It's a depressing fact of life that we all age, and as We age we tend to deteriorate through general wear and tear. This is just as true for chemical plants as it is for people. 'Plants are getting older and they're not being replaced, by and large,' says Martin Goose, a former process safety specialist at the UK Health and Safety Executive(HSE) and now consultant and adviser to the Institution of Chemical Engineers (IChemE). 'Other than the new LNG (liquefied natural gas)import terminals and associated plant, I can't think of any really major new chemical plant built in the UK for several decades.'

Over the past few years, the HSE has become increasingly concernedabout the ageing of the UK's industrial plants, releasing several reports on the issue. Its main concern is that plants can become more prone to accidents as they age and their infrastructure starts to degrade. An HSE report on ageing plants published in 2010 revealed that ageing was an important contributory factor in 96 major accidents occurring in European chemical plants between 1980 and 2006, representingalmost 30% of all major industrial accidents over that time.

Ageing plants

As plants age, pipework and reaction vessels corrode and crack, support structures crumble, mechanical components wear away, and electrical systems begin to fail, all of which could potentially lead to a major accident. The explosions and major fire that occurred at the Buncefield oil storage depot in Hemel Hempstead in 2005 have been blamed on a faulty level gauge and safety switch that allowed oil to overflow from a storage tank.

To make matters more complex, chemical plants all age differently,again like people, which makes defining an ageing plant quite difficult. Most chemical plants are designed to operate for around 25 years, meaning that most chemical plants in the UK are now operating beyond their original design life and can therefore be objectively classedas ageing. But just because a plant is old doesn't automatically mean it's in danger of failing or that an accident is imminent.

The classic ageing profile for a piece of mechanical equipment is time-dependent, with the chance of failure changing in a defined way over time. The chance of failure may rise steadily as the mechanical equipment ages or it might be highest at the beginning and end of theequipment's lifetime, forming the so-called bathtub curve. But this isn't what actually tends to happen in the real world.

'You think that plant equipment is going to fail linearly, but it doesn't,' says Mike Burrows, managing director of the UK maintenance monitoring company, Monition. 'Only 11% of failures occur in a time-dependent manner, 89% of all mechanical failures are random in nature.' As a consequence, it is very difficult to predict how long a piece of equipment will keep operating.

This was demonstrated by SKF Group, a Swedish industrial maintenance company, which measured the time to failure of 30 identical bearing elements placed under the same operating conditions. The company found that while some bearings failed within 15 hours, others lasted 10times longer or more, with one operating for 300 hours.

But it is not just inherent physical differences between seeminglyidentical bits of equipment that determines how long they last; it'salso differences in how they're used and managed. 'Some equipment, if properly managed, has an unlimited lifetime,' says Richard Gowland,technical director of the industry-funded European Process Safety Centre.

Much of the time, though, equipment is far from properly managed. 'Quite often, what we find is that probably 80% of failures are a result of operational conditions,' says Burrows. 'Most machines are stop-started under load, they may not be lubricated properly, they'll be tampered with, they'll have things dropped on them.'

This combination of physical and operational differences ensures that not all old plants are ageing and not all ageing plants are old; you can't define an ageing plant purely based on its age. As a consequence, the HSE has adopted the following definition for an ageing plant: 'Ageing equipment is equipment for which there is evidence or likelihood of significant deterioration and damage taking place since new, or for which there is insufficient information and knowledge available to know the extent to which this possibility exists.' The significance of deterioration and damage relates to the potential effect onthe equipment's functionality, availability, reliability and safety.Just because an item of equipment is old does not necessarily mean that it is significantly deteriorating and damaged. Ali types of equipment can be susceptible to ageing mechanisms.

So ageing plants are not necessarily old plants, but plants that are adversely affected by corrosion, wear and tear, and general deterioration. It is this deterioration that can put a plant at risk of equipment failure and accidents. Inevitably, the rate of deterioration can be very different. Chemical plants that work with naturally corrosive substances, such as acids or chlorine, are going to be more at risk of corrosion than those that work with more benign substances. External conditions, such as exposure to wind and rain, will also affectthe rate of deterioration.

Maintenance management

No matter how fast a plant deteriorates, however, it doesn't causea problem as long as that deterioration is well managed, which meansregular maintenance. But maintenance costs money and may involve shutting down parts of the plant. All of this adds to operational expenses, so that maintenance can account for as much as 40% of a plant's total operational budget. Not surprisingly, owners of chemical plants try to walk a fine line between keeping maintenance activities to a minimum while also ensuring that the plant operates safely.

'Maintenance is something that companies don't want to spend more money on than is necessary,' says Goose. According to a recent surveyconducted by the conference organiser easyFairs ahead of the UK maintenance conference MAINTEC 2012, while two-fifths of interviewed maintenance managers said their boards saw maintenance as an invaluable part of the business, one-fifth said their boards saw it as merely a 'necessary evil'.

