This article, written by TWI Technology Fellow John Wintle, was originally published in the November 2015 edition of Managing Aging Plants magazine.
It is now over ten years since an inspector from the UK's Health and Safety Executive Hazardous Installations Directorate (HSE HID) called Harry Bainbridge came to me and said that there were a lot of plants in a poor physical condition around and what industry needed was a guide to manage ageing. The result was the publication of Research Report 509 – Plant Ageing: Management of Equipment Containing Hazardous Fluids and Pressure (2006). The rest, as they say, is history.
Of course, the subject matter was not new and not confined to the hazardous chemicals and refining sector. By the end of the 1990s the International Atomic Energy Authority had set out the principles of ageing management and published a series of guides to the management of ageing of different nuclear components and the European ageing aircraft programme had been running for several years. For the process industries, HSE Research Report 509 changed a mind-set in their approach to integrity management, as the management of ageing became a holistic interdisciplinary activity.
Historically, engineering plant was designed and constructed to function without major failure or repairs for a pre-determined design life. The design life was based largely on prior experience of similar constructions in similar duty and provided designers with a target from which corrosion allowances and fatigue lives could be conservatively assessed. Plant inspection was largely qualitative, prescriptive, periodic and judgemental, designed to confirm that the design assumptions had not been breached by service conditions and misuse beyond the design specification. If some deterioration was found, it was normal to repair the damage or replace the component rather than to attempt a fitness-for-service assessment or re-rating of the degraded component.
A combination of technological development and economic factors brought about the management of ageing. Low prices for oil and bulk chemicals, competitive pressures from Far East producers and a lack of investor confidence forced Europeans and Americans to adopt leaner operations and to keep equipment in service when previously it would have been replaced with new. The need to operate older equipment, combined with developments in understanding degradation mechanisms, inspection planning practices and non-destructive testing (NDT) technology, fitness-for-service assessment and repair techniques, created the conditions for a change in thinking from a ‘find and fix’ culture to a ‘predict and preserve’ mentality.
The understanding of corrosion degradation mechanisms and rates has largely been driven by experience of plant operation and failures. It is generally difficult to replicate plant conditions over the long durations involved in the laboratory through accelerated testing. A catalogue of degradation mechanisms for the petrochemical and refining industry was published in API RP 579, but a comprehensive guide of degradation mechanisms for a wider range of material–environment combinations has yet to be produced. The dependence of the occurrence and rate of progression of degradation on a wide range of process and product composition variables and material factors, such as temperature, trace elements, microstructure and hardness, may make such a task intractable.
There have been significant developments in the understanding of degradation for particular material–environment combinations. Conditions for the avoidance of external corrosion of stainless steels from pitting and atmospheric-induced stress corrosion cracking in marine environments are now becoming established. The susceptibility of ferritic materials and welds to attack in corrosive environments and the problems of hydrogen-induced cracking are becoming better known.
Operators often need to rely on measures of wall loss, pitting depth and cracking over a period of service to determine the corrosion rates for particular plant conditions. Tools such as trending analysis and statistical techniques are vital to make appropriate predictions. Even so, the ability to characterise damage for analysis in its true geometric complexity is still in the early stages. Experience of plant failures informs us that the causes of degradation can occur throughout the life of a plant. Defects introduced during fabrication or installation can be the root cause of failure in service, while problems during commissioning can result in premature ageing.
The possibility of fatigue cracking in welds over extended periods of cyclic service should be assessed using modern fatigue design methods. The bathtub curve of ageing and failure rate remains a useful model but may not reflect the reality of modern plants where quality management of design and construction reduce early life failures, and interventions during service can yield extended periods of reliable operation.
Risk-based inspection (RBI) is an inherent part of the management of ageing. From the mid-1970s regulators in the UK were placing the responsibility for maintaining health and safety on the shoulders of industry that created the risk and set high-level goals for the prevention of dangerous occurrences. In 1989 the Pressure Systems and Transportable Gas Containers Regulations removed any prescriptive requirement for periodic inspection by requiring simply that owners of pressure systems should have a written scheme of examination drawn up or certified by a competent person that would ensure the detection of a defect that could give rise to danger.
This requirement forced operators to start to think about the particular locations in a plant where degradation mechanisms might cause a defect and where a defect could give rise to danger. As the regulator did not prescribe inspection intervals, operators were encouraged to consider the mechanisms and rates of degradation for each particular plant component and to select intervals that were appropriate to reasonably address the risk from a defect by timely detection.
The Institute of Petroleum’s code of practice in the 1980s allowed inspection intervals to be extended for some equipment based on favourable experience and technical understanding, but the new regulation gave this opportunity to all operators of pressure systems. Some sectors preferred to keep to the fixed inspection intervals that they had traditionally used as the costs and knowledge required to adopt RBI were beyond their means and capability.
The move towards RBI gathered pace during the 1990s. The American Petroleum Institute gave RBI a major boost following a joint industry project which resulted in the publication of Base Resource Document 581 and Recommended Practice 580. In the UK, the HSE published Research Report 363 on RBI practices in 2001, and this was followed by similar publications from industry-led bodies such as EMMUA. Other European regulators have been slower to recognise RBI as the preferred approach and it is only now that practices are beginning to change.
