In North America today, the use of plastic pipe for a wide variety of applications is becoming more common. This document wishes to summarize some of the basic facts about plastic pipe to provide a better understanding of where, how and why it should be considered for use. Various aspects related to current construction problems will be addressed.
plastic types
Polyvinyl Chloride (PVC) – PVC, also commonly known as vinyl, is perhaps the best known and most widely used of all plastics for pipe and fittings in construction today. It offers users one of the best balances of tensile strength vs. economy used for various pressure and non-pressure applications in both above ground and underground applications.
PVC resin is derived from 2 natural resources on earth, namely natural gas and salt. Ethylene derived from natural gas combines with the chlorine component of the salt to form the resin building block. The chlorine component is actually a byproduct of a process by which sodium is extracted from natural salt for use in caustic soda for soaps, detergents, and other applications.
PVC is perhaps one of the oldest plastics used for pressure piping since post WWI Germany. Being one of the most rigid plastics, PVC is popular for use as electrical conduit, process pipe, commercial drain, irrigation pipe, and underground municipal pipe such as water and sewer pipes. It is usually joined by solvent welding or bell and spigot with joints.
Other uses for PVC in our daily lives include flooring, patio furniture, siding, medical tubing, and small items like credit cards.
Chlorinated Polyvinyl Chloride (CPVC): CPVC is a second member of the vinyl family and is essentially a modified version of PVC. CPVC contains some additional chlorine in its molecule to improve resistance to chemicals and higher temperatures. Therefore, the pipe is used for hot water supply, higher temperature applications in industrial facilities, or for fire sprinkler piping in residential or light commercial applications.
Its properties are very similar to those of PVC as it has rigidity and high tensile strength; however, it is usually 3-4 times more expensive than PVC. Many times, CPVC will still be competitive with metals or other specialty plastics for specialty chemical applications. It is normally joined by solvent welding or by using flanged joints.
Polyethylene (PE) – This is perhaps the most common plastic for everyday use and is certainly growing in popularity for many piping applications. PE is derived 100% from ethylene and its lack of chlorine is what gives it greater flexibility than vinyls but at the cost of less tensile strength. Therefore, its popularity for pipe has been primarily in applications where flexibility is desired, such as marine intakes or outfalls, pipes for rural water wells, underground irrigation, road culverts, and landscaping or agricultural drainage.
It is resistant to most solvents and therefore cannot be joined by solvent welding. Thus, it is mainly joined by mechanical couplings or by thermofusion.
PE pipe is generally supplied in coils or straight lengths.
Cross-linked polyethylene (PEX): PEX is a modified form of PE that has its molecular bond strengthened to improve physical properties such as resistance to heat, chemicals, and cracking. It is joined primarily by mechanical connections and is used today primarily as small diameter (3/8 to 1 inch) pipe for potable water or hydronic (hot water) heating. PEX is also available with an external oxygen barrier layer for hydronic heating or as an integral part of composite pipe with an aluminum layer centered on the pipe wall for increased rigidity and strength.
Polypropylene (PP) – Again, PP is commonplace for everyday plastics, but a bit more specialized for pipes. It is very similar to PE but offers better chemical and temperature resistance. As a result, its most common piping application is for corrosive laboratory waste drainage. It can be joined by mechanical joints or heat fusion. Heat fusion is preferred for acid waste to ensure 100% reliability of pipe joints. The second most popular application for PP today is high purity water, such as deionized water for medical use or ultra-medical manufacturing.
Acrylonitrile butadiene styrene (ABS): ABS is a softer plastic than PVC but more rigid than PE or PP. It is dominant in the use of residential drain and vent pipes offering economy and ease of installation.
ABS is known for having better than average impact resistance in colder temperatures and is therefore commonly used for car bumpers and other similar applications.
Only recently has ABS pipe been available for pressure piping applications that acts as an alternative to PVC or CPVC by offering improved impact and abrasion resistance, but reduced tensile strength and chemical resistance. Applications here include transporting mining slurries, coolants, and chilled water pipelines.
Benefits and limitations
Plastic pipe offers many benefits to users that have contributed to its steady growth in North America over the past 50 years. These include being lightweight, easy to assemble, excellent hydraulics, chemical resistance, corrosion resistance, low labor costs, and rigid industry standards.
However, as with all piping materials, there are some factors that will limit the use of plastic piping and which designers should always be aware of, such as pressure rating (typically OK to 250 psi), temperature limits (above 200F is often a concern), and building code issues (caution for air chambers, vertical shafts, and high-rise buildings).
In short, plastic piping has become a dominant player in mechanical piping in North America. A proper understanding of the different plastics, as well as their benefits and limits, will help ensure trouble-free installations.
