Condition monitoring for SAFETY-CRITICAL SYSTEMS

The key to choosing the correct technology lies in Failure Mode and Effects Analysis (FMEA), a tool that enables the identification and prevention of process or product errors before they occur.

BY: SIMON FRIDLYAND

Atime-based approach to maintenance is appropriate for only 11% of the assets in a typical plant. For many components, the most likely time for failure to occur is immediately after maintenance has been performed -- either because the existing or replacement parts have hidden defects, or because the repair work was performed improperly.

Certain items, such as brake shoes, chains and sprockets wear at a rate that is strongly correlated to usage hours, but other items -- most notably electronic instruments -- do not behave that way at all.

In a recent project, we were involved with the design of an HVOF (High Velocity Oxy-Fuel) control system, part of a thermal spray process. The process consists of a robot that is positioned in a booth. The booth is equipped with several interlocked doors and an exhaust and make-up ventilation system. The fuel is a mixture of hydrogen and oxygen. Nitrogen is used as well for purging processes. In other words, this process is quite dangerous from the points of view of fire, explosion and robot safety. It relies heavily on electronic instruments, such as various sensors and controllers, to run the process.

Should the electronic instruments fail, the following conditions may take place:

• Substantial or total process interruption

• Significant safety risks, such as a fire, toxic leaks or an explosion

• Significant repair costs.

Clearly, the costs and repercussions of failures on such assets dictate continuous monitoring and multiple conditionmonitoring technologies.

The key to choosing the correct technology lies in Failure Mode and Effects Analysis (FMEA) studies, where the types of failures are categorized and the failure mode is ascertained.

FMEA is a proactive tool, technique and quality method that enables the identification and prevention of process or product errors before they occur. The FMEA process recommends the hazard assessment technique described in the accompanying tables (Severity, Probability and Detectability).

FMEA can help identify and eliminate concerns early in the development of a process. It is a systematic way to examine a process prospectively for possible ways in which failure can occur, and then to redesign the processes so that the new model eliminates the possibility of failure.

Using FMEA techniques, a qualified designer will be able to choose control-critical electronic components that will not fail. This is very important for systems like the HVOF control system.

Simon Fridlyand, P. Eng., is president of S. A. F. E. Engineering Inc., a Toronto-based company specializing in industrial health and safety issues and PSR compliance. He can be reached 416-447-9757 or simonf@safeengineering.ca.

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