The ASME and API design codes and standards for pressurized equipment primarily focus on providing guidelines for the design, fabrication, inspection, and testing of new pressure vessels, piping systems, and storage tanks. These codes, however, do not address the deterioration of equipment while it is in service. During subsequent inspections, deficiencies due to degradation, or even those present from the original fabrication, may be identified.
To address this concern, Fitness-For-Service (FFS) assessments are conducted. These assessments are quantitative engineering evaluations that aim to demonstrate the structural integrity of an in-service component, even if it contains a flaw or damage. The first step in an FFS assessment, as outlined in API 579-1/ASME FFS-1, is to identify the type of flaw and the cause of the damage. This process is crucial and also serves as the initial step in performing a Risk-Based Inspection (RBI) study, as per API RP 580.
During an FFS assessment or RBI study, it is essential to determine not only the cause of the observed damage or deterioration but also to assess the likelihood and extent of potential future damage. Flaws and damage may result from pre-existing conditions before a component enters service or can be induced during service. The root causes of deterioration might include inadequate design considerations, such as materials selection and design details, or interactions with aggressive environments and conditions encountered during normal or transient service periods.
One significant challenge in performing FFS assessments or RBI studies for refining and petrochemical equipment is the vast diversity of material/environmental condition interactions. Refineries and chemical plants consist of multiple processing units, each exposed to unique combinations of aggressive process streams, temperatures, and pressures. In general, the following types of damage are commonly encountered in petrochemical equipment:
a) General and local metal loss resulting from corrosion and/or erosion
b) Surface-connected cracking
c) Subsurface cracking
d) Microfissuring/microvoid formation
e) Metallurgical changes
It’s important to note that each of these general types of damage may be attributed to one or more specific damage mechanisms. Furthermore, these damage mechanisms manifest under highly specific conditions, including the material used, the process environment, and the operating conditions of the equipment.
Damage Mechanisms Affecting Fixed Equipment in the Refining as per API RP 571
API RP 571, a recommended practice document, serves as a valuable resource by offering general guidance on the most common damage mechanisms affecting alloys commonly used in the refining and petrochemical industry. Its purpose is to introduce the concepts of service-induced deterioration and failure modes. This guidance is instrumental in assisting plant inspection personnel in various ways:
- Identifying Likely Causes of Damage: API RP 571 provides insights into the probable causes of damage within the industry. This information aids inspection personnel in understanding the potential threats to equipment integrity.
- Developing Inspection Strategies: It helps in the development of effective inspection strategies. By recognizing the likely damage mechanisms, inspection plans can be tailored to focus on areas most susceptible to deterioration.
- Establishing Monitoring Programs: The document suggests monitoring programs that can be implemented to ensure the ongoing integrity of equipment. This proactive approach helps in early detection and prevention of issues.
The summary provided for each damage mechanism in API RP 571 furnishes fundamental information necessary for conducting two critical assessments:
- FFS Assessment (API 579-1/ASME FFS-1): The information can be used as a foundation for Fitness-For-Service assessments, which are quantitative evaluations conducted to determine if equipment with flaws or damage can remain structurally sound and operational.
- RBI Study (API RP 580): It supports Risk-Based Inspection studies by providing insights into the likely causes of damage and helps in prioritizing inspection efforts based on risk.
The document comprehensively covers damage mechanisms that are commonly encountered in pressure vessels, piping, and tankage within the refining and petrochemical industry. While it provides valuable guidance, it’s important to note the following:
- Not Exhaustive: The descriptions of damage mechanisms are not exhaustive and may not cover every possible situation encountered in the industry.
- Consultation May Be Needed: In some cases, particularly those involving unique degradation modes and failure mechanisms, it may be necessary to consult with an engineer experienced in the specific area.
- Safety and Industry Standards: The document incorporates information from major incidents in the refining industry and aims to align with applicable API documents, industry standards, and best practices. It should serve as a guidance tool but not be considered the sole technical basis for assessing and analyzing damage mechanisms.
API RP 571 plays a crucial role in enhancing the understanding of damage mechanisms within the refining and petrochemical industry. It aids inspection personnel in making informed decisions regarding inspection, assessment, and maintenance strategies to ensure the safety and reliability of equipment.