April - Library

AMMJ Lessons Learnt In 45 Years of Condition Monitoring 26
Case study 4
The new graduate engineer hooked up the accelerometer via the long cable reel to the signal conditioning/
readout instrument. He reported that turbine vibration was 55mm/s rms – over 10 times greater than what might
be expected! Before panic set in, we found that he had used the cable to connect the accelerometer to the
instrument. It was an ordinary shielded co-axial type, intended to be used from the instrument to an analyser. Low
noise cables are required from charge output accelerometers to avoid tribo-electric boosting of the output to give
a spurious high vibration reading.
Case study 5
Lesson #13 Check, and recheck, critical data values if any look to be unusual.
Using an innovative approach, site trim balancing
was conducted on a 120MW generator rotor.
The coupling between turbine and generator
was unbolted, and faces held apart. The exciter
was connected to run as a motor, with the rolling
torque provided using the overhead crane and
a rope wrapped around the rotor. (Appropriate
design checks had been made).
After reassembly, run-up proceeded as normal,
until when nearing normal service speed
generator bearing vibration suddenly jumped
so much that the floor shook and dust fell from
the rafters! The operator tripped the machine.
Subsequent attempts at run-up were no different.
A challenge for the vibration team! The gear
was set up with the analyser set to PEAK HOLD
mode. The extreme vibration was revealed as at
19 Hz – the first critical speed of the rotor. It
was noticed that the vibration started soon after
the auxiliary oil pump was stopped, so it was
left running and the unit was eventually put into
service. The 19Hz vibration was still evident, and
could be varied in amplitude by changing the oil
temperature.
Bearing dimensions and clearances were found, and the bearing wedge pressures (giving shaft loading) and oil
temperatures noted to calculate the Load Parameter. The resulting plot on a bearing stability assessment chart
showed that the operating range was well outside the “recommended” area (ESDU 1966), as shown in Figure 5.
The only variable that could be changed permanently was the length of the bearings (to increase the specific
loading). Surprisingly, the spares in the store were found to be shorter, as were those on the adjacent “identical”
machine! Bearing changeover was the cure. This was a strange case, as this machine had operated 17 years
without this problem. The vibration team gained superhero status for this success. (Beebe, 2002).
CM by performance analysis – the big bucks
Case study 6
Fig 5 Bearing stability chart showing effect of oil viscosity
and bearing length
Load parameter W’
Original
operation
Operation
when
bearings
modified
I had developed performance tests for both types of 200MW machine at Hazelwood with useful outcomes and
continued this at Yallourn W. The methods followed had since been published (ASME, 1970). On one unit, our
tests were run before the official acceptance tests.
Routine tests on a 350MW unit showed a small but significant decline in performance. Prior to a planned outage, a
steam forced cool was conducted. This procedure is used to bring the machine to standstill more quickly by cooling
the turbine metal, rather than allowing slow natural cooling. The inlet steam temperatures were slowly decreased
over some hours during offloading. Testing after return to service showed that the performance has returned to its
initial level. Close examination of the data concluded that there was some restriction in the intermediate pressure
section, deduced to be from blade deposits (Beebe, 1978).
Soon afterwards, the OEM site manager met with the plant manager to tell him that as the first unit had
reached 2 years of service, it was time to arrange a major outage and stripdown. When asked the reason for the
recommendation he was told that an inspection after two years was standard practice in the OEM’s country. The
manager had been my boss and mentor in my initial job, so was well versed in CM! He did not support an overhaul
given our vibration and performance condition assessment, and the machine continued to operate for 17 years
before its high pressure section was opened.
Lesson #14 Take OEM recommendations into careful consideration,
but do not follow them blindly.
10
1.0
0.1
Oil 71°C
Oil 40°C
Lines of
increasing
constant b/d
Increased risk of halffrequency
whirl
0.1 0.5 0.9
Eccentricity ratio
Recommended
area
Bearing
too short
Vol 24 No 2