Materials of Construction for Refinery Units

Metallic materials used in the manufacture of equipment for the petroleum refining industry are subjected to a wide variety of potential damage mechanisms, the most common being corrosion and environmental stress corrosion cracking. Stress corrosion cracking mechanisms are associated with wet corrosion processes, and the most common ones, affecting metals and alloys used in oil refineries, are described. Dry corrosion affects large areas of refineries and (as the name implies) does not require the presence of water. The dry corrosion process also includes oxidation, carburization, and metal dusting, but the most relevant for oil refining is attributed to the presence of sulfur, naphthenic acids, or both.

This chapter presents the issues involved in selecting the proper material for construction of refinery units that will assist in corrosion control.

Keywords

Materials selection; metals and alloys; ferritic alloys; ferrous alloys; pipes; pipelines

2.1 Introduction

To accomplish the conversion of crude oil feedstock to saleable products, a refinery is a large industrial complex with extensive piping to transport different fluids between large reactor units (Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2014). The complexity of refineries depends on the type of crude oil being processed and the type of products being manufactured. Oil refineries typically process between 100,000 and 2,000,000 barrels of crude oil per day into saleable petroleum products. Because of this high-throughput capacity, most refinery processing units are operated continuously, which makes process optimization and process control very desirable.

The relative complexity of a refinery is determined by the number and nature of the process units. Refining begins with the desalting/dewatering of the petroleum feedstock(s) followed by fractionation (distillation) into separate boiling-range fractions: the resultant products are directly related to the properties of the crude oil. Beyond the desalting operation, refineries are classified into (1) simple refineries, which include distillation, catalytic cracking, and distillate hydrotreating units, (2) complex refineries, which—in addition to the units at simple refineries—include catalytic reforming, alkylation or polymerization units, and gas processing operations, and (3) very complex refineries, which—in addition to the units found in complex refineries—include a hydrocracking unit, a coking unit, an asphalt plant, and—often—a petrochemical plant (Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2014). Each unit in the refinery is subject to common corrosion problems (Table 2.1). From the refinery, the products are transported to product distribution centers or terminals. Pipelines are used to transport petroleum to the refinery and also to transport petroleum products to the various distribution centers.

Metallic materials used to manufacture equipment for the refining and gas processing industries are subjected to a wide variety of potential damage mechanisms, the most common being corrosion (Chapter 1). Safe operation of a refinery or gas processing plant depends on understanding these corrosion degradation mechanisms, making the proper material selection, devising corrosion control, creating inspection programs for early detection of problems, and monitoring material performance.

2.2 Metals and Alloys

There are many diverse applications for the stainless and heat-resistant alloys throughout the range of temperatures encountered in the refining, petrochemical, and gas processing industries (Tillack and Guthrie, 1998). A wide variety of iron- and nickel-based materials are used for pressure vessels, piping, fittings, valves, and other equipment in refineries and petrochemical plants. The most common of these is carbon steel in which the main interstitial constituent is carbon in the range of 0.12–2.0% w/w. The term carbon steel may also be used in reference to steel that is not stainless steel. As the percentage of carbon increases, steel has the ability to become harder and stronger through heat treating, but this is accompanied by a lowering of the ductility along with a lower melting point and lower weldability.

Furthermore, drilling for crude oil and natural gas, as well as the production, refining, storage, and transportation inflicts corrosion on the equipment. For example, acid-bearing fluids used during drilling operations can corrode the tubing through which they flow. The presence of hydrogen sulfide, other sulfur compounds, and mineral matter can induce corrosion in pipelines either chemically or through attrition—problems that have affected many pipelines transporting crude oil and natural gas from the wellhead to refineries.

2.2.1 Metals

Metals used for unit constriction should be durable and corrosion resistant. Most refining and petrochemical processing equipment—as well as gas processing equipment—is designed and fabricated to the requirements of the American Society of Mechanical Engineers (ASME) or equivalent pressure vessel and piping codes of other countries. These codes establish the basis for and the setting of allowable stresses. Thus, the mechanical properties of a material are usually the first criteria used in the selection process. This is especially important for applications at temperatures in the creep range in which a minor difference in operating temperature can significantly affect the load-carrying ability of the material.

Without adequate corrosion resistance (or corrosion allowance), the component will fall short of the minimum design life desired. In the refining and petrochemical industries, this is typically set at 10 years or more. The additional cost usually associated with choosing increased corrosion resistance during the selection process is invariably less than that due to product contamination or lost production and high maintenance costs due to premature failure.

However, unlike mechanical properties, there are very few, if any, codes governing corrosion properties of metals. For some applications or services, recommended practices have been published by organizations such as the American Petroleum Institute and NACE International. Small variations in the composition of a process stream or in operating conditions can cause very different corrosion rates. Therefore,...