tee assemblies must have received qualification training prior to installing a tee assembly. UGI instituted a certification program for employees who install mechanical tapping tee assemblies, which included a written examination and a practical test. However, when the tee assembly involved in the accident was installed in 1998, federal regulations concerning operator qualification training standards for installation of tee assemblies did not exist, and UGI did not have a training program for installing them. At that time, UGI was relying solely on the written installation instructions shipped with the tee assembly. NTSB Tee Assembly Installation Testing The NTSB experimented with installing several exemplar PermaLock tee assemblies and found the following. Locking Sleeve The NTSB found that certain techniques can alter the tee assembly installation, particularly affecting whether the locking sleeve attaches to the main. For example, when the NTSB used 4.5-inch and 7-inch length wrenches, the level of resistance was felt in the wrench handles as the cutter assembly was driven down the tower. Specifically, it was possible to feel the following installation stages: • the cutter tool piercing the main; • the cutter tool being driven through the wall of the main; • the locking sleeve forming threads into the main; • the outer lip portion of the locking sleeve making contact with a manufactured stop near the bottom of the tower; and • the cutter tool separating from the locking sleeve, which was also audible. However, when using a 19-inch length wrench, changes in resistance were difficult to feel via the wrench handle during the installation stages. NTSB testing demonstrated that as the cutter assembly was driven down the tower with the longer wrench, the outer lip portion of the locking sleeve was easily driven below the stop, leading to an incorrectly installed locking sleeve and potential damage to the threads in the main. Sensing resistance on the handle of a wrench indicates the correct positioning of a PermaLock tee assembly during the installation stages. In particular, sensing that the locking sleeve is resting against the stop near the bottom of the tower is critical to knowing that a locking sleeve is properly attached to a main. Therefore, knowing which wrench length will best facilitate the ability to feel increasing resistance is safety-critical information necessary to correctly installing a PermaLock tee assembly. 4 <<>> Using the supplied depth tube is also essential to correctly installing a PermaLock tee assembly. The depth tube is inserted into the tower, on top of the cutter assembly. The depth tube provides the installer a visual indication of the vertical travel distance of the cutter assembly, thereby, allowing the installer to visually verify the point at which the locking sleeve attaches to the main. When the locking sleeve attaches to the main, the depth tube will be flush to 0.125 inch above the top of the tower. Knowing to use the depth tube and how to use it are also critical to reliably ensuring the correct and safe installation of a PermaLock tee assembly. Detailed installation instructions for each specific tee assembly version are necessary to eliminate confusion or misinterpretation during installation. For tee assemblies with a locking sleeve, such instructions must specify (1) the exact tools that are most effective for attaching a locking sleeve to a main and (2) the use of a depth tube to verify that attachment. Therefore, the NTSB concludes that more detailed installation instructions for PermaLock mechanical tapping tee assemblies that specify the exact tools to be used and emphasize what should be sensed while using those tools are necessary to ensure correct installation and prevent gas leaks. The NTSB recommends that Honeywell update its PermaLock mechanical tapping tee assembly installation instructions to specify the exact tools that should be used during installation and explain what an installer should sense while using those tools throughout the installation process. For example, specify which size wrench to use when installing a PermaLock tee assembly, and explain what should be sensed via the wrench handle when the locking sleeve reaches the stop at the bottom of the tower indicating the locking sleeve is attached to the main. Nylon Bolts According to a Honeywell representative, the locking sleeve is the primary means of securing a PermaLock tee assembly tower to a