Polymer Water Pipe Fitting Failures
Charles C. Roberts, Jr., Ph.D., P.E.
Acetal copolymers are thermoplastics produced by the polymerization of aldehydes yielding unbranched polyoxymetheylene chains. These materials have been used in the manufacture of plumbing fittings for several years. Acetal resins are among the strongest and stiffest of thermoplastics. They are easily molded into complex parts and are relatively cost effective to manufacture. Unfortunately, over time, it has been found that acetal fittings can fail as a result of environmental influences. Figure 1 shows a failure of a water T fitting which caused a large loss at an industrial facility. The body of the T fitting fractured at a stressed area near a tube coupling. As usual, the fitting failed over a weekend and was not discovered until the following Monday.
Figures 2 and 3 show views of several cracks that had developed in the T fitting body. One of the cracks grew sufficiently large, causing unstable crack growth and failure of the fitting.
Acetal copolymer has been found to be susceptible to aqueous solutions containing hypochlorite ions (chlorine) at a level usually found in domestic drinking water supplies. These oxidizing agents encourage environmentally assisted cracking of the plumbing fitting, especially in areas of mechanical stress concentration, such as threaded areas or sharp bends in the polymer. Additional mechanical stress arises from the manufacturing process (molding) and connection with other plumbing fixtures. High temperatures accelerate the failure process. (A familiar similar phenomenon is the deterioration of automobile tires. Oxygen atoms from the air cross-link with the long flexible polymer chains making the tire outer surface much stiffer. When the tire flexes, cracks form in the outer surface, eventually causing tire failure.)
The cracking in the plumbing fitting shown in Figures 2 and 3 is typical of environmentally related cracking. Chemical analysis of the fracture surfaces revealed evidence of chlorine deposits. It was determined that the chlorine atoms oxidized the polymer chains, causing chain cleavage (separation of the polymer chain).
should be noted that some manufacturers are warning against using acetal fittings
in high chlorine concentration environments. Other manufacturers claim that
their acetal products are more resistant to chlorine. Material degradation of
acetal plumbing fittings in normal chlorinated environments can be considered a
design defect as a result of improper material selection. In some instances,
the product may contain several sharp features that result in high mechanical
stress concentration, which is an ingredient in environmentally assisted
cracking, also a design defect. Losses
from failures of acetal plumbing fittings are at the root of one of the largest
class actions in the