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[November 2006]
Introduction
The Institution of Mechanical Engineers seminar ‘The Management of Plant Ageing’, held on 1 November 2006 marks the launch of a new Research Report from the Health and Safety Executive (HSE) on Plant Ageing: Management of Equipment Containing Hazardous Fluids or Pressure. A few years ago, HSE recognised that the goal setting regulations for pressure systems and other non-pressure equipment containing hazardous fluids needed support in relation to the management of plant ageing issues. Greater access was required to the improved understanding of ageing issues and management practices, together with new assessment, inspection and repair methods, that have developed in recent years. At the same time, the pool of trained and experienced people managing industrial plant was perceived as reducing and there was a need to capture knowledge and good practice for a new generation. No single reference source for advice on ageing issues was available,
and so the HSE decided to initiate a research project where this material could be gathered.
The HSE appointed TWI to manage the project in 2002 and a team was formed with a range of asset management expertise in 2003. In addition to HSE and TWI, this comprised representatives from ABB, Allianz Cornhill Engineering, and SCS (INTL). The work was carried out over 2004/2005, and finalised with a limited peer review in 2006.
The management of plant ageing requires consideration of a number of factors: safety, damage, consequences of failure, people and confidence. Around these, there are many technical and managerial issues to take into account, and it is easy to lose sight of what these are and how they relate. The work undertaken sets many of these issues into a coherent structure, that, it is hoped, users will find helpful and easy to use.
What the guide covers
The guide is in four main sections, plus Appendices. The first section covers the scope and awareness of ageing issues, where users can find out if it is likely to be helpful. The second section is about getting organised in terms of responsibilities, management, people (including competencies) and data. Finding and identifying ageing is addressed in the third section, and deals with the stages of life that equipment may pass through, indicators of ageing, and inspection and non-destructive testing (NDT). The fourth section considers the options when ageing damage is found or changes made, and includes fitness-for-service assessment, repairs, and revalidation. Three Appendices contain: audit tool questions, where an organisation may analyse its capability in managing plant ageing, a range of illustrative case studies, and an appendix on the selection and assurance of NDT.
The guide is relevant to a wide range of equipment types used for the containment of hazardous fluids and/or pressure (e.g. pressure vessels, boilers, piping, hoses, storage tanks etc). Fluids may be hazardous for many reasons, many of which are covered by statutory provisions (steam, pressurised fluids, hot/cryogenic fluids, dangerous substances (flammable or oxidising), substances hazardous to health or environment). Ageing equipment is where there is evidence or reasonable likelihood of
degradation (e.g. corrosion, fatigue etc) or where there is insufficient knowledge or information to know the extent to which this possibility exists.
While being general, the guide covers a wide range of degradation mechanisms (e.g. corrosion, fatigue) and damage types (e.g. effects like thickness loss, cracks), which tend to be equipment and environment specific. Manufacturing and welding defects introduced during fabrication are an initial source of damage that can grow during service by other mechanisms. Damage to equipment can also be induced during service by mechanisms such as corrosion, fatigue and stress corrosion cracking, and can be quantitatively measured by the metrics such as wall thickness loss, crack number/depth, leaks etc. Growth rates during service is very variable, and can be influenced by a range of life-dependent factors, including transients, process conditions, invasive work and repairs.
Four stages of life
During its life an item of equipment may pass through four loosely defined stages: Stage I (‘Initial’) is post commissioning and is when issues resulting from design and manufacturing faults, installation and commissioning, and early operations may become evident. There may be higher rate of ageing damage occurring as the equipment beds-in, and indicators of things may become problems as the equipment gets older. Experience and confidence in performance is being gained, and reinforced
from the first in-service examination.
By Stage 2 (‘Maturity’), the equipment has reached steady state conditions and extended operating periods with minimum intervention may be possible. The equipment is well within its design limits and damage not yet evident, or insignificant or within predicted limits. Knowledge of original manufacture and operating history is still available. Routine maintenance and risk based inspection to confirm expectation may be all that is required.
When design limits are approached and there is evidence of degradation and damage, the equipment enters the third stage of its life (‘Ageing’). Degradation rates increase and are less predictable, and changes in ownership, use and process conditions may occur that and result in gaps in knowledge and records and require the equipment to be revalidated. Proactive integrity management is now needed for life extension, which may involve a more quantitative approach to inspection and NDT combined with fitness-for-service and remanant life assessments, modifications and repairs.
In the fourth and final stage (‘Terminal’), damage is accumulating and accelerating and the equipment is beyond normal design limits and experience and approaching safe operating limits. Major repairs,
refits or re-rating are required, and until these are done, the damage must be closely monitored with frequent inspection. More complex fitness-for-service assessments to justify safe continued operation may allow time for an economic analysis to decide the replacement strategy. The stage is
characterised by close control over equipment, with major investment decisions involving wider business issues to consider.
Revalidation
During its life, an item of equipment may see many changes. While pressure equipment is designed for a specific range of application, the safe uses for non-pressure equipment are often not as well defined or documented. When changes occur that are beyond the design basis there is a need for revalidation. These can include physical modifications and repairs, changes in operating or process conditions, external loads and supports, or in regulations and codes. Industry experience, failures and
new research can also be reasons for revalidation of existing equipment.
The guide describes the technical and other actions needed to revalidate equipment for changes in design temperature and pressure. Reassessment of equipment is usually required when there are
changes to contents and environment, or when equipment is purchased second hand or brought in from storage. It is important to stay up-to-date with technological and industrial developments. Sources of reports from major failure investigations, research and development, and opportunities for sharing experience and best practice are highlighted.
Final remarks
The guide is offered in the spirit of being helpful advice, but is in no way prescriptive or totally comprehensive. It is intended to assist thinking about ageing issues and to promote safe and effective management of plant ageing. While it has received a limited peer review and reflects current
knowledge and experience, it offers the opportunity to be expanded and revised when greater feedback has been obtained.
Acknowledgement
The authors of the ’Guide’ would like to thank all the many individuals and organisations that have contributed to its development. They also acknowledge the support of the Health and Safety Executive, and particularly Harry Bainbridge as Project Officer.
John Wintle, TWI
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