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Growing old gracefully

SKF Microlog data collector/analyser
SKF Multilog on-line system DMx
SKF Machine Condition Advisor (MCA)
SKF Machine Condition Inspector (MCI)

2015 July 01, 09:00 GMT

Nothing lasts forever, but correct maintenance and other measures can help to extend the useful working life of a manufacturing facility.
The passage of time catches up with everybody – and everything. In manufacturing, factories are built to last, but even steel and concrete wear out.
The way in which plants age can vary, but it is typically seen through the same mechanisms: corrosion, stress cracking and erosion. Add fatigue, instrument drift and the effect of temperature fluctuation, and a shiny new facility – and the machinery within it – will soon begin to show its age. While this process is inevitable, it can be controlled, slowing the ageing process and extending the life of plant assets.
Just as a single machine has parts that age and wear out, so a factory contains machines that do the same. The challenge for maintenance and site engineers is to maintain maximum plant efficiency.
It should be pointed out that an ‘ageing’ plant is not necessarily ‘old’. The Health & Safety Executive (HSE) says: “Ageing is not about how old your equipment is; it is about its condition, and how that is changing over time. Ageing is the effect whereby a component suffers some form of material deterioration and damage – usually associated with time in service – with the increasing likelihood of failure over the lifetime.”
When considered in this way, it means that the effect can be counteracted. Plant engineers are not simply facing the march of time – but fighting the onset of ‘apparent’ age. Just as humans are able to extend their lives by healthy eating and moderate exercise, so a manufacturing plant can maintain efficiency through careful treatment.
“There are many examples of very old plant remaining fully fit for purpose, and of recent plant showing evidence of accelerated or early ageing – due to corrosion, fatigue or erosion failures,” it says.
What rots?
There are four main types of asset that are subject to ageing: primary containment systems, such as tanks and pipework; process safeguards, such as pressure relief valves or overflows; electrical control and instrumentation (EC&I) systems, such as leak detection or CCTV monitoring; and structures, including safe work areas and access routes like ladders and gantries.
The vast majority of ageing mechanisms are due to physical deterioration – such as metal corrosion, material fatigue, weathering (including expansion/contraction due to changes in temperature) and stress cracking. There are a few others that relate to electrical equipment, such as instrument drift and detector poisoning. Between them, these and other mechanisms can cause potential failure in just about every component found across the plant. 
Corrosion is a typical example. It can take many forms – general, localised or pitting corrosion, for example. It also has a number of causes, from the simple action of moisture through ‘sweet’ corrosion (caused by carbon dioxide dissolved in water), ‘sour’ corrosion (hydrogen sulphide solution) and even microbial corrosion.
In this case, the signs of corrosion are well-known and are an obvious sign of ageing. However, it should be noted that a rusty surface does not indicate imminent failure of a part.
“Rust is merely a sign that the equipment is ageing,” says HSE. “The rate of this ageing process and its importance in risk terms are parameters the plant operator should be concerned with.”
Corrosion can be prevented – through measures like coatings or cathodic protection. Also, susceptible components can be monitored and controlled through four processes: identification (of components at risk); detection (locating the corrosion); quantification (how serious is the corrosion); and, finally, assessment (of implications for equipment integrity).
Under stress
There are also lesser known forms of corrosion – which are harder to spot and therefore more likely to cause irreparable damage to systems. Stress corrosion cracking (SCC), for example, can be caused by high chloride or sour (hydrogen sulphide-rich) environments, in combination with physical stress.
SCC is very hard to detect without the help of non-destructive testing (NDT). Thankfully, there are some indicators for SCC: pitting corrosion at welds in corrosion resistance alloys is often a precursor to SCC, as cracks begin at the bottom of pits. Also, these materials may exhibit rusty staining with no apparent source. It has actually come from within a tight stress corrosion crack.
SCC can affect a wide variety of alloys, but can be mitigated at the design stage by ensuring the lowest possible stress levels. Because it is unpredictable, SCC is very difficult to monitor and control. If found, it should be repaired – or measures taken to avoid the consequences of failure, says HSE.
Machine health
Physical ageing, such as rusting, is most relevant for a factory’s infrastructure. But in a typical engineering facility, the most important assets are the machine as these make the end products and consequently generate the profit. A rusty, leaking pipe may take some time and money to fix; but a machine out of service can stop the production line – which is far more serious.
For this reason, plant managers need to keep a close eye on machine health. Condition-based maintenance (CBM) can be an effective, efficient way of keeping tabs on machinery in the fight against ageing – eliminating the need to use time-based methods for routine service tasks.
Traditional maintenance is often carried out at set intervals, regardless of the actual condition of machinery. During this process, engineers often find that there is no need to change lubricants, seals or other consumable items. If operating conditions change, it can also lead to parts failing before a scheduled service.
By monitoring the operation of critical systems in real time, CBM can identify potential wear or faults in moving components as they develop – and can schedule service or repair intervals at a time that best suits the operational capacity of the plant.
Monitoring tools typically measure changes in vibration – especially on rotating shafts – or changes in operating temperatures in both mechanical and electrical systems. In each case, handheld units with either remote probes or permanently fixed sensors can be used. The advantage of permanent installation is that readings are taken continuously, and provide true preventative maintenance in real time. This is where many advances in technology are being made.
Assessing assets
CBM is an important element of improved asset management – the way in which elements of the plant can be assessed.
For larger, more critical assets – where safety implications, production interruptions, difficult or dangerous access, and the costs of failure are significant – permanent online monitoring is more suitable and reliable than handheld data collection. Typically, these applications include process segments such as pulp and paper, steel, mining, petrochemicals and energy supply.
In some instances, permanent on-line systems are used in tandem with handheld or periodic data collection instruments, giving a more holistic approach to reliability and productivity, with round-the-clock machinery monitoring. For equipment based in harsh, remote or unsafe locations – such as nuclear power plants or water treatment works – an on-line system is a necessity, providing the most up-to-date asset data and analysis.
Data itself is gathered and transmitted via permanent sensors, which can be hardwired to junction boxes or connected wirelessly. In each case, data is normally routed to a centralised computer system running an advanced management and data analysis tool.
On-line systems are usually configured to provide data states that can be defined as Surveillance – providing up-to-the-minute information – or Protection, which helps owner-operators optimise the longer term operation of key production assets.
Surveillance monitoring systems are often configured for periodic automated data collection using multi-channel, wireless or a series of single channel transmitters. They communicate directly with a software package or directly with a PLC or DCS for trending, alarm and machine shutdown, to improve machine reliability and increase personnel safety. Typical applications are motors, gearboxes, cooling towers, ID & SD fans, boiler feed water pumps, and reciprocating compressors.
Protection systems, meanwhile, are classified as real time systems, and remotely monitor and protect critical machinery. Their overall purpose is to provide rapid and safe machine shutdown capabilities rather than periodic data collection.
These would normally be used to support and monitor high speed machines and the type of critical assets found in, for example, the power generation or petrochemicals industries. Installation and configuration is identical to that for surveillance systems.
Degradation of plant and equipment through well-recognised ageing mechanisms is an important challenge for industry to solve. But by monitoring and controlling these processes, plant managers can ensure that they keep ahead of these detrimental effects – and help the plant to avoid the problems normally associated with old age.
This article was originally published in the July 2015 edition of Maintenance and Engineering.

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