Thermoplastic and thermoplastic lined RTP equipment (Dual Laminates) have been used for years in the Chemical Process Industry. As with all types of materials the proper use and application are important to long service life. Compiled in this paper are several of those applications that have been successful and those that have not. Reasons for either success or failure are given and how to avoid problems in the future is discussed. One last item for discussion is that of stress relieving of thermoplastic dual laminate equipment.
APPLICATIONS
Five applications are presented the specifics of each application given, service or design conditions, the findings after being in service, and a summary of conclusions based on the observations is given for each.
The five applications include concentrated sulfuric acid containment tank, high purity HC1 acid heat exchanger, loose and bonded ECTFE liners for an emergency chlorine scrubber inlet, ETFE duct liner for an incinerator, and one dual laminate damaged in storage.
Sulfuric Acid Containment Tank
A unique opportunity was presented to us years ago; a steel mill coke plant generated sulfuric acid and had recently installed solid polypropylene tanks to handle the effluent of acid. Unfortunately upon first fill these tanks failed almost immediately. As it turned out the design was not adequate to properly and safely handle to loading of the tank and the fluid.
The specifics of this tank are: rectangular supported from the top flange, the size was 96" long by 72" wide by 100" deep, the liner was 3 mm PVDF backed with type II FRP structural laminate and reinforced with structural steel tubing.
Service conditions for design: 90% by weight of sulfuric acid (H2SO4) at a temperature of 170 ° F. The tank was installed outdoors at a steel mill coke plant.
Approximately one year later, a call was placed by the operating personnel to the tank manufacturer. The tank was showing signs of failure and the internal baffle had collapsed. When personnel arrived on site to inspect the tank and observe the operation, the tank was indeed failing. The interior baffle had begun to separate from the sidewalls and was leaning away from the vertical. The comers and welded joints of the liner were cracking and discoloring. When the operator was asked what the operating conditions were his answers explained it all. The sulfuric acid concentration was greater than 96% by weight and the temperature was in excess of 210 ° F.
Observations of the liner revealed that the sheet material appeared to be in good condition while the areas where it had been welded were not doing as well. Picture 1 shows the condition of the tank wall, comer, and welds. The structural FRP was beginning to deteriorate and soften. The structural steel tubing frame had not yet been exposed to the acid.
This tank was replaced using Alloy 20 and has been in service since. The following conclusions were reached as a result of this unfortunate failure. Welded areas can react differently than the original sheet used to produce the equipment. Be aware of this when consulting a corrosion guide for piping. Observe the material limitations of chemical concentrations and temperatures. Know when not to use thermoplastics and dual laminates.
High Purity. HCI Acid Heat Exchanger:
The production of microelectronic components requires high purity acids; one of those acids is hydrochloric acid (HC1). One process to purify HC1 is to vaporize the HC1 to remove all impurities. Conventional metallic heat exchangers cannot be utilized, as they tend to give up minute elements that further contaminate the acid. Common materials used are fluoropolymers and the proces