Diagnostics and digital systems: Employing predictive maintenance saves money

Businesses are created for the purpose of generating profits. As such, all investments need to have a positive return to be justified. The incentive or driver for that justification is different at each stage of a project’s life cycle.

For example, during the construction phase when the project is spending capital, the incentive is to complete the project as quickly as possible for as little money as possible so that production can start and the company can begin to recoup some of its money. Diagnostics are less important during most of this phase, except during commissioning and startup when digital communications significantly reduce the time required for loop check-out and device calibration. Since the instrumentation and control system is one of the last parts of the project to be installed, any time savings here will result in the unit starting up sooner, signifying the transition to the operating phase of the project life cycle.

Because a typical facility (or at least the control system portion of it) has a life cycle of approximately 20 years or more, operational expenditure savings represent the majority of the life cycle costs.

The largest impact to positive cash flow in any facility is lost production, and the most expensive form of lost production is an unscheduled outage. Facilities rely on instrumentation and control systems to warn them of deviations from normal or desired operations so that they can take appropriate preventive actions. To keep the equipment in a facility operating, operators perform maintenance, which can get expensive. In a typical facility, 40 per cent of manufacturing costs are spent on maintenance. Any opportunity to reduce that expense without impacting reliability can be directly added to the bottom line as profit.

Research by the ARC Advisory Group has shown that 50 per cent of maintenance is corrective. Because this form of maintenance is typically the result of breakdowns, it cannot be properly planned for and therefore is almost always the most expensive way of doing business. There are some exceptions for certain types of equipment where running to failure makes the most economic sense. Generally, though, corrective maintenance is 10 times more costly than preventive maintenance and, therefore, almost every facility has some form of preventive maintenance program.

Preventive maintenance really means scheduled maintenance–every piece of equipment gets checked at predetermined intervals to be sure that it is operating within defined tolerances. If it has drifted toward a limit, it is repaired and calibrated. If the change is not significant, it is left alone until the next scheduled look.

Fortunately (or unfortunately) typically 60 per cent of the time there is nothing that needs to be done to the device. In some cases, making a change can have a negative impact on device reliability because of the associated wear and potential for early failure. However, because preventive maintenance is easy to set up and provides such a benefit when compared to breakdown maintenance, it is used 25 per cent of the time. The new diagnostic capabilities of today’s digital systems make predictive maintenance possible. Predictive maintenance uses the ability of the devices to continuously monitor themselves with self-checking algorithms to detect any deviations from predetermined limits. This data is then communicated over the network to a central location, typically a server or other computer, where more powerful software can use the information to accurately predict how much run life is left in a piece of equipment.

This allows the equipment to run for the maximum time possible with minimal impact to the operation of the facility. The ARC data reveals that predictive maintenance is five times less costly than preventive maintenance practices. Figure 1 shows the relationship in savings between the different forms of maintenance activity.

The same reasoning applies not only to the purchase and installation of digital field devices and control systems, but also to the use of network diagnostic tools. If the network is unable to communicate the information between the field and the control system, the system will not be able to control the process.

In the next issue, we’ll talk about the importance of standards and how you can make a difference in their development.