Many gas utilities and distribution companies can date their operations back to the 19th century, especially in the United States. At that time, gas was used for lighting, and the pipes were cast iron, and its particular qualities and characteristics defined the operational procedures required for its use and maintenance.

Subsequently, the use of steel pipe, and the technologies it gave rise to, characterized yet another distinct phase in the evolution of natural gas distribution. In the post-World War II era, the growing use of polyethylene (PE) plastic pipe for gas distribution would define another era.

PE did have several key advantages over these other types of pipe: it was less expensive to acquire and install, and it was also easier to repair in the field, handle and store. Its cost advantages over steel were of particular interest to many utilities (Table 1). PE pipe had a reliable, long life and lower operating and maintenance costs, primarily because it did not need cathodic protection. The first installation of PE gas distribution pipe took place in Caney, Kansas, in 1959, with pipe manufactured by Phillips Petroleum. By the mid-1960s, utilities began to make the transition from steel and cast iron to PE pipe.

DuPont and Phillips Petroleum were two of the early main developers of PE pipe. Phillips developed butt fusion in 1955, and shortly thereafter installed pipes in oil field gathering applications. It was about this same time that engineers developed the extrusion process, which made PE pipe economical to manufacture.

Beginning in the mid-1960s, Dupont offered their Aldyl “A” medium-density, high-melt index piping system. The high melt index (meaning easy melt flow when heated) made it possible to hand socket fuse through 4-in., and apply saddle fittings by hand. Operators and contractors generally did not use butt fusion with machines on pipes below 4-in., in the early days. Soon thereafter, Phillips marketed their high-density Driscopipe 7000 product line, which had a low melt index (meaning that melt flow was more viscous when heated), and was joined by butt fusion and saddle fusion using machines.

About the same time, Plexco and Nipak offered a medium-density piping system with a melt index between the previous products described. Hand socket fusion could be performed through 4-in., and saddle fusion could be done by hand, but most users butt fused pipes above 2-in., and used saddle fusion machines for the application of tapping tees and branch saddles. The pipe and equipment manufacturers helped spur the transition to PE pipe by promoting its use.

These efforts initially met resistance, because most utility officials felt that steel was sturdier and safer. Thus, manufacturers had to sell the safety side of PE pipe. Also, some of the earlier PE pipes were produced from resin materials that did not have the environmental stress crack resistance of today’s materials, and were thus susceptible to slow crack growth.

By the early 1970s, PE pipe was made from higher-performing resins, and was introduced into the gas distribution market. Utilities generally considered these an improvement over the first materials; and, at $0.60/lb, it was inexpensive, and could be run for miles at a relatively small cost. These products turned out to be considerably higher performing than the previous generation of PE pipe. One of the greatest benefits to come about and spur growth in the use of PE pipe was Phillips Petroleum’s development of the heat fusion process, including sidewall fusion tapping tees for hot tapping PE mains.

But there were other challenges in the early years of plastic pipe. One of the first products was the Dupont Aldyl “A” pipe. Over the years, a large amount of this type of pipe was placed into service. Phillips Driscopipe and Plexco also sold large quantities of orange medium-density and black high-density pipe. Because all of the materials had unique joining procedures, and also different melt indexes, or measurements of melt flow, there were often challenges in joining these materials together.

In addition, the multi-colored versions of the early PE pipe caused confusion, and created an environment for potential problems. The Dupont Aldyl “A” was colored “ochre” (a pinkish tan); the Plexco and Nipak pipes were orange; and the Phillips Driscopipe was black. For a time, a significant amount of both gas distribution pipe and telecommunications pipe were orange, and the need for distinguishing colors soon became evident.

In time, the industry became more standardized. Yellow PE pipe came into use to conform to the universal standard of color coding for underground gas lines. Today, medium-density PE gas pipe is colored yellow; high-density PE pipe is either black with yellow stripes, or black. These colors conform with the International Color Standards for gas pipe.

Mechanical fittings were developed for close tie-in work, and repair of third-party damage. Some of these were designed for sealing only, and some for sealing and pull out resistance. Later, electrofusion was developed as another heat fusion method for joining PE pipe. This process involved a type of fitting in which wires were embedded in the fitting itself. In this process, the wires were heated electrically to melt the inside of the coupling and the pipe OD to join the two components without movement. This eliminated the problem caused by the different melt indexes.

A key innovation came with the advent of the coiled PE pipe. It eliminated a large number of joints, and allowed utility crews to plow the pipe in, something that could not be done with steel. With PE pipe in a coil, a crewman could attach it to a plow, and pull it into the ground. Coiled PE pipe dramatically increased the cost savings.

