School of PE Blog

Pipelines have always fascinated me since this topic offers both history and science. Some of the earliest piping in the United States was installed not long after the Thirteen Colonies achieved independence from British rule. For example, Baltimore Gas and Electric (BGE) was established in the early 1800s as the first gas company in America, and Maryland was one of the Thirteen Colonies. But there is also the science of the materials used in pipeline construction. Cast iron was used in early gas mains and water mains and the field of materials science is based on applied chemistry. When reviewing old gas records (mains, services, meters), I have jokingly told co-workers to "remember the timeline" when trying to determine the material used for mains and services during certain time periods.

Material strength and properties are topics that you also frequently see on both the Fundamentals of Engineering (FE) and Principles and Practice of Engineering (PE) exams, as well as everyday life, so I would recommend reading about different materials and their applications and brushing up on your chemistry too! For example, aluminum (Al) is a relatively cheap, lightweight metal that offers decent robustness, which is why you see aluminum foil in grocery stores, and it is also the choice material for gas meters (aluminum casing is protective of internal components, but still light enough to carry without excessive exertion). Aluminum is commonly found in the Earth's crust, so it can be readily extracted for use, and aluminum can also be found on the periodic table (atomic number 13)! I passed the PE Mechanical (Thermal and Fluid Systems) exam, and I certainly saw my fair share of questions involving water, steam, and pipe flow, including their associated material properties. You will specifically see questions about material properties and behavior on the PE Metallurgical and Materials exam.

Wrought iron was also used as an early pipeline material. I think of this time period like a blacksmith era since before the 1900s, there was no assembly or mass production. Wrought iron is heated and then forged into a particular shape; it can be reheated and reworked until the desired shape is achieved for use, like a blacksmith skill. Cast iron is formed by melting iron in furnaces and then pouring into molds to make the castings (hence the name, "cast iron"). Cast iron has more than 2% carbon (also found on the periodic table!), so its carbon content differs from steel. Because cast iron has a higher carbon content, it is less metallic compared to both iron and steel. Carbon is a non-metal, so adding more non-metal content to a material will cause the material to exhibit non-metal properties. Steel is a ductile, malleable, and more weldable material whereas cast iron is brittle and has less weldability. So, if you add enough carbon to steel, it will eventually morph into cast iron (the higher carbon content is also more prone to cracking, so it is easier to weld steel than cast iron).

In the past, cast iron was the choice material for water and gas mains (referencing the historical timeline, ductile iron was not conceived until the 1950s). Since ductile iron was not available in the early 1900s and cities and towns were populated, there needed to be some sort of piping network for gas heat and water usage. Because cast iron is brittle, it was only really used for lower pressure mains (the brittle material characteristics cannot handle the higher pressures). This also explains why ductile iron, steel, and polyethylene (PE, Plastic) have superseded cast iron as the piping material for water and gas mains. These materials are overall superior to cast iron since bursting would occur if cast iron was utilized for higher pressure pipeline applications.

Steel in its most basic form is an alloy; that is, steel is a combination of iron (Fe) and carbon (C). And, of course, iron is found on the periodic table (but you have probably already heard this enough from me by now). Both steel and oil are staples of the world economy. Steel can be tailored and customized with different alloying elements (e.g., titanium, chromium, cobalt). Transition metals are located between metals and non-metals on the periodic table; the name comes from these metals transitioning from metals to non-metals. And because they are transitioning, they are more customizable, so most alloying elements are transition metals. Steel's economic and customized potential enabled more opportunities for it to grow as a viable pipeline option.

Over time, steel fabrication methods improved, enabling the development of higher strength steels for pipelines. Welding steel became more prominent in the 1930s, and this led to increased variety and quality. However, the steel industry was largely dominated by World War II in the 1940s, and most of the steel produced was used in the war effort. Continuing the timeline, the 1950s was a bad decade for steel pipe. The world had been decimated by war, and obviously, everything did not fully recover overnight. And with all the best steel focused on military applications, there was not much good steel remaining for pipelines.

Working in the natural gas industry, I have seen mostly steel pipe used for gas lines in the 1960s and into the early 1970s. Galvanizing is the process of applying protective zinc coating to steel or iron pipe; the coating prevents rusting and corrosion. Galvanized pipe was also commonly used in the 1960s for water pipe too. Different types of coatings were applied to protect pipe, and this was a better alternative to bare steel since, as its name implies, bare steel did not have any coating, so it was more susceptible to corrosion, leaks, and integrity loss. But remember, this was 1950s leftover steel being used for pipeline in the aftermath of the Second World War. Industry needed to keep moving forward, but resources were scarce.

Poor welding practices can also cause defects such as cracking, porosity, and gouges. Most bare steel has been replaced in the industry by today's standards. This was a priority since leaks were frequent and bare steel pipe needed immediate remedy, otherwise there was an increased risk of pipe failure and gas explosions. But steel pipe is still widely used today for steam lines and higher-pressure mains (e.g., transmission lines). The higher tensile strength is critical for handling high pressures without bursting failure. Other methods such as post-weld heat treatment (PWHT) have been developed to improve welding quality and integrity. Because steel pipe was subject to corrosion and required additional techniques (e.g., welding), companies and material scientists were still trying to pursue other piping alternatives. In the mid-1960s, utilities started transitioning towards plastic pipe. Different fusion techniques were being developed to connect pipe segments together without the need for welding, and equipment manufacturers helped to usher in the new plastic era. Aldyl-A pipe was first conceived by DuPont as a new plastic alternative to steel pipe, and it became commercialized in the early 1970s. Since plastic offered a new solution for gas mains, Aldyl-A was installed nationwide in the United States in the budding plastic years.

Over time, however, a couple of issues arose with the new Aldyl-A pipe. There was confusion over the color code of the plastic pipe. Before yellow became the industry standard for gas pipe, different colored versions of Aldyl-A pipe were installed, and other plastic pipes were orange (telecommunications lines were also orange). The bigger issue was the Aldyl-A material integrity. As Aldyl-A was implemented into the market, DuPont later discovered in the 1970s that the Aldyl-A was prone to cracking and rupture due to excessive temperature settings when manufacturing the pipe. Because crack propagation was more apparent, DuPont issued a nationwide memo to alert contractors and utility companies of the issue and worked to develop an "improved Aldyl-A" in the 1980s. Remember, original Aldyl-A was 1960s technology at the time, so materials testing was not as refined as we see it today.

Plastic pipe today is largely produced from polyethylene (PE) and polypropylene (PP) materials. This has superseded Aldyl-A which has been discontinued, and provisions have been implemented to replace Aldyl-A pipe as a precaution. But there certainly continues to be new eras ahead as some manufacturers are looking into potentially developing plastic pipe for high pressure gas transmission applications (they're currently using steel due to the high-pressure conditions), among other ideas. Regulations are continuously updated, and new technologies are always being explored, so I would recommend continued reading about both the industry and School of PE blog posts.

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