Non-metallic pipe materials

Non-metallic pipes such as HDPE, PVC or FRP are commonly used and may be utilized to repair or replace steel lines. However, a direct comparison of material properties between thermoplastics and metal is undesirable due to the variations in application and associated material responses.

Temperature, environment and duration of loading may alter the stress–strain response, rupture strength and ultimately strain capacity. Moreover, mechanical properties vary from one class of thermoplastic material to another (e.g. between PVC and HDPE) and also within the same type of material, depending on its constituents and the manufacturing process.

4.2.2.1. High density polyethylene

Although not a markedly less expensive material in itself (in larger pipe sizes), high density polyethylene (HDPE) is much less expensive to install than CS piping due to its light weight and ease of handling (see Fig. 4). It is immune to service water corrosion, is highly resistant to fouling and can withstand normal operating temperatures up to 60°C, including short term accident transients up to 80°C.

HDPE used in buried pipe applications is typically bimodal, non-cross linked HDPE, as opposed to cross linked HDPE which is typical of chemical tank applications. Cross linked HDPE uses thermoset resins that cannot be fused. Non-cross-linked HDPE uses thermoplastic resins that can be fused. Proper material selection, control of manufacturing (typically extrusion) and field fabrication are important to reliable applications.

HDPE piping has numerous positive characteristics when compared to metal piping. For installation these include it being lightweight, easier to bend, cheaper to install, easier to install via trenchless technologies and

having joints that are as strong as or stronger than the pipe itself (see Fig 5). Once in service the piping does not corrode, maintains optimum flow rates (e.g. is not susceptible to tuberculation or scale) and has fewer leaks (e.g. excellent water hammer characteristics and virtually no breakage due to freezing) [21].

HDPE is suitable for use in low temperature, low pressure process fluid applications and other industrial services throughout the balance of plant (BOP). The American Society of Mechanical Engineers (ASME) Code Case N-755-1 [23] has been written for Section III [24] and Section XI [25] applications. Where the code case is not directly applicable due to changes in joining techniques and/or material properties, some utilities have been successful in obtaining approval to use such piping by invoking additional sampling and in-process destructive testing during pipe installation [26].

HDPE is sensitive to slow crack growth (SCG), improper fusion and ultraviolet (UV) radiation.

Fabrication is performed by thermal fusion and can be performed more rapidly than steel welding. HDPE pipes or fittings can be joined (welded) together by many methods of heat fusion or by mechanical flange fittings. Types of heat fusion joints include butt fusion, saddle/sidewall fusion, socket fusion and electrofusion. HDPE creeps under load, so flanged joints need to be retorqued.

Fusion introduces heat into the elements to be joined, then applies pressure under controlled conditions between the heated elements, causing the polyethylene to fuse together. After cooling, the joint area is as strong as or stronger than the pipe elements themselves. Butt fusion, saddle/sidewall fusion and socket fusion welds are similar in that each applies heat to the areas to be joined using some kind of heating element. Force is then applied to fuse the elements together and to hold them immobile until they cool. Electrofusion uses a coupling with embedded heating wires that are supplied with an electric current to produce heat. Expansion of the piping elements inside the coupling produces the force to fuse the elements together where they are held immobile until cool. Figure 6 shows a typical fusion machine.

4.2.2.2. Polyvinyl chloride

PVC is a widely produced plastic. It is commonly used in construction because it is often cheaper and easier to install and maintain than traditional materials such as copper, iron or wood in piping and applications requiring rigidity (e.g. door and window frames, partitions, siding) (Fig. 6). In electrical applications it can be plasticized for use in flexible PVC-coated wire and cable.

Rigid PVC is stronger and stiffer than HDPE, thus PVC pipes require longer bending radii, but also less material to achieve or meet desired strength levels. PVC pipes are stiff enough to permit direct connection to mechanical valves, non-plastic fittings and various other water and wastewater appurtenances (Fig. 7) and

FIG. 5. High density polyethylene is flexible (courtesy of EPRI) [22].

have been used in applications such as underground fire line repair in nuclear power plants (e.g. Ontario Power Generation’s Pickering Nuclear Generating Station).

