For years, process plants have relied on non-metallic piping as a cost-effective alternative to stainless, alloy and other expensive metallic piping in applications posing high corrosion rates or requiring stringent cleanliness standards.
Plastic piping affords excellent resistance to attack by many chemicals, including most acids, alkalis and salt solutions. Such piping comes in schedule-40, schedule-80 and other common sizes, with wall thickness usually corresponding to that of steel piping. In addition, some plastic piping, e.g., PVC, is offered with standard dimension ratio (SDR) ratings, which mean the piping system will maintain a more-or-less uniform pressure rating at a specified temperature regardless of pipe diameter.
However, some issues — e.g., thermal movement and other thermal effects, and liquid hammer — demand more attention with plastic piping than with commonly used metallic piping. Most plastic piping materials exhibit a relatively high coefficient of thermal expansion. Elevated temperatures may seriously affect plastic piping; for some materials, pressure/temperature ratings drop substantially at temperatures above 50ºC. So, plastic piping should not be located near steam lines or other hot surfaces. When liquid flow in a piping system stops suddenly (for instance, because of a quick-closing valve), a pressure surge known as liquid hammer (or often water hammer) develops and can easily rupture a plastic piping system.
Plastic piping systems usually require closer support spacing than metallic ones, particularly at elevated temperatures. Proper support for plastic piping is essential. As a very rough indication, allowable span is around half that of equivalent metallic piping (same size, schedule, etc.); check the values of allowable span published by the plastic piping’s manufacturer or consult a specialist. Supports and hangers can be clamps, saddles, angles, or other standard types; supports should have broad, smooth bearing surfaces, rather than narrow or localized contacts, to minimize the danger of stress concentrations.
Vibrations can damage most plastic-piping systems. Therefore, you must properly assess the piping under dynamic forces and apply mitigation techniques as needed. For instance, plastic piping connected to a large pump might experience high power vibrations, which might necessitate a vibration isolation device. However, such vibration isolators may pose operational or reliability problems, so install them only in special applications where they really are required and no other solution is viable.
Non-metallic piping systems most often rely on a fitting, such as a tee, to provide a branch connection; the fitting usually provides adequate strength to sustain the internal and external pressure of the piping. A branch connection made directly on a pipe weakens the pipe at the location of the opening; unless the wall thickness of the pipe sufficiently exceeds that required to maintain the pressure, you must provide added reinforcement. Assess the amount of reinforcement per applicable codes and specifications.
Several types of thermoplastics are available as piping. So, let’s look at those most commonly used.
Polyvinyl chloride (PVC) is stronger and more rigid than many other thermoplastics and has relatively high mechanical strength, tensile strength and module of elasticity. It is both lightweight and low cost, and demands little maintenance. Additionally, various solvent cements or other methods can fuse PVC pipes together to create permanent joints that are virtually impervious to leakage. Moreover, the material exhibits excellent chemical resistance to a wide range of corrosive liquids. However, PVC requires careful installation to avoid longitudinal cracking and over-belling, and certain liquids such as aromatics and some chlorinated hydrocarbons can damage it.
The allowable span for PVC piping generally should not exceed around 40%–50% of that of equivalent steel piping; the allowable span increases more slowly with diameter compared to steel piping systems. As a very rough indication, for typical steel piping systems, the allowable spans are 3 m, 5 m and 7 m for 2-in., 6-in. and 10-in. piping, respectively. You also can conservatively estimate the allowable span of steel piping via S = 2 D½ where D is the pipe diameter in inches and the span is in meters. In contrast, for PVC piping, allowable spans are 1.8 m, 2.5 m and 3 m for 2-in., 6-in. and 10-in. piping, respectively; you conservatively can estimate the allowable span of PVC piping via S = 1.4 D⅓, again with diameter in inches and span in meters.
Chlorinated PVC (CPVC) offers higher heat resistance than PVC; because of its excellent corrosion resistance at elevated temperatures, CPVC finds use at temperatures up to 90°C versus the normal limit of 60°C for PVC. However, it is more expensive. Therefore, CPVC primarily gets selected where such benefits are required, such as in certain chemical or relatively hot liquid services.
CPVC shares most of the features and properties of PVC; it, too, is readily workable, including by machining, welding and forming. However, CPVC requires specialized solvent cement. (For more on such piping, see: “Put CPVC Piping In Its Place.”)
CPVC is more ductile than PVC — allowing greater flexure and crush resistance. Because of its mechanical strength, CPVC is a viable candidate to replace many types of metal pipes in conditions where susceptibility to corrosion limits metal’s use.
Polypropylene (PP) is rugged and unusually resistant to many chemical solvents, bases and acids. It is one of the lightest plastics used in piping systems and comes in various forms. For example, fiber-reinforced-polymer (FRP) wrapped piping combines the excellent chemical resistance of PP with the mechanical strength of FRP.
Strains And Stresses
Thermoplastic piping requires flexibility analysis that incorporates appropriate elastic behavior. In many systems, the strains generally will produce stresses of the overstrained (plastic) type, even at relatively low values of total displacement strain. Often, the displacement strains (those due to thermal movements) will not cause immediate failure but may result in detrimental distortion. Progressive deformation may occur upon repeated thermal cycling or with prolonged exposure to elevated temperatures.
Piping layout often offers adequate inherent flexibility through changes in direction, wherein displacements chiefly produce bending and torsional strains of low magnitude. The amount of tension or compression strain (which can produce larger reactions) usually is small.
