Cross-linked polyethylene, commonly referred to as PEX, has exploded onto the scene throughout North America with use in a variety of plumbing and heating applications. While the product seems relatively new, it has actually been around for many years. Companies like DOW Corning in the United States and others in Europe began development on cross-linking polyethylene as early as the 1960s, and it has evolved into one of the most highly engineered, highly scrutinized and highly successful fluid distribution materials available today.

The plumbing and heating industries have evolved over the years when it comes to fluid distribution piping. Few people still rely on a horse and carriage as their primary form of transportation due to the automotive industry's evolution from the simplistic automobiles of yesteryear all the way to ultra-modern cars with high-powered, quiet and efficient engines, cruise control and navigational systems. Piping systems have followed a similar course, with each generation providing advantages over the next.

Old Generation--Rigid Piping Systems

The first products used for widespread fluid distribution were rigid piping systems, starting with galvanized steel and evolving to copper and CPVC. While these types of systems offered a solution for the problem of how to effectively deliver water throughout a building, they also had many complexities and inherent problems. Galvanized pipe and copper are heavy and rigid materials that are difficult to work with. They are comparatively difficult to cut and require a fitting anywhere that the piping needs to make a turn or branch. The joining methods are also very complex and require a great deal of time to assemble. There is a certain amount of liability involved with copper joining systems, as they often require open flames to be used close to combustible materials on the job site, resulting in a multitude of fire damage claims. Because these piping systems are difficult to work with, the types of systems installed are limited to running larger diameter lines as directly as possible to the areas they are serving and branching or "teeing" off from, with smaller lines to each fixture. This type of installation, commonly referred to as "branch and tee," can be susceptible to temperature and pressure drops when multiple fixtures are used simultaneously (like a toilet and a shower), especially if the required flow rates and pressures are not properly taken into consideration.

Additional ill effects with rigid piping systems include noise and water hammer. As water rushes through the system, metal pipes amplify the sound, possibly reaching throughout the structure. Additionally, rigid pipes promote water hammer because there is no way for the system to dissipate the pressure being back-lashed through the pipes as a valve is closed rapidly. The result is a vibration of the pipes as they clang around and try to absorb the pressure. Water quality and taste are also affected as the lines slowly corrode away, particularly in aggressive water areas. Rigid piping systems are extremely prone to freeze damage as the materials provide little room for expansion, typically resulting in widespread failures throughout a system when freezing occurs. While copper and galvanized piping systems do provide a means to deliver water throughout a building, they have many inherent shortcomings and are very difficult and expensive to install.

CPVC (chlorinated polyvinyl chloride) was one of the first attempts to develop a suitable plastic material for fluid distribution. CPVC was certainly lighter and demonstrated the strength characteristics necessary for water distribution, but it did very little to correct many of the problems associated with copper and galvanized steel, such as water hammer and freeze damage. Additionally, the joining system, which requires glue and primer, generated a whole new list of concerns. When applied correctly, the glue and primer will fuse the pipe and fittings. However, in reality this can oftentimes be difficult, especially on dirty job sites or in colder weather where the glues are susceptible to freezing and the joints can be brittle or not adhere very well at all. With CPVC, you require just as many connections as with galvanized steel or copper systems. As a result, CPVC is just another rigid piping material with many of the same shortcomings as its predecessors.

While copper and CPVC were an improvement over galvanized steel systems, there was still plenty of room for improvement. Just as these products were developed to deal with the problems faced by installers and consumers using galvanized steel, a new system was needed that took things a few steps further.

The Next Generation--PEX Piping Systems

A lightweight and efficient to install system that was strong enough to withstand rigorous plumbing and heating applications was needed. Engineers realized that the piping material utilized in most heating and plumbing applications did not require burst ratings of several thousand psi. Hence, PEX was developed. PEX is an acronym for cross-linked (X) polyethylene (PE). It is essentially polyethylene (a thermoplastic that consists of a series of ethylene hydrocarbon chains) that has undergone a chemical or physical reaction that causes the molecular structure of the PE chains to link together. This creates a three dimensional structure between the molecular chains, similar to a chain link fence. The cross-linking creates a material that performs extremely well at high temperatures and pressures, making it perfect for plumbing and heating applications. Properly manufactured PEX tubing demonstrates excellent characteristics at elevated temperatures, particularly in the areas of tensile strength, resistance to deformation, creep resistance, abrasion resistance, impact strength and chemical resistance. PEX is manufactured very efficiently with very stable pricing.

There are several ways to manufacture PEX tubing. All of the processes must pass the stringent ASTM F876 standard and have to be certified by the National Sanitation Foundation (NSF) in accordance with Standard 14 and 61, which essentially state that the products are sanitary and suitable for use in potable plumbing applications. As a result, all of the commercially available PEX products that pass the ASTM and NSF standards are well suited for plumbing and heating applications. Certain manufacturing processes allow for enhancements of the material, such as increased strength beyond the ASTM standards, increased UV resistance and increased chemical (chlorine) resistance, and it can even be color coded to further ease installations. There are also several types of joining methods for PEX. One of the easiest, strongest and most widely used is the insert and crimp system. In this joining process, a copper ring is slid over the pipe, and a brass fitting with raised barbs is inserted into the tubing. A crimp tool then compresses the ring and the PEX down onto the barbs, creating a leak-proof seal. The entire process takes only a few seconds, and the connection is permanent.

The insert and crimp fitting system is very cost effective and is not affected by cold temperatures, so a perfect connection is achieved every time. The tools even allow for adjustment as they wear over time. This is determined by the use of a gauge to check the crimped fittings, and adjustments can be quickly performed with an alIen wrench.

PEX tubing is a very flexible product that provides some distinct advantages over rigid piping materials. Water hammer, which plagues rigid piping systems, is simply absorbed into the flexible pipe wall of PEX. Additionally, when a PEX system freezes, the tubing will expand and then return to its original size when thawed out. This saves a tremendous amount of money when you consider how costly it is to repair rigid piping systems after they freeze and rupture. A PEX system also requires as much as 90% fewer fittings than rigid piping systems, further reducing installation time and costs. PEX is corrosion proof, so it will not corrode in the most aggressive water conditions and can even be installed directly in concrete with no effect on the product, as is popular with radiant floor heating systems. PEX can be installed in a conventional branch and tee manner or a home run system, where each plumbing fixture can have its own dedicated line designed to eliminate temperature and pressure fluctuations. In short, PEX was developed to specifically correct all of the problems inherent to a rigid piping system, and it is installed in a fraction of the time it takes to install galvanized steel, copper or CPVC.

In every industry, technology provides avenues for the enhancement of the products being used. Cars were a dramatic improvement over a horse and carriage for transportation. Technology has even further enhanced automobiles to the point that they are not only a more efficient means of transportation but essential to our way of life. Piping systems have gone through a similar evolution, culminating in a series of products, each one a little better than the next. PEX tubing is the next generation of high quality engineered piping systems that will help to shape our lives, making housing more affordable with better system performance and increased comfort.