If the PEX manufacturer’s installation rules are not followed, failures can result. One of the most common installation defects is sharply bending the pipe to make a directional change. Sharply bending the tubing causes the tubing to kink. The kink in the tubing places localized stress at a spot in the wall of the tubing. Stress in the PEX material facilitates oxidative degradation of the PEX causing it to become brittle at the high-stress point.
A CASE STUDY
The following case study is the property of Plastic Expert Group and cannot be copied or distributed without prior permission. Our case analysis does not imply that this specific plastic failure will happen systematically. Every case is unique and should be treated accordingly.
PROBLEM: Defective PEX pipes with expansion joints leak causing water damage
There are a lot of different types of PEX pipes and fittings. The three types of PEX are PEXa, PEXb, and PEXc. PEXa and PEXb are the most common. There are several different types of fittings used to assemble a PEX piping system. The most common fittings are brass and plastic. Because of the potential for corrosion (dezincification) of brass fittings, plastic fittings are generally preferred. Once the installer decides on whether to use brass or plastic fittings, he or she then has to decide whether to use standard size fittings or use expansion fittings. As shown in the figure below, expansion fittings are larger in diameter than the inside of the pipe.
In order to be able to insert the larger diameter expansion fitting into the end of the PEX pipe, the end of the pipe is expanded using an expansion tool like shown in the figure below.
Once the fitting is inserted into the expanded tubing, the tubing shrinks onto the fitting. There are potential problems with the use of expansion fittings including 1) the teeth on the tool damages the inside of the tubing by placing indents into the inside surface, and 2) the stress/strain on the expanded tubing are high as revealed in the finite element stress analysis below.
The photograph below shows an image of an example of a typical PEX failure at a connection to an expansion fitting.
Degradation and embrittlement of PEX piping are accelerated by stress. The pipe is designed to
handle the stress of normal water pressure. However, stressors are additive. The use of expansion fittings places significant additional stress on the wall of the pipe making the pipe more prone to chemical attack and degradation by chlorinated water. The image below shows an image of the inside surface of the pipe installed using an expansion fitting. Notice the damage to the inside surface caused by the teeth of the expansion tool and the cracks that formed on the inside surface resulting from embrittlement of the PEX material accelerated by the additional stress of the expansion. The use of fittings that are the same diameter as the inside of the PEX pipe and crimp fitting results in a more reliable joint because of the lower stress.