PE could also be installed through insertion. Operators could insert the coiled PE pipe in old cast-iron and steel mains. With these technologies, crews could insert the new pipe into the old one, without disrupting pavement or service.

Houston Natural Gas, the forerunner to Entex (today’s CenterPoint Energy) began installing PE pipe in 1968. Prior to 1967, the preferred material for both companies’ distribution lines was Grade B steel. In 1976, Entex acquired the gas distribution properties of Houston Natural Gas, and its use of PE pipe has been ongoing since then.

By the mid-1970s, the major gas distribution companies were rapidly transitioning to PE, and there were both medium and high-density products to choose from. Most utilities started with smaller sizes of pipe, mostly 2-in., installed in subdivisions. They still used steel pipe in any service that had elevated pressures – anything above 50 or 60 psi, and any size above 6-in. The larger diameters of 8 and 12-in. were not available for PE pipe until the 1980s, when most gas distribution companies began to use PE for distribution service.

The Plastic Materials Committee was formed within the American Gas Association (AGA) to advance the understanding and application of PE piping systems. It was unique in that in addition to the gas industry users, key technical representatives from the resin, pipe, fitting, equipment providers, and research laboratory consultants were welcome members. Many of the supplier representatives were also members of the Plastics Pipe Institute (PPI). These associations were beneficial in meeting the needs of the industry, and promoting communication within it. For example, in 1981 the PPI issued Technical Note (TN) 13, “General Guidelines for Butt, Saddle, and Socket Fusion of Unlike Polyethylene Pipes and Fittings.” This answered an industry request to join different medium density materials to each other, as well as to join medium to high- density materials. In more recent years, the gas industry requested one generic procedure be established and agreed to by all the suppliers, in order to simplify the training and conformance to regulations. The PPI responded with Technical Report (TR) 33, “Generic Butt Fusion Joining Procedure for Polyethylene Gas Pipe” and TR 41, “Generic Saddle Fusion Joining Procedure for Polyethylene Gas Pipe.”

By the mid-1980s, a number of natural gas companies had begun using 12-in. medium-density polyethylene (MDPE) and high-density polyethylene (HDPE) in their gas distribution systems. Lone Star Gas Company in Texas began using 12-in. polyethylene in 1983, and National Fuel Gas Distribution Corp. in New York began installing 12-in. in 1987. Washington Gas Light Company began installing 12-in. PE pipe in 1988. Cinergy installed its first 12-in. MDPE in 1998, when approximately 2,500 ft was inserted into 16-in. standard pressure cast iron.

By the late 1980s, PE pipe was regarded as a proven, dependable and cost-effective means of delivering natural gas to homes and businesses. Entex increasingly turned to PE pipe for new construction, a trend that was reflected in its installation ratio of plastic-to-steel over a 10-year span. In 1987, the percentage of steel pipe was 60.2, and the remaining 39.8% was PE pipe. By 1997, 89.7% of the pipe Entex installed was PE.

Two types of PE pipe became the mainstays: high-density PE-3408 and medium-density PE-2406. Many utilities opted for MDPE, which was produced by manufacturers such as Plexco, Uponor, Phillips and CSR Polypipe. A number of polyethylene resins products became a well-known part of this process, including Phillips TR-418, Solvay Fortiflex K-38-20-160, Chevron 9300T/P24BC, and Novacor HD-2100-A.

New technologies invariably imposed new operational procedures on the industry. Utilities had to develop safety procedures to combat the potential for static electricity as an ignition source, for example. Wrapping the pipe in burlap that had been soaked in soapy water prior to working on the affected area was one effective technique; using grounded tools was another. Neither procedure was particularly expensive or time-consuming, nor did it alter the fact that the increased use of polyethylene plastic pipe resulted in tremendous cost savings and greater operational efficiency.

By the late 1990s, the average installation cost of 2-in. plastic pipe was easily 40% less than that of 2-in. steel. In addition, PE pipe offered much better flow characteristics than steel, and the need for expensive cathodic protection systems was eliminated.

Another aspect related to cost-effectiveness was the amount of time required to instruct employees in the methods and procedures involved in joining PE pipe. In less than half the time required to qualify someone on the most basic of steel welding techniques, an employee could be taught the basics of PE pipe joining, including butt, socket and saddle-fusion applications.

The PE piping systems in use today are more reliable not only because of high-performance resins, but also due improvements in training; fusion and handling equipment; and a broad range of specialized repair fittings, including electrofusion. Also, it is possible to squeeze off the flow of gas in an emergency situation, due to the flexibility of PE pipe. Added to these considerations was the fact that PE was much more reliable than its mechanically coupled predecessor used from the 1920s through the 1950s. It had become readily evident that PE pipe had positively affected natural gas distribution operations.

