Experience of Carbon Steel in Petroleum Refinery Asset Integrity Approach
Chidambaram Subramanian
Officer Grade A
(Metallurgy & Inspection)
Inspection Department

Large number of static equipments fabricated by variety of carbon steel class of grades for chemical, petroleum, petrochemical, power and utility industries. Although third generation High Entropy Nano Composite Steels (HEC Steel) and composite materials were developed recently, still carbon steel is candidate material for pressure vessel, boiler, concrete structures and pipes etc. The author claims his own experience with carbon steel material selection, fabrication, corrosion, testing, characterization and evaluation. Few case studies with carbon steel were presented in brief for validating material suitability in petroleum and power industry. The weldability and welding process were also discussed in brief with respect to repair procedures of static equipments made of carbon steel

Static Equipments Material Requirement

As far as petroleum industry is concerned the material behavior with respect to chemical resistance in various service conditions like atmospheric, salt water, sour water, boiler steam, acid handling, petroleum processing and petroleum products is considered important for material selection. The weldability of chosen material is another important factor with respect to fabrication of all static equipments by various welding techniques. Material selection plays a vital role for static equipments which undergoes repair and replacements during turn around. Fundamentally material selection and design is specified in various codes and standards which clearly define specifications for large number of petroleum industry applications. American Society of Mechanical Engineers(ASME)-Boiler and Pressure Vessel codes(BPV), National Association of Corrosion Engineers (NACE), Indian Boiler Regulations(IBR), Oil Industry Safety Directorate (OISD), American National Standard Institute (ANSI) and American Petroleum Institute (API) were different codes specifies procedures for construction of equipments, procedures for in service inspection and procedures for inspectionduring shut down after several years of refinery processing. The length of any hydrocarbon or steam fluid services in static equipment without degradation and deterioration depends on material selection and fabrication. The factor of safety is considered important factor for design which prevents static equipments from unpredictable causes. Prior history and its empirical data, experience and judgment based on industry data are major factor contributing to factor of safety. Equipments design against static loads in refinery may subject to dynamic loads due to various engineering reasons like induced & applied stressors, damping and vibrations. All codes and standards design against dynamic conditions with superimposing dynamic loads on static loads for specified conditions.

Carbon Steel Candidature

Among various classes of steels, carbon steel is most used material for construction of petroleum refinery static equipments like pressure vessel, columns, boilers, heat exchangers, condensers and pipelines. The carbon steel is candidate material of concrete structures for withstanding heavy loaded Liquefied Petroleum Gas (LPG) mounded bullets supports, crude & refinery products storage tanks bottom supports and column & pressure vessel fire proof supported structures. Almost 98% of refinery equipments constructed from different grades of carbon steel. The various advantage of using carbon steel in refinery equipments is low cost, intermediate corrosion resistance, availability, easy to recycle, excellent weldability, erosion resistance, oxidation resistance at intermediate temperatures etc .

Table 1 shows chemical composition of carbon steel 1010 grade

Also carbon steel shall be used up to 450 deg C high temperature service conditions and -29 deg C low temperature service conditions. Various classes of carbon steel grades available and with ranges of alloying compounds like manganese, silicon, phosphorus and sulfur specified. One such carbon steel chemical composition 1010 is shown in table 1. The steels supplied in various forms like plate, pipe, structural bars, wire rod, sheet metal, galvanized steels, pre-stressed rods & bars for refinery static equipment construction and repairs.

Brief Examples in Refinery

The corrosion is multifaceted phenomena generally depends on steel composition, service fluid, velocity, flow rate, temperature, environment conditions, metallurgical structure, stress applications etc. As shown in figure 1 large amount of energy spends on iron oxide to producing single ton of steel which revert back to original configuration i.e. formation of iron oxide on carbon steel plate, pipe, tube, vessel of refinery static equipments after certain time frame. Every codes and standards ensures safe life design philosophy for all specific cases of refinery processing. The carbon steel undergoes various forms of corrosion specific to refinery processing units. Although carbon steel is sufficient material used for general utility piping certain conditions like non draining structures and dead ends in pipelines will severely affected by corrosion before design life due to stagnant conditions irrespective of service and environments. In specific cases like boilers although design life is higher, IBR specify mandatory material evaluation and assessment for every 8800 hours of operation approximately every year. After several hours of boiler operation if any flaw detected shall be evaluated under scope of fitness for service. The detected flaw were recorded and inspected every year generates specific data of flaw characteristics in extended time frame. The generated data's and detailed inspection further increases confidence for full proof structural integrity of boilers. Other examples like LPG mounded bullets and storage tanks were one among static equipments subjected to full proof structural integrity. Another classic example of corrosion in refinery is distillation column overhead pipeline made of carbon steel and corrosion resistance is ensured by altering the service environment (corrosion inhibitor).

