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Probability and Consequence PUBLIC ACCESS

Petroleum and Power Businesses Enhance Efficiency by Weighing Their Risks and Keeping Watch. A New ASME Standard Makes that Method of Inspection Available to a Wider Group of Industries.

[+] Author Notes

Dgniel Sharp, an engineering adflilinistrator for ASME, is the secretary of the Post Construction Subcommittee on Inspection Planning. Daniel Peters is a senior consultant with structural lntegrity Associates and a member af the ASME Post Construction Subcommittee tee on inspection Planning. David Mauney, recently retired from Strtutural Integrity Associates, is past chairman of the ASME Inspection Planning subcommittee and co-author of the ASME Risk-Based Handbook on Equipment Life Management.

Mechanical Engineering 131(03), 38-41 (Mar 01, 2009) (4 pages) doi:10.1115/1.2009-MAR-4

This article reviews features of a new American Society of Mechanical Engineers (ASME) standard named PCC-3-2007 Inspection Planning Using Risk-Based Methods, which aims to make method of inspection available to a wider group of industries. Petroleum and power businesses enhance efficiency by weighing their risks and keeping watch. PCC-3 provides guidance by listing the many parameters that influence the damage mechanisms as well as many parameters that can influence the rate of damage. The standard provides a table with listings of possible damage mechanisms along with their definitions, common attributes, and references. PCC-3 provides planning details to develop consequence scenarios with different associated probabilities. The framers of ASME’s standard believe that proper applications of the new PCC-3 can lead to major cost savings and allow for application of technically based engineering judgments on expenditures for large capital, inspection, and safety improvements for most industries.

Most people who see a power plant take its safe operation for granted. Inside it there are major componentsincluding vessels, valves, and piping-that operate under extreme pressure for years at a time. Yet the plant and others like it around the world run reliably and raise no alarm.

It’s probable that no one knows how many pressure vessels and miles of pipeline are in use. Every day they give us the electricity, fuel, and steam that light and shape the modern world. And news photos from Manhattan in July 2007 show a gaping hole in Lexingtoh Avenue and remind us of what can happen when a steampipe fails under pressure.

The managements of many power plants and their counterparts in the petroleum industry have methods for keeping operations safe and reliable~ They know the risks posed by different parts of their work sites-the likelihood and COl).sequences of failure of every major piece of equipment. They keep a close eye on the big-risk areas and react to early signs of impending failure.

The practice is called risk-based inspection. It lets companies weigh the risks posed by" their equipment so they know how often they should inspect each key component and how to deal with the unexpected. It makes more effective use" of resources and usually requires fewer shutdowns. Plants operate safely, and companies save money.

The practice of risk-based inspection is less common outside petroleum and power generation businesses. That is the reason for a new ASME standard, which is designed to introduce risk-based inspection practices and planning to a wider range of industries.

In the normal course of events, inspectors responsible for the safety and reliability of plants have a seemingly endless list of hazard areas to review, and all of these areas are given equal weight. Large parts of a plant or refinery are shut down so that the inspectors can do their jobs. Product flow and service are diminished, and the company loses the use oflarge amounts of capital. Risk-based inspection planning is intended to minimize the loss of capital while controlling the risk in a plant.

The new American Society of Mechanical Engineers standard, PCC-3-2007 Inspection Planning Using Risk-Based Methods, was specifically developed for planning and executing risk-based inspection of fixed pressure-containing equipment and components. Equipment covered ranges from piping and boilers to pumps and compressors, from heat exchangers and furnaces to storage tanks and valves. Risk-based inspection planning takes the concepts of REI and applies them to such issues as frequency of inspection.

This new standard is the result of the hard work by approximately 40 volunteers representing such industries as power, petrochemicals, and pulp and paper, who participate on ASME’s Committee on Post Construction and its Subcommittee on Inspection Planning.

ASME’s risk-based inspection standard was modeled largely on concepts that started in a document known as API-580, published by the American Petroleum Institute. That document and similar principles are used in the petroleum and power industries. An agreement with API permitted ASME to develop a parallel standard on risk-based inspection for general use.

ASME has also produced supporting documents for this effort. One of the most important of these supporting documents is Center for Research and Technology Development, Volume 41, Risk-Based Methods for Equipment Life Management: An Application Handbook, A Step-by-Step Instruction Manual With Sample Applications (CRTD-41).

