take only seconds to collect, sample, and average in order to
depict specific frequencies that identify defects such as misalignment, imbalance, looseness, rubbing, oil whirl, etc. Field-device based analysis leads a specialist to a specific defect,
enabling faster decision-making.
In the turbine startup scenario described previously, the
phone rings and an operator wants to know “what’s going on?”
He is advised to get out of the high vibration zone immediately
by backing off the load or decreasing speed and then slightly
increasing bearing #2 lube oil pressure. Another suggestion is
increasing the lube oil temperature.
First, the operators reduce the steam load and then go to
work on the lube flow and temperature. They are unable to
pinpoint an oil flow issue, so they increase the oil temperature
a few degrees in order to change the viscosity of the oil around
the bearings. When the operators try once more to start the
turbine, it comes up to operating speed without a hitch. What
was going on in there?
There certainly was a malfunction, which the maintenance
technicians quickly and confidently diagnosed as oil whirl at
bearing #2. This abnormal condition of bearing lube oil instability can lead to bearing damage, seal damage, or even loss of
performance. The cyclic vibration caused by this phenomenon
can actually bend the rotor, resulting in a fatigue failure. At
best, without an integrated online monitoring technology, the
safety shutdown system will save the machine but provide no
answers for the boss when he starts with the questions.
So, how is it that the maintenance staff seemed to have
all the answers, while everyone else was wondering and asking,
“What is oil whirl? Why does it happen? And how did maintenance know how to fix it?”
Here’s their secret. They recognized the presence of “oil
whirl,” the name maintenance workers give the first stage of
fluid instability around the shaft or rotor of a machine. This
type of fluid instability might be between the shaft and a bearing pad, or it might occur between the rotor and a seal. Oil
whirl, although bad, is actually a stable form of fluid instability. When oil whirl becomes more severe and unstable, it transitions into “oil whip,” causing violent full contact between
shaft and bearing.
Figure 1
There are many reasons oil whirl occurs. Some are process
related and others are machinery health related. For the most
accurate diagnosis, it is best to correlate information from both
process control and vibration monitoring.
Process parameters such as oil flow, oil temperature, and oil
pressure can affect lube oil stability of a multi-ton machine that
is spinning 60 times per second, essentially hydroplaning on a
thin film of lube oil with clearances as small as a human hair.
By monitoring these parameters of the turbine in combination with online vibration monitoring, technicians have
access to the essential information needed to let operating
personnel know what’s going on as they try to start precision, high-cost machinery. Abnormal conditions will show up
during the transition period long before they become serious
enough to trip the unit or cause a catastrophic event.
The condition of the machinery can also help initiate the
oil whirl condition. For example, if the rotor is slightly mis-aligned, the shaft may be nearly suspended, and not properly
supported by the middle bearing. Since forces acting on the
rotor have now changed, the performance of the lube oil at this
bearing location may also be negatively impacted.
When advanced data plots from online vibration monitoring are combined with process control information, the
evidence is available to capture the criminal before the crime
occurs. Technology exists today to actually replay a turbine
startup, just as video surveillance and instant replay aid in solving crimes.
