Abstract
The National Transportation Safety Board investigated numerous accidents involving the release of natural gas from polyethylene pipelines. The industry manufactures polyethylene pipes and fittings in many shapes and they can be installed/joined in a variety of ways. New grades with improved material properties, such as high-density polyethylene (HDPE) and ultra-high-molecular-weight polyethylene (UHMWPE) have been introduced that show improved material properties compared to their medium-density polyethylene (MDPE) predecessors. However, improved material properties by themselves do not guarantee extended service life of an installed pipe or fitting. Simple errors, whether unintentional, unknown, or unaware by the installer, can lead to a gas leak and possibly a serious accident that causes injury or loss of life. These errors can significantly reduce the service life of a pipe or fitting.
The NTSB Materials Laboratory examined several polyethylene service tees that were involved in an accident as a result of a natural gas leak. Examples of gas leakage resulting from failure to follow best practice at installation will be mentioned. Certain service tee assemblies are joined to pipe segments by mechanical methods while others are joined by fusion methods, such as heat fusion or electrofusion process. An example is cited where a manufacturer of a mechanical tapping tee made several versions of the same design that required specific installation instructions that were different from their predecessors. When several versions of the same tee assembly are manufactured by the same company, one should be aware of identifiable features that differentiate one version from another and then carefully use installation instructions that apply to the model that will be installed. Another example is cited where a service tee that was joined by the heat fusion process had been contaminated prior to bonding.
Learning from mistakes that are made during installation, through examples, provides a foundation from which we can correct our procedures and avoid repeating the same mistakes in the future.