In most facilities, a large percentage of the total risk will be concentrated in a relatively small percentage of equipment. The selection of equipment to be included in an assessment is based on the specific objectives of the risk assessment. Screening of equipment is typically conducted to identify higher-risk components, which will receive more-detailed risk analysis. PCC-3 provides key questions that should be answered about each piece of equipment to aid the screening process. Additional sitespecific questions will also need to be developed.

For each piece of equipment to be analyzed, a variety of data and information should be collected. The data coll~cted provide the information needed to assess potential damage mechanisms, potential failure modes, and scenarios of failure. PCC-3 discusses nine data categories with more than 60 sub categories. One category, for example, addresses site conditions and includes nine subcategories, including population density, corrosive atmospheres, wind, and seismic activity.

Risk is defined as the possibility of suffering a harmful event. In engineering terms, risk is defined as: Risk = Probability of an event x consequence if the event happens On the most basic level, risk-based inspection serves to help users of pressure equipment assess levels of risk to various pieces of equipment and then prioritize inspection of them based on the associated level of risk.

Since risk equals the probability of fail,ure times the consequence of failure, it is important to know how to determine the probabilities and consequences. The probability of failure is based on identifying the active and plausible damage mechanisms along with their rates of damage and the effectiveness of inspection programs. This is used to identify the damage mechanism.

PCC-3 provides guidance by listing the many parameters that influence the damage mechanisms (materials of construction, operating and upset temperatures, fluid composition, etc.) as well as many parameters that can influence the rate of damage (contaminants, phase of fluid, pH, flow velocity, etc.). The standard provides a table with listings of possible damage mechanisms along with their definitions, common attributes, and references. It also provides a screening table with specific conditions under which these various damage mechanisms may occur. It provides a table listing inspection methods that can be used to detect each damage mechanism with the highest probability of occurrence.

The standard recommends that compames consult experienced materials or corrosion engineers to obtain the best possible analysis. After the damage mechanisms are identified, specific failure modes are linked with each damage mechanism (such as cracking, localized or general metal loss, or metallurgical damage). For example, under-insulation corrosion can lead to metal loss. The metal loss could be generalized corrosion causing wall thinning and eventual loss of containment. The loss of containment could range from a small hole to a large rupture. The metal loss could also be localized corrosion, resulting in pitting. The pitting corrosion can lead to small leaks, or if the pitting is aligned, it may result in a catastrophic rupture. Another example is creep failure. Creep occurs at high temperatures when a metal deforms slowly and continuously under load. This deformation can lead to internal cracking and eventually a rupture. The probability for each failure mode should be identified. Failure modes may impact the magnitude of the consequences.

Daniel, Sharp, an engineering administrator ASME, is the secretary of the Post Construction Subcommitee on inspection Planning. Daniel Peters is a senior consultant with structural lntegrity Associates and a member of the ASME Post Construction Subcommittee on Inspection Planning. Mark Tanner, a principal engineer with M&M Engineering Associates, also is a member of the ASME Post Construdion Subcommittee on Inspection Planning. David Mauney, recently retired from structural lntegrity Associates, is past chairman of the ASME Inspection Planning Subcommittee and co-author of the ASME Risk-Based Handbook on Equipment Life Management.

Since risk equals the probability of failure times the consequence of failure, it is important to know how to determine the probabilities and consequences.

The probability of failure for a given damage mechanism and piece of equipment can vary, depending on whether the risk assessment is qualitative or quantitative. Qualitative RBI analysis is dependent upon using descriptive information based on engineering judgment and experience to determine a probability and consequence offailure. Quantitative RBI is based on providing individual plant components with a numerical measure of the ch.ance offailure, from 0 to 1.0. Neither method is considered better than the other, and risk-based inspection programs may contain a mixture of qualitative and quantitative analysis.

Risk assessment techniques are best thought of on a spectrum of resolution. At one extreme is the qualitative assessment, which is usually verbally based. These are based on generalized relative rankings. Qualitative methods are generally used on simpler systems that have limited numbers of components. When systems become large or complex, it may become necessary to use a quantitative approach to risk analysis.