Inspection and NDT technology
Developments in inspection and NDT technology also created the platform for the management of ageing. Ultrasonic testing, with the introduction of advanced transducers and computerised signal processing over the past 30 years, has improved the reliability and quantification of detection, sizing and characterisation of defects in plant. Long-range ultrasonic technology, time-of-flight diffraction and other screening techniques such as eddy currents, acoustic emission and thermography and magnetic induction methods have added to the armoury of NDT that can be deployed in particular circumstances. The PANI trials in the 1990s still showed deficiencies in the practical implementation of industrial NDT and these led to a series of guidance documents covering the main techniques.
Fitness for service methods
If physical or functional degradation due to ageing mechanisms is to be managed then it is necessary to know the impact of the degradation on the performance of the component and to assess its fitness-for-service now and in the future given that further degradation may occur. For active components like pumps, valves and motors, a functional test checked against specification may be all that is required. Deterioration in passive equipment like vessels and structures is harder to assess since a fall-off in performance is not usually evident until final failure.
The evolution of fitness-for-service assessment procedures is aligned to the development of fracture mechanics of crack-like defects. It was largely driven by the requirement to evaluate defects in welded fabrications, particularly for nuclear pressure vessels, and later by the need to determine the fatigue lives of offshore welded structures, pipelines and risers. The effect of non-crack-like defects, such as localised corrosion, pits and gouges requires a different approach, and methods are still evolving. Modern finite element stress analysis is allowing the modelling of real defects and the ability to have a seamless interface between NDT and fitness-for-service assessment is a real prospect.
API Recommended Practice 579 is a compilation of fitness-for service assessment methods applicable for refinery plant designed to the ASME Boiler and Pressure Vessel Code. It contains many useful methods including those for assessing hydrogen and creep damage and equipment damaged by fire. However, the methods need to be translated for equipment designed to other codes where different safety margins and criteria apply. More generalised fitness-for-service procedures have been standardised in publications such as BS 7910 and FITNET.
Ageing management process
While ageing is usually associated with the physical deterioration of equipment, its scope also encompasses non-physical issues and other changes with the passage of time. For example, obsolescence may mean that original parts are difficult to replace, whereas changes in standards or technology can challenge the adequacy of the original construction. Lack of knowledge of the physical condition in uninspectable areas or loss of original design data and key operating records can undermine the confidence necessary for continued safe production.
Against this background a philosophy for the proactive management of ageing has evolved. In some respects it can be regarded as a management- of-change process. The management of ageing is a process that requires foresight of changes that will occur in the future from the extrapolation of trends based on historic data and the application of technical knowledge.
In its essentials the ageing management process can be broken into five steps. It requires an organisation to recognise what assets it has, understand what they do, know how important they are, determine what condition they are in and how fast this may change in the future, and evaluate what happens should they fail. On this basis an ageing management programme can be created to understand, predict, minimise, find and remediate the effects of ageing, with appropriate feedback. Making this happen in a busy company is not easy and requires senior management to set the policies and create the roles to champion its implementation.
Ageing management is an activity requiring a team of plant engineers and technical experts in fields such as materials technology, integrity management, inspection and NDT. Some companies have appointed managers with a wider asset management remit to co-ordinate and direct the management of ageing, supported by systems engineers who are the custodians of particular assets and design authorities who are the discipline experts concerned with design intent functionality and fitness-for-service.
Link to asset management and asset integrity management
The ageing management process forms part of a wider process called asset integrity management, which itself is a subset of the global process of asset management. British Publicly Available Specification (PAS) 55-1 defines asset management as systematic and co-ordinated activities and practices through which an organisation optimally and sustainably manages its assets and asset systems, their associated performance, risk and expenditures over their lifecycle for the purpose of achieving its organisational strategic plan.
It translates the organisation's strategic plan and policies into long-term objectives and plans and activities over the lifecycle of the asset portfolio where assets are created or acquired, utilised, managed, maintained, renewed, decommissioned and disposed. Asset integrity management is the process by which an asset’s integrity is maintained to deliver required levels of functionality, availability, reliability, survivability and interdependence. As well as considering the management of ageing, integrity management focuses on maintaining asset integrity through operation controls, planned maintenance, testing, refurbishment, upgrading and replacement.
It is difficult to find references to ageing within the context of asset management. For many types of assets, ageing is regarded as a natural process that is accommodated through periodic replacement once the asset no longer performs to its required level or sustains some level of failure. This position on ageing is, however, usually not tenable for physical assets in the high hazard industry sectors, such as the petrochemical and nuclear industries, where the risk of failure cannot be tolerated and components are not easily repairable or replaceable. In these sectors the proactive management of ageing becomes a necessity and a benefit.
The need to manage ageing plant is not new. The ability to do so effectively has improved markedly over the past 20–30 years with developments in understanding materials behaviour and inspection and NDT technology and fitness-for-service assessment procedures. These technical developments are allied to the evolution of the formalisation of systems for lifetime asset management and more specifically the management of asset integrity.
Different industry sectors are managing ageing in a way that is consistent with their context, character and culture, although the basic principles are common across all sectors. Industry forums and networks through different media are vital components for learning from experience and sharing knowledge and information within and across industry sectors. The launch of Managing Aging Plants magazine is to be welcomed and will surely make an important contribution to this field.
To find out how TWI's expertise can help support the life extension of your company's ageing assets, contact us.