Sulfidation corrosion failure: An alert for users of refineries in the United States and and the world that were built before 1985!

The purpose of Refinery Industry Corrosion Control is to provide you with an overview of refinery process units, descriptions of specific processes, and the opportunity to identify and examine corrosion and metallurgical problems that can occur in process units. There are many aids to this, including guidance documents that have been developed to help the user determine the possibility of having a certain damage mechanism present in their system. In this sense, several documents have been developed that help: API 571, API 570, API 574, API 578, which contain practical guides to guide the activity of technical inspection and evaluation of the facilities, which help to avoid failures of equipment and process units. This time it was a failure that determined the need to revise the reference documents for the inspection and created an alert for users of refineries built before 1985 in the world.

On August 6, 2012, the Chevron U.S.A. Inc. Refinery in Richmond, California (“the Chevron Richmond Refinery”) experienced a catastrophic pipe rupture in the #4 Crude Unit. The incident occurred from piping referred to as the “4-sidecut” stream, one of several process streams exiting the refinery’s C-1100 Crude Unit Atmospheric Column.1 The pipe rupture occurred on a 52-inch long component of the 4-sidecut 8-inch line (the 52-inch component). At the time of the incident, light gas oil was flowing through the 8-inch line at a rate of approximately 10,800 barrels per day (bpd).

The report highlights the following technical findings. (An in-depth discussion appears in the Chevron Interim Report.): “The rupture of the 4-sidecut piping resulted from the 52-inch component being extremely thin due to a damage mechanism11 known as sulfidation corrosion. Sulfidation corrosion, also known as sulfidic corrosion, is a damage mechanism that causes thinning in iron-containing materials, such as steel, due to the reaction between sulfur compounds and iron at temperatures ranging from 450°F to 1,000°F. This damage mechanism causes pipe walls to gradually thin over time.”

Sulfidation corrosion causes pipe walls to thin, which eventually leads to the need to replace the thinned piping. Chevron determines the date for replacing thinned piping by using a piping “Minimum Alert Thickness” and a piping “Minimum Required Thickness” . When piping reaches its Minimum Alert Thickness, an engineering evaluation is triggered to calculate the piping’s Minimum Required Thickness, or the lowest thickness that can withstand the pressure and structural stresses of the piping circuit, to determine whether the piping must be replaced immediately or if replacement can be safely delayed. This evaluation may result in the lowering of the Minimum Alert Thickness to 0.1-inch. Evaluation of the inspection thickness data obtained on the 4-sidecut piping during the 2011 turnaround indicated that the 4-sidecut piping would thin below its 0.14-inch Minimum Alert Thickness before the next turnaround scheduled for 2016. A minimum structural thickness value of 0.036-inch had been calculated for a small piping component within the 4-sidecut piping earlier during the turnaround. This 0.036- inch value was applied to the full length of the 8-inch 4-sidecut piping circuit. This calculation was used as a technical justification to reduce the 8-inch 4-sidecut Minimum Alert Thickness to 0.1-inch, and the piping wall thickness was predicted to stay above this Minimum Alert Thickness until after the next turnaround. The 4-sidecut line was therefore allowed to continue operating with replacement scheduled for the next turnaround in 2016. API RP 574: Inspection Practices for Piping System Components provides users with a default minimum structural thickness of 0.11-inch for piping with a diameter of 8-inches—which can be used as the Minimum Required Thickness for piping in lieu of detailed engineering calculations. Chevron performed a detailed calculation to determine the 4-sidecut Minimum Required Thickness and the API RP 574 default minimum structural thickness was not used. However, had Chevron used the API RP 574 default minimum structural thickness value of 0.11-inch as the 4-sidecut Minimum Required Thickness, the remaining life of the piping circuit would have been predicted to be less than ten years, and a turnaround planning group discussion should have been triggered to discuss replacement options for the 8-inch 4-sidecut piping. Such a discussion could have resulted in the decision to replace the 8-inch 4-sidecut piping during the 2011 turnaround, and the August 6, 2012, pipe rupture could have been prevented.

What is Sulfidation? According to the third edition of API 571 it is the corrosion of carbon steel and other alloys as a result of their reaction with sulfur compounds in high temperature environments.

Sulfidation corrosion, also known as sulfidic corrosion,6is a damage mechanism61 that causes thinning in iron-containing materials, such as steel, due to the reaction between sulfur compounds and iron at temperatures ranging from 450°F to 1000°F. For pipe walls, this damage mechanism causes gradual thinning over time. Sulfidation corrosion is common in crude oil distillation, where naturally occurring sulfur and sulfur compounds found in crude oil feed, such as hydrogen sulfide, are available to react with steel piping and equipment. Process variables that affect corrosion rates include the total sulfur content of the oil, the sulfur species present, flow conditions, and the temperature of the system. Virtually all crude oil feeds contain sulfur compounds; therefore, sulfidation corrosion is a damage mechanism present at every refinery that processes crude oil. Sulfidation corrosion can cause thinning to the point of pipe failure when not properly monitored and controlled.