There are several methodologies for quantitative probability analysis. PCC-3 provides guidance on both an objective approach and subjective approach to quantitative probability analysis and provides an appendix dedicated to quantitative analysis. Objective quantitative probab~lity analysis is based on hard data, such as failure history data or failure frequency. It is especially helpful when there is a statistically significant amount of data to call upon. Subjective quantitative probability analysis is based on quantified intuitive opinion or expert elicitation. It is useful when estimating the probability of rare events. Both methods apply a numerical value to the probability of failure, and follow the rules of the mathematical theory of probability.

The ASME publication CRTD-41, the application handbook, also discusses these methods and provides examples on application of the methods in great detail. The handbook is used to develop an inspection program for pressure vessels, piping, and other equipment in power plants or in other industrial facilities. The handbook helps users by providing guidance in framing inquiries, collecting and analyzing data, determining failure ,probability vs. time, and determining the correct sequence and timing for a series of major maintenance activities.

The second part of the risk calculation is consequence. The consequence is estimated based on the possible failure modes of equipment. Consequences are typically categorized into three areas: health and safety, environmental, and economic.

Health and safety consequences can range from no injuries to minor injuries requiring first aid to a large number of fatalities. Environmental consequences can range from minimal environmental impact to long-term impact. Economic consequences involve the cost of lost equipment, revenue, and the costs of all other possible damage and liability.

PCC-3 provides many details (toxic fluid releases, flammable fluids releases, environmental effects, and business interruption effects). One such detail is flammable effects. For instance, there are six possible outcomes for a release of flammable fluids: safe dispersion, jet fires, explosions, flash fires, fireballs, and pool fires. The magnitude of the consequence will depend on the volume of fluid released, the release rate, dispersion of fluid, and detection and isolation time. PCC-3 provides planning details to develop consequence scenarios with different associated probabilities.

Once the probabilities of specific damage mechanism and failure modes are determined, along with probabilities of specific consequences, risk for the equipment can be determined.

Based on the rankings of items, the risk management process begins. For risk considered unacceptable during the analysis, there are various mitigation categories that can be considered: inspectionlcondition monitoring, consequence mitigation, probability mitigation, and decommissioning. PCC-3 provides extensive details on risk management through inspection activities, consequence reduction, and probability reduction.

Once the probabiLities of specific damage mechanism and faiLure modes are determined, aLong with probabiLities of consequences, risk for the equipment can be determined.

National Thermal Power Corp. in India is implementing not only risk-based inspection, but risk-based major maintenance management across its complete power system. Risk-based major maintenance management includes all aspects of maintenance in risk assessment and not just inspection. This is no small effort. NTPC is the third-largest power company in the world, and the largest in India, with 105 power units and 29,894 megawatts of installed capacity.

Carolina Power and Light Co. (now Progress Energy) implemented risk-based major maintenance management that included risk-based inspection of its boiler tube components across its fossil power fleet in 1993. At the time, the fleet had 19 power units generating about 6,600 MW. Applying the whole spectrum of risk-based methods for the replacement planning of these 140 major tube components resulted in savings of more than $70 million that would have been spent under conventional engineering planning, where risk-based analysis was not used.

Effective long-term use of the implementation of PCC-3 on a widespread basis throughout industry will require two things: acceptance by local jurisdictions and major insurance carriers of the fundamental principles of a risk-based inspection, and further education of the engineering community.

Standards are typically developed in response to widespread existing practice. In the case of risk-based inspection, the standard was developed without widespread industry acceptance of the techniques. After the first publication of this standard, time is needed to gain widespread acceptance. The rate at which this happens is based on the interest of the industry in risk-based inspection and therefore in the standard.

Probabilistic design and assessment is not common today in most industry practice or in university curriculums. There are only a limited number of qualified analysts at a handful of companies actively practicing risk-based assessment. Engineering schools do not actively teach the concepts of risk or probabilistic analysis in general engineering curriculums, even though the benefits of this methodology can be significant in all areas of industry.

Much of the application of statistics, as a subset of probability theory, is focused on manufacturing principles in schools, and not on operational and maintenance issues. Risk and probabilistics are part of most business administration programs around the world. This means that engineers, if able to express themselves in these terms, can communicate more effectively with corporate decision makers.

The framers of ASME’s standard believe that proper applications of the new PCC-3 can lead to major cost savings and allow for application of technically based engineering judgments on expenditures for large capital, inspection, and safety improvements for most industries.

Copyright © 2009 by ASME
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