Today across North America, the use of plastic pipe for a wide variety of applications is becoming increasingly common. This paper wishes to summarize some of the basic facts about plastic pipe to give a better understanding of where, how and why it should be considered for usage. Several aspects concerning construction issues of today will be addressed.
Types of Plastic
Polyvinyl Chloride (PVC) -- PVC, also commonly referred to as Vinyl, is perhaps the most well-known and widely used of all plastics for pipe and fittings in construction today. It offers users one of the best balances of tensile strength versus economics being used for several pressure and non-pressure applications in both above-ground and under-ground applications.
PVC resin is derived from 2 natural resources of the earth, namely natural gas and salt. The natural gas derivative ethylene is combined with the chlorine component of salt to form the resin building block. The chlorine component is actually a by-product of a process whereby the sodium is being extracted from natural salt for use in caustic soda for soaps, detergents and other applications.
PVC is perhaps one of the oldest plastics being used for pressure pipe as far back as Post-WW1 Germany. Being one of the more rigid plastics, PVC is popular for usage as electrical conduit, process piping, commercial drains, irrigation piping and underground municipal piping such as water mains and sewers. It is commonly joined by means of solvent welding or gasketed bell and spigot.
Other uses of PVC in our daily lives include flooring, patio furniture, siding, medical tubing and small items such as 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 extra chlorine in its molecule for enhanced resistance to chemicals and higher temperatures. Thus, 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 that of PVC by having rigidity and high tensile strength however, it is typically 3-4 times more expensive than PVC. Many times, CPVC will remain competitive with metals or other specialty plastics for specialized chemical applications. It is normally joined using either solvent welding or by use of flanged joints.
Polyethylene (PE) -- This is perhaps the most common plastic for everyday usage and certainly growing in popularity for many piping applications. PE is derived from 100% ethylene and its lack of chlorine is what gives it higher flexibility than the vinyls but at the expense of a reduced tensile strength. Thus, its popularity for piping has been mainly on applications where flexibility is desired such as marine intakes or outfalls, tubing for rural water wells, underground irrigation, road culverts and landscaping or farm drainage.
It is resistant to most solvents and thus cannot be joined using solvent welding. Thus, it is mainly joined by mechanical couplings or through heat fusion.
PE pipe is typically supplied in coils or straight lengths.
Cross-Linked Polyethlyene (PEX) -- PEX is a modified form of PE that has its molecular bond strengthened to enhance physical properties such as heat, chemical and crack resistance. It is mainly joined by mechanical fitting connections and is used primarily today as small diameter (3/8 to 1 inch) tubing 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 tubing with an aluminum layer centered in the pipe wall for added rigidity and strength.
Polypropylene (PP) -- PP is again a common item for everyday plastics but a bit more specialized for piping. It is very similar to PE but offers better chemical and temperature resistance. As a result, its most common piping application is for corrosive waste drainage for laboratories. It may be joined by either mechanical joints or heat fusion. The 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 usage or ultra-sanitary manufacturing.
Acrylonitrile Butadienne Styrene (ABS) -- ABS is a softer plastic than PVC yet more rigid than PE or PP. It is dominant in usage for residential drain waste and vent piping offering both economics and ease of installation.
ABS is notorious for having better than average impact resistance in colder temperatures and thus is commonly used for car bumpers and other similar applications.
Only recently has an ABS pipe been available for pressure pipe applications acting as an alternative to PVC or CPVC by offering improved impact and abrasive strength yet reduced tensile strength and chemical resistance. Applications here include mining slurry transport, refrigerants and chilled water piping.
Benefits and Limits
Plastic pipe offers many benefits to users which have helped lead to its steady growth in North America over the last 50 years. These include having light weight, ease of assembly, excellent hydraulics, chemical resistance, corrosion resistance, low labor costs and rigid industry standards.
However, as with all piping materials, there are some factors which will limit the use of plastic pipe and of which designers should always be aware such as pressure capacity (typically OK to 250 psi), temperature limits (over 200F often a concern), and building code issues47A (caution for plenums, vertical shafts, and hi-rise).
In summary, plastic pipe has become a dominant player in mechanical piping across North America. A proper understanding of the different plastics as well as their advantages and limits will help to ensure trouble-free installations.
Gordon Lefort is a Professional Engineer (Mechanical) employed with IPEX Inc., Canada's largest plastic pipe manufacturer and has over 20 years experience with the design and installation of mechanical plastic pipelines.