PVC’s properties are highly temperature dependent. As shown in Fig. 8, PVC’s impact strength drastically decreases with temperature. The blue area in Fig. 8 reveals a significant decrease in impact strength as the temperature gets colder; the PVC is becoming increasingly brittle. This phenomenon is one that is not seen with metals and may be overlooked when designing with plastics. The green curve presents the PVC modulus as a function of temperature. The green dashed line shows the transition zone towards a dramatic decrease in stiffness (around 50°C in this example).

PVC can be enhanced with additives in order to give it higher impact strength at low temperatures; however, this results in lower stiffness (the green curve will tend to move to the left, lowering the temperature when stiffness is lost). Thus, a compromise between impact strength and stiffness must be made, depending on application requirements.

FIG. 6. Butt fusion weld machine for high density polyethylene pipe (courtesy of EPRI) [21].

FIG. 7. An example of polyvinyl chloride pipe (Blue Brute) suitable for new construction or as a repair/replacement material for connecting to old cast iron pipe (courtesy of Ipex Inc.) [27].

4.2.2.3. Acrylonitrile-butadiene-styrene pipes

Acrylonitrile-butadiene-styrene (ABS) pipes are widely used for drain, waste and vent piping. They can be used to a very limited extent for small diameter pressure piping.

Design methods and procedures are essentially the same as those for PVC pipes, with the appropriate elastic modulus used for calculating pipe stiffness and the appropriate hydrostatic design stress for pressure pipe design.

4.2.2.4. Other thermoplastic pipes

In addition to the thermoplastic piping materials discussed previously, there are other types of thermoplastic piping materials which are used to a lesser extent. These materials include polybutylene (PB), cellulose acetate butyrate (CAB) and styrene rubber.

4.2.2.5. Fibre-reinforced plastic

Fibre-reinforced plastic (FRP) is a composite material made of a polymer matrix reinforced with fibres. The polymer is usually an epoxy, vinylester or polyester thermosetting plastic while the fibres are usually glass, carbon, basalt or aramid.

In contrast to metals, where the fracture process is known to result from nucleation and subsequent growth of a single dominant crack, fibre-reinforced composite laminates are characterized by the initiation and progression of multiple failures of different modes. Consequently, there are many more potential failure modes for composite laminates than for metallic materials. They thus have to be analysed in detail for better understanding of their failure [29].

FRP allows for the alignment of thermoplastic glass fibres to suit specific designs. Specifying a reinforcing fibre orientation can increase polymer strength and deformation resistance. Details on FRP properties can be found in Ref. [30].

Some fire codes, such as NFPA 1 [31] and NFPA 30 [32], allow non-metallic piping to be used underground for flammable liquid transport in small diameters. Underground steel piping would require CP systems and inherent periodic testing. FRP piping is therefore a cost effective alternative to underground steel piping with CP.

TABLE 5. PROPERTIES OF GLASS AND CARBON FIBRES [33]

Type of fibre Density

(g/cm³) Tensile strength

(MPa) Modulus of elasticity

(MPa) Melting point

(°C)

Glass

E glass 2.52 2400 73 000 700

R glass 2.55 3600 86 000 800

S glass 2.50 3400 88 000 840

Carbon

High modulus 1.90 2000 500 000

Temperature limit 230

High strength 1.75 2500 240 000

Aramid 1.45 3200 133 000

Boron 2.63 3200 420 000

FIG. 8. The influence of temperature on polyvinyl chloride properties [28].

4.2.2.6. Glass reinforced plastic

Glass reinforced plastic (GRP) or glass-fibre reinforced plastic (GFRP) is a fibre reinforced polymer made of a plastic matrix reinforced by fine glass fibres. It is lightweight, extremely strong and robust. Although its strength properties are somewhat lower than carbon fibre and it is less stiff, the material is typically far less brittle than carbon fibre and the raw materials are much less expensive. Bulk strength and weight properties are very favourable when compared to metals and it can be easily formed using moulding processes.

Different types of glass are used for reinforcing fibres, each type being distinguished by its chemical composition. Some key glass fibre properties are given in Table 5. Low-alkali E glass is used most frequently for reinforced plastics while R and S glass have better mechanical and thermal properties.

4.2.2.7. Clay pipe

Vitrified clay is very corrosion and abrasion resistant. Because of its inherent low strength, vitrified clay piping is used for non-pressure applications only. It is brittle and subject to impact damage; therefore, special care in handling is required.