Polymer Water Pipe Fitting Failures
By
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.
Figure 1
Figure 2
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.
Figure 3
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).
It
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