Plastics as Materials of Construction for Chemical Plants

Plastics are being increasingly used as corrosion-resistant materials for chemical plant construction. They can be divided into two broad classes  

Thermoplastic materials, which soften with increasing temperature for example, polyvinyl chloride (PVC) and polyethylene. 

Thermosetting materials, which have a rigid, cross-linked structure for example, the polyester and epoxy resins. 

Details of the chemical composition and properties of the wide range of plastics used as engineering material can be found in the books by Butt and Wright (1980) and Evans (1974). 

The biggest use of plastics is for piping sheets are also used for lining vessels and for fabricated ducting and fan casings. Mouldings are used for small items such as, pump impellers, valve parts and pipe fittings. 

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Lead was one of the traditional materials of construction for chemical plant but has now, due to its price, been largely replaced by other materials, particularly plastics. It is a soft, ductile material, and is mainly used in the form of sheets (as linings) or pipe. It has a good resistance to acids, particularly sulphuric. 

A development of interest to the chemical industiy is the tubular precipitator of reinforced-plastic construction (Wanner, Gas Cleaning Plant after T1O2 Rotary Kilns, technical bulletin, Lurgi Corp., Frankfurt, Germany, 1971). Tubes made of polyvinyl chloride plastic are reinforced on the outside with polyester-fiber glass. The use of modern economical materials of construction to replace high-maintenance materials such as lead has been long awaited for corrosive applications. 

The susceptibihty of resins, reinforcing fibres and composites to aqueous and non-aqueous environments has been reviewed. It is clear that with careful material selection and composite construction, reinforced plastics are very durable in aggressive conditions. One of the most severe environments, aqueous acids, can be designed to use the ideas presented here. This practice is implemented in industry, as demonstrated by the durability of GRP in environments encountered in chemical plant and conduit for corrosive effluents. 

These materials may be divided into two classes those which occur naturally, such as wood, stone, and raw rubber, and those which are produced synthetically, such as glass, graphite, stoneware, plastics, and synthetic rubber. Though metals are of prime importance in the construction of chemical plant, non-metals possess certain qualities, which, in spite of their low thermal conductivity and physical strength, make them a preferable material. For example the transparency of glass or the resilience of rubber are often important. 

One main area of application for utilizing the media resistance of plastics is in machine and plant construction. Over several decades the chemical industry, transportation, water management, and others had positive experiences with the utilization of plastic materials. At the same time, the industry knows the limitations of these materials and lessons have been learned from damage events. This is reflected in the catalogues of requirements. Main areas of use for reinforced thermosets as well as for engineering thermoplastics include ... 

In the construction of machines, plant and products, materials are selected because they have particular physical and chemical properties. Wood, metals, concrete, plastics and other substances all have their uses but there are limitations as to what they can do and how long they can do it. Properties may change with use, temperature, operating atmosphere, contamination by surrounding chemicals and for many other reasons. It is necessary to know the properties of the materials and how and why they have been used so that an assessment can be made of whether likely changes in the properties may give rise to hazards. 

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