The power of prediction

Modern instrumentation and control systems are allowing many users in the process industries to adopt true, predictive maintenance regimes for the first time. However, says Mark Allinson of ABB, this strategy can only work if the frontline instruments are properly applied and maintained

These days, maintenance is increasingly about heading off trouble before it starts. This predictive approach is possible due largely to the advent of more intelligent control systems and instrumentation. Predictive maintenance is not just about preventing catastrophic failures. Across the entire range of process industries, where product specifications are extremely tight and the pressure to minimise operating costs enormous, the ability to spot early signs of deteriorating performance or outof- specification products can make the difference between profit and loss.

Controlling and measuring process variables is just one part of the picture, however. The condition of the field instruments and their ability to provide accurate and repeatable measurements will also have a bearing on plant performance and product quality. For this reason, it is advisable to use systems that not only gather as much information as possible from the process, but which can monitor the status of field instruments and, if necessary, flag up the need for maintenance.

This is where fieldbus technology comes into its own. Fieldbus is a generic term that refers to a single-cable network that can replace conventional means of transporting data between field devices and a control system, such as the traditional mA loop. A digital fieldbus provides communication between industrial control systems and field instruments.

Today's smart field instruments contain valuable status and performance information that cannot be accessed via an analogue link. When a fieldbus is used, however, these variables can be retrieved and a whole new world of possibilities opens up for predictive maintenance, advanced diagnostics, asset optimisation and management.

For example, in an analogue architecture, device failure can, at best, manifest itself as a 'limit violation' alarm somewhere in the process, which could either show a true process excursion or an instrument failure.

Fieldbus status signals allow operators to immediately identify if the alarm is due to a malfunctioning device, and obtain an indication of the cause.

However smart your controls are, any preventive maintenance will be meaningless unless the readings coming back from the field instruments are reliable. Choosing the right devices, installing them correctly and treating them with respect is essential.

Calibration is a good example. Many pressure transmitter vendors claim their products require recalibration only every five years, but the figures quoted are based on a specific set of conditions that may have little to do with conditions on site.

Top tips for maintenance Different types of instrument will require the user to make different kinds of periodic checks and adjustments, but there are some general rules worth bearing in mind to help keep the need for field instrument maintenance to a minimum.

Always try to install instrumentation for easy access, so it can be cleaned, calibrated, repaired or replaced as easily as possible.

Install instruments in a suitable environment. Factors such as high temperatures and humidity will often shorten their working life.

Be on the lookout for low-maintenance options. Some types of equipment are designed with built-in protection against arduous environments. For example, it may be worth opting for a type of electronic transmitter which is 'fully-potted', meaning that the unit is completely encapsulated in, for example, epoxy resin. This makes the transmitter less vulnerable to vibration, contamination ingress and changes in ambient temperature that can cause drift in some other models.

Another key maintenance tip is to conduct regular performance surveys of your plant.

Checking the performance of compressed air plant and in particular keeping an eye out for leaks, for example, has been estimated to typically help UK companies to save 30% in energy costs.

As more companies move towards predictive maintenance, advanced instrumentation and control systems provide the early warning systems that enable users to apply automatic condition monitoring to many production lines for the first time.

Investing in this technology makes sense if you consider the potential cost of unplanned downtime if things go wrong. However, it's vital to protect your investment by ensuring that your planned maintenance programme includes the instruments themselves.

ABB offers a range of high quality instrumentation to help deliver top levels of plant performance, as well as highly trained personnel to help maintain it. Specialist sales and application engineers can guide you through the options to ensure you get a suitable instrument for the job in hand.