Achieving Long-Term Reliability With Plastic Plumbing Systems

Be sure to consider these factors to ensure long service life for your customers’ CPVC and PEX systems

Achieving Long-Term Reliability With Plastic Plumbing Systems

Cracking indicative of a Stage 3 PEX failure.

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Following manufacturer-recommended installation procedures is essential for any residential plumbing system. But there are also other factors that need to be considered to ensure long-term reliability.

Take the two most used plastic plumbing materials, PEX and CPVC. They have different operating specifications and can perform differently depending on local water conditions. In addition, a recently discovered new cause of failure associated with PEX piping now needs to be considered during material selection, system design and installation.

Water Compatibility

Most plumbers are aware that PEX is subject to some degree of degradation from chlorinated water. PEX is manufactured to compensate for this degradation over its expected lifespan, but there are several known conditions that can accelerate the process and cause premature failure.

These conditions include temperatures exceeding 140 degrees F, water pressure exceeding 80 psig, and water with an oxidative reduction potential (ORP) above 825 mV. Any of these conditions can result in a Stage 3 failure, which is a pure oxidative failure characterized by multiple cracks along the length of the pipe.

Of these three conditions, ORP may be the least understood and the most important. When this condition was first identified, an ORP above 825 mV was considered rare. Since then, municipal water treatment practices have continued to evolve with many municipalities increasing the concentration of disinfectants and/or introducing chlorine dioxide, both of which can increase the ORP of the water flowing through the pipe.

The result is that an ORP above 825 mV could be fairly easy to achieve in domestic plumbing systems today. It’s also possible that, even if a plumber is reasonably confident that ORP levels at a site are below 825 mV, the municipality could change its water treatment practices, driving the ORP above the threshold. This uncertainty makes it difficult to know how well PEX will perform in local water conditions. CPVC, by contrast, is immune to chlorine degradation in drinking water and can deliver reliable performance in aggressive water conditions.

Expansion and Contraction

When installing plastic piping it’s important to account for expansion and contraction using expansion loops, offsets, and changes of direction. A best practice is to avoid the use of restricting hangers or restraints near the joints used to accommodate expansion, as well as leaving adequate space between the expansion loop and any fixed structure. Since PEX pipes expand nearly three times more than CPVC, larger or more frequent compensation for expansion and contraction will be necessary with a PEX system. 

Because these expansion loops are designed to bend and flex when the pipe expands, PEX can present an additional set of challenges when typically degraded pipes are bent or flexed in service. Recently, a new failure mode in PEX piping has been discovered that looks distinctly different from traditional Stage 3 failures resulting from chlorine degradation. These failures exhibit cracks that run around the circumference of the pipe. They were initially misdiagnosed in the field as being caused by physical damage to the pipe during installation. But when analyzed in a lab environment, they were found to have a more complicated cause. 

Circumferential cracking on a PEX pipe.
Circumferential cracking on a PEX pipe.

The interior wall of the pipe had a very thin degraded layer that was consistent with the degree of degradation that would be expected for the pipe’s time in service, which was 10 to 12 years. Based on the depth of degradation, the pipe should have been able to remain in service without experiencing a failure for many years. So why did the pipe fail?  

Minimal chlorine degradation was observed inside a pipe that experienced a failure after 10 to 12 years in service.
Minimal chlorine degradation was observed inside a pipe that experienced a failure after 10 to 12 years in service.

Analysis of the failure found that stress-corrosion cracking originated in the thin embrittled layer that had been minimally degraded by chlorinated water and then propagated via slow crack growth under mechanical stress. When any piping, including PEX, is bent out of its natural state, it places the pipe under mechanical stress, as was the case in this sample where the failure occurred in a bent section of the pipe. 

This is an important finding for those using or considering PEX. Up to this point, the most common causes of premature PEX failure were the result of conditions that accelerated the rate of chlorine degradation in the pipe. Now, however, failures have been observed even when the rate of degradation would otherwise be expected to support a long service life.

The analysis suggests that when an already stressed PEX pipe is degraded, the risk of stress corrosion cracking may be lower because the stress would be considered static. However, when a PEX pipe that has experienced even minimal degradation is subjected to new bending stress from expansion and contraction or manipulation, the risk of cracking is significantly higher.

This issue requires further investigation and analysis and may warrant revisions to PEX installation and servicing guidelines to ensure PEX piping does not experience mechanical stresses after it is installed. For more information on the analysis that uncovered this cause of failure, check out this technical article

UV Light

CPVC piping can experience some discoloration and reduced impact strength from exposure to UV light but will not lose any of its pressure-bearing capability. Nevertheless, exposure should be limited whenever possible. CPVC pipe should not be stored in direct sunlight prior to installation and CPVC pipe that is directly exposed to sunlight following installation should be painted with a compatible latex paint to minimize the impact of UV light on the piping material.

In PEX piping, UV light exposure can contribute to system failures. When PEX piping is exposed to UV light, it consumes the chlorine-inhibiting antioxidants which protect the pipe from chlorine induced oxidation. As these chlorine inhibitors get consumed, the piping is left vulnerable to attack from chlorine in the water. If the material is exposed to excessive UV light prior to, during or after installation, it could make the pipe susceptible to accelerated chlorine degradation resulting in premature pipe failure. Different brands and styles of PEX generally have different UV ratings, with colored PEX typically having a higher UV rating than clear or translucent PEX. When switching between red/blue PEX and clear PEX, the UV exposure rating can be reduced by as much as 80% from 6 months to 1 month.

A Final Word

While both PEX and CPVC are widely used and accepted plastic plumbing materials, there are many differences between the two that must be considered during material selection, system design and installation. One of the most significant, yet rarely considered of these is that CPVC is immune to chlorine degradation while PEX experiences some degree of degradation when exposed to chlorinated water. Under ideal conditions, PEX can provide years of reliable service, but the various factors that can accelerate the chlorine degradation process or otherwise result in premature failure are often difficult or impossible for a plumber to control. 

About the Author

Jonathan Simon is the North American residential plumbing manager for Lubrizol Advanced Materials Inc., the parent company for FlowGuard Gold Pipe and Fittings. For 60 years, FlowGuard Gold Pipe and Fittings has provided reliable hot and cold water plumbing systems to residential and commercial buildings.



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