4th September 2009
Written by: Neil Croft
Published in: Speciality Chemicals Magazine, Process Industry Informer
Mechanical Integrity - A Case of Too Little, Too Late?
Would you sail on a twenty year-old ship if you knew that it hadn’t been surveyed in its life, simply because it was built for the environment it was designed to operate in? Or would you fly in an aircraft if you knew it never underwent routine inspection? Our cars are designed and manufactured to transport us from A to B, yet we still service them regularly and put them through
In recent years there has been, arguably, an almost unhealthy shift towards the suitability of Secondary and Tertiary containment and other mitigation issues rather than a focus on Primary asset integrity. This shift in emphasis, led to a large extent by the various reports coming out of the Buncefield investigation, has made operators focus their dwindling resources on whether or not they can mitigate the likely consequences of an event, rather than what they need to do in the first place to prevent an event’s occurrence.
All too often chemical manufacturing companies believe that because the materials of containment were chosen on account of their suitability for their contents (be they liquid or gas) and operating environment during the plant’s design, degradation is not an issue.
The current economic situation has meant that an increasing number of chemical companies have been forced to cut back on investment and personnel with the inevitable negative impact on routine maintenance and inspection. At the same time even more pressure is being placed on existing machinery and equipment to deliver competitively-priced products in a declining marketplace. It is therefore more vital than ever that chemical companies are able to demonstrate the mechanical integrity of their ageing plant. Indeed the HSE has recognised the issues surrounding plant ageing and is placing increased emphasis on the subject.
At this juncture it is perhaps worth highlighting that the term ‘ageing’ in this context relates to the accumulation of damage and the increasing likelihood of failure over time rather than the physical age of the plant. Ageing is often associated with time in service, but this is not always the case. There is no defined time for the onset of ageing – even new plant and equipment can exhibit signs of ageing if the design is not suited to the process or the environmental conditions. The actual chronological age of an item is potentially the least important factor. How often have we waited for a train, only to be told it’s either delayed or cancelled because it has suffered a mechanical failure? Now contrast this with the number of times you had to wait for a 50+ year-old stream train to arrive on any of many heritage railways operated by volunteers?
“But inspection costs money!”, I am often told, with operators complaining that the expense is not just the actual cost of the work, but also the loss of availability of the asset should internal inspection be necessary. More often than not they claim to find no defects. So why undertake routine inspection? The answer is simple. We should not forget that loss of containment due to lack of mechanical integrity can cost companies an order of magnitude more than the cost of the inspection itself, as the BP Grangemouth Petrochemicals Complex in Falkirk found out following a major accident in June 2000. The organisation was fined £750,000 when a redundant 3 inch branch to the main transfer line between two distillation columns failed during start-up in the Fluidised Catalyst Cracking Unit. The result was the release of 13 tonnes of highly flammable Naphtha which subsequently ignited and caused considerable fire damage to the plant. In addition, asbestos contamination occurred due to damage to vessel and pipework insulation.
What caused the branch to fail? - progressive vibration fatigue as a result of a combination of inadequate support and a prolonged period of service which saw a greater number of start-ups than was normal, putting increased pressure on the system. Could this have been prevented? Even allowing for hindsight, the answer is ‘most probably’, had a systematic degradation study been conducted. Vibration fatigue and failure of ‘small bore’ tees is well understood and it is highly likely that such locations would have been highlighted for thorough inspection.
Statistically speaking, you are most likely to suffer hazardous substance release via piping or pipework elements. It’s perhaps surprising to note, then, that these systems are more often than not excluded from inspection under insurance-based Written Schemes of Examination (WSE) - even when a risk assessment has been undertaken. As a result it can be several months or years (if ever) before the piping and pipework systems are inspected, by which time deterioration can have set in and, depending on the substances conveyed, the risk of release increased. Incidentally it is also worth bearing in mind that even if the substance contained does not pose any immediate threat in itself (e.g. water) the effect of its release on adjacent equipment could be a different story altogether.
Common causes of loss of containment in pipework include the failure of supports, leakages at bolted flanged joints, corrosion under lagging, steam trap failure, failure of vulnerable in-line items, over pressure, modifications where the parts or materials specified are unsuitable or not as per the design intent, leakage where the pipes meet other instrumentation and leakage brought on by fatigue, vibration, or loss of insulation alongside the more obvious failures such as loose fittings, erosion and mechanical damage.
The introduction of the Pressure System Safety Regulations (PSSR) in 2000 do go some way to guarding against major accidents, but they are principally concerned with the risk of release of stored energy through system failure. In addition those sites that operate under the COMAH Regulations have a general duty (Regulation 4) to ensure all necessary measures to prevent major accidents and limit their consequence, although the regulations make no specific reference to pipework. It is likely, therefore, that on the majority of UK chemical sites no pipework inspection is taking place.
Of greater concern is that, on a large number of installations, inspection of vessels and process equipment that doesn’t fall under PSSR is not taking place at all. As a consequence, if nothing is done, this lack of inspection will in all likelihood translate into numerous loss of containment incidents. Should a tank containing toxic material fail, it is likely kill, injure or cause significant damage to the environment. But what is the likelihood of this happening?