main. A saddle O-ring, made from an elastomer, is located under the tower of the tee assembly (see figure). When the tower is attached to the main, the saddle O-ring forms a seal between the tower and the main that prevents gas from escaping out of the tee assembly. If the locking sleeve within the tower is not attached to the main, additional tensile stress is placed on the four Nylon bolts, which are used to join the tower and the base together around the outer surface of the main. The four Nylon bolts then become the fail-safe feature holding the tee assembly in place. When the locking sleeve is not attached to the main, fracture of one or more of the Nylon bolts can be sufficient to allow a gap to open in the saddle O-ring area between the tower and the main where gas can escape out of the tee assembly. The mechanical behavior of bolts fabricated from polymers, such as Nylon, is different than metallic materials, such as stainless steel. Nylon bolts have lower tensile strength and creep resistance, and they are susceptible to failure by slow crack growth, a failure mode not encountered in traditional metal alloys (such as in the case of the bolt-joint design of the tee assembly involved in the Millersville accident). 10 Honeywell tee assembly installation instructions do not warn about the consequences of slow crack growth and stress relaxation due to creep, which can result from overtightening the Nylon bolts.11 The NTSB concludes that Nylon bolts are susceptible to slow 10 (a) Tensile strength refers to the amount of stress a material can endure before fracturing. (b) Creep is the deformation of a material when exposed to constant stress for an extended period of time. Creep resistance refers to the ability of a material to resist deformation when subjected to constant stress for an extended period of time. 11 Stress relaxation is the reduction in stress in plastic material when subjected to constant strain. Deformation can progress until eventual fracture of the material. 5 <<>> crack growth when overtightened. Therefore, the NTSB recommends that Honeywell specify in its PermaLock mechanical tapping tee assembly installation instructions a not-to-exceed torque limit for Nylon bolts and have that value checked and adjusted with a torque wrench immediately after installation. UGI Remediation Program The UGI remediation program refers to repairing or replacing PermaLock tee assemblies. 12 Specifically, the UGI Gas Operations Manual, Section 70.40.20, “Remediation of Perfection Plastic Mechanical Tapping Tees,” states that the tee assembly Nylon bolts are subject to breaking, and the locking sleeve may not have attached to the main during installation.13 UGI remediation includes replacing the Nylon bolts, checking the attachment of the locking sleeve to the main, gathering information on the tee assembly installation and site conditions, and handling any leaks that have occurred. 14 When a tee assembly is excavated and found with one or more fractured Nylon bolts but shows no evidence of a gas leak, UGI replaces all of the Nylon bolts with type 316 stainless steel bolts, flat and lock washers, and nuts. When a tee assembly is found leaking gas at its base, UGI cuts out the segment of the main with the tee assembly and replaces it with a new main segment and a new approved tee assembly. Since 2007, UGI has remediated or replaced about 4,000 PermaLock tee assemblies, more than a 1,000 of which have been remediated or replaced since the July 2, 2017, Millersville accident. UGI also stopped installing PermaLock tee assemblies about 10 years ago, choosing instead to install electrofusion tee assemblies in their place. 15 Related Incidents and Investigations Out of concern that incorrectly installed PermaLock tee assemblies may be a widespread safety issue, the NTSB reviewed related incidents and investigations and found the following. UGI Excavation, 2006 On September 2, 2006, a natural gas explosion destroyed a residence at 39 Apple Blossom Drive, in West Lampeter Township, Pennsylvania. UGI excavated a PermaLock tee assembly that was leaking gas from the main in front of the residence. The tee assembly had two fractured bolts. UGI contracted JANA Laboratories, Incorporated (JANA), to examine the tee assembly; JANA 12 The remediation program applies to all three versions of the PermaLock tee assemblies. Because the first two versions of the tee assembly do not contain a locking sleeve, the position of the internal cutter tool is not checked. For all versions, the Nylon bolts are replaced. 