Figure 1: Figure1 shows Uniform corrosion, Pitting corrosion, Cavitation damage, Stress Corrosion, Hydrogen Cracking of Static Equipments made of Carbon steel. Top Centre shows Life Cycle of Carbon steel

Figure1 shows Uniform corrosion, Pitting corrosion, Cavitation damage, Stress Corrosion, Hydrogen Cracking of Static Equipments made of Carbon steel. Top Centre shows Life Cycle of Carbon steel The Cavitation damage in steam drum and steam pipes of boiler grade steel were designed with superimposing dynamic stress on static load. Biological stability of material is another important requirement for material selection in refinery , because of hydrocarbon processing contains sulphur and dissolved oxygen gases which produce bacteria on material while in operating or in stagnant conditions. Although carbon steel is less resistant to biological degradation, carbon steel pipe or plate with internal coating is highly resistant. Prevention of equipments from general atmospheric corrosion lasts for many years by painting and coating with few micron layers on exposed industrial atmosphere. Therefore with proper selection and design based on qualitative and quantitative assessments the various grades of carbon steel is candidate material for refinery static equipments.

Repairs and Replacements

NACE specifies various forms of corrosion tests of material subjected to corrosive service like sour gas, amine and high total acidic number crudes. If in case flaw were developed beyond the code and standards accepted criteria permanent repair procedures were employed. Repair procedures of carbon steel constructed static equipments are less complex than other grades of high alloyed steels. Repair and replacements in static equipments made of carbon steel is comparatively easier than any other material grades. Repair arises due to various causes like low wall thickness, pitting corrosion, erosion, and oxidation damage from crude refining processing conditions. In major cases, static equipments was carbon steel cladded with higher grade materials like stainless steel for corrosion resistance rather than constructing whole equipment from higher grade which is economically cheap and viable for refiners. NACE specifications for corrosion coupon which monitors corrosion rate in refinery equipments are also made of carbon steel available in round and flat shaped specimens. The corrosion coupon installation is major corrosion monitoring technique for service water, steam, sour water and distillation column overhead services which provides a result corrosion rate less than 1.5mpy (miles per year) after appropriate process design conditions. Several Non destructive testing like ultrasonic thickness gauging, profile radiography are useful for evaluating remaining wall thickness, corrosion coupons provide a process related remaining wall thickness. Gas tungsten arc welding and shielded metal arc welding were mostly employed welding techniques for repairs and replacements made of carbon steel. Heavy wall thick pipe may employed by submerged arc welding process technique. The welding of carbon steel is complex phenomena and structural integrity of equipments directly depends on quality of weldments. The electrode and filler wire for welding carbon steel is equivalent material. The structural integrity directly depends on welding quality and workmanship. The welding of carbon steel is complex in nature due to difficulties in achieving homogeneity of welded sections adjacent to original base metal. Therefore design considerations were taken into account to avoid any kind material failure due to non-homogeneity. The carbon steel suspected hydrogen induced failure which shall be controlled by pre heating before welding. The homogeneity shall be achieved by post weld heat treatment near to base metal. Cumulatively, welding process includes pre heating and post weld heat treatment for ensuring quality assurance. Some Non destructive tests(NDT) like radiography, magnetic particle inspection and ultrasonic flaw detector were quantitatively measure flaws in weld joints of carbon steel. The hardness is only mechanical property of material measured for carbon steel structures, vessels, pipes for field joints of static equipments. The hardness shall vary from 120 BHN to 200 BHN (Brinell hardness number) irrespective of any grade of carbon steel fabrication.