To answer this question Mechanical Integrity Programmes (MIP) should be devised and implemented to address the issue of inspection. Companies often fall into the trap of assuming that they already undertake an effective inspection and maintenance programme. However more often than not they are doing sufficient to comply with regulations such as PSSR, but are falling short in other areas not covered by specific legislation. Knowing what, when, where and how plant should be inspected is the key to an effective MIP, the core principles of which should be:
- A system which is tailored to the process fluids and materials of construction in use
- A system designed to ensure that all foreseeable risks/failure modes are identified, assessed and - where necessary- maintenance plans developed
- A programme which is sustainable in terms of resource and competency
- A system robust enough to reassure the operator and regulator that the asset remains safe to operate
The first step to assuring mechanical integrity isn’t to rush out and start removing lagging, scaffolding tanks or undertaking expensive NDT. It is much more basic (and less expensive.) In a nutshell, you need to ensure that you fully understand what your physical assets are. In summary the first steps should be:
- Updating the plant’s Piping & Instrumentation Diagrams (P&IDs) to ‘as-built’ status
- Identifying all assets from the up-to-date P&IDs
- Comparing the asset list with the P&ID asset register and identifying errors
- Generating an accurate plant asset register
The next step is to identify those assets that are safety critical, i.e. which equipment within the plant will have the greatest overall impact were it to fail. Information from other process hazard assessments should be reviewed as part of this process and, where appropriate, their findings used to assist in the setting of inspection priority. If effective Layers of Protection Analysis has not been undertaken and appropriate steps taken to manage containment, relatively small leakages of chemicals such as chlorine can be potentially lethal, as can the release of combustible hydrocarbons.
Once identified, safety critical assets should be highlighted on the corrected asset listing and the necessary inspection plans be developed and scheduled within the site’s computerised maintenance management system (CMMS). The last element is to complete the development of a detailed site piping register. At the completion of the process, the operator is significantly nearer to being able to answer the ‘what’ they need to inspect question however, the identification of safety critical items is only half the story.
The next step is to tackle the degradation issues and assess the probability of a loss of containment incident occurring as a result.
Pipes and pipework have already been cited as common sites for potential release events, however almost every element of the plant’s equipment is susceptible to deterioration or damage. For example, storage tanks can suffer from either internal or external corrosion due to environmental factors such as damp or sea air. This can cause the walls to become thin and leaves them open to the increased possibility of cracking. Thin walls can also relent on account of mechanical damage such as denting.
We have already touched on the PSSR 2000 Regulations. By their very nature, boilers and other pressure vessels (including natural gas and LPG under compression) are already highly regulated and are prone to the same types of degradation as storage tanks and vessels i.e. corrosion, wall thinning, vibration and other stress-related damage. However, plant operators should also be looking beyond the boundaries of specific regulations to ensure mechanical integrity. The substances contained within the vessels (e.g. ammonia) can be responsible for corrosion, especially if these are variable. Other factors impacting on the mechanical integrity can include pressure cycles, changes in temperature, batch cycling and even cleaning.
Similarly, the very devices that are designed to protect equipment against overpressure are not immune to ageing in the form of condensation, fouling and calibration inaccuracies, so should always form part of an inspection regime. Operators and maintenance personnel should constantly be vigilant for visible signs of degradation and be made aware of the consequences of turning a blind eye to seemingly insignificant faults.
Assessing plant and equipment degradation can be time consuming and laborious, however the process is essential and should be undertaken by a team of competent and experienced engineers and operations personnel. During the degradation workshop process the team needs to include a review of all Loss of Containment events, however small, and their root causes. In addition, the site’s current process safety management systems should also be put under the microscope. The outcome of this work should be the development of detailed Written Schemes of Examination covering all the pipework systems in use across the site (both PSSR and non-PSSR).
At this point the operator should be in a position to answer the ‘What’, ‘When’ and ‘Where’ questions for each identified system. This leaves the question of ‘How?’
‘How?’ should be tackled in the final phase, during which the Mechanical Integrity Programme should be concerned with the creation of effective inspection and work plans. If the site has no previous data then the first tasks should be to generate baseline inspection reports. The findings of these initial surveys (which can be as simple as straightforward external visual inspections) should be used to reassess the WSE developed in the previous steps. At this stage it may also be appropriate to look at the development of necessary emergency procedures in the event of a major incident.
In cases where the degradation mechanisms are understood and the inspection and maintenance strategies are matched to the life stage of the equipment, there is no reason why such equipment can not be stretched well beyond its planned retirement date.
Responsibility for safe operation ultimately rests with the senior decision makers in the company. Those in charge must demonstrate that all risks are, and will continue to remain, as low as reasonably practicable and an effective Mechanical Integrity Programme should help to ensure that this is the case.
Neil Croft is Head of Safety Engineering at HFL Risk Services Ltd. For further information please call 0161 304 5902 or visit www.hflrisk.com
