Basic tools of the vibration trade

Of the traditional vibration condition-monitoring approaches and techniques from industry (paper mills, refineries,

power plants), less than 20% are applicable to wind for various reasons. Condition monitoring approaches used in test and measurement are also largely not feasible. So if these approaches are largely not useful, then what is common in wind?

There are many vibrations in a wind turbine, some which are normal at operating conditions, while some indicate a degraded part or less-than-ideal operating condition. How do you tell what is normal and what is meaningful for predictive maintenance? It all revolves around a frequency and its amplitude, and the measurement in which they occur.

As you would expect, different vibration measurements are used to help simplify the task of condition monitoring

in a wind turbine. This article is by no means a complete explanation of these measurements or a replacement for vibration certification, but merely an explanation of the different types of measurements. There are several vibration measurements, each for determining severity and root cause of the vibration. Let’s discuss the different types and their purposes in a measurement overview.

These are derived from a measurement and converted into a spectrum by a process called fast Fourier transform (FFT). They are processed into various types of spectrum measurements, each used for different frequencies and stages of failure detection. These are the most common vibration x and y graphs used.

This is an early detection measurement used primarily for bearings. Several companies have names for this type of measurement. There are about six or seven different names for a demodulated signal or enveloped signal derivative.

What is a demodulated signal? Simply put, a low-frequency vibration is suppressed and a higher frequency or defect

frequencies are “folded over” so that they show in a spectrum. Normally a demodulation or enveloped signal is used in bearing detection, lubrication verification, and early gear wear in wind turbines. This spectrum is designed for higher frequency events.

This measurement is the rate of change in velocity predominately used for high frequency analysis. It’s good because of its response and is used for gear and bearing measurement in wind turbines. Gearmeshing frequencies can  sometimes be quite high in frequency for the simple reason that the number of teeth multiplied by the rpm can occur at higher frequencies. Furthermore, to catch its multiple harmonics, the frequency captured will likely be a factor of 10 or more of the gearmesh frequency.

Velocity
Velocity is the most basic measurement probably because it is so useful. It is focused on lower frequencies. What happens at lower frequencies in a wind turbine? Aside from tower and yaw movement, velocity is useful in determining misalignment, imbalance, looseness and advanced wear beyond the above measurements. There are many other purposes for velocity. Suffice to say, there are many low-frequency vibrations in a wind response and is used for gear and bearing measurement in wind turbines, but only a few indicate impending unreliability.

Time wave form
A time-wave form looks like a seismograph measurement because, essentially, it’s vibration over time. It is another tool used usually to look for impacting over time at a given measurement location. Because of the impacts of gear meshing (sometimes periodic), time-wave forms are helpful in confirming what is seen in the vibration spectrums and vice versa. These vibrations over time are most commonly processed into acceleration time waveforms. This is excellent for most locations in the gearbox of wind turbines.

Trend
Even a casual observer can make sense of a trend line. This elementary tool requires no vibration certification. However it can also be the least specific. If a vibration is trending upwards, there is likely an issue with that specific frequency or component. The trend must be highly qualified when used on a variable load, variable-speed asset that runs periodically. Specific frequencies can also be trended as well.

Comparative analysis
This is helpful in comparing several similar components or drive trains to one another. In a wind farm or fleet where
there are several turbines for comparison, it’s obvious why comparative analysis is beneficial. It becomes a “one of these is not like the others” approach. Comparative analysis, depending upon the software platform, can compare
trends to one another or spectrums to spectrums, either overlaid or in a stacked comparison. Looking at a normal vibration measurement tells a lot when it is overlaid on another that has 10 times the amount of vibration. A waterfall plot is also useful in contrasting the vibration signatures of several turbines for the same reason – a picture is worth a thousand words. The plot is less technical and easy for most people to comprehend when there is an issue because it is clearly visible.

Overalls
Probably the most beneficial, potentially misleading and easily misunderstood measurement for wind, is a vibration
overall. This is a summation of all of the vibration in a measurement. A vibration overall is potentially problematic because a discreet frequency’s amplitude can increase tenfold while the overall will not significantly increase. Generally speaking, overalls are used as alarming thresholds and with proper set-up can be beneficial
monitoring a fleet.

There are many standards (ISO) for vibration overalls in basic industrial applications where it deals with a constant load, constant-speed application. Wind turbines do not fall in that category. A standard has been constructed for wind turbines in which the vibration overall is applicable for turbines sized from kilowatts to 3 MW. But be careful. This is like comparing a mini cooper to a semi-truck. They both vibrate the same, right?

In general, it is important to understand there are different vibration measurements for detecting different components and failure modes. Despite some companies best marketing efforts, there is no one magic measurement. What is consistently required is a competent, wind experienced, certified vibration analyst working in concert with proper vibration measurement set-ups and parameters. The result: A change in maintenance culture from a reactive to a proactive. WPE