NTSB Finds Incorrectly Installed Tapping Tee Caused Explosion

A locking sleeve was not attached to the gas main at the Millersville, Pa., site of the July 2017 explosion, and two of four nylon bolts needed to secure the tee to the main were broken.

The National Transportation Safety Board has determined that an improperly installed gas connection that allowed natural gas to seep into a single-family house was the probable cause of a July 2017 explosion in Millersville, Pa. The explosion at 206 Springdale Lane killed one person and injured three others, destroyed the home, and significantly damaged six neighboring homes, one of which was subsequently condemned.

NTSB reported that central to its investigation was the installation of a Permalock mechanical tapping tee, the connection between the 2-inch gas main and the home's individual gas line. NTSB's examination of the tee assembly involved in the accident revealed the assembly was incorrectly installed because a locking sleeve, described as "an important piece of hardware that served as an attachment between the tee and the gas main," was not attached to the main. Because the locking sleeve was not attached, additional stress was placed on four nylon bolts that hold the tee assembly in place.

The safety board determined two of the four nylon bolts broke while in service in a manner consistent with slow crack growth. The incorrect installation of the tee, combined with the fracture of the two nylon bolts, allowed gas to escape from the tee assembly.

The board's report includes the timeline leading up to the explosion. A neighborhood resident smelled gas that morning and reported it to the local gas utility, UGI Utilities, Inc. A UGI technician arrived and requested a response crew from the company because he measured high gas readings and confirmed a leak, saying he measured readings of 80 percent gas in the sewer and 98 percent gas over the tee. UGI called Emergency PA One Call to mark the underground utilities because an excavation would be necessary, and a UGI senior supervisor assembled a response crew and went to the scene. Once there, he called 911 to summon fire department personnel to the scene. During this time, the technician spoke to a resident of 202 Springdale Lane but was unable to get the occupant of 206 Springdale Lane to answer the door. As the gas main was being excavated, the 206 Springdale Lane occupant responded, and the technician told her to evacuate. She requested permission to leave in her personal vehicle and, though the technician did not allow her to use her powered garage door opener, he allowed her to start her vehicle and leave the area.

NTSB's report says electrical power to the neighborhood remained on at the time of the explosion, which was 12:32 p.m. UGI Dispatch contacted the local electrical utility at 12:59 p.m., asking it to shut off power to the neighborhood.

The explosion killed the technician, who was near the home's gas meter when it occurred. Two gas employees who were excavating the main were injured, as was a third person.

The report was issued Feb. 25. It reiterates four safety recommendations that NTSB had issued in June 2018 to the Pipeline and Hazardous Materials Safety Administration and Honeywell International Inc. stemming from the investigation. The recommendations to Honeywell ask the company to update its Permalock tapping tee assembly installation instructions to specify the exact tools that should be used during installation and also to specify in the instructions a not-to-exceed torque limit for nylon bolts and have that value checked and adjusted with a torque wrench immediately after installation.

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