An improperly installed gas connection allowed natural gas to seep into a single-family house and resulted in a deadly 2017 explosion in Millersville, Pennsylvania.

The National Transportation Safety Board (NTSB) said the explosion killed one person and injured three others, destroyed the residence and significantly damaged six neighboring homes, one of which was later condemned.

Central to the NTSB investigation was the installation of a tapping tee, the connection between the gas main and the home’s individual gas line.

(Learn more from Engineering360 about standards for tapping tees used for a variety of applications.)

Likely cause

NTSB’s examination of the tee assembly involved in the accident showed the assembly was incorrectly installed. A locking sleeve — a piece of hardware that serves 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 NTSB found that two of the four nylon bolts fractured when in service in a manner that it said was consistent with slow crack growth. The incorrect tee installation, combined with the in-service fracture of the two nylon bolts, allowed gas to escape from the tee assembly.

According to the NTSB report, the natural gas main on the residential street was made from a polyethylene material that was installed in 1995. The maximum allowable operating pressure (MAOP) of the system main serving the street was 60 psig. The service line to the destroyed house was installed in 1998, and was constructed of 1/2-inch polyethylene pipe.

Aerial image showing the aftermath of the natural gas explosion. Source: NTSBAerial image showing the aftermath of the natural gas explosion. Source: NTSBThe mechanical tapping tee was installed at the same time as the service line. The tee and service pipe had been in service for 19 years when the accident occurred, NTSB said. The pipe between the main and the meter at the residence was 50 ft long. The service line to the residence was inserted inside a 1 in polyethylene pipe that served as a protective jacket. The jacket ran from the outlet of the service tee to the meter.

Event timeline

At 10:26 a.m. on July 2, 2017, a neighborhood resident walking in the area smelled natural gas and reported it to the local gas utility, UGI Utilities Inc. About half an hour later, according to the NTSB timeline, a UGI service technician arrived to investigate and confirm the gas leak.

An additional UGI technician and a senior supervisor arrived shortly thereafter. A Lancaster Area Sewer Authority employee arrived on the scene in response to UGI’s Emergency PA One Call to mark the location of the sewer lines. About 15 minutes before the explosion, UGI personnel directed the resident to evacuate. About 12:15 p.m., UGI requested fire department support. The first fire truck arrived at 12:28 p.m. and assumed a standby position.

One utility technician was near the gas meter for the residence when the explosion occurred, about four minutes after fire trucks arrived. At the time, two gas employees who were digging at the main in front of the home had fully excavated the plastic main and were ready to squeeze-off the plastic main line. The technician closest to the gas meter was killed, and three others were injured.

In June 2018, the NTSB issued four safety recommendations to the Pipeline and Hazardous Materials Safety Administration and Honeywell International Inc. stemming from the investigation. The recommendations were intended to prevent the incorrect installation of mechanical tapping tee assemblies in gas distribution systems.