What’s your standby philosophy?
Adopt one that delivers maximum reliability.
Equipment, machinery and other physical assets are primary revenue generators, which is why there are standard practices for minimizing the exposure to risk associated with equipment or system failure. This is especially important for rotating equipment.
Christopher Gaspard, the reliability practices lead of the Asset Performance Group – a consultancy based in Burlington, Ont. (with an office in Calgary) covered redundancy in a presentation to maintenance professionals.
“A duty/standby operating philosophy is integral to achieving a plant’s asset management goals. It specifies operational modes for plant equipment in redundant configurations,” he explained.
There are essentially two philosophies: 100:0 (with variances of different ratios) and the 50:50. In this context, “duty” means the main operating unit (motor, pump, fan) and “standby” means the redundant, or second replacement unit.
Gaspard noted rotating equipment is designed for long, uninterrupted service. Nevertheless, the following factors influence effectiveness when running duty and standby machines:
• Switching equipment “on” introduces more failure modes.
• Shafts and seals undergo increased wear during start and stop phases.
• There’s an increased likelihood for multiple failures when duty and standby run parallel.
• Start-up procedures are often complicated, therefore it’s deemed convenient to keep a standby running once it’s started.
• Human intervention introduces more failure modes.
The 100:0 philosophy means that the duty machine is run 100% on run-to-failure and that the standby is only switched on after failure occurs. The rationale is often “that’s what we have always done.” The spare is a guarantee only in the event that the duty machine fails.
The 50:50 philosophy provides an alternative: half of the time the duty is run, and half of the time the standby is run. Alternating the two means that assets accumulate only 50% of total system run time, and it’s good to know both duty and standby work when needed. Also, wear and tear is distributed over both machines, multiplying the effective system lifetime; plus defects are worked out during the warranty period.
Is there an ideal duty/standby practice? The answer depends on five important factors:
• Are there hidden failures of the standby equipment?
• What are the inspection and testing intervals?
• What are the established start-up and shutdown procedures?
• What’s the process fluid or material residence?
• What are the scheduled maintenance tasks?
Adopt a duty/standby philosophy that works for you; apply it consistently; track equipment deficiencies; and optimize the switching ratio to yield significant benefits.
This article is a synopsis of a presentation made at the MainTrain maintenance, reliability and asset management conference, convened by the Plant Engineering and Maintenance Association of Canada (PEMAC).