As evidenced by the chemical analysis performed on the Chevron 4-sidecut piping post-incident, carbon steel piping components within a single circuit can contain varying percentages of silicon, resulting in a large variation in sulfidation corrosion rates by component. Current corrosion inspection guidance documents allow for the measurement of pipe thickness at a minimal number of permanent Condition Monitoring Locations (CMLs) along the piping length. These CMLs are most frequently placed on elbows and fittings because higher turbulence in these areas usually results in the fastest metal loss. However, due to details of the manufacturing process, carbon steel elbows and pipe fittings, even when manufactured to the ASTM A53B specification, generally contain relatively high percentages of silicon. When measurements are taken only at high silicon-containing fittings, the measurements can fail to identify high corrosion rates within a pipe circuit occurring within low-silicon, straight-run piping components.

API Recommended Practice (RP) 939-C Guidelines for Avoiding Sulfidation (Sulfidic) Corrosion Failures in Oil Refineries describes the challenges when attempting to inspect carbon steel lines susceptible to sulfidation corrosion. The recommended practice states that older ASTM A53B piping, such as the Chevron piping that failed on August 6th, creates a “major inspection challenge” and “unless the refinery is fortunate enough to have located an inspection point on that particular [low silicon] section of pipe or fitting, it is very difficult to detect the thinning component.” It states that in some applications, carbon steel will appear to be adequate based on measured corrosion rates until failure occurs at some undocumented or unidentified low-silicon component...

This case showed that although API has published several codes and recommended practices, these documents provide some information on sulfide corrosion, the information and guidance is varied and inconsistent. The investigation therefore recommended revising API RP 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry to increase awareness of the characteristics of sulfide corrosion and refer users to specific API standards that provide important information to prevent catastrophic rupture of low silicon carbon steel pipes. It also determined the need to revise API 570: Pipe Inspection Code: Inspection, Classification, Repair and In-Service Alteration of Piping Systems to incorporate language consistent with API RP 939-C: Guidelines for Preventing Sulfide (Sulphide) Corrosion Failure in Petroleum Refineries, to increase awareness of the characteristics of sulfide corrosion. Another recommendation that emerged from this investigation was to revise API RP 578: Material Verification Program for New and Existing Alloy Piping Systems to require users to establish and implement a program to identify carbon steel piping circuits that are susceptible to sulfide corrosion and may contain low silicon components. These circuits have the potential to contain carbon steel components that were not manufactured to the American Society for Testing and Materials (ASTM) specification A106 and may contain less than 0.10 weight percent silicon content. Finally, it was recommended that API RP 574: Inspection Practices for Piping System Components (3rd edition) be revised to incorporate API RP 939-C: Guidelines for Preventing Sulfidation (Sulphide) Corrosion Failure in Petroleum Refineries and to follow the requirements of the leak response protocol established in API RP 2001: Refinery Fire Protection.

As a result of these events, the following was considered for inclusion in the third edition of API 571 “The silicon content of carbon steel can significantly affect its susceptibility to sulfurization. It has been found that carbon steels with an Si content of less than 0.10% suffer varying and often higher rates of sulfidation corrosion than carbon steel with a silicon content above this level (carbon steels with higher Si contents remain susceptible to sulfidation, but low Si content steel may suffer higher rates). ) Silicon steel has more than 0,10 % Si, while non-silicon steels generally do not. There have been a number of incidents in the refining industry in which ASTM A53 grade low Si pipes suffered significantly more metal loss than ASTM A106 grade pipes or standard carbon steel pipe fittings or flanges welded to A53 pipe.”

The API 570-year 2018, I add in the paragraph 5.6.3 CML Allocation: sulfidation corrosion (provided that it is a uniform liquid phase with no naphthenic acid and the piping circuit does not contain low silicon CS, see 5.12 and API 939-C). In paragraph 5.12 he also cites the following: In lines in older process units operating above 500 °F (260 °C) and subject to sulfidation corrosion, carbon steel piping containing less than 0.1 wt % silicon can corrode at significantly higher rates than higher silicon carbon steels (modern “silicon-killed” process). For piping systems / circuits that have been identified in sulfidation corrosion service that may contain older low silicon carbon steels, consideration should be given to conducting inspection of each piping segment in order to identify the worst-case corrosion rate / limiting component. After about 1985 to 1990, most purchased pipe became double stamped, and hence the low-silicon issue diminished for piping purchased and installed after that time frame. Inspection techniques that can be useful for finding susceptible components under insulation include real time radiography, GWT, and PEC. Inspection plans for sulfidation corrosion should be in accordance with API 939-C.

An alert for users of refineries in the United States and the world that were built before 1985:

Carbon steel piping components in refineries throughout the U.S. are susceptible to highly variable sulfidation corrosion rates. Carbon steel piping is manufactured to meet certain specifications, including American Society for Testing and Materials (ASTM) A53B,17 ASTM A106,18 and American Petroleum Institute (API) 5L.  ASTM A53B and API 5L do not contain minimum silicon content requirements for carbon steel piping, while ASTM A106 requires the piping to be manufactured with a minimum silicon content of 0.10 weight percent. As a result, manufacturers have used different levels of silicon in the carbon steel pipe manufacturing process.

Thus, sulfidation corrosion rates could vary depending on the manufacturing specification for silicon content in the carbon steel installed in refinery processes. In the mid- 1980s, pipe manufacturers began to simultaneously comply with all three specifications, so most carbon steel piping purchased since then for refinery operations likely has a minimum of 0.10 weight percent silicon content. However, over 95 percent of the 144 refineries in the U.S., including the Chevron Richmond Refinery, were built before 1985. Therefore, the original carbon steel piping components in these refineries likely contain varying percentages of silicon, so they may experience highly variable sulfidation corrosion rates.