Fending off the attacks
Pipe systems for aggressive media

The chemical industry routinely handles acids and alkaline solutions as well as mixtures containing those substances. Digestion agents used in metallurgy applications, solutions used for surface treatment, and neutralization, regeneration or precipitation agents used in water and waste treatment are all examples of media that have to be safely transported, monitored and controlled. An operational strategy that includes the use of thermoplastic materials can reduce investment and maintenance costs.

Plastics such as PVC-U, PVC-C, PP-H, PVDF and PE exhibit good resistivity to aggressive chemicals, and this is one of the main reasons why they are used in plant engineering applications. Metals on the other hand are susceptible to attack resulting from electrochemical reactions that take place in aqueous media containing electrolytes. Measures taken to combat the various manifestations of corrosion include increased wall thickness, the use of stainless steel with higher alloy content and the use of special materials such as titanium, tantalum and others. In many industrial applications where process conditions are moderate (temperatures normally remaining significantly below 100 °C and pressure rising to only a few bars), engineers have had good experience with pipes made of thermoplastics.
Even plastics show signs
of corrosion
Despite the fact that plastics are not susceptible to corrosive electromechanical reactions, other chemical and physical processes can take place that must be considered during selection of a suitable material. These processes may be viewed as corrosive phenomena even if they are caused by other mechanisms. Swelling and the formation of stress cracks are very significant problems, but engineers can reduce or eliminate the risks associated with these phenomena by selecting the right materials. In addition to specific material
suitability that can be determined qualitatively using resistivity lists and quantitatively using reduction factors, the other factors to consider are as follows:
temperature and pressure,
the complete composition of the media (including concentration),
exposure time (alternating operation, empty pipes),
the influence of natural soundings (external installation, UV radiation), and
mechanical stress (resulting from connections, thermal expansion and installation).
Mechanical stress can be minimized by the design implementation and by using a suitable type of connection. Proper consideration must also be given to monitoring and legal requirements. Ease of installation is normally one of the positive design characteristics of plastics. Low material densities and the availability of a wide range of accessories make it possible to perform pre-assembly, installation and maintenance work with less effort and thus often at lower cost compared to materials made of metal.

The performance characteristics of the types of thermoplastics that are most often used in industrial applications are compatible with the types of mixed media that are used in surface treatment technology and in wastewater treatment. Aqueous acid and alkali solutions as well as bleaching and saline solutions are the most common transport media. System solutions including material and connection technology are also available that can handle super pure chemicals (for example electronic grade quality levels). Most acids and alkaline solutions are regarded as hazardous and are categorized as water pollutants. The effects of some substances including hydrofluoric acid and chromic acid are so toxic that special safety precautions have to be taken. One solution to the problem is Georg Fischer’s Contain-It-Plus double pipe system that provides safe transport of hazardous media even through areas where sensitive operations take place. The company has a team of experts to help customers select the right material and pipe system for a given application. This experience base enabled the company to provide support during implement a simple pipe system with exhaust suction for permeating substances, for example. The development of system solutions continues to advance. PVDF and the bead and groove free WNF welding process now make it possible to offer systems suitable for handling oxidizing media.
Standard solutions
and special solutions
The standard polymers that are used for the most common applications are the semi-crystalline thermoplastics PP-H, PVDF and PE-HD and the amorphous thermoplastics PVC-U and PVC-C in combination with sealing materials EPDM, FPM, PTFE and other special materials.
Materials such as ABS, PP-n, PVDF HP and PB are used in other applications and industries including:
special chemicals and fine chemicals,
the chemical trade,
surface technology (e.g. galvanizing),
mining,
paper manufacturing,
textile and leather industry,
food and drinks industry,
water and waste water treatment.
A one-stop shopping approach offers another dimension. Besides valves, pipes and fittings, customers also have a source for modern valve and control drive technology, sensors, transmitters, process controllers and installation accessories. Minimizing the number of interfaces makes it easier to manage projects and implement system solutions where all of the pieces work well together. Training is available as required for those who process, plan or use the systems, for example connection technology training.