Philippa Moore joined the Structural Integrity Department at TWI in 2003, after completing her PhD jointly with the University of Cambridge and TWI. Philippa is a Senior Project Leader and runs projects involving fitness-for-service assessment and fracture mechanics testing, particularly for the oil and gas industry. As a qualified European Welding Engineer, Philippa is also one of TWI's duty welding engineers.
John Wintle is Consultant Engineer for Structural Integrity, and Programme Manager for the Design and Engineering Core Research Suite. He specialises in projects involving integrity management of equipment in high hazard industries. John is a Fellow of the Institution of Mechanical Engineers and vice chairman of their Pressure Systems Group.
The purpose of the guide is to increase awareness of the factors to consider when managing ageing equipment and to help those responsible to understand and assess the risks of accumulated damage and deterioration. The guidance is at a general level, and can be applied to a wide range of static equipment and associated machinery. The guide was launched on the 1st November 2006 with a seminar at the IMechE, and is publicly available from the HSE. Philippa Moore and John Wintle describe the new guide Plant Ageing: Management of Equipment Containing Hazardous Fluids or Pressure that has been commissioned by the Heath and Safety Executive (HSE) to assist UK industry to maximise the life of their existing equipment, in safety.
The HSE is concerned that there is an increasing amount of ageing equipment in UK industry containing hazardous fluids that, if released, would threaten health and safety. TWI was appointed to lead this project to compile the Ageing Plant guide, exploiting TWI's experience, particularly in structural integrity, fitness-for-service and welding engineering, along with a team of partners from industry. ABB Engineering's knowledge of site asset management and inspection was combined with experience of engineering insurance from Allianz Cornhill, and consultancy on non-destructive testing on-site by SCS (INTL) Ltd.
Awareness of ageing
Ageing is not about how old your equipment is. It is about what you know about its condition, and how that is changing over time. An awareness of the risks of accumulated damage and deterioration is vital in order to ensure ongoing equipment structural integrity and safety, while running it beyond the original stated design life or changing its operating conditions. Some of the issues to consider are highlighted in Figure 1.
The four stages of equipment life
One model for understanding ageing is to consider equipment life in four stages. It is important to appreciate which stage equipment may be in its lifetime.
Stage 1 is 'Initial' when the equipment has been commissioned and installed. Failures such as valves leaking, shake-down issues or faults not anticipated during design can occur.
Stage 2 is 'Maturity' as equipment operates within its design limits. Routine maintenance is carried out and the probability of failure is lowest.
Stage 3 is 'Ageing' when the equipment's design life is approaching or has been used, or when degradation becomes evident, and a justification must be made for continued service. Equipment needs regular inspection, and repairs or modifications may be necessary.
Stage 4 is the 'Terminal' stage, when accelerating and accumulating damage to equipment means that close monitoring is required or the duty is reduced since no further service can be justified on safety or financial grounds.
The four stages of equipment life are illustrated in Figure 2, which shows the probability of failure over equipment lifetime for a population of equipment. The risk of failure of an individual component from accumulated damage normally rises over time, but this can be reduced periodically by appropriate maintenance, inspection and repair of damaged areas in Stages 2 and 3 of equipment life.
Identification of ageing
The symptoms of ageing cover a large number of indicators and damage mechanisms. Many of these will be obvious to day-to-day operators when equipment starts to suffer from leaks, vibration, drips, noise, repeated problems, or breakdown. Other symptoms of deterioration such as wall-thinning, wear, corrosion, or cracking may need to be detected using non-destructive testing (NDT). With a good awareness of ageing, inspection for the latter should be considered when one or more of the former set of symptoms is evident.
Understanding the expected degradation and damage plays a big role in tailoring the type of inspection carried out. Just three examples are: fatigue cracking under cyclic loading at high stress locations such as welds; stress corrosion cracking (if the combination of material and environment is susceptible), or erosion at elbows in systems carrying particulates in fluid. TWI carries out assessments of in-service cracking or damage as part of its failure investigation service.
