Transient Analysis Spots Turbomachinery Issues

by | Feb 8, 2011 | Asset Management


The other night I was talking to some engineering students before a college basketball game at one of Austin’s famous watering holes. They were complaining about all the mind-bending math they were having to endure. Time domain/frequency domain, Fourier analysis–that kind of fun stuff.

What reminded me of this was an article I discovered by Emerson’s Deane Horne, Analyzing transient events, from Turbomachinery International magazine. Rotating equipment has natural resonant frequencies through which the equipment has to pass on startup, shutdown, and sometimes even some operating ranges. Some of this mind-bending, Fourier analysis can help determine these frequencies.

In the article, Deane notes that high-speed turbines present a special set of challenges since they rarely operate in steady-state conditions. He writes:

Loads, pressures, temperatures and vibration are changing rapidly, especially during startups, shutdowns and “bumps” in the night.

It’s during these transitions or “transient events” where much can be learned about the health and performance of this turbomachinery, especially the bearings. The challenge has been the speed and volume of information that needs to be capture to do meaningful analysis. Deane shares:

…transient analysis technology meets that challenge by automating the process throughout the critical period. Live views give analysts a decision-making tool that can be accessed from another location by dialing into the system. Any machine that is being monitored by protection systems can be upgraded to transient analysis technology.

As we highlighted in yesterday’s machinery protection post, overall protection is important and transient analysis is not a substitute. Deane describes transient analysis:

…turbine engineers and operators have easy access to continuous, real-time vibration information, allowing them to closely monitor the condition of turbines during critical startup and shutdown periods. Several different plots of live data are available on control room monitors, giving decision-makers an exact picture of what is happening within the machinery.

In transient analysis-based packages such as AMS Machinery Manager, these real-time plots include orbit, shaft centerline Bode, Nyquist, waveform, spectrum, and cascade. These help to spot:

…specific defects, such as oil-whirl, rubbing, unbalance and looseness.

Deane shares an example of how this transient analysis can spot problems that potentially damage the turbomachinery. Normally on startup, a turbine will pass through a resonant frequency where the vibration will increase then decrease. Instead, it continues to increase. The view of a Bode plot:

…shows that the critical resonance speed of the machine was changing – it was shifting to the right. For the critical to shift, damping, stiffness or mass must be changing. It is reasonable to assume that mass did not change, but that damping or stiffness of the system has changed.

A turbine specialist versed in the clues provided by these plots would conclude that the issue is related to loading or misalignment, which increases the stiffness and shifts the critical resonance speed. Spotting the cause and fixing it before the machine further damages itself can prevent outages of key turbomachinery and unplanned downtime.


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