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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 |

s.c.christofi@helpe.gr

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

https://www.orbit-magazine.com/wp-content/uploads/2014/11/Figure-6.png

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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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