Risk-based inspection of pressure vessels

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Problem statement: During shutdowns for regular inspections, petrochemical plants experience prolonged production downtime, which affects production efficiency. During the regular inspection of pressure vessels, tasks such as cleaning, replacement, installing or removing insulation, removing or installing catalysts, and regenerating catalysts all incur significant costs. For the vast majority of important pressure vessels, the cost of these auxiliary tasks is much higher than the direct inspection costs, and companies carry out these tasks merely for the purpose of conducting regular inspections on the pressure vessels. This raises a series of new questions for the testing agencies: what kind of testing is necessary while ensuring safety? How can we conduct inspections that ensure the safe use of pressure vessels, while at the same time reducing downtime for businesses and saving on associated costs? We know that the safety of pressure vessels is not an isolated issue; it is closely related to factors such as the vessel itself, system-related elements, usage conditions, and environmental factors. Judging from accident cases, most pressure vessel safety accidents are related to the use and management of the vessels. If the manufacturing quality of the pressure vessel is excellent and the design conditions fully meet the operational requirements, then the inspection of safety accessories becomes even more important. We know that normal corrosion-induced thinning of pressure vessels can be predicted and easily detected, whereas complex failures such as hydrogen embrittlement and stress corrosion require specific conditions to occur. The purpose of inspecting pressure vessels is to reduce risks. For a given pressure vessel, is the inspection procedure planned to be necessary in order to reduce those risks? Can the inspection procedure play a role in reducing risks? In response to these two issues, many international organizations have conducted extensive research and exploration, introducing the concept of risk-based testing. As soon as this concept was introduced, it was immediately well received by enterprise users. It has currently been widely used in developed Western countries. Risk-Based Inspection: Risk-Based Inspection (RBI) is an inspection concept that has seen rapid development internationally in recent years. It is currently widely used in developed Western countries. The risk inspection program established by the American Petroleum Institute (API) was initiated in May 1993 by an industry-funded group, which is organized and managed by API, with the aim of researching practical methods for risk inspection. In 2000, the American Petroleum Institute published the public document API 581 Risk Based Inspection. It defines risk as the product of the likelihood of failure and the consequences of that failure. The diagram below shows the risk matrix defined by API 581; the vertical axis represents the likelihood of failure, which is classified into levels 1 to 5 based on its magnitude. The horizontal axis chart represents the consequences of failure, which are classified into grades A to E based on their severity. The consequences of a failure in terms of images are determined by the amount of medium that may leak as a result of the failure and the properties of that medium. Four factors are taken into consideration (in API 581, these four factors are referred to as four scenarios) ； 1. Combustion events (thermal radiation and explosive shock wave overpressure) 2. Toxic release ； 3. Environmental pollution ； 4. Business interruption. Regarding the possibility of failure, API 581 summarizes the failure mechanisms in petrochemical equipment and classifies them into the following 8 modes. 1. Thinning Technical Module ； 2. Stress Corrosion Cracking Technical Module ； 3. High Temperature Hydrogen Corrosion (HTHA) Technical Module (High Temperature Hydrogen Technical Module) ； 4. Furnace Tube Technical Module ； 5. Mechanical Fatigue (Pipes Only) Technical Module ； 6. Brittle Fracture Technical Module ； 7. Equipment Linings Technical Module ； 8. External Damage Technical Module. The sum of all possible failure scenarios constitutes the total failure probability, while the sum of the consequences of each failure scenario represents the total failure consequences. The product of the total failure probability and the total failure consequences is the risk. Such analysis is conducted for each filtered equipment item; in API 581, an equipment item can be a single piece of equipment, a pipeline, or a part of a piece of equipment (such as the tube side and shell side in a heat exchanger). In the inspection procedure, the equipment components to be inspected are rated based on their risk level. In each case, once a risk is identified, there are alternative options to reduce it. Once the components of equipment risks are known, it is possible to reduce the likelihood of equipment failure by employing appropriate inspection methods such as online monitoring. Additionally, the consequences of failures can be mitigated by installing relief systems such as fire suppression systems and isolation systems. Inspection of pressure vessels can reduce the likelihood of their failure. For each mechanism of failure, APl 581 classifies inspection methods into five effectiveness levels: highly effective, moderately effective, basically effective, poorly effective, and ineffective, and specifies what is required for each level of effectiveness for every mechanism of failure. The table below shows the effectiveness of different inspection techniques. Note that there is no technique with the highest effectiveness for all types of pore damage, nor is there any inspection technique that is most effective for every type of damage. Images. However, for most types of damage, there are various inspection techniques available, each of which can improve the effectiveness of the inspection. For example, in the case of inner wall corrosion, ultrasonic thickness measurement data will be more effective when combined with visual inspection ； Failure modes such as creep, microporosity, and deformation are not easily detected by any single testing method; however, repeated measurements using techniques such as ultrasonic thickness testing, radiographic inspection, and dimension measurement can make the data more reliable. By combining the quantified level of risk with the quantified effectiveness of the testing procedures, it is possible to assess the contribution of these procedures to risk reduction and to optimize them. The principles for optimization are as follows: 1. If the planned testing procedures are not sufficient to reduce risk, then increase the effectiveness level of the testing or shorten the testing cycle ； 2. If a testing procedure with a high level of effectiveness is used yet no risk reduction is achieved, then the effectiveness of the testing should be reduced, or the testing interval extended. With the above analysis results, it is possible to assess whether our inspection plan addresses the failure mechanisms of the equipment components and whether it truly helps to reduce the likelihood of failures. If reducing the inspection ratio or eliminating certain inspection items does not affect the effectiveness of reducing the likelihood of failures, then the inspection ratio and number of items can be decreased. This can **significantly reduce the amount of auxiliary work for businesses, resulting in substantial economic benefits. According to foreign statistics, for a refinery with a single function, implementing RBI can save one million dollars in maintenance costs per year.