COMMON CAUSES OF FAILURE OF AWWA C900 PVC PIPES

Polyvinyl chloride (PVC) piping is the highest volume of all plastic piping products. Rigid PVC pipes are highly ductile when properly manufactured. Their long term service life of PVC pipe has been determined to be more than 50 years.

PVC pipes are generally ductile when exposed to long term stress, but their inherent toughness is rate dependent.

They can undergo catastrophic failure if exposed to high speed stress events. Therefore, forensic failure analysis studies should include tests to investigate which of these factors contributed to the cause of failure.

There are occasional brittle failure of PVC pipes and fittings. PVC pipes occasionally fail due to a number of reasons including:

• Defective pipe manufacture – The main manufacturing defects that cause PVC pipes to fail include incomplete resin fusion, incomplete fusion of extrusion knit lines (also called “weldlines” or “spider-lines”), and inhomogeneity of the filler content
• Defective installation – The main installation errors are over-belling and improper soil conditions.
• Defective operation – The main operational defects include over-pressurization due to water-hammer and liquid column separation due to improper pipeline venting.

WHAT IS C900 PVC PIPE?

C900 is the American Water Works Association (AWWA) standard for cast-iron-pipe-equivalent outside diameter PVC pressure pipe and fabricated fittings covering nominal pipe sizes from 4 inches through 12 inches. C900 pipes and fittings must comply with the Safe Drinking Water Act requirements, meaning for potable water transmission and distribution. The C900 standard does not include injection molded PVC fittings.

1. INCOMPLETELY FUSED EXTRUSION KNIT LINE

PVC pipes are manufactured by an extrusion process. PVC powder is melted and forced through a circular die under high pressure. The center of the die-face is a solid metal plug held in place by thin metal webs or vanes. The vanes slice the melted plastic as it flows through the circular slits in the die. The plastic then fuses back together to form a solid pipe wall.

Occasionally, the knitting process doesn’t take place completely, producing what are called “weak extrusion knit-lines.” Therefore, it’s common for pipes to fail with a straight longitudinal split running down the full length of the pipe because it follows the path of least resistance, which is an imperfect knit-line.

When a pipe fractures at a knit line, the fractured pipe will curl up on itself with the two fracture surfaces overlapping each other. This indicates that the pipe was manufactured with high residual stresses in the wall of the pipe. Pipes manufactured with high residual stresses are much more likely to fail than pipes with low residual stress.

2. DEFECTIVE INSTALLATION

Photograph of a PVC pipe failure caused by over-belling.

Most large diameter PVC pipe joints are bell/spigot. The bell contains a rubber gasket which seals against the inserted spigot end of the pipe. Insertion of the spigot end of one pipe into the bell end of an adjacent pipe is an extremely delicate process and must be done properly or failure is likely.

“Over-belling” is the most common installation defect that leads to PVC pipe failure. If the spigot is inserted too far, it makes direct contact with the PVC bell, causing the bell to split as shown in the image above. Our forensic PVC experts can determine if defective installation caused the failure.

“Our highly experienced forensic scientists can conclusively determine if a PVC pipe failure occurred because of defective knitline using optical microscopy.”

Dr. Duane Priddy, Plastic Expert Group, Founder & CEO 

FORENSIC PVC FAILURE ANALYSIS PROCESS

A typical PVC pipe failure analysis involves: