A journey through 30 years of vibration analysis on large turbines: a history of progress in technology and experience

G. Boon1, K. De Bauw

2

1 GDF Suez, Research and Innovation Division,

1and 2 Place Samuel de Champlain, 92930, Paris, France

e-mail: gustaaf.boon@gdfsuez.com

2 LABORELEC, Structural Integrity Assessment and Monitoring Department,

Rodestraat 125, B-1630, Linkebeek, Belgium

Abstract In this presentation, we will make a journey along some main evolutions in the available technology and

the power generation market environment that have enabled the introduction of efficient vibration analysis

techniques in the condition monitoring of large steam and gas turbines as a tool for an early detection and

identification of emerging mechanical problems. Besides the availability of more sensitive and reliable

instrumentation, the progress in digital signal processing and information technology has proved to be a

main driver.

With the increasing flexibility requirements of the machines, the need for a reliable estimation of the

remaining lifetime of the units is today the most important challenge for condition monitoring. This

requires the introduction of more specific instrumentation, together with model-based analysis in a

multidisciplinary approach. But at the end, a profound understanding of machine design principles and the

physical laws of mechanical vibration will always remain as important as the most powerful analysis

technique.

1 Introduction

Laborelec is a main partner of the GDF-Suez power generation divisions for different condition

monitoring activities, of which vibration monitoring is a very important one. Laborelec vibration

specialists have an on-line remote access to more than 80 local vibration monitoring systems, monitoring

more than 120 shaft lines of large steam and gas turbines. The widespread implementation of a similar

monitoring system on a large diversity of machine types in the last 25 years has an important added value

in the troubleshooting activity. We dispose today of more than 1000 machine-years of analysis experience.

Besides the lateral vibration analysis, other techniques are often used in parallel, such as the measurement

of stator part displacements for assessing alignment variations or modal analysis for examining resonance

excitation of machine elements. In specific cases also on-line torsional vibration monitoring is used. Rotor

dynamic analysis supports our experts for more complex analyses of the dynamic machine behaviour.

In the past 30 years, our working methodology and tools in vibration analysis have evolved together with

the technology evolution and the landscape of the power generation market. In this presentation we look

back on the main evolutions that have marked this period. The importance of these evolutions are

illustrated with some typical case studies.

But even when vibration analysis is today accepted as a mature technique, the changing power generation

market introduces new challenges for condition monitoring. The flexibility of our units is often at least as

important as their availability. This has influenced the machine designs, leading to more complex shaft

line layouts and a trade-off between robustness and capability for flexible operation. A combined

1

assessment of material and mechanical behaviour is necessary to estimate the reliability, the maintenance

strategy and the remaining lifetime of the units in their actual operating conditions. This opens

perspectives for the introduction of new technologies and analysis approaches in our daily condition

monitoring activities and therefore also for vibration analysis.

A reflection on the past evolution often helps in identifying priorities for new challenges. Let us therefore

start with a brief historical journey, starting at the ISMA Seminar of 1981.

2 Bringing vibration analysis from the laboratory to the power plant

2.1 Available tools and methods 30 years ago

During the 1970’s, several theoretical methods and measurement techniques became available for

supporting a more in-depth analysis of vibration problems. Especially the development of dynamic signal

analyzers for experimental modal analysis and more general spectral analysis was an important innovation

and enabled a transition from laboratory to the field. In parallel with this, there was a very interesting

progress in sensor technology. The introduction of a reliable non-contacting shaft vibration measurement

sensor technology was essential for vibration monitoring and analysis on large turbomachinery. The fast

progress in the domain of vibration analysis in the period from 1965 to 1980 is very well illustrated in

several articles published in the 40th Anniversary edition of Sound and Vibration Magazine ([1,2]). At that

time, the most important theoretical concepts were available, but the processing capabilities of the

measuring equipment was often a limiting factor for efficient vibration analysis and monitoring in the

field.

Laborelec was also experimenting with the use of different vibration analysis techniques for

troubleshooting of vibration problems in the Belgian power plants. The use of a co-quad calculation

algorithm on the data sampled with an external speed trigger was first implemented for low-speed

balancing of steam turbine rotors during overhauls [3]. This technique was a main input basis for

Laborelec’s permanent vibration monitoring approach in the Laborelec Vibration Monitoring System

(LVMS). The positive experiences with hammer excitation in impact testing for the field validation of the

modal response of large structures lead to a regular use of this technique during the analysis of complex

vibration problems. The applications ranged from the investigation of turbine blade resonances up to the

study of large turbine foundations [4].

2.2 The economical context of power generation in the 1970’s and 1980’s

The economic growth in the 1970‘s and 1980’s led to a steady increase in the need for electrical power.

There was a continuous activity in construction of new power plants of an increasing size. The 300 MW

fossil fuel fired units and nuclear plants commissioned in the 1970’s and 1980’s were designed as base

load units, with availability as a main concern. The older steam turbines from the 1960’s became more

regularly operated at variable load or even in start-stop mode. These machines were not conceived for this

operation mode and had never been operated as such.

Unexpected behaviour during the start-up of the older machines and problems of balancing with the CP2

generation of 1000 MW nuclear units commissioned in 1984 (Doel 4, Tihange 3), together with the

economic importance of a reliable generation, created a context with a need for an adequate vibration

problem solving group, that could also challenge the manufacturers.

Bringing together at the right moment a number of people with an interest in the technological

development of measurement technology, signal processing and numerical data treatment, with people that

have a feeling for practical technical solutions and the implementation of new technologies in pilot

applications, supported by the power plants, allowed Laborelec to develop in a short period a state of the

art monitoring system, using the available technology at the edge.

2 PROCEEDINGS OF ISMA2012-USD2012

2.3 The introduction of permanent vibration monitoring

The LVMS answered the existing need for improved monitoring with an adequate measurement

technology and continuous data recording. The economical context described in the above paragraph

together with experiences of some incidents that clearly proved the added value of shaft vibration

monitoring for troubleshooting of complex vibration problems accelerated its development.

A first prototype of a computerised monitoring system using shaft vibration measurements was installed in

1985 during the commissioning of the Doel 4 nuclear power plant. The system used a Hewlett Packard

HP6940 measuring unit where different signal processing cards could be plugged in. Up to 32 channels

could be sampled at 16 times per revolution, which leads to sampling rates of 25.6 kHz with a rotating

frequency of 50 Hz. Taking into account the overspeed of the turbines, this was just in the possibilities of

A/D converters available at that moment. To pilot the acquisition with a single phase reference pulse, the

speed was first calculated with enough precision by using a programmable pulse generator and counter,

determining the generated pulses during one revolution. The sampling rate was then programmed on

another card as the multiple of the rotating speed. The acquired samples were fed through a 16 channel

GPIO parallel interface to a computer, a first generation of HP9000 workstations. All data treatment was

done in a calculated digital way. The computer (with 1megabyte memory) calculated from these samples

via the co-quad algorithm the real-time 1st and 2

nd harmonic amplitude and phase and represented them in

vector diagrams. Also the sampled time signal and the spectrum of the individual sensors, based on a

sampling of 16 samples per rotation, could be shown. Orbit plots and attitude curves (shaft position in the

bearings) were also implemented. Every start and stop of the turbine was continuously recorded, which

was quite a challenge with 31 stops in the first operating month. These important transient data were

automatically stored on a 20 megabyte disk, which was the maximum available at that time. After each

stop a back-up of the data was made on floppy disks, of which a full box was needed every time. But

relevant historical data for analysis during a period of months and years became available, and was kept

ever since.

Figure 1: Measurement principle of the Laborelec Vibration Monitoring System

The system was very successful in assisting the vibration experts in their analysis and in parallel with

improved calculation capacity and disk space of the workstations, it was further improved with intelligent

alarm monitoring, efficient data handling and an automated back-up routine. The fast implementation of

these features was possible thanks to the completely digitalised data processing. Very soon, in 1986 a

management decision was taken at EBES and Intercom (at that time the main electricity producers in

Belgium) to implement the system on their entire fleet. At the beginning of the 1990’s, already more than

KEYNOTE 3

50 systems were in operation in the different Electrabel1 plants. A user interface with color display, and a

keyboard for interrogating the system were developed to be installed in the control room, as shown in

Figure 2. The standardised monitoring of the different turbine types in different operating modes has

enabled Laborelec to gain a large experience with the analysis of vibration incidents in a very short time

period. Some early examples are described in the reference document [5].

Figure 2: User interface of the Laborelec Vibration Monitoring System (LVMS) in 1988 (left)

and for the Windows-based system since 2000 (right)

At the end of the 1990’s, a Windows version of the Laborelec Vibration Monitoring System (LVMS) was

developed. The first aim of this development was to transpose the existing vibration monitoring system on

a new hardware platform, in order to assure its continuity. A second aim was to improve the performance

of the system by incorporating the experience gained with the first generation of systems in the newer

version. This led to the implementation of an improved alarming strategy and the adding of several new

functions for measurement and analysis. Both features help the users of the system to interpret changes in

the vibration behaviour more correctly and to reduce the analysis time significantly.

Additional improvements of the system were possible thanks to the use of modern information

technology. The most important is certainly the availability of remote monitoring, permitting an on-line

expert's advice in case of important changes of vibration behaviour. Other important improvements

concern the speed of data-acquisition and analysis and the user-friendliness of the system. Today,

Laborelec vibration experts have an on-line access to the vibration behaviour of more than 120 gas

turbine, steam turbine and large pump shaft lines in 13 countries. The basic measurement principle of the

system is however still the same as nearly 30 years ago.

2.4 Developments in vibration protection systems

Since the introduction of reliable shaft vibration measurements in the mid 1970’s, the protection of

turbomachinery for excessive vibration has also known a large evolution. Before 1990, most systems were

based on amplitude monitoring. The introduction of digital signal processing in vibration protection

systems around 1990 has enabled a cost reduction and the introduction of more complex functions, such as

vector alarming or trip multiplication in transient speed ranges. It is however only at the end of the 1990’s

that the digital signal processing in vibration protection was deployed to its full capacities for vibration

protection in power plants. This has improved the reliability of the vibration protection, with concepts

1 In 1991, EBES, Intercom and Unerg were joined to create Electrabel.

4 PROCEEDINGS OF ISMA2012-USD2012

such as sensor grouping and voting becoming common practice. This evolution went together with the

development of more complex shaft line configurations, such as single-shaft CCGT units, which also

needed more complex control algorithms.

At that time, a limited number of vibration monitoring systems such as LVMS were already using a more

intelligent vibration alarming for over more than 10 years for the warning of the operators and even for

trip recommendations into the control systems. The reliable transfer from these concepts to protection

systems was therefore not only a matter of technology, but also of priorities and awareness with the

turbine manufacturers. Since an unplanned trip of a single shaft CCGT unit was to be avoided at any price,

turbine control engineers adopted the available know-how on vibration monitoring also as a best practice

for vibration protection.

Figure 3: Vibration protection instrumentation as used in the late 1970’s (Bently Nevada 7200 series)

and since the late 1990’s (Bently Nevada 3500 series)

In modern power plants, there is an increasing trend of embedding the vibration measurements directly in

the process control systems, with the use of plug-in cards in the process computers. This facilitates an

even stronger integration of vibration measurements in the process control logic, besides their standard

function of machine protection.

3 Technological developments that changed vibration analysis

It is not easy to make a complete list of technological developments that have changed the activity of

vibration analysis on large turbomachinery. We will however identify some important developments of

which we believe that they made a difference in the development of vibration analysis towards a proven

condition monitoring technique. It is often the interaction between information technology and vibration

theory that enabled a significant progress and created added value.

3.1 The introduction of reliable shaft vibration and phase reference measurements

In the early 1970’s, velocity pickups on the bearings were still the standard measurements on large

turbomachinery. By the mid 1970’s, proximity probes based on the eddy current measurement principle

had gained a universal acceptance as the preferred method for monitoring the mechanical condition of

large turbomachines with fluid film bearings. Today, these measurements are considered as a necessary

element of machine protection for shaft lines with fluid film bearings, and are mentioned as such in the

relevant standards (e.g. API 670). The pioneer role of Donald E. Bently in developing a reliable

measurement cannot be overrated in the development of vibration monitoring and rotor dynamic analysis

in general.

The use of shaft vibration measurements enabled a better insight in the working principles of fluid film

bearings and the causes of shaft vibration. Especially the improved understanding of subsynchronous

KEYNOTE 5

instability problems led to more reliable machine and bearing designs [2]. The use of eddy current sensors

also enabled the introduction of a reliable phase measurement, which is essential for the combined use of

amplitude and phase information in a “vector” vibration measurement. The unreliable optical phase

sensors were not suited for a permanent use.

The importance of shaft vibration analysis for an early detection and reliable solution of machine

malfunctions is well demonstrated in several case studies [5, 6]. These measurements enable an improved

identification of common machine malfunctions such as rub contacts, different types of unbalance sources

or instability problems. Their added value for the detection of excessive shaft vibrations that risk to remain

unnoticed by standard absolute bearing vibration sensors is also well illustrated in the following case.

During certain load and ambient conditions, it was noticed that SGT5-2000E gas turbine shaft lines could

show significant shaft vibration amplitudes at subsynchronous frequencies near the turbine bearing. The

absolute casing vibration measurement installed near the exhaust duct didn’t measure this phenomenon

due to their standard vertical mounting direction and the used filter band on these type of accelerometers.

It was presumed that combustion dynamics and acoustical resonance were the main excitation sources of

this phenomenon. A rotor dynamic model was developed to understand the dynamics of these gas

turbines designs better.

A horizontal harmonic load was modelled near the axial location of both silo combustion chambers and a

frequency sweep of the load enabled to visualize the rotor interaction. This indicated a resonance

behaviour near 18.7Hz (Figure 4). From the Campbell diagram one could conclude that this frequency

coincides with the first horizontal bending mode at operating speed. Moreover, the calculation showed that

this mode was very poorly damped, which means that already a small excitation force could drive the shaft

line towards large rotor deflections.

This analysis clearly explained the sensitivity of the unit to external forces introduced near the mid-span

area, especially when the frequency of the acoustic resonance of the silo combustion chamber area

matches with an eigenfrequency of the gas turbine rotor, as is the case for this type of units.

Figure 4: Excitation of a bending mode through acoustic resonances in a GT

combustion chamber leads to subsynchronous vibrations

Excited bending mode at 18.7 Hz

Gas turbine

Generator

6 PROCEEDINGS OF ISMA2012-USD2012

3.2 The cost reduction of digital data processing technology

The cost reduction and downsizing of digital data processing technology enabled the transition of

powerful vibration analysis techniques to the field. The introduction of the Laborelec Vibration

Monitoring System in the 1980’s that is described above is a good example. The system enabled a

permanent and simultaneous real-time monitoring of a large number of measurement channels. This made

it possible to perform more powerful vibration analysis at all times and to compare different machine

signatures on a more regular bases. Experience with solving a vibration problem on one unit was

transferred more easily to another unit.

Figure 5 and Figure 6 illustrate how the vibration amplitudes of the low pressure rotor of a 160 MW steam

turbine were decreased with a one-shot dynamic balancing correction that was implemented during a

planned stop. After years of stable base load operation, the unit was operated in a more regular start/stop

regime. Due to the vicinity of a 2nd

bending critical speed of the low pressure rotor, the vibration

amplitudes on both sides of this rotor had always been fairly high, but still below the alarm threshold of

120 µm pk-pk in a stabilised condition. An increase of the amplitudes had been noticed in the last years

due to small unbalance changes caused by erosion of the last stage turbine blades. In the first 5 to 10

minutes after a start, high shaft vibrations were encountered due to the sensitivity of the dynamic

characteristics of the shaft line through the nearness of the critical speed. The shaft vibrations would

sometimes exceed the danger level of 180 µm pk-pk just after the run-up and thus cause an unplanned trip,

combined with an unavailability of sometimes several hours. This situation was not acceptable anymore in

the more flexible operation of the unit. Based on a thorough analysis of the vibration behaviour and

experience with other units, a balancing proposal with a couple of balancing masses in opposition at both

rotor ends was defined and implemented at a planned stop of the unit. The mounting of the masses was

possible through holes in the casing that gave access to the balancing planes at both rotor ends.

The action resulted in an important reduction of overall vibrations at nominal speed and a disappearance

of the overshoot behaviour of the shaft vibrations that was typically encountered with the former

unbalance condition of the rotor. Since the unbalance excitation of the 2nd

bending mode of the rotor was

nearly completely eliminated, also the overshoot behaviour had disappeared. An additional grinding of the

turbine blades in the weeks following the balancing actions caused a noticeable change of the vibration

behaviour, yet with only a very limited impact.

Before the introduction of a permanent vibration monitoring, a similar problem would have needed the

removal of the rotor for low speed balancing during an overhaul, as described in [3], with an important

additional overhaul cost. Thanks to the availability of relevant historical data and experiences with

balancing on different comparable units, a minimal intervention during a planned overhaul was sufficient

to solve the problem.

Figure 5: Instrumentation overview of a 160 MW steam turbine unit. The unit is equipped

with relative shaft vibrations near all bearings.

KEYNOTE 7

Figure 6: Reduction of the 1st harmonic amplitude through a dynamic balancing correction on the LP rotor.

3.3 The development of information technology

Another very important result of the reduction of digital processing and the rapid developments in the

information technology was the integration of more complex data treatment routines inside vibration

monitoring systems. Besides the calculation and visualisation of important parameters of the vibration

behaviour, computer-based vibration monitoring systems were able to integrate more additional data

treatment in real time. This led for example to the implementation of more intelligent alarming based on

the operating conditions of the unit. Since the vibration behaviour of steam and gas turbines is often

highly load dependent, this made a more sensitive yet still reliable vibration alarming possible.

The increased possibilities of data storage enabled a continuous availability of historical data. In the early

1990’s, the 20 megabyte hard disk of the LVMS was already capable of storing a relevant history of up to

more than 1 year, thanks to an intelligent event-driven data storage algorithm. But for every analysis a

local tape backup needed to be made and analysed. With the transition to modern information technology,

a remote follow-up of the measurement systems became possible. Since more than 10 years, all Windows-

based LVMS systems are integrated on the company Wide Area Network. The possibility to move the

data and not the people has tremendously increased the efficiency of field support to the plants. Since

several years, Laborelec has organised a 24/7 support for its LVMS, giving the plants a permanent access

to remote expert advice in case of important changes in the vibration behaviour. For some machine types

where balancing corrections are needed in the startup phase after an overhaul, balancing proposals can be

provided immediately to the power plant, thus reducing the startup time with hours to even days.

Compared to the tape backup interventions on the 20 megabyte hard disks in the 1990’s, several gigabytes

of data are securely available on mirrored disks in the power plant, and are also automatically transferred

to centralised servers through network connections. Laborelec experts have an immediate access to years

of historical data on all surveyed units. With the migration of its LVMS to a Windows-based platform, we

also developed a data migration program, enabling a transfer of the old data formats to the new Windows

format. This makes that a continuous history of vibration signatures for more than 20 years is immediately

available for some of the monitored units. This facilitates the comparison of vibration signatures of

different machines, which is essential for more complex data analysis and for knowledge transfer between

experts.

High vibrations after every startup

Amplitude reduction

through balancing

Effect of additional

grinding of blades

8 PROCEEDINGS OF ISMA2012-USD2012

3.4 Data interfacing with other information platforms

About 10 years ago, a reflection on condition monitoring was held within the power generation division of

GDF-Suez. Thanks to the development of the data processing technology, more powerful monitoring

techniques were being introduced in the plants. This was very important with the increasing complexity of

the turbine technology. However, the increasing machine complexity also required a more

multidisciplinary approach in order to increase the efficiency of the condition monitoring. As an example,

the interpretation of the changes in the dynamic behaviour of gas turbines requires a combination of

information related to the internal combustion process and the vibration behaviour. The high temperatures

in the combustion chambers and at the turbine inlet are also a determinative factor when examining the

impact of the dynamic behaviour of machine elements on their mechanical integrity.

Since the end of the 1990’s, several data interfacing protocols have found their way to the power plant

information systems, enabling a real-time exchange of data between condition monitoring and control

systems. In many plants, a global data platform gathers today simultaneously data from process control

systems and condition monitoring platforms. This enables an enhanced multidisciplinary approach in the

analysis and the use of complex statistical algorithms for early detection of emerging deviations from the

normal operation signature, which is facilitating the evolution towards the embedding of expertise in

condition monitoring systems or adaptive process control systems. This is a necessity for supporting the

operator when confronted with malfunctions of the complex modern turbomachinery. Experts have access

to the different monitoring and process systems via a Diagnostic Center at Laborelec.

3.5 Rotor dynamic modelling as a support tool in vibration analysis

With the increasing complexity of shaft line layouts, the understanding of their dynamic behaviour is also

becoming more complex. Besides a more multidisciplinary approach, the root cause analysis of occurring

problems also necessitates the use of more developed analysis techniques. The capabilities of rotor

dynamic analysis have significantly improved since the availability of shaft vibration measurements and

have led to a better understanding of the interaction between rotor vibrations and fluid film bearings. This

type of analysis was however often restricted to the machine manufacturers or to universities and research

institutes collaborating with the manufacturers in specific research tasks.

The increasing processing capabilities in the information technology has enabled a transition of modelling

from mainframe computers to personal computers. A rotor dynamic or finite element calculation that

required hours of calculation time on a mainframe computer 10 to 15 years ago can be executed today on a

personal computer in a few minutes. This evolution has made these tools economically available on a

larger scale. Especially rotor dynamic analysis has found its way to a larger forum of vibration experts,

thus also increasing the insight in vibration problems and the reliability of the proposed solutions.

Some examples of cases where rotor dynamic modelling was used as a support tool in vibration analysis

by Laborelec are discussed in [7]. Even when certain phenomena are known from the experience of

vibration analysis, additional information can be obtained from the model results. This mainly involves

issues linked to the bearing-rotor interaction. The increasing complexity of the shaft lines, together with

the need of fitting machines into complex existing processes and the short time to market of a design often

leads to design tasks being subcontracted to different teams or companies. Bearing design is for example a

task that is typically outsourced. The merging of the different results together with the pursuit of an

economically optimised design can lead to shaft lines that are less robust in their dynamical behaviour. A

feedback between vibration measurements and rotor dynamic analysis can identify the design margins

more effectively.

A recent example of the added value of rotor dynamic analysis for increasing the reliability of a defined

solution is shown in a field balancing action on a 1100 MW steam turbine in a nuclear power plant. The

generator of the shaft line was exhibiting important vibrations due to the presence of shorted turns in the

rotor, leading to vibration amplitudes reaching alarm levels on the exciter rotor bearing at the end of the

shaft line. In order to improve the vibration behaviour of the unit, Laborelec examined the possibility of

KEYNOTE 9

improving the vibration behaviour with an on-site balancing action. Past unbalance changes on the exciter

rotor due to shifts of its coupling during grid incidents were found to have had a pronounced effect on the

generator vibrations. It was therefore suspected that the generator was running near a 2nd

bending critical

speed, which would mean that a balancing correction on the generator couplings would be able to

influence the generator vibrations significantly. While the shaft vibrations indicated a vibration behaviour

at nominal speed where the exciter rotor was clearly out of phase with the turbine end of the generator

rotor, the stator vibrations indicated a behaviour that tended to be more in phase at both generator ends.

This was somehow in contradiction with the presence of a 2nd

bending critical speed near nominal speed.

A rotor dynamic model of the shaft line was developed in order to have a better understanding of the mode

shapes influencing the behaviour of the shaft line at nominal speed. The two mode shapes that are the

closest to the nominal speed are shown at Figure 7. The 2nd

bending critical speed was identified to be near

28,2 Hz, but with an important damping of about 15%. This means that it still influences the behaviour at

nominal speed. From the unbalance response analysis it was defined that an unbalance correction of at

least 1 kgm would be needed at each coupling to have a significant effect on the complete generator. It

was not possible to install this correction during a short planned intervention, since this would require a

cooling down of the unit for opening the bearing pedestal between the LP2 rotor and the generator. The

analysis also showed that the coupling at the exciter end would have a higher influence on the vibration

behaviour, since it tended to be further away from the nodes of the mode shape. This coupling was freely

accessible on the shaft line. Based on the rotor dynamic calculation, the experience with the grid incidents

leading to shifting of the exciter couplings and the practical feasibility of mounting a mass during a

weekend stop, a balancing proposal was defined. The aim was to reduce the vibrations of the exciter

bearing and to obtain a decrease of the vibrations near the generator exciter end bearing. This would lead

to a more reliable operation of the unit until the next opportunity to exchange the generator rotor.

Figure 7: Mode shapes for the damped critical speeds influencing the vibration behaviour

at nominal speed of a 1100 MW steam turbine shaft line

During a weekend stop, an unbalance correction of 425 gm was applied on the exciter coupling. The

correction enabled to reduce the exciter bearing vibrations with about 10%, which was clearly visible, but

less than was expected. The angular position of the correction was however very effective. An unplanned

outage of the unit about a month later created the opportunity to improve the balancing proposal. The mass

was increased to 720 gm and the angle was modified with 15°. The result of both balancing corrections on

the 1st harmonic vibration amplitudes of the bearings at the generator turbine and exciter end (bearings 5

and 6) and of the exciter bearing (bearing 7) is shown in Figure 8. The desired reduction of the amplitudes

near bearing 7 through the balancing corrections is evident on the trend. The effect of the 2nd

balancing

correction was stronger than was expected. A more detailed analysis of the results showed that this was

partially due to the appearance of an additional shorted rotor turn during the restart after the standstill.

10 PROCEEDINGS OF ISMA2012-USD2012

Figure 8: Effect of the balancing corrections on the relative shaft vibration amplitudes

In this case, the use of a rotor dynamic model confirmed the feasibility of the proposed solution of

applying a balancing correction on the exciter rotor as a temporary solution to reduce the generator

vibrations and gave an input for the estimation of its order of magnitude.

Also manufacturers are looking for an improved support of commissioning engineers through more

effective balancing procedures. The paper [8] describes tests performed by Siemens to determine a more

reliable unbalance identification in the field through rotor dynamic analysis. The approach is already used

for improving the balancing process in the balancing facility, where the conditions are often more

repeatable than in the actual shaft line configurations in the power plants.

3.6 Modal analysis and operational deflection shape analysis

The developments in modal analysis and especially the tools for visualizing the modal deformations of

structures are important in the analysis of resonance vibration of structures. This is certainly the case for

more complex structures. Figure 9 shows for example a mode shape obtained during an impact test on a

compressor blade of a gas turbine. The relatively thin compressor blades show a large number of very

poorly damped resonance frequencies, which are often situated relatively close to each other in “families”

of modes. Identifying the different modes can be important in the root cause analysis of blade damages.

Modal identification routines simplify the identification of the different adjacent modes.

A particularly helpful tool for measurements on rotating machinery is the Operational Deflection Shape

(ODS) analysis. The visualisation of the actual deformation of a structure under the operational unbalance

force excitation gives a better insight in which modes are dominating the behaviour at nominal speed. A

good example of the added value can be found in the identification of modal deformations of complex

structures such as machine foundations. The parallel analysis of the operational deflection shape of the

foundation, resonance modes obtained with impact tests at standstill and transient vibration data enables

the identification of the most important resonance modes of a foundation in a relatively short time period.

Several softwares are today available on the market that can visualize modal deformations or ODS results

very quickly from a large number of transfer function data formats.

1st balancing 2nd balancing

bearing 7

bearing 5

bearing 6

MVar

Vibration increase due to

shorted rotor turn incident

KEYNOTE 11

Figure 9: Modal deformation of a compressor blade obtained with an impact test

4 New challenges and trends in condition monitoring

4.1 A changing power generation market

Throughout the last 30 years, the power generation market has known some very important evolutions.

A first important evolution is the internationalization of the market. This has led to a grouping of different

power producers, formerly often focusing on a national market, into a small number of companies that

dominate the international market of the larger fossil fuel and nuclear power plants. The plant focused

organisation of operation and maintenance activities was absorbed in large matrix organisations, where

tasks such as asset management and maintenance are also managed on a centralised level. This did not

always facilitate the implementation of condition monitoring for predictive maintenance purposes in the

field, since the added value is often noticed the clearest with the person involved in both the operation and

maintenance of a particular machine.

A second important evolution concerns the introduction of new plant concepts, such as Combined Cycle

Gas Turbine (CCGT) plants or Combined Heat and Power plants (CHP). In the pursuit of optimum

efficiency for a minimal investment, these plants often use “tailor-made” modifications of existing

machine designs, where a trade-off needs to be found between machine robustness and efficiency. Single

shaft designs, where a gas turbine and steam turbine are coupled to the same generator, often by using a

clutch coupling, leads to challenging rotor dynamic designs in order to be able to control the dynamic

behaviour of the shaft line in every load configuration. The higher power density inside the machines also

increases the risk for instable dynamic behaviour in situations where unbalance forces and fluid forces are

becoming equal in magnitude.

The increasing availability of renewable energy in the power generation portfolio and the short term

changes of fuel prices have led in the last 5 years to a very rapidly increasing demand for flexibility of the

plants. This emphasizes even more the need for turbines that are capable of providing large power steps in

a very short time. The turbines must be able to withstand important thermal transients, which leads to

lighter and consequently less robust designs. While the availability and reliability of a turbine was the

most important requirement in the past, its capability for flexible operation is today often even more

important, and sometimes to the detriment of the mechanical condition of the units.

12 PROCEEDINGS OF ISMA2012-USD2012

4.2 New challenges for condition monitoring

The condition monitoring (CM) process can be divided into 4 major tasks:

The fault detection capability of the CM technique: a first requirement of a CM technique is

that it is capable of detecting an emerging fault soon enough in order to be able to planify the

necessary maintenance actions for correction in due time. The P-F interval, which is the time

between the detection of an emerging fault situation and the actual failure, needs to be

sufficiently large.

Figure 10: P-F interval in for a condition monitoring technique - ref [8]

The fault identification capability of the CM technique: a CM technique must be capable of

identifying the type of malfunction that is occurring. An analysis should be able to indicate

the most probable fault type (e.g. unbalance, bearing instability, alignment,...).

The fault localisation capability of a CM technique: after having identified the type of

problem, the identification of the most probable location is a next important feature of a CM

technique, in order to planify only the necessary maintenance interventions.

The capability of a CM technique to identify the fault development mechanism: in order to

avoid a fault to repeat itself, the mechanism of its development must be understood in order to

eliminate the root causes.

Once these 4 requirements are being fulfilled in a condition monitoring process through the use of 1

particular or the combination of several CM techniques, a prevention of future faults becomes possible.

This process is illustrated in Figure 11.

Figure 11: The different steps of a Condition Monitoring Process

The development of the vibration analysis on rotating machinery led to a technique that proves to be very

effective on the first three steps in the CM process (fault detection, identification and localization). These

enable a reduction of the unplanned downtime of the machines. The identification of the fault mechanism

requires however a multidisciplinary approach, where the investigation of process parameters, several

different CM techniques and mechanical damages also have an important role. In today’s market demands

towards increased flexibility margins of the turbines, a reliable identification of the fault mechanism and

the risk assessment of fault development for a given operation strategy are increasingly important.

Condition Monitoring has therefore to develop today more than ever on this capability.

Condition Monitoring will need to develop more towards a “Residual Lifetime Monitoring”. The plant

operators and asset managers need a continuous assessment of the impact of their operation strategy on the

consumption of residual lifetime of their unit. This must enable them to identify at a given moment the

best available unit for the actual power demand. Even if a certain operation will lead to damage of a unit,

it could still be economically the best choice to operate this unit due to other factors such as fuel price,

Faultdetection

Faultidentification

Faultlocalisation

Faultdevelopmentmechanismidentification

Faultprevention

KEYNOTE 13

startup time, etc... The operator mainly wants to know “how much it hurts” when he will push the startup

button of his turbine.

4.3 Technologies that will make a difference

While lateral vibration measurements are a mature technique, interesting developments are continuously

being made with respect to the measurement technology, for different types of applications that are

important for the troubleshooting of complex vibration problems in the field.

A first evolution that we would like to mention is the important development in optical measurement

technology. In the last 10 years, several optical techniques were introduced and have become common

practice in the field. 2D or 3D topographical measurements are used to follow-up the position of

foundations and machine alignment variations with an ever increasing accuracy. Laser displacement

measurements enable a continuous vibration and displacement measurement at locations that are difficult

or dangerous to access with other sensor types. Optical accelerometers are today a standard for vibration

measurements in an environment with high electromagnetical disturbances, such as on the end windings of

generators [11]. Taking into consideration the rapid increase of technology in optics and its potential in

applications for complex sensing and imaging, this domain has to be followed with a particular interest.

Another interesting evolution is the miniaturisation of the sensors, leading to very compact accelerometers

and MEMS technology. Together with the evolutions in wireless data transmission, this will facilitate the

embedding of physical sensors at more relevant locations inside a machine, and the introduction of more

intelligence at the sensor level. This will certainly boost the capabilities for condition monitoring

purposes. Robust sensors with a minimal external signal processing are needed for a use in harsh

environments, such as for example off-shore wind turbines.

As we have already seen that the evolution of information technology has had a tremendous impact on the

evolution of vibration monitoring, this will continue to be the case in the future. Besides the possibilities

to store and analyse ever increasing datasets, the continuous increase of data processing capabilities

enables an improved measurement and analysis of rapidly varying signals at a reasonable cost. A good

example are applications based on high speed pulse train variations, such as tip timing technology for

turbine blade monitoring or torsional shaft vibration monitoring. The risk for torsional vibration excitation

increases with the complexity of the electrical grid and the flexibility of both power generation and

consumption, due to the coexistence of different power sources on the electrical grid. In large steam

turbines, the risk for excitation of blade vibrations and consequential damage is more important than ever,

due to the higher power density at every turbine stage and the huge stresses at the blade roots, as well as to

the possible interaction between torsional vibration resonance modes and the fundamental vibration modes

of the last stage low pressure steam turbine blades. A reliable monitoring of these phenomena at a

reasonable cost will rapidly be accepted as a standard condition monitoring tool.

Thanks to their decreasing costs and their increasing user friendliness, modelling techniques will continue

to find their way from the desktop of machine designers to the troubleshooting engineers. The simulation

of the impact of specific process conditions on the dynamical behaviour of a machine can be a tremendous

help in root cause analysis. The identification of critical locations with a model can also help in identifying

optimised locations of sensors, that will be able to “measure where it hurts” This will only improve the

capabilities of condition monitoring techniques and decrease their implementation cost.

14 PROCEEDINGS OF ISMA2012-USD2012

5 Conclusions

In this presentation we have highlighted some important milestones in the history of vibration analysis on

large turbines towards its present status of a mature and reliable condition monitoring technique. In his

historical overview of condition monitoring [2], John Mitchell states that “the key to success is not

technology but the awareness of value, resources and time”. In the past 30 years, we also learned that

joining the right persons at the right moment with a challenging machine problem boosts the introduction

of new technologies and competences. While the technological possibilities of vibration analysis and the

layout and operation of machines have evolved continuously, the physics behind mechanical vibrations

have fortunately remained the same. The understanding and identification of these fundamental physics in

an existing problem has, is and will always be as important as the measurement technology itself. The

actions that have brought this expertise to the measurement systems have therefore always been the most

successful ones.

The most important future challenges for vibration analysis lie in the integration with other condition

monitoring techniques towards a reliable “residual lifetime monitoring”, that will enable a safe and

durable use of the assets in a flexible power generation portfolio. Interesting new technologies are

emerging in order to assist in this transition, but the laws of physics will always remain the same....as old

wine in new barrels.

Acknowledgements

The authors wish to thank the different managers of Laborelec and its historical mother companies that

have supported in the past 30 years, and still today, the initiatives to bring the right techniques, persons

and problems together to develop vibration monitoring within GDF SUEZ to an up to date technology,

and to give the priority to the related activities. This support enables us to continue to evolve in this

exciting environment.

References

[1] D. L. Brown, R. J. Allemang, The Modern Era of Experimental Modal Analysis, Sound and Vibration

Magazine, 40th Anniversary Issue, January 2007, pp. 16-25.

[2] J. S. Mitchell, From Vibration Measurements to Condition Based Maintenance, Sound and Vibration

Magazine, 40th Anniversary Issue, January 2007, pp. 62-75.

[3] G. D’Ans, P. Bulens, Low-speed Two-Plane Balancing of Turbine Bladed Wheels: Co-Quad and

Swept-Sine Revisited, paper presented at ISMA, Leuven (1981).

[4] G. Boon, Comparison of Finite Element Computations with Modal Analysis for Large Turbine-

Generator Foundations, paper presented at ISMA, Leuven (1981).

[5] G. Boon, F. Van Zeveren, J. De Raedt, Experiences with a Continuous Vibration Monitoring System

on Turbogenerators in the Belgian Power Plants, paper presented at the AUSY-MODI workshop,

Florence (1991).

[6] K. De Bauw, S. Grégoire, Practical Experiences in Avoiding Downtime of Turbogroups through

Efficient Vibration Monitoring, Proceedings of the Symposium zur schwingungsdiagnostischen

Überwachung von Kraftwerkturbosätzen, Potsdam (2006).

[7] K. Matthys, K. De Bauw, M. Perucchi, Rotor Dynamic Modelling as a Powerful Support Tool for

Vibration Analysis on Large Turbomachinery , Proceedings of the 8th IFTOMM International

Conference on Rotor Dynamics, Seoul (2010).

KEYNOTE 15

[8] B. Lüneburg, U. Ehehalt, R. Oswald, U. Kotucz, Vibration Identification on large Steam Turbine

Rotor Trains for Power Generation, Proceedings of the 8th International Conference on Vibrations in

Rotating Machines – SIRM 2009, paper ID 45,Vienna (2009).

[9] J. Moubray, Maintenance Management – A New Paradigm, article available at

http://www.maintenanceresources.com/referencelibrary/rcm/maintparadigm.htm.

[10] D. Frerichs, Überwachung der Wickelkopfschwingingen von Generatoren mit faseoptischen

Beschleunigungsaufnehmern, Proceedings of the Symposium zur schwingungsdiagnostische

Überwachung von Kraftwerkturbosätzen – Methoden, Nutzen, Erfahrungen, p. 395 – 428, Referat nr.

17, Potsdam (2006).

[11] B. Lüneburg, Ch. Keune, J. Bienert, N. Darsow, Validation of the Torsional Layout Procedure of

Single Shafts with Synchronous Clutches for Power Generation, Proceedings of the 9th International

Conference on Vibrations in Rotating Machines – SIRM 2011, paper ID 29,Darmstadt (2011).

[12] D.E. Bently, Ch.T. Hatch, Fundamentals of Rotating Machinery Diagnostics, Bently Pressurized

Bearing Press, 2002, ISBN 0971408106.

16 PROCEEDINGS OF ISMA2012-USD2012