13 The UGI Gas Operations Manual became effective February 16, 2015. 14 According to the UGI Gas Operations Manual, to determine the depth of a locking sleeve and whether it is attached to a main, an installer should attach a wrench to the cutter tool and count the number of turns and measure the torque value as the cutter tool is driven down the tower. The number of turns made on the wrench and measured torque values can then be compared to those specified in the UGI manual. 15 Electrofusion tee assemblies have built-in electric heating elements. They are used for welding a plastic tee assembly to a plastic pipe. 6 <<>> issued its examination results in report 07-4063, dated July 2008.16 The report does not indicate the reason for the bolt fractures. However, X-ray inspection results in the JANA report showed that the locking sleeve did not appear to be attached to the main. These findings are similar to those in the Millersville accident. NTSB Delegated Investigation On December 9, 2009, a natural gas explosion occurred at 9140 Grey Pointe Drive, in Knoxville, Tennessee. 17 The resulting fire destroyed the brick three-story residence, causing more than $1 million in damages, including about $55,000 of damage to the Knoxville Utilities Board (KUB) gas utility system. One person died, and two people sustained serious injuries. Three nearby houses were also damaged and later condemned. The NTSB delegated the Tennessee Regulatory Authority (TRA), Gas Pipeline Safety Division, to investigate the accident and submit its findings to the NTSB.18 The TRA investigation revealed that the gas leak stemmed from a PermaLock tee assembly, and two of its four Nylon bolts were fractured. In 2014, the NTSB determined that “the probable cause of the Knoxville, Tennessee, accident was natural gas leaking from an incorrectly installed, bolt-on service tee. The natural gas migrated into the single-family structure where it accumulated and ignited.” The NTSB further stated that— because PHMSA had taken significant action to address plastic pipe issues before the accident, and the KUB took significant corrective actions to identify and correct plastic pipe installation deficiencies in its system, no safety recommendations were proposed, and no further action [was] taken on this accident. To ensure public safety, the KUB decided to remove all PermaLock tee assemblies from service throughout its distribution system. The KUB initiated a replacement program, which involved locating and replacing about 1,875 tee assemblies that KUB had purchased and installed between 1997 and 2001. The KUB excavated about 12,000 service connections to find and replace the tee assemblies.19 The replacement program cost about $4 million, not including KUB employees’ salaries and benefits. Wilson Boro, Pennsylvania, 2017 On August 15, 2017, as a result of a natural gas leak, UGI excavated a PermaLock tee assembly in front of a single-family residence at 2253 Firmstone Street, in Wilson Boro, Pennsylvania. The tee assembly was the third version. Although the NTSB is not investigating the 16 JANA is located in Aurora, Ontario, Canada. 17 NTSB Director, Office of Railroad, Pipeline and Hazardous Materials Investigations letter to the Public Docket regarding the “Delegated Pipeline Accident Report, Knoxville, Tennessee,” dated June 19, 2014. A copy of the letter is available in the NTSB Docket Management System, search for accident ID DCA10FP001. 18 In 2017, the TRA was renamed the Tennessee Public Utility Commission. 19 See the Report of Natural Gas Safety Inspection Report #11-211, issued by the Gas Pipeline Safety Division, Tennessee Regulatory Authority, which is available in the NTSB Docket Management System, search for accident ID DCA10FP001. 