A typical example is shown in Figure 3, where TWI was called in to investigate the cause of cracking at the weld between a small bore forging attachment and an 18" diameter pipe, after leaks had been detected. In this case, a recent change in loading had caused fatigue crack propagation from a weld toe with a sharp profile, and premature failure. A greater awareness of ageing mechanisms may have identified the increased risk of cracking.
The type of NDT inspection carried out will depend on the stage of the equipment within its life cycle. In Stages 1 and 2 the inspection will simply confirm the expected condition of the equipment, typically using techniques such as routine manual ultrasonic testing (UT) and magnetic particle or dye penetrant testing. In Stages 3 and 4 when equipment is in an unknown condition, it's necessary to measure flaw sizes quantitatively, and deterministic inspection is done, possibly using methods such as automated UT techniques.
The scheme of examination must therefore be reviewed at suitable intervals, as the equipment passes through the stages of its life when a different approach to the inspection becomes necessary. Understanding the applications and limitations, with respect to probability of detection and the flaw sizing error, of all the available inspection methods is necessary when preparing the written scheme of examination for equipment. These are discussed further in the guide.
Addressing ageing
With a good awareness of the risk of equipment ageing, and knowing what damage to look for and how to detect it, those responsible for equipment containing pressure or hazardous fluid should be in a good position to address any ageing damage that is found. There are a number of mitigating actions to choose from, and sometimes more than one will be chosen:
- Do a fitness-for-service, or remaining life assessment
- Remove the damage, possibly without further repair
- Repair (may be either a temporary or permanent repair)
- De-rate service conditions
- Monitor the vessel to ensure rate of damage accumulation does not compromise safety
- Scrap the equipment
Fitness-for-service (FFS) assessment
A fitness-for-service assessment can be a good first step to determine what further action is necessary. An FFS assessment can be carried out to assess ageing damage such as corrosion, fatigue cracking, locally thinned areas, ordents and gouges - the guide discusses the FFS assessment of a range of damage types. FFS assessments can determine whether damage is tolerable and may be left, or needs to be repaired. Alternatively, it can determine a safe lower service stress for which the flaw is tolerable.
The expected failure mode is an important consideration - whether it is likely to suffer ductile or brittle fracture, and whether the equipment will leak-before-break, all of which will affect the way the damage is addressed. TWI has significant expertise in carrying out FFS assessments, particularly using the procedures in BS 7910. For crack-like flaws, this procedure plots the FFS assessment point on a Failure Assessment Diagram (FAD), which shows graphically whether the flaw and/or conditions are safe or not (an illustration of this is given in Figure 4).
Repair
When surface and near surface flaws are found, the most common approach is to remove the flawed material by grinding. Skill is required to ensure a smooth profiled groove is made. After NDT to check that all the flaw has been removed, the remaining wall thickness can be measured to determine whether it is still sufficient to retain the hazardous contents, without further repair welding. For weld repairs, appropriately qualified and experienced welding personnel are essential, since when making weld repairs, the potential for defects to be introduced may be greater than when the item was new. TWI often provides advice on suitable repair welding procedures. Whenever repairs or modifications are made, it is important to document the work carried out. This may affect future inspection requirements, and the period in which the repair or modification may remain in service.
Revalidation
Repair is not necessarily the most cost effective method of dealing with damage to equipment. Consideration can also be given to the possibility of revalidation to justify changes to the operating conditions of the equipment, which can then preclude the need for any repair. Examples where revalidation for changes of service conditions or equipment modifications are particularly critical are: the operation of ferritic steel equipment at lower temperatures where the material might show a significant drop in fracture toughness; changes to the contents or environment such that unforeseen damage mechanisms could occur (eg stress corrosion cracking); or physical modifications that would limit access to critical locations for inspection.
When to determine the end of equipment life
Equipment will not be able to continue in service forever, and ultimately will need to be scrapped or replaced. The decision for when this will occur is often based on financial considerations, when the cost of ongoing operation,repairs and revalidation, along with the combination of risk and consequences of failure, becomes greater than the cost of replacement.