Where piping lacks inherent flexibility or is unbalanced, you must provide additional flexibility by one or more of the following means: bends, loops or offsets; swivels or similar; and other special devices and arrangements such as flexible joints. Choose corrugated, bellows or slip-joint expansion joints only where other solutions aren’t feasible.
While different codes and specifications give some general guidance to ensure adequate flexibility, they often don’t provide either specific stress-limiting criteria or particular methods for stress analysis of non-metallic piping systems. This is because of the significant difference of the stress-strain behavior of non-metals versus metals. In particular, Poisson’s ratio varies greatly for the various plastic materials and temperatures; the simplified formulae used as the piping code design basis for stress analysis of metallic piping may not be applicable or valid for some non-metals. Certain codes require the piping system layout to provide substantial flexibility to ensure minimizing displacement stresses; while this approach should allow for a high degree of safety, it isn’t always cost effective. The reality is stress and flexibility analysis of non-metallic piping often depends more on the engineer’s experiences and knowledge of specific non-metallic piping under study.
Temperature is an important parameter. Each thermoplastic generally has a fixed maximum service temperature, which identifies the upper limit to which pipe may be heated without damage. When heated above this temperature, the pipe will soften and deform. Upon cooling, it will harden to the deformed shape and dimensions.
Another important factor for plastic piping is the long-term hydrostatic strength. This — which serves the basis for the piping’s design pressure — is determined by finding the estimated circumferential stress that, when applied continuously, will produce failure of the pipe after around 100,000 hours (say, about 11 years of continuous operation) at a specified temperature. In addition, design calculations generally include a service factor that takes into account certain variables together with a degree of safety appropriate to the installation. The service factor most often reflects long system life (say, about 40 or 50 years). This design method usually doesn’t include the fittings, joints or cyclic effects such as liquid hammer. Most pressure ratings for thermoplastic pipes are calculated assuming a water environment; so, adjustments usually are needed for other fluids. As the temperature rises, the pipe becomes more ductile and loses strength; therefore, you should decrease the rating to allow for safe operation. These factors differ for each pipe material.
Aging can degrade the physical and chemical properties of plastic piping, and generally depends upon temperature. The changes can occur naturally through normal atmospheric or temperature fluctuations and ambient light, or can develop because of conditions in the process, such as elevated temperature of the fluid in the piping. One way to determine the onset of aging is to measure thermal stability (oxygen induction time) using differential scanning calorimetry.
Fire conditions greatly accelerate the degradation of plastics. In the early stages of a fire, most plastics melt and lose their structural shape and strength. As the temperature rises, they chemically decompose, often releasing toxic chemicals. This decomposition happens at a lower temperature than ignition. By the time ignition occurs or is possible, a relatively long period of chemical emission has elapsed. When thermoplastic pipe burns, it releases smoke and toxic gases, provides heat that increases the intensity of a fire, and may offer a path for flame to spread along its length. All organic materials are flammable but this is particularly true of polyolefins. It is well proven that many polymers actually are difficult to ignite; addition of flame retardants can further impede ignition.
Continuous application of load on a plastic material creates an instantaneous initial deformation that then increases at a decreasing rate. This further deformation is called creep. Removing the load at any time leads to an immediate partial recovery followed by a gradual creep recovery. However, if the plastic is deformed (strained) to a given value that is maintained, the initial load (stress) created by the deformation slowly decreases at a decreasing rate. This is known as the stress relaxation response. The ratio of the actual values of stress to strain for a specific time under continuous stressing or straining commonly is referred to as the effective creep modulus or effective stress-relaxation modulus. Time significantly affects this modulus. Experience shows that all plastic pipe will creep — with the actual extent influenced by time of loading, temperature and environment. Therefore, standard data-sheet values for mechanical properties may not suffice for some design purposes. The stress-strain responses of plastic reflect its viscoelastic nature. The viscous, or fluid-like, component tends to dampen or slow down the response between stress and strain.
Different piping codes identify special protective considerations when using non-metallic piping systems. For instance, some codes recommend safeguards and protection against possible impact because plastic piping systems often are vulnerable to accidental impact or similar damaging situations; consider safeguards for any above-ground plastic piping from which a spill or leakage can pose safety or environmental hazards. Take into account the lack of ductility and poor resistance to thermal and mechanical shock of some plastic piping systems and provide proper protection. In addition, during design incorporate methods to minimize the build-up of potentially dangerous electrostatic charges in piping that handles electrically non-conductive fluids.
Plastic fittings present a special problem; the geometry of some fittings can result in complex stress patterns that offer some stress concentrations and amplify the apparent stress cycle. A seemingly harmless pressure cycle thus can produce a damaging stress cycle that eventually can cause fatigue failure. This issue is particularly important in the case of branch fittings such as tees. In addition, the existence of stresses from other sources — for example, bending stresses induced by flexing under hydraulic thrust in improperly supported systems — can aggravate the situation. Because the design of plastic fittings isn’t completely standardized, consult fittings manufacturers for recommended derating factors for cyclic loading conditions. Usually you must consider plastic fittings separately from plastic pipes regarding dynamic loading, cyclic analysis and fatigue.
AMIN ALMASI is a rotating equipment consultant based in Sydney, Australia. E-mail him at firstname.lastname@example.org.