Good Vibrations

After my previous blog post on the NERC-CIP Plant Tour, colleagues asked questions about  the systems mentioned. One of the questions that took some time to answer, and required a lot of explanation, was regarding vibration monitoring systems, specifically the Bently system. While the term “vibration monitoring” seems to be self defining, there are some nuances to be aware of. Bently systems measure vibration in the rotating shaft through a set of very accurate eddy current sensors, super advanced versions of the road loops that detect your car at a stop light. This allows the Bently to give a much more accurate measurement of vibration, and to also measure it directly at the shaft rather than by inferring it from other sources.

In broad strokes, the question was “Is Vibration Monitoring a critical piece of plant operation?” Basically, my colleague was unsure of exactly how the vibration monitoring system he was looking at fit into the whole control scheme at the plant. My answer was, of course, the usual engineer answer of “It depends”, which I’m aware is thoroughly frustrating to the supposedly standardized world of information security.

The reason the Vibration Monitoring question ‘depends’ is because of the varied implementation of vibration monitoring schemes in use today, the different types and criticality of mechanical equipment, how it is wired into the control system, and how operations staff take cues from the vibration monitoring system. I’ve been to some plants that won’t start up without a functioning vibe, and I’ve been to one that muted the speakers on the vibration HMI and stuck to their existing readouts. Sometimes, vibration monitoring is used as a planning tool, data accessed on a quarterly basis to determine wear on the turbine, and then forgotten until the next quarterly review comes along.

So, when might vibration monitoring be critical?

First, the more modern your plant, the more likely the vibration monitoring system is critical. For example, GE 7FA turbines now come fully integrated with a set of Bently sensors, logic, and alarms within their console, and can be configured for automatic throttle back and safe shutdown when levels of vibration are encountered during operation. Earlier installations of GE turbine were fitted with the Bently sensors as part of a protection scheme, and even earlier ones were retrofitted into place as a standalone system to monitor performance related to vibration in a historical manner.

Second, the more often your plant starts up and shuts down in any given time period (referred to as ‘cycling’ in the business), the more likely that vibration monitoring plays a critical role. Startups and shutdowns take a heavy toll on equipment due to thermal processes beyond the scope of this blog post. The best analogy I’ve found is the Centennial Light, a light bulb that is claimed to be in near continuous operation for 110 years. This bulb has lasted this long due to few heating and cooling cycles that wear down the element. This relationship is analogous to wear and tear on power generation equipment, and excessive wear and tear shows up as increased vibration during startup and shutdown. Hence, monitoring vibration allows you to monitor the health of your equipment, and justify proactive action.

Third, if vibration monitoring is treated as an equipment protection mechanism, it’s is overwhelmingly critical. Because the sensors are being used to monitor for conditions which would necessitate the fast shutdown of operations, interference with those protective mechanisms can have reliability consequences. Intentional reconfiguration of those settings would be even worse, as you could force an outage by imitating a high vibration signal, or potentially damage equipment (or have it taken out of service by policy) by squelching the excessive vibration signal.

As a rule of thumb, if you can trace any outputs from a vibration monitoring device back to the larger control system, it is likely critical. If those outputs are used in shutdown, startup, or steady state operation logic, it is overwhelmingly critical. And if the vibration system is wired directly into hardwired trip points, it is definitely critical.

If you have any other questions regarding the criticality of vibration monitoring systems, please post them here.

title image by badgreeb RECORDS