Petroleum refineries convert crude oil into many products and support the manufacture and development of essential products for various industries. The equipment in these facilities faces significant corrosion challenges both during processing operations and in the handling and storage of products. While the most common refined products are motor fuel and gasoline, refined crude oil has many uses, including heating, electricity generation, chemical feedstock, and building blocks for plastics.
By Srikumar Chakraborty, ex ASP/SAIL, Freelance Consultant
Petroleum refineries face different types of corrosion at the various stages of production depending on the interaction between the product, equipment and the environment. For example, in sour water stripping, the presence of hydrogen sulfide (H2S) and ammonia (NH3) creates a hostile environment conducive to multiple types of corrosion. Sudden failures in petroleum refineries receive a great deal of attention due to the scale and severity of the consequences, which include disruptions to operations, including unplanned shutdowns, leak repair in situ and loss of production for repair and maintenance. However, if proper mitigation methods are not undertaken promptly, units may also suffer from general corrosion, localised pitting corrosion, hydrogen-induced cracking, etc.
New build hydrogen plant
For this new-build project in Texas, the global engineering company for industrial gases will build, own and operate a new hydrogen plant that will supply clean hydrogen and other industrial gases to the neighboring blue ammonia manufacturing site. The new complex will include autothermal reforming with carbon capture and a large air separation plant.
Sulzer was brought in at the very early planning stages to provide support with pump specifications and layout. Although the new plant will be built from the ground up, the overall footprint of the site is quite restrictive, and this has influenced some of the design features. A competitive tender was issued for two pump-turbine skids, each with a back-up pump to cover any unplanned maintenance interventions. The two pump trains will work in parallel, making reliability a key requirement.
Complex processes
Petroleum refineries are complex industrial facilities that break crude oil products down into various components, which are then selectively reconfigured into new products. Refined petroleum products are derived from crude oils through a series of processes such as catalytic cracking and fractional distillation. Stainless steels are used in the crude distillation units for protection against elevated-temperature sulfidation, primarily against attack by corrosive naphthenic acids.
The most important processes in the refinery are catalytic cracking and functional distillation. In both processes, equipment is manufactured from stainless steel for uninterrupted production and corrosion protection.
- Catalytic cracking– Large hydrocarbon molecules are broken up or ‘cracked’ into smaller, more valuable hydrocarbon molecules. The cracking process takes place in the presence of a catalyst that promotes the breaking of large molecules in such a way as to maximise yield. Catalytic cracking produces a full range of hydrocarbons, from methane to residue.
- Fractional distillation – The function of fractional distillation is to refine and separate (as well as join or split) the various lengths of hydrocarbon chains to create various petroleum products from different distillates.
The efficient functioning of equipment in the refinery is closely dependent on its design, correct material specification, and timely identification of potential trouble areas. Vital equipment prone to mild, medium, and severe corrosion includes vessels, tanks, pumps, pipeline fittings, heat exchangers, compressors, and fired heaters.
Corrosion at the processing stages
Various types of corrosion mechanisms may occur to equipment in the various processing stages. These include:
- Cracking,
- Corrosion fatigue,
- Low/high-temperature corrosion,
- Hydrogen flaking,
- Corrosion under insulation,
- Metal dusting,
- Carburisation,
- Graphitisation.
The extent of damage can lead to catastrophic failures in oil refineries. Key design factors to consider when designing equipment include the geometry of the components, and the fluid flow and velocity in the refinery, which are controlled to prevent the accumulation of sour water and other corrosive substances.
Choosing a construction material that can withstand the harsh environment of the refinery process is critical. Corrosion in refineries can be mainly classified into low and high-temperature corrosion, causing:
- Erosion-corrosion – In the equipment body.
- Weight loss – The density of the metal in the equipment is likely to be reduced for several reasons. The presence of oxygen can result in a molecular state where metallic oxides can easily slough off the parent body.
- Stress corrosion cracking – Usually occurs in certain specific alloy environments, which are induced by the combined effect of tensile strength and environment.
- Corrosion fatigue – Premature fractures due to cyclic stresses are imposed on equipment/components in a corrosive environment, causing pitting corrosion and initiating fractures.
- Galvanic corrosion – Occurs when two metallic materials in the equipment with different nobilities (electrochemical potential) are in contact and are exposed to an electrolytic environment. The difference in metallic potential causes corrosion.
Corrosion problems faced at processing stages in petroleum refining.
Areas affected by corrosion
The most sensitive and corrosion-prone equipment, such as pipelines, valves, storage tanks and vessels exposed to medium to high temperatures, should be constructed from stainless steel. Equipment in areas where lower temperature products are handled may be fabricated using carbon steel or low alloy steels containing chromium – molybdenum. Grades such as AISI 4130/35/40 coated with anti-corrosion paint for equipment and human safety are commonly used.
