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Picking the Best Thermoplastic Lining
Viral Vora & Gary Denis For protecting the interior of pipes with thermoplastic lining, plastic is an economical alternative to exotic alloys. Picking the appropriate one requires careful analysis of the contained fl uid, cost and performance factors. The article deals with various types of plastic materials as liners for pipes and how to choose the best amongst them.

Carbon-steel pipes, when unprotected, cannot stand up to the corrosive properties of many chemicals. Specialty alloys can offer some protection against corrosion but many introduce other obstacles, not the least of which would be a hefty price tag. One way around this is to protect the interior of pipes with a thermoplastic lining. There are several types of lining materials to choose from. Picking the appropriate one requires careful analysis of the contained fluid, cost and performance factors.

By understanding how different thermoplastics perform in chemical plant applications, engineers will specify the highest-performance lining for that installation, while getting maximum value within the constraints of an installation or retrofit budget.

The best lining for a particular installation is not necessarily the most expensive. There are several reasons to favour pipes made from carbon steel. These pipes are available in standard sizes and lengths, which makes them inexpensive to buy and install. Carbon steel also has high structural strength, making it ideal for many industrial applications. However, it must be noted that aggressive chemicals often encountered corrode this metal, significantly decreasing its lifespan.

Pipes made of other metals - such as stainless steel, titanium, molybdenum, Hastelloy and Inconel - are less susceptible to corrosion. However, here too, these often suffer from oxidation and chloride-ion corrosion. Their inner-film layer may deteriorate quickly when contacted by abrasive liquids and slurries. In addition, alloy metal pipes often add to the installation cost.

Introducing the Liner
Thermoplastic lining is sometimes an economical alternative to alloy metal. In the Chemical Process Industries (CPI), the primary thermoplastic lining materials (Table 1) are polypropylene (PP), polyvinylidene fluoride (PVDF) and polytetrafluroehtylene (PTFE).

PP, a common nonfluorinated thermoplastic, is used in various applications such as packaging and pipe linings. This plastic resists many corrosive fluids, is excellent in caustic environments, and is low in cost. When in continous use, PP has an upper temperature limit of approximately 90 degrees Celsius. However, it cannot be used with most chlorinated solutions or with oxidising acids.

PVDF is a fluropolymer thermoplastic. The entire class of such thermoplastics, including chlorinated varieties, resists chemicals and does not corrode. They are stable at high temperatures and are much less flammable than other materials. In most cases, they do not burn when exposed to open flame. Fluoroplastic lining materials, such as PVDF and PTFE are chemically inert, even after years of exposure to concentrated acids. PVDF fills the middle range of thermoplastic lining material, both chemically resistant and applied cost. Very resistant to halogen and acid-containing fluids, it can be used with temperature up to 150 degrees Celsius without significant loss of mechanical properties, especially tensile strength, abrasion resistance and heatdeflection. Being a fluropolymer, PVDF has excellent overall chemical resistance, however it is not suitable for amine, ester and ketone solutions.

An impact-modified grade of PVDF is a relatively new lining material. This copolymer resin shares many characteristics with PVDF homopolymer but has greater flexibility and elongation properties. This grade is preferred for lining pipes and tanks subjected to wide variations in temperature. Such pipes tend to have high-stress areas at flanges and joints, due to expansions and contraction difference between the thermoplastic and the steel substrate. As a result, flexible PVDF is often used in coldweather installations. An additional property of flexible PVDF is increased stress-crack resistance in high PH solutions. PTFE is a fluropolymer thermoplastic, renowned for its low coefficient of friction, that offers protection against corrosion. This polymer has the highest amount of fluorination. As a result, it exhibits the broadest range of chemical inertness and can withstand environments up to 220 degrees Celsius. However, its continuous use is limited by low tensile strength, as well as high vapourtransmission rates that can lead to vaccume collapse of the linings under temperature and pressure.

Consider the Properties
In chemical facilities, there is no such thing as a typical installation. The appropriate lining material will depend on the chemicals, temperature, pressure and flowrate. Liningmaterial characteristics that will determine suitability include chemical resistance, temperature limits, pressure capacity, permeability, flexural modulus, and percent elongation at break. See all the tables for these basic properties. When handling diverse compositions of corrosive wastes and slurries, give first priority to abrasion and toughness (tensile strength) characteristics. The strongest of the thermoplastic for pipe lining, PVDF hompolymer, can be used as a stand-alone pipe. As shown in the Table 2, the tensile strength order, best to worst, is: PVDF homopolymer, PVDF Copolymer, PP and PTFE.

Chemicals tend to pass through these plastics on a molecular scale. The measure of this is permeability, the rate at which various liquids and gases penetrate plastics. PTFE is one of the most inert materials known, but gases pass through it at many times the rate of PVDF or PP. Table 3 compares the gas permeability of PVDF homopolymer and PTFE. Confirming experiments with corrosives shows that they pass through PTFE at a high rate. Weep holes often need to be added to PTFE liners to avoid back-pressure collapse in many lined-pipe applications, especially in halogenated environments. Weep holes are not required in either PVDF or PP installations. Flexural modulus and percent elongation at break determine the amount of strength a thermoplastic has before stress-related cracking can occur. Table 2 shows that PP, PTFE and PVDF copolymer tolerate a high percentage of expansion and contractions within the steel substrate. This is shown by the materials high elongation at break values. Table 4 is a comparison of PVDF homopolymer (rigid) and PVDF copolymer (less rigid). These measurements are done on new formulations, but as plastic ages, it stiffens. Longer life is achieved by starting at high flexibility. Figure 1 is a pipe section lined with PVDF homopolymers, showing that complex surfaces can be fully protected.

Guidelines for Choosing Raw Material for Liner
In most cases, design engineers can follow these basic rules of thumb when choosing a thermoplastic lining.

• Use PTFE where temperatures exceed 150 degrees Celsius and permeability is not an issue.
• Select PVDF homopolymer when high mechanical strength (stand-alone pipe or solid valves and pumps).
• Use flexible PVDF copolymer in acidic or halogenated service for both pipe and vessels. It combines low permeability with high tensile strength and good elongation characteristics. Applied cost falls between PP and PTFE.
• PP or mild-steel lines are generally acceptable with caustic solutions. PP s acid resistance depends on temperature and concentration. PP is the lowest-cost material of construction.

While no single lining is right for any plant, careful selection and use of PP, PTFE and PVDF will meet most of today s requirements. Many factors are involved in choosing the plastic type, but a basic knowledge of the characteristics will help designers in selecting these critical components.