Alloy chemical process piping

Figure 7.1 Alloy chemical process piping being prepared for welding away from the process hazards in a welding shop.

In the chemical processing industry, for example, fluoropolymers are selected for their resistance to chemical attack. They serve as linings for carbon steel vessels, and for piping and other fluid handling components. They provide durable, low maintenance, and economical alternatives to exotic metal alloys. In these applications, fluoropolymers also offer thermal stability for use at high temperatures. And because they do not react with process streams, they help prevent contamination of products. 

Formation of black powder (BP) is one of the reasons of internal corrosion in pipelines. BP resnlts from chemical reactions between components of the transported gas (HjS and COj) and primarily the iron (Fe) portion of ferrous pipe alloys. In some cases, microbial action exacerbates the ongoing chemical process. The presence of water is key to supporting biological and certain chemical corrosion phenomena leading to BP. The formation of BP in pipelines may be mitigated by the following ... 

Although tantalum is by far the most widely used of the refractory metals to impart corrosion resistance to chemical-process vessels and components, there are some applications where molybdenum has been used with great success. Molybdenum support structures have replaced graphite in the processing of high-purity alcohols. Molybdenum-rhenium alloys, first developed because of their vastly improved ductility at low temperatures and in the recrystallized condition, have been used as vessel lining and piping components for the manufacture of Freon replacements. ... 

Alloy A1-6XN finds applications as chemical process vessels and pipelines, condensers, heat exchangers, power plant flue-gas scrubbers, distillation columns, service-water piping in nuclear plants, and foodprocessing equipment. 

Corrosion in metallic components occurs when pure metals and their alloys form stable compounds with the process fluid by chemical reaction or electrochemical processes resulting in surface wastage. Appreciable corrosion can be permitted for tanks and piping if anticipated and allowed for in design thickness, but essentially no corrosion can be permitted in fine mesh wire screens, orifice plates and other items in which small changes in dimensions are critical. Rates of corrosion can be heavily affected by temperature changes and whilst a material of construction may be suitable at one temperature, it may not be appropriate for use at a higher temperature with the same process fluid. 

Stainless steel 316L material used for piping and equipment shows considerable corrosion resistance because of the beneficial effect of molybdenum on the surface properties. It is also observed that the surface treatment (pre-reduced, polished, passivated and chemically treated surfaces) of stainless steel equipment and piping reduces the corrosion process in seawater applications. The corrosion resistance of stainless steel in seawater applications can also be enhanced by bulk alloying the stainless steel with nitrogen, chromium, molybdenum and nickel by converting the stainless steel into super austenitic stainless steel. From leaching studies it is also observed that the release of iron, chromium and nickel from the super austenitic stainless steel to seawater is considerably... 

Pulp and paper industrial equipment design consists of proper material seleetion in conjunction with the process chemicals and prevention of stagnant fluids in the process equipment. In the absence of corrosion, low-alloy carbon steel would be the material of choice if corrosion were not a problem. However, for many processes, stainless steel and even nickel-base and titanium alloys are required for better performance in corrosive environments. At present, current US paper mills are constructed of about one-third carbon steel and two-thirds stainless steels. There are several grades in the group of stainless steels. The relative cost of the stainless steels is dependent on the concentration of the major alloying elements (Cr, Ni, Mo), the volume produced, and the form in which it is supplied such as tube, pipe, plate, or block. It is useful to note that stainless steels are 10-20% more expensive than low-alloy carbon steels. 

While any plastic material, irrespective of its chemical composition and character, may be made into pipe or tubing, by far the greatest amount of pipe is made from thermoplastics (TPs) that are adaptable to extrusion processes (Chapter 5). Specialty pipe is made in small amounts from TS materials such as phenolic and polyester, but very large of commercial pipe is made from polyethylene, polyvinyl chloride, acrylonitrile butadiene copolymers, and acrylonitrile butadiene styrene types of alloys. Specialty tubing in relatively small amounts is made of acrylates and acrylate copolymers, as well as other transparent materials (Chapter 3). 

Carbon and low-alloy steels are probably the most commonly used materials for pipes handling water, petroleum products, and some chemicals. Reference 1 provides a summaiy of the different specifications used for pipelines. Steel tends to corrode by both pitting and uniform surface deterioration [2]. Steel must usually be protected from corrosion both on the inside from the material being carried and on the outside from corrosion by the atmosphere, soil, or water that surrounds the pipe. External corrosion protection is provided by material selection, selective backfill, barrier coatings, stray current control, and cathodic protection. Internal corrosion protection can be provided by inhibitors, coatings, design process control, and materials selection. 

This alloy is used for heat exchangers in chemical-, petroleum-, and food-processing industries, as well as piping. 

---