7 <<>> Wilson Boro incident, UGI provided the tee assembly to the NTSB for examination. The NTSB determined that the locking sleeve was not attached to the main, and two of the four Nylon bolts were fractured. The findings about the condition of the tee assembly are similar to those of the tee assembly involved in the Millersville accident. Ongoing UGI Gas Leak Survey Incidents After the Millersville accident, UGI instituted an accelerated gas leak survey to more quickly identify gas leaks resulting from incorrectly installed PermaLock tee assemblies throughout its distribution system. As of June 12, 2018, UGI had found 19 tee assemblies leaking gas in its distribution system, many of which were discovered during the excavation process. All of the leaks were associated with fractured Nylon bolts.20 An incorrectly installed tee assembly can leak gas several months, years, or decades after installation. Thus far, none of these recent gas leak incidents identified by UGI’s leak survey have caused any personal injuries or property damage. PHMSA Advisory Bulletin, 2008 Because of ongoing concerns about mechanical couplings and related appurtenances failures, PHMSA issued advisory bulletin ADB-08-02 in 2008. It advised owners and operators of gas pipelines to consider the potential failure modes for mechanical couplings used for joining two pipes together. The bulletin cautioned that “failures can occur when there is inadequate restraint for the potential stresses on the two pipes, when the couplings are incorrectly installed or supported, or when the coupling components such as elastomers degrade over time.” Further the bulletin warned operators to precisely follow manufacturer installation instructions to prevent future accidents. Distribution Integrity Management Program Title 49 CFR Part 192, Subpart P, requires gas distribution pipeline operators to have a distribution integrity management program (DIMP). 21 Specifically, operators are required to develop a DIMP that identifies threats, actions to mitigate the hazards associated with those threats, and measures to reduce risk as well as their frequency. A DIMP should include an engineering evaluation of any reinforcement or remediation of a PermaLock tee assembly to ensure its longevity and leak-free service. As part of a DIMP, an operator should have an understanding of the design of and best practices for installing a mechanical tapping tee assembly. Because UGI is not the only operator to have detected gas leaks stemming from PermaLock tee assemblies due to incorrect installation, additional action is needed to ensure that such tee assemblies are installed correctly. Manufacturer installation instructions for tee assemblies must precisely convey best practices, and operators must clearly explain those best practices to their employees installing the tee assemblies. The NTSB 20 UGI did not conduct examinations to determine whether the locking sleeve was attached to the main in these 19 instances. 21 Operators had until August 2, 2011, to establish their DIMPs. 8 <<>> concludes that Honeywell’s installation instructions, on which UGI relied, are too brief and imprecise to ensure the correct installation of the PermaLock mechanical tapping tee assembly. Therefore, the NTSB recommends that PHMSA work with state pipeline regulators to incorporate into their inspection programs, a review to ensure that gas distribution pipeline operators are using best practices recommended by the manufacturer in their DIMPs, including using the specified tools and methods, to correctly install PermaLock mechanical tapping tee assemblies. Frequently Asked Questions PHMSA provides written clarification of the pipeline safety regulations (49 CFR Parts 190-199) in the form of frequently asked questions (FAQs). The FAQs help clarify, explain, and promote better understanding of how to comply with the regulations. The NTSB notes that the FAQs for preparation of DIMPs focus primarily on system information that the operators maintain; the FAQs do not reference external sources of information for threat identification, such as advisory bulletins, safety alerts, or NTSB investigation reports. The NTSB concludes that referencing the use of external sources of information for threat identification in the FAQs for preparation of DIMPs would help gas distribution pipeline operators recognize and better understand how to reduce potential incidents or accidents. Therefore, the NTSB recommends that PHMSA reference the use of external sources of information for threat identification in its FAQs for preparation of DIMPs. Recommendations To the Pipeline and Hazardous Materials Safety Administration: Work with state pipeline regulators to incorporate into their inspection programs, a review to ensure that gas distribution pipeline operators are using best practices recommended by the manufacturer in their distribution integrity management programs, including using the specified tools and methods, to correctly install PermaLock mechanical tapping tee assemblies. (P-18-1) Reference the use of external sources of information for threat identification