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Locating and prioritizing replacement of polyethylene pipes in advance of mounting legislative action

A half century after its coast-to-coast installation in U.S. residential communities to deliver natural gas, Aldyl A polyethylene pipe is now being identified and removed in several states due to its potential vulnerabilities.

In general, there are three distinct ways polyethylene (PE) pipe may fail while in service: rapid crack propagation, ductile rupture, and slow crack growth. Certain vintages of Aldyl A pipe are known to demonstrate lower resistance to stress intensification,  in particular. Failures of Aldyl A pipe and fittings have been linked to  and potential hazards to property and the public. 

In September 2023, the Pipeline and Hazardous Materials Safety Administration (PHMSA) moved to revise federally mandated Distribution Integrity Management Programs (DIMP) for utilities and natural gas distribution operators to include Aldyl A and other PE pipes under a  stating "that operators must identify the threats posed by specific material types in their pipeline system." The U.S. House of Representatives is also considering a  that would require widespread identification and removal of Aldyl A pipes.

The history of Aldyl A: recognizing brittle slow crack growth in polyethylene gas pipe and fittings

In the 1960s, DuPont introduced a new called Aldyl A to the market. This PE pipe was lighter, easier to install, less expensive, and more corrosion resistant than its metal counterpart. Over the years, DuPont produced several iterations of this salmon-colored PE pipe resin, including changes in colors and resin formulations, until it was acquired by Finnish product maker Uponor in 1991.

Pre-1973 Aldyl A, identifiable by its color, is widely considered more vulnerable compared to later formulations. By contrast, current PE resins take advantage of modern advances in polymer science such as bimodal molecular weight distributions and increased short-chain branching to increase resistance to slow crack growth.

What amounts to thousands of miles of natural gas pipelines that include Aldyl A are dispersed among utilities across the U.S., which may be significantly challenged to accurately locate and rank the risk of their vintage PE pipes.

In the decades since, Aldyl A and other early PE gas pipe resin formulations have proven susceptible to brittle-like, slow crack growth failures. In 1998, in response to a series of pipeline incidents in Iowa, Texas, and Puerto Rico, the National Transportation Safety Board (NTSB) released a  on "Brittle-Like Cracking in Plastic Pipe for Gas Service," which included information pertaining to Aldyl A and other PE pipe manufactured from the 1960s through the early 1980s describing its vulnerability to stress intensification. Several more bulletins and recommendation documents followed. 

Then, in March 2023, Aldyl A again made news after a  at a Pennsylvania candy factory. The high-profile incident led to both the current bill in the U.S. House of Representatives and another  identifying an Aldyl A fitting with slit fractures as the root cause, stating that, "natural gas was leaking from a DuPont Aldyl A service tee that was installed in 1982." 

Today, as the possibility of federal regulations loom, many utilities and pipeline operators are working to remove Aldyl A in tandem with existing infrastructure replacement programs. However, there is a critical problem: knowing where it is. 

Challenges for locating installed Aldyl A pipe

While some organizations may have detailed records or may have completed voluntary assessments, many others rely on documentation from the 70s and 80s, when speed of installation took priority over cataloguing now-crucial details on the of pipes. These details are vital to identifying the level of risk and potential vulnerability for installed pipe. 

In 2011, the  asked utilities to assess 15,000 miles of their pipeline to identify Aldyl A and found that most were lacking information about the manufacturer or installation year that could help assess its PE formulation or remaining lifetime service expectancy. 

What amounts to thousands of miles of natural gas pipelines that include Aldyl A are dispersed among utilities across the U.S., which may be significantly challenged to accurately locate and rank the risk of their vintage PE pipes. Although its replacement has not been formally mandated, Aldyl A pipe presents a known risk — especially in certain geographies, conditions, formulations, and production dates — for both utilities and the communities they serve. 

Assessing and prioritizing Aldyl A pipe removal through risk assessments 

Given the current context and DIMP requirements to maintain pipeline integrity, some utilities and states are getting a head start on removing Aldyl A in advance of federal regulations, including , , , , , , and . 

For instance, in New Jersey, a regional utility began implementing a five-year, $200 million infrastructure program in 2022 , including vintage coated steel and Aldyl A plastic mains. Over the past decade in Washington,  has been working to replace or remove approximately 355 miles of pipeline. As of 2021, it had completed 165 miles. 

For more states to prepare for potential federal law and increasing regulatory scrutiny — and achieve safer operations and communities — risk assessments can support developing clear action plans around existing Aldyl A pipe. 

1. Documentation is the first step. When available, relevant information may be spread throughout a utility's documentation or filing system or be in paper form at multiple locations with multiple versions and updates over the years. The quality of the documentation on distribution pipelines may also vary, creating challenges in meeting the traceable, verifiable, and complete (TVC)  comparable to those that PHMSA has aspired to for steel transmission systems. A well-developed process is needed to effectively collect, digitize, and review data.

   When TVC documentation is lacking, as can be the case for old or legacy systems, a methodology to rank the quality of the documentation and resolve discrepancies may be necessary to begin identifying priorities. External information resources may also prove useful.  and  have released bulletins to support industry in identifying both Aldyl A and other plastic piping and components that pose a heightened risk of material failure.

   Since 2000, the American Gas Association, PHMSA, and other industry and regulatory leaders have maintained , which collects self-reported information from utilities on leaking detected in plastic pipe and fittings within their networks. Participation in the secure database remains voluntary and confidential. The objective of the initiative is to identify possible performance issues at a broad scale, which originally included risks associated with the susceptibility of older plastic piping systems, such as Aldyl A, to brittle cracking mechanisms. Leveraging external information resources can support stakeholders in zeroing in on potential risks and top-line priorities. 

2. Bridge knowledge gaps. Exploratory excavation may prove to be the only way to conclusively identify the composition of a pipe in question if other avenues are exhausted. If little is known about the location of pipelines, it is imperative that utilities consider the relative risk of accidentally striking the pipeline during excavation as they develop their Aldyl A identification and replacement plans.
3. Assess geographical factors. Once the likely locations of pipes are identified, considering their locations is critical to prioritizing removal. Several geographic factors may intensify stress on Aldyl A, resulting in microscopic defects that can precede larger cracks and dangerous leaks. Some factors to consider in ranking pipes for priority replacement include:

• Those installed in rocky soil
• Those exposed to frost heaves from freezing temperatures
• Those subject to differing expansion/contraction rates from fittings made of dissimilar materials
• Whether the pipe has been subject to excavation damage, unstable slopes, or seismic activity
• Whether its placement involves potential sewer cross-bores

In light of the above, an effective path for utilities may be a thorough review of pipeline records, careful field assessments of lines with insufficient or incomplete documentation, and prioritization of replacement based on geographic factors likely to influence stress intensification.