Polypropylene (PP) and PVDF lined pipe experienced frequent failures in spent (60-78%) sulfuric acid after about seven to ten years in service. The failures were due to exceeding the design temperature when mixing 98% acid where high temperature is generated which then created thermal stress in the liner and bolted flared joints and developed cracks and product leaks. The PP was replaced by Polytetrafluoroethylene (PTFE) lined pipe using proper bolting torque. PTFE has been in service for more than 7 years without any failure.
Plastics are widely used in industries, especially petrochemical plants due to their good chemical resistance to most of the aggressive chemical products. It is important during the design stage to consider all possible operational cases that may occurred during operation in the field such as changes in the temperature, concentration and other process parameters. This study helps in reducing failure and personnel injury specially when dealing with hazardous chemicals such as acids or caustic. For example, if carbon steel material was selected for 98% Sulfuric acid but there is a possibility of getting diluted acid into the system in the future. Then it is beneficial to consider this possible change and use better material such as alloy steels or lined pipe in order to prevent failures and injuries. Evaluation of the proper material selection in at the design stage may cost some money but in the long term, it pays back by reducing failures and frequent replacement. Therefore, process and material engineers should work together during the design stages in order to select the proper material that gives the design life without failures.
The common plastic or plastic lined carbon steel materials used in industries are: ~
1. Polyethylene (PE) - This material can be joined by thermal welding. It is widely used up to approximately 60°C especially for water service application. It is attacked by strong oxidants and is susceptible to environmental Stress Cracking (ESC) by some inorganic (48% hydrofluoric acid) and organic chemicals (notably detergents, wetting agents, alcohol, ketones, aldehydes, and organic acids). Chlorinated solvents and strong oxidants are to be avoided.
2. Polypropylene (PP) - This material is closely related to PE; they are both members of polyolefins group, composed only of carbon and hydrogen, its maximum temperature is around 107 °C and has melting point of 166°C. PP is stronger and somewhat more chemical resistance than (PE). However, it is susceptible to ESC in hot brine solutions and in 98% sulfuric acid.
3. Polyvinylchloride (PVC) - This material is very popular for water use, particularly in the presence of chlorine or other oxidants. Chlorinated PVC (CPVC) is used for hot-water services. Both PVC & CPVC have excellent resistance to both acids and alkalis at low concentration, but their solvent resistance is very limited.
4. Polyvinylidene Chloride (PVDC) - has improved strength, hardness, and chemical resistance, especially to organic solvents, mineral acids, and oxidants.
5. Fluorocarbons - These are the most versatile and important group of plastics for process industries.
They have excellent corrosion resistance to almost all chemicals.
Polytetrafluoroethylene (PTFE) provides adequate heat stability up to 260°C. Fluorinated ethylene propylene (FEP), Chlorotrifluoroethylene (CTFE), Polyvinylfluoride (PVF), Polyvinylidene fluoride (PVDF), Ethylene Chlorotrifluoroethylene (ECTFE) and Perfluoroalkoxy (PFA), are types of fluorocarbons which have different properties and uses. Fluoroplastics have a low coefficient of friction, especially the fluorinated resins, giving them unique non-adhesive