Managing corrosion through plastics

Long service life, minimum downtime and environmental compliance are critical requirements in today's highly competitive industrial environment. These factors can be particularly challenging in the pulp and paper industry where corrosion can decrease equipment life, interrupt production and create potential environmental issues.

Piping systems must be able to withstand harsh and highly corrosive chemicals, as well as having good abrasion resistance and lower thermal conductivity in different manufacturing steps. These include such processes as bleaching plant operations, recovery of pulp and chemicals, and waste water treatment.

The process of bleaching requires use of many different corrosive chemicals. As a result, various materials continue to be used to reduce or prevent the process of corrosion. While many materials are available today, metals continue to be the primary materials of choice for pulp and paper producers.

Though bleaching chemicals are very corrosive to normal steel, use of polymeric materials seems to be very limited due to a lack of understanding about their compatibility to different process chemicals under manufacturing conditions.

One material gaining wide acceptance and recognition in the European pulp and paper industry is Corzan PVC-C (post-chlorinated polyvinyl chloride). Its combination of corrosion and chemical resistance, mechanical strength, low installation cost and easy maintenance make it ideal for different applications within pulp and paper production processes.

Moreover, glass-reinforced Corzan PVC-C, a cheaper alternative to expensive high grades of steel (such as 254 SMO and titanium steels), offers an enhanced mechanical strength with no brittle fracture at low temperatures, while retaining pressure rating capacities even with the temperature increase (up to 100 degree C). Allowable support spacing can be doubled easily. The system is easy to maintain during its long service life.

In general, Corzan PVC-C industrial systems are chemically inert to most mineral acids (including sulphuric, chromic, phosphoric and hydrochloric), bases and salts, as well as aliphatic hydrocarbons. Moreover, most common pulp and paper bleaching liquors do not have any significant effect on Corzan PVC-C.

When non-metals replace metals in process applications for corrosion reasons, the mechanical strength of the system becomes a key design consideration. In order to ensure safe and reliable operation, non-metals must provide sufficient mechanical strength to meet design criteria, such as maximum operating pressure, strength at elevated temperatures, equipment service life and structural integrity.

Many non-metals do not have the tensile strength to meet the pressure requirements of process applications, especially at high temperatures. Corzan PVC-C piping systems are pressure rated for operation up to 93 degree C, and can be expected to maintain their pressure-bearing capabilities for over 50 years.

Corzan PVC-C appears to have lower thermal expansion coefficient than other widely used plastic materials, such as polypropylene (PP) and polyvinylidene fluoride (PVDF):

PVC-U0.08mm/mdegreeC<br> Corzan PVC-C0.07mm/mdegreeC<br>ABS0.1mm/mdegreeC<br>PP0.15mm/mdegreeC<br>PVDF0.12-0.18mm/mdegreeC<br>

This results in less expansion of pipe when chemical media runs, greater distance between support hangers and therefore reduces the total installed cost.

Corzan PVC-C industrial systems components can be fabricated with all of the most common techniques for thermoplastic fabrication, such as hot gas welding and butt fusion welding. Piping is most often joined by solvent cement and primer, which brings down the installation cost. In addition, Corzan PVC-C's relatively low density means that the equipment is lightweight, enabling installation without costly moving equipment.

Gulnara Omarbekova is a Corzan market development specialist at Noveon Europe in Brussels