Keeping maintenance activities to a minimum doesn't just make sense from a financial point of view, but also from a health and safety point of view. Maintaining a chemical plant can be rather risky, whichisn't surprising when you think it often involves working with pipesand reaction vessels that usually house hazardous or explosive materials. A study by the Belgian Maintenance Association found that maintenance technicians were more then twice as likely to suffer an accident resulting in disability than the average chemical company employee.

Historically, there have been two basic types of maintenance: reactive, which involves fixing equipment after a failure, and periodic, which involves conducting maintenance on some kind of fixed schedule.

Reactive maintenance can be very costly because it may require replacing an expensive piece of equipment and shutting down the plant while the equipment is being fixed or replaced. Periodic maintenance isdesigned to fix or replace equipment before it fails, therefore removing the need for reactive maintenance.

The downside with periodic maintenance is that it runs the risk ofreplacing equipment that doesn't need replacing. So a bearing designed to operate for 10 years will be replaced after 10 years, even if it is still working perfectly well. 'In the chemical industry, there'sa hell of a lot of maintenance done that is unnecessary,' says Burrows.

Now there is a third option: predictive maintenance. This utilisesa variety of non-destructive testing (N DT) technologies to monitor equipment and infrastructure to determine whether it is still in goodworking condition. These technologies allow maintenance managers to focus their maintenance activities on equipment that is faulty or close to failure. As a result, they can fix or replace equipment before it fails, while leaving equipment alone that is still operating perfectly well, even if it has exceeded its normal operational lifetime. In the long term, predictive maintenance can prove to be much cheaper than other forms of maintenance.

Studies have shown that refining and petrochemical plants that have adopted predictive maintenance have been able to reduce their maintenance budgets by up to 50%. Mike Burrows at Monition, which offers its predictive maintenance expertise to chemical plants around the world, says that they have also been able to cut maintenance costs in chemical plants by up to 50%.

As well as reducing costs, predictive maintenance can also help toimprove productivity, by both reducing the amount of time that a plant needs to be shut down and by helping to make equipment run more efficiently. According to the easyFairs survey, even in these tough economic times, 55% of maintenance managers were planning to increase their investment in predictive maintenance in 2012.

Non-destructive tests

The simplest form of NDT used in predictive maintenance is vision,albeit enhanced with cameras and microscopes. Tiny cameras and videocameras are sent down pipes and into reaction vessels to look for cracks and corrosion. Other NDT technologies can monitor the state of mechanical equipment and infrastructure without physically interactingwith it, allowing the equipment to be studied while still operating.

For example, ultrasonic analysis monitors the high frequency soundwaves released by mechanical equipment, which can be an indicator ofcomponent wear, fluid leaks and growing cracks. Vibration analysis monitors the vibrations in mechanical equipment generated by the movement of its component parts, with abnormal vibration indicating component wear. Thermal analysis monitors the heat generated by mechanical equipment and can thus be used to detect component wear, corrosion and electrical faults.

In addition, a variety of analytical techniques, including infra-red spectroscopy, can be used to analyse the oil used to lubricate mechanical equipment. Such analysis can determine the condition of the oil, ensuring that it is providing sufficient lubrication and doesn't contain a high level of contaminants. It can also help to determine the wear of the machine by measuring the concentration of metals, suchas iron and copper, released into the oil by wear.

These tests can be used as part of a regular monitoring programme to highlight equipment not working properly or in danger of failing. In this case, it's important to keep accurate long-term records of the analyses, so that it's easy to see if some mechanical equipment is releasing more heat or emitting a different range of sound frequencies than normal. 'If a plant has an adequate amount of inspection and tests done on it, as long as the records have been kept properly, thenyou do get a good idea of the trends,' says Goose.

Equally, the tests can be used to diagnose why a piece of equipment isn't working properly. Oil analysis, for example, will give early warning that some of the components in a piece of mechanical equipment are wearing, while abnormal vibration indicates that the componentsare closer to failure; and if they are generating lots of heat, thenthey are just about to fail. The tests can also reveal that the reason the components are wearing is because the machine isn't sufficiently lubricated.

Chemical plants tend to carry out their own regular tests, but call in a company, such as Monition, to determine why a piece of equipment isn't working properly or to do a more detailed analysis of the plant. Plants also often operate a combination of predictive and periodic maintenance, using the findings of predictive maintenance to alterthe schedule for periodic maintenance. So rather than service a piece of mechanical equipment every year, predictive maintenance may showthat it's perfectly safe to service it every two years.

In this way, although the chemical plants in developed world countries like the UK are getting older, they aren't necessarily ageing.

In brief

* The UK Health & Safety Executive reports that ageing infrastructure was a contributory factor in 96 major accidents occurring in European chemical plants between 1980 and 2006

* But chemical plants age differently and not necessarily on a predictable time-scale

* Whatever its cause, deterioration of mechanical components need not be a problem if the plant is regularly maintained

* Predictive maintenance, which uses a range of analytical tests, combined with routine maintenance can stop the ageing process

Jon Evans is a science writer based in Chichester, UK

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