Corrosion Prevention: Properly Apply Plastic Piping

Use of such piping requires careful consideration of a number of issues

By Amin Almasi, rotating equipment consultant

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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.

Plastic Options

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 D, 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.

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  • My 2 cents… I’d like to further emphasize that fire fighting systems should not have polymer-based piping. They are excellent for chemicals with which there are no compatibility issues. However, I have some issues when they are used or recommended for water. While water per se is not the problem, it's usage for fire water systems becomes a default by extension. This is the area of concern as these pipes cannot withstand fire conditions and will collapse. The point of use may not be exposed to fire, but the piping layout will be through hazardous areas. The extent of damage due to a fire can be controlled by water but what if the water itself is unable to reach the point of use. Regards, Jatin D Shah Finornic Chemicals (India) Pvt. Ltd. Editor's Note: This comment was posted on behalf of Jatin D Shah by Traci Purdum, senior digital editor, Chemical Processing.


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