in your frequently asked questions for preparation of distribution integrity management programs. (P-18-2) To Honeywell: Update your PermaLock mechanical tapping tee assembly installation instructions to specify the exact tools that should be used during installation and explain what an installer should sense while using those tools throughout the installation process. (P-18-3) Specify in your PermaLock mechanical tapping tee assembly installation instructions a not-to-exceed torque limit for Nylon bolts and have that value checked and adjusted with a torque wrench immediately after installation. (P-18-4) 9 <<>> BY THE NATIONAL TRANSPORTATION SAFETY BOARD ROBERT L. SUMWALT, III EARL F. WEENER Chairman Member T. BELLA DINH-ZARR Member Adopted: June 18, 2018 10 ## Provenance - Official: Yes - Source: - Source ID: `ntsb-pipeline` - SHA-256: `00f7f401ced3684ac763e74c2ea3e803238a6b0f1d76791cbe162833f4ec86d6` - Retrieved: 2026-08-20T04:57:25.499Z - Exported: 2026-08-23T19:16:57.853Z - Document slug: `ntsb-case-dca17fp006` ### Source metadata ```json { "recordKind": "ntsb_pipeline_investigation", "mkey": 95479, "ntsbNumber": "DCA17FP006", "completionStatus": "Completed", "caseClosed": true, "hasSafetyRecommendation": true, "eventType": "Accident", "accidentType": "Leak/explosion/fire", "operator": "UGI Utilities", "pipelineType": null, "location": { "city": "Millersville", "state": "PA", "country": "USA", "latitude": null, "longitude": null }, "reportNumber": "PAB1901", "probableCause": "The National Transportation Safety Board determines that the probable cause of the natural gas explosion at 206 Springdale Lane was an improperly installed mechanical tapping tee that leaked and allowed gas to migrate into the house where it ignited.", "events": [ { "eventNumber": 3, "sequenceNumber": 3, "eventCode": "700170", "tier1Number": "700", "tier1Name": "Post-release", "tier2Number": "170", "tier2Name": "Explosion", "isDefiningEvent": false }, { "eventNumber": 4, "sequenceNumber": 4, "eventCode": "700160", "tier1Number": "700", "tier1Name": "Post-release", "tier2Number": "160", "tier2Name": "Fire (post-release)", "isDefiningEvent": false }, { "eventNumber": 5, "sequenceNumber": 5, "eventCode": "600800", "tier1Number": "600", "tier1Name": "Emergency response", "tier2Number": "800", "tier2Name": "Emergency response", "isDefiningEvent": false }, { "eventNumber": 2, "sequenceNumber": 2, "eventCode": "700460", "tier1Number": "700", "tier1Name": "Post-release", "tier2Number": "460", "tier2Name": "Evacuation", "isDefiningEvent": false }, { "eventNumber": 1, "sequenceNumber": 1, "eventCode": "400450", "tier1Number": "400", "tier1Name": "System operating", "tier2Number": "450", "tier2Name": "Product leak/release", "isDefiningEvent": true } ], "findings": [ { "findingNumber": 3, "sequenceNumber": 3, "findingCode": "01010117", "findingTier1Name": "Pipeline", "findingTier2Name": "Pipeline handling/service", "findingTier3Name": "Operating information", "findingTier4Name": null, "findingModifierName": "Related operating info", "findingReportText": "Pipeline - Pipeline handling/service - Operating information - Related operating info", "findingText": null, "inProbableCause": false }, { "findingNumber": 4, "sequenceNumber": 4, "findingCode": "01010108", "findingTier1Name": "Pipeline", "findingTier2Name": "Pipeline handling/service", "findingTier3Name": "Operating information", "findingTier4Name": null, "findingModifierName": "Capability exceeded", "findingReportText": "Pipeline - Pipeline handling/service - Operating information - Capability exceeded", "findingText": null, "inProbableCause": false }, { "findingNumber": 2, "sequenceNumber": 2, "findingCode": "02032099", "findingTier1Name": "Personnel", "findingTier2Name": "Experience/knowledge", "findingTier3Name": "Knowledge", "findingTier4Name": null, "findingModifierName": "Not specified", "findingReportText": "Personnel - Experience/knowledge - Knowledge - Not specified", "findingText": null, "inProbableCause": false }, { "findingNumber": 1, "sequenceNumber": 1, "findingCode": "01021502", "findingTier1Name": "Pipeline", "findingTier2Name": "Pipline systems/equipment", "findingTier3Name": "Fittings", "findingTier4Name": null, "findingModifierName": "Malfunction", "findingReportText": "Pipeline - Pipline systems/equipment - Fittings - Malfunction", "findingText": null, "inProbableCause": true } ], "caveat": "Official NTSB investigation data. 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