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STEAM TURBINE SEAL RUB
Steam Turbine Seal Rub
Vibration data helps to identify a steam turbineseal rub.
November 19, 2014
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Sotirios Christofi | Deputy Manager, Head of Mechanical Maintenance, Thessaloniki Refinery |
Executive Summary
This case history describes how the vibration data provided by System 1 software helped to identify
the criticality of, and determine an appropriate course of action following severe vibration events
experienced by a Steam Turbine driver of a Recycle Gas Compressor in an Oil Refinery of Hellenic
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Petroleum SA in Thessaloniki, Greece.
Some months after its commissioning, the turbine instrumentation started indicating brief periods of
elevated vibration. These spikes in high vibration gradually appeared more and more frequently and
with higher and higher observed amplitudes. In August 2012, the observed vibration closely
approached turbine trip limits. Based on the measurements shown by System 1, the operations and
maintenance staff decided to wait until the next plant shut down to shut down the turbine for
troubleshooting. The turbine was in operation for another six months, with occasional periods of high
vibration being noted.
In February 2013, the Turbine was disassembled and inspected. The findings indicated that the reason
for the peaks was carburized oil in the vacuum chamber, which touched the turbine shaft from time to
time. The oil had leaked through the oil labyrinths, which had worn unevenly because of
misalignment.
Steam Turbine Nameplate Data Construction
Power at shaft, rated / normal : 1485 / 1150 kW
Shaft speed, rated/normal : 9250 / 9466 rpmSteam flow, rated/normal : 24020 / 18660 kg/h
Steam inlet pressure / temperature : 40 barg @ 380 C
Steam exhaust pressure / temperature : 10 barg @ 250 C
The drawing in Figure 1 shows a cross section of the Non-Drive End (NDE) part of the turbine. The
bearing lubrication oil is contained by the oil labyrinth. Any oil leaking past the lip seals of the oil
labyrinth normally flows to the drain. Any evaporated oil fumes are evacuated from the space
between the oil and steam labyrinth by the vacuum separation system.
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Figure :
Turbine cross-section drawing Non-Drive End
The Vacuum Chamber therefore collects flow suctions from two directions: the steam leaking from the
steam labyrinth seal and the evaporation, if any, of the oil leaking from the oil labyrinth seal. A similar
arrangement exists on the Drive-End (DE) of the turbine.
Vibration Instrumentation
Figure 2 shows a System 1 view of steam turbine monitoring instrumentation. In addition to
Temperature and Keyphasor* (speed and phase reference) monitoring, there are proximity probes
VE-14375X/Y and VE-14376X/Y that measure the horizontal and vertical distance between the inner
surface of DE and NDE bearings and the shaft. These non-contacting proximity transducers measure
the vibration of the turbine rotor within the clearance of its oil-film bearings.
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Figure 2:
Steam turbine vibration instrumentation communicating with System 1 software
Sequence of Events
The steam turbine was commissioned in January 2011 and started normal operation in September
2011.
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Publisher: Ge Oil & Gas
Editor: Emma Morgan
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Figure 3:
System 1 vibration trend, showing occasional spikes of high vibration that occurred over a
period of two weeks. Note: The Alert alarm set point (57 microns) is indicated with a dashed line.
A closer evaluation showed that the brief recurring periods of high vibration started at normal levels of
10 to 15 microns, climbed quickly to a peak value as high as 50 to 80 microns, and then rapidly
dropped back down to normal levels. Each event lasted from about 5 to 15 minutes (Figure 4). The
peaks appeared at intervals ranging from 8 hours to 3 days. During some of the high vibration
occurrences, operators who were near the turbine reported that they could feel and hear significant
increases in machine vibration.
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Figure 4a and 4 b:
This System 1 plot show the phase and amplitude (both unfiltered and 1X-filtered)
trends. The Orbit plot shows the behavior of a typical brief period of high vibration. In this example,
vibration amplitude rose from normal levels (8 microns) to a peak value of 74 microns in about 4
https://www.orbit-magazine.com/wp-content/uploads/2014/11/figure-4b.png -
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Figure 5:
The orbit plots during the intermittent vibration peaks show changing phase angles, a classic
indication of rub caused by thermal bow with a wandering hot spot.
https://www.orbit-magazine.com/wp-content/uploads/2014/11/figure-5b.pnghttps://www.orbit-magazine.com/wp-content/uploads/2014/11/Figure-5c.png -
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Figure 6:
Carbonized oil is visible in the vacuum chamber.
Figure 7:
This photo shows uneven (one-sided) wear of oil labyrinth rings
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Conclusions and Further Actions
Based on the inspection results, the original diagnosis of an intermittent light rub was verified. The
most probable failure scenario was summarized:
Misalignment caused uneven wear on one side of the oil labyrinth rings.
Excess oil leaked into the space between the oil rings and the vacuum chamber.
The oil vapor mixed with high temperature steam in the vacuum chamber.
The oil heated until it formed hard, carbonized coke, which built up until a rub occurred.
Within a few minutes, the carbon deposits were abraded away through the rubbing action until the rub
disappeared. But the leaking oil would form more coke and the cycle would eventually repeat itself.
Addressing the Root Cause
In order to prevent the recurrence of a similar incident in the future, the turbine misalignment was
investigated.
The turbine had initially been correctly aligned in the shop while installed on its skid-style foundation.
No additional on-site alignment had been performed. No apparent causes of the subsequentmisalignment were found, but the turbine manufacturer reported that despite the rigidity of the
foundation skid, similar misalignments can sometimes happen during the first months of the operation
of a new turbine. In order to avoid similar incidences in future it is deemed necessary to carry out hot
alignment of equipment after 3 to 6 months of operation.
Tags: Measurement, Monitoring, Oil & Gas, Performance-based Maintenance
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