Natural Gas Explosion Killed 2 People and Destroyed Residence in Pennsylvania.

Pennsylvania: National Fuel Gas Distribution Corporation (National Fuel) probed the ground near the residence for gas leaks. Combustible gas-to-air readings were found in two areas: one area was adjacent to the affected residence, and the other area was directly over the failed pipe on the front lawn. After the explosion, National Fuel excavated the front lawn. The excavation revealed a leaking butt-fusion joint in a 2-inch-diameter plastic main line pipe.

A butt-fusion joint is made by the following process. A heater plate is inserted between two pipe ends, melting the material on each end. The heater is then removed, and the pipe ends are pushed together under applied force. The pipe ends remain under applied force as the pipe ends cool and fuse together. The butt-fusion process forces excess molten plastic onto the inside and outside of the pipe surfaces. Because both sides of the pipe are molten, two visible beads are formed on the exterior and interior of the pipe joint. When this material cools and solidifies, it is called a fusion bead.

 National Fuel began excavating the accident site before National Transportation Safety Board investigators arrived. During the excavation, National Fuel accidentally severed the plastic pipe within a few feet of the butt-fusion joint. Because of this mishap, Safety Board investigators could not determine the undisturbed position of the pipe to assess its bending. Only a small ligament remained connected at the butt-fusion joint, making it impossible to directly test the strength of the joint.

The Safety Board’s examination of the butt-fusion joint linked to the explosion indicates that the bead widths were not uniform. The bead width on one side of the butt-fusion joint ranged from 0.0535 inch to 0.1135 inch. On the other side of the joint, the bead width ranged from 0.0745 inch to 0.1060 inch. The total bead width ranged from 0.1315 inch to 0.2045 inch. The joint was found to have been visibly mitered, having about a 2° angle. Mitering can concentrate stresses. No evidence was found indicating that the joint was significantly misaligned. 

 At the same location where the external bead width was thinnest, the bead was distorted. The external bead on one side of the joint was larger than the bead on the other side. The boundary between the two beads was indistinct. The internal bead was also distorted, with the bead on one side of the joint higher than the other side. On one side of the joint, the internal bead had numerous pockmarks. On the other side, small globules of plastic adhered to the bead, which is consistent with material being pulled from one bead and deposited on the other bead when the joint was made.

The fracture between the two pipe ends closely followed the plane of the center of the butt-fusion joint.  The fracture surfaces at the butt-fusion joint were examined using optical microscopy and an environmental scanning electron microscope. The fracture surface exhibited voids where the internal and external beads were distorted. Fracture surface features indicated that the crack initiated near the center of the pipe wall within the butt-fusion joint, followed by a slow progressive crack growth. Ductile ribbons of plastic pipe material present at the inner and outer edges of the fracture surface indicated that the crack did not initiate at the notches of either the internal or external beads.

Studies of butt-fusion joints subjected to elevated temperature and high pressure indicate that a fracture occurring in the butt-fusion area will normally start on the pipe wall surface at the notch of the internal bead or, if bending is involved, the fracture would be expected to start at the outer pipe wall surface. However, the initiation of the fracture in the accident pipe near the center of the pipe wall is consistent with inadequate fusion of the joint.

The butt-fusion joint linked to the explosion was made with a 500-foot length of 2-inch coiled pipe that was butt fused to a previously installed 2-inch coiled pipe using the McElroy 14. According to the National Fuel butt-fusion procedure, the fusion required the fuser to apply 15 to 20 foot-pounds with the machine, which would have resulted in a joining force on the pipe ends of 165 to 215 pounds. A 500-foot roll of 2-inch plastic pipe weighs approximately 315 pounds. Depending on the placement of the pipe roll, the drag caused by the weight of the pipe roll could have been negligible or significant. The written procedures of Uponor, National Fuel, and McElroy did not account for drag force; however, McElroy’s onsite training program provided instructions to fusers so that they could determine in the field the drag force for a butt-fusion joint. Among the four National Fuel interviewees, there was variation in the level of knowledge regarding drag force and the need to compensate for its effects.

Since the accident, the PWPoly Corporation has acquired Uponor. The combined businesses of PWPoly Corporation and Uponor operate under the name USPoly Company (USPoly). Today, USPoly continues to manufacture medium-density pipe, but it no longer manufactures the type of pipe involved in this accident.  

The National Transportation Safety Board determines that the probable cause of the leak, explosion, and fire in DuBois, Pennsylvania,  was the fracture of a defective butt-fusion joint and the failure of the National Fuel Gas Distribution Corporation to have an adequate program to inspect butt-fusion joints and replace those joints not meeting its inspection criteria.