The role of stainless steel
The presence of at least 10.5% Cr in stainless steel creates a passive layer of chromium oxide, providing effective corrosion resistance. The other alloying elements in stainless steel improve corrosion resistance in other mediums, such as in marine environments, which are of concern in the oil and gas industry. In the refining process, oil extraction is done through high-pressure tubing and then collected in a separator vessel. Extracted crude oil is then transported via pipeline. The whole process is quite challenging; hence, high-grade material is important for equipment and machinery to enhance sustainability.
Material selection
Material selection has a significant impact on the operability, economics, and reliability of petrochemical refinery plants. Considerations for material selection include design, construction material, fabrication quality, operating conditions, maintenance practices, and environmental factors. Stainless steel AISI 304 or 316, duplex and Precipitation Hardening grades are most suitable for manufacturing process equipment.
Chemical composition (%) of popular stainless steel used to manufacture equipment.
| Type | C | Mn | Si | S | P | Cr | Ni | Mo |
| 304 | 0.08max | 2.0max | 0.75max | 0.03max | 0.045max | 18-20 | 8-10.5 | – |
| 316 | 0.08max | 2.0max | 0.75max | 0.03max | 0.045max | 16-18 | 10-14 | 2-3 |
Mechanical properties
| Type | Tensile strength | Yield strength | Hardness | Elongation | Liquidus | Solidus |
| 304 | 505MPa | 205MPa | 123 BHN | 70% | 1450°C | 1400°C |
| 316 | 480MPa | 170MPa | 149 BHN | 40% | 1400°C | 1370°C |
Fabrication of stainless equipment
Stainless steel can be fabricated using any of the traditional forming and shaping processes. Rolled austenitic stainless steel as plate/sheet, spun or welded or seamless tube/pipe, hot
or cold forged valves, and fittings are common products for manufacturing refinery equipment.
Austenitic stainless steel is most commonly used for fabrication in shape as per design.
Austenitics can be cold-worked to improve hardness, strength, and stress resistance. It can also be heated to be shaped, but returns to its original strength when cooled.
Stainless steel/plate is a highly weldable material, and a welded joint can provide optimum corrosion resistance, strength, and fabrication economy. The welding process for stainless steel varies depending on the thickness; three common welding processes are resistance welding, MIG and TIG.
Typical types of corrosion
Uniform corrosion – Typical causes of corrosion are exposure to corrosive chemicals in the equipment, surroundings and environment, and poor aeration. It is also more common in steels with unwanted inclusions or manufacturing defects. For example, manganese-sulphide manganese often initiates pitting.
Adequate oxygen availability may help to reduce the rate of pitting corrosion or halt it entirely, depending on the cause. The molybdenum in stainless steel increases its ability to resist pitting corrosion. Other considerations include pH, chloride concentration and the temperature of the environment in which the products are exposed.
Crevice corrosion – Occurs when a crevice between the stainless steel and another material allows chloride compound to concentrate or prevents proper oxygen levels to regenerate the oxide layer formed and typically found near or within the gap between two metals. Crevice corrosion can also occur between metallic and non-metallic surfaces.
Weld decay corrosion – Occurs in petroleum refineries in very large joints in pipelines or tanks. Localised corrosion takes place as preferential weld corrosion. This is the result of damage in weld joints where a galvanic couple is established between the parent pipe and the weld in the heat affected zone (HAZ), or the weld metal, due to the different chemical composition of welding rods. Welding in pipelines, tanks etc. is a critical process where metal components are combined to create durable structures. However, the quality of the welding equipment can significantly impact the outcome of the welding process and overall quality of the end product.
Intergranular corrosion – Results from heating stainless steel to a specific temperature range, often between 550°C and 850°C. This causes chromium to precipitate from the stainless steel and reduces the ability of the passivation layer to form.
Conclusion
Stainless steel is essential in petroleum refineries by enabling productive results while protecting processing equipment and offering a better lifecycle cost than lesser-quality materials. Stainless steel has been proven to have a lower cost over time than other materials due to its superior durability, corrosion and heat-resistant properties, and hygienic benefits.
The creation of a corrosion protective layer with a minimum of 10.5% Cr and approx. 3% Mo prevents many types of corrosion. In addition to large items such as tanks and pressure vessels, numerous small stainless items such as valves are used throughout a refinery for their resistance to erosion and corrosion. Stainless stems, gates, and seats assure trouble-free operation.
Corrosion in the petroleum industry not only damages processing equipment but also causes changes in product properties and mechanical behaviour. Global losses due to corrosion in the oil and gas industry are estimated to be about 3-4% of global GDP.
Source:
AISI/ ASTM, Welding Journal
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