Besides plowing, utilities increasingly used boring, or horizontal directional drilling (HDD), to install PE pipe. By the 1990s, gas utilities were using HDD to install PE gas distribution pipe in shallow and short bores (e.g. typically less than six feet deep and less than 500 ft long). These pipes were generally small-diameter (2, 4, or 6-in.). In such drilling applications, utility engineers had to address general design requirements, while leaving the details of specific projects to be handled by an experienced, trained crew of drillers and installers.

Some of the key parameters the drillers had to take into account were locating crossing utilities (water, sewer, power, and communication), selecting drilling fluids consistent with the soil formation, reaming the borehole, controlling inadvertent fluid returns (migration of drilling fluid to the surface), properly sizing the breakaway swivel to prevent over-straining the pipe during pullback, handling coiled pipes, and making tie-ins.

There were some installation problems discovered in the early days. These included the possibility of rock impingement and the natural shifting of the ground, which could place stress upon the pipe or fitting. Specific installation guidelines had to be employed to lessen the potential for these occurrences. Later, plastic pipe would be designed to safeguard against these events. There was also the possibility that third parties might “kink” the line during subsequent construction and restrict the flow of gas, but this was also a consideration in steel installations, and incidents of this type were not common enough to discourage the use of PE pipe.

Most utilities used butt, saddle, and socket methods to join PE pipe, and electrofusion was also used for socket and saddle applications. An entire industry grew out of the need for fittings and fusion equipment. Manual and hydraulic butt fusion equipment were continually developed by McElroy Mfg. as the size of the pipe grew larger. Socket and saddle fusion tools, including electric temperature-controlled heaters, were developed by McElroy to replace the gas-fired heaters. Other fusion equipment manufacturers included Rigid, which was sold to T.D. William-son, and later merged with Christie Pipeline Contractors to form Connectra. Electrofusion fittings and controls were developed by Dupont (and later sold to Uponor), Friatec, Innogas, and Central for close quarter tie-ins and repair work. Mechanical fittings were developed by Dresser, Perfection, RW Lyall, and Raychem.

In addition to rock impingement, there were other guidelines and considerations specific to PE pipe installations. Compliance to the critical bending radius specified by the manufacturer was a key requirement, in addition to guidelines for scratch depth in identifying pipe too damaged to be installed. And in all fusion operations, proper cleaning of joining surfaces was essential. And as with all pipe, maximum allowable operating pressure could not be exceeded. Many utilities only used PE pipe in intermediate-pressure (<60 psi) applications.

By the late 20th century, utilities were undertaking massive projects designed to update and renew their distribution systems, by replacing their cast iron and bare steel with PE pipe. One example was Atlanta Gas Light Co. (AGL), which in 1999 embarked upon a 10-year project to replace more than 2,300 mi of bare steel and cast iron pipe throughout the State of Georgia. Some 200 mi of bare steel and 30 mi of cast iron pipe would be replaced annually until all existing pipe is retired, which made it the largest distribution pipe-replacement project in the U.S. at that time. In most cases, the older pipe was replaced with 2, 4, 6, and 8-in. MDPE, although bare steel mains were replaced with epoxy-coated steel where high-pressure mains were involved.

In April 2001, Cinergy embarked upon a 10 to 15-year project to replace some 1,460 mi of cast iron and bare steel in high-density residential areas. The project involved two of its gas utility companies, Cincinnati Gas & Electric Co., and Union, Light, Heat & Power Co., in Ohio and northern Kentucky. It targeted high-risk cast iron mechanical joints with 4, 6, 8 and 12-in. diameter, and 1950s-vintage pipe for replacement. Some of the cast iron pipe systems dated back to the 19th century. In many cases, the utilities chose to replace the older pipe with 2, 4, 6 and 8-in. PE pipe, often via HDD.

U.S. Department of Transportation (DOT) statistics indicate that as of year-end 2002, there were nearly 526,000 mi of PE main and over 35.8 million PE services installed in the systems of over 1,400 gas companies in the United States. Industry statistics also indicate that an additional estimated 39,000 mi of PE main and almost 2.1 million PE services are installed each year. Today, PE pipe has impacted every aspect of modern natural gas distribution operations.

Looking toward the future, manufacturers will no doubt be able to produce PE pipe that can be used at elevated pressures. With the advent of new resins coming onto the marketplace, operators may in the future be able to use PE for the higher-pressure service areas, either though insertion in steel pipe or through direct burial. Recently, the U.S. DOT approved the use of PE pipe (made on or after July 10, 2004) at pressures up to 125 psi. And, with the advent of newer materials and higher pressures, there may be opportunities for the use of PE pipe in gas transmission applications.