Getting organised for managing ageing
The management of ageing equipment requires knowledge and expertise across a wide range of disciplines. The competencies required include:
- Awareness of safety codes and regulations
- Pressure/chemical plant engineering
- Familiarity with the equipment concerned
- Design and construction codes
- Metallurgy and corrosion
- Inspection, NDT and maintenance
- Structural integrity
- Welding engineering
- Management, teamwork and organising
- Communication skills
Not all these will be found in one person. Instead, a team of people possessing these skills is required, be it all in-house, or through selected sub-contractors. For example, TWI Industrial Members can exploit TWI's expertise on many of these subjects.
The guide covers much more on the topic of 'getting organised', but the principal issues are covered by the following questions:
Who is responsible?
Responsibility may be shared by more than one person. For pressure equipment, the Pressure Systems Safety Regulations Approved Code of Practise defines responsibilities for the 'Competent Person' and 'Duty Holder'. If applicable,make sure you understand these terms, and know who fills these (or equivalent) roles.
What competencies do they have?
Competence is both having and demonstrating the necessary knowledge, skills and experience to do a particular task within a specific context. How are individuals' competencies demonstrated?
Is there a company culture of problem awareness?
The person who notices the problems may not be the one who understands their implications. A culture of communication, and shared responsibility, without blame, can be effective.
What is known about the history of the equipment?
Who knows about it and how is information transferred or passed down? A lack of knowledge about the equipment's condition is a risk factor for ageing.
When will the equipment's design life end?
Has an end-of-life date been set and what strategies will be used to manage the equipment up until, and possibly beyond this date?
Human factors
Frequently the people who are responsible for equipment containing hazardous fluid or pressure are the most critical link in its safety management. For equipment entering the 'Ageing' stage of life, often those responsible for its installation may have retired or moved on and its safety and structural integrity could be managed by those without first-hand knowledge of the equipment's full history. A serious risk is that a lifetime of knowledge about pieces of equipment can be lost, and careful succession planning strategies need to be considered to transfer and retain this precious history.
It is also harder to recruit skilled workers, as there is a declining source of engineering skills from traditional UK industries (such as mining or shipbuilding). Many young recruits will be graduates who may lack industrial experience. There is an increasing need to train staff on the job to give them the skills needed. Experience can be transferred both informally, by mentoring schemes between older and younger staff, or formally through training courses. The training school at TWI offers a range of welding, inspection and integrity related training courses, and can also offer bespoke training courses either at TWI or at the client's location.
Knowledge management
Documents and records also need good management. The principal tool for this is the equipment inventory, so you know exactly what equipment you have got. Good inventory control will also record any modifications and repairs. This,in combination with a complete set of inspection and maintenance records, can be invaluable. Skilled interpretation of the data stored in these records allows trends to be identified, and hopefully, future problems to be averted. The transfer of paper records onto computer systems should be done with caution, since cost savings incurred by reducing the amount of data stored may be at odds with the need for detailed records. Equipment records can also be lost when changing ownership or maintenance contracts. Once information has been lost, it cannot be recovered.
The plant ageing guide - a summary
The Plant Ageing guide: Management of Equipment Containing Hazardous Fluid or Pressure was launched on 1st November 2006 with a seminar at the IMechE in London, and the guide is now publicly available from the HSE. The purpose of the guide is to increase awareness of the factors to consider about managing the integrity of ageing equipment, and to help those responsible to understand and assess the risks of accumulated damage and deterioration. The guide also contains a number of other features, including:
- Audit tool providing a list of questions that HSE inspectors and others can use to assess the management of ageing equipment.
- Glossary of terms and definitions.
- Bibliography of useful references and websites.
- Descriptions of the available non-destructive testing and inspection techniques, along with their applications and limitations.
- Lists of indicators (symptoms) of ageing to look for, risk factors that might promote ageing, and damage mechanisms that could be occurring.
- Set of Case Studies illustrating many aspects covered in the guide.
Ageing equipment will continue to be an issue for TWI Member companies. This guide will help develop a strategy to manage ageing equipment safely. Where further expertise is required, TWI is available to help.
Acknowledgement
The work presented in this article has been sponsored by the UK Health and Safety Executive. Thanks also to Neil Henry of ABB Engineering Services, Shaun Smalley of SCS (INTL) Ltd, Glyn Amphlett of Allianz Cornhill Engineering and Harry Bainbridge of HSE for their assistance in compiling the Guide.
