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48 ORBIT [Vol.25 No.1 2005] Gas Turbine Vibration Monitoring – An Overview Mel Maalouf Applications Engineer, Bently Nevada ® Products GE Energy [email protected]

Gas turbine vib monitoring

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Page 1: Gas turbine vib monitoring

48 ORBIT [Vol.25 N o . 1 2 0 0 5 ]

Gas Turbine Vibration Monitoring– An Overview

Mel MaaloufApplications Engineer, Bently Nevada® ProductsGE [email protected]

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[Vol.25 No.1 2005] ORBIT 49

Introduction

For well over 30 years, the Bently Nevada® prod-uct line has been used by gas turbine manufactur-ers and end-users to monitor these importantmachines. More than perhaps any other type ofturbomachinery, gas turbines have undergone themost dramatic evolution in the past severaldecades, with increased firing temperatures, usageof advanced – even exotic – composite and alloyedmaterials; extremely sophisticated cooling mecha-nisms for components such as blades, nozzles, andtransition pieces; innovative methods to reduceemissions; and significant increases in overall ther-mal efficiencies. Not surprisingly, these technolog-ical innovations have made gas turbines the primemover of choice for many power generation andmechanical drive applications. Today, there is anextremely large installed base of these machinesaround the world.

However, while gas turbines have unquestionablyevolved in sophistication, the basic approach tovibration monitoring on these machines hasremained largely unchanged. What has changedover the last several years is the additional meas-urements available today that complement BentlyNevada vibration monitoring products, such ascombustion instability monitoring, hazardous gasdetection, flame detection, exhaust gas tempera-ture diagnostics, thermodynamic performancemonitoring, and overspeed detection.

This article reviews the basics of monitoring vibra-tion on gas turbines, providing recommendations,and discussing some of the new measurements nowavailable. It begins with a review of the two broadclassifications of gas turbines: aeroderivative andindustrial.

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A P P L I C A T I O N S

Compressor TurbinePower

(or Reaction)Turbine

Driven Machine(pump, generator, etc.)

Gas Generator

Gas Turbine

FIG. 1 | NOMENCLATURE USED FOR MANY AERODERIVATIVE GAS TURBINES

Aeroderivative Gas Turbines

Aeroderivative gas turbines (sometimes called aeroderiv-ative engines – the terms are used interchangeably inthis article) are derived from aircraft applications. Tomeet the needs of the aerospace industry, aeroderivativegas turbines have primary emphasis on power-to-size/weight ratios, since smaller, lighter engines reducethe weight of the aircraft, and therefore fuel consump-tion. With few exceptions, aeroderivatives use rollingelement, rather than fluid-film, bearings. Some alsoemploy concentric shafts, allowing the various com-pressor and turbine stages to run at different speeds,and making them better able to produce the large pres-sure ratios required in certain engine designs.

Obviously, the primary function of a gas turbine whenused on jet aircraft is to generate the thrust necessaryfor propulsion. This is accomplished by channeling thehot gases through a nozzle and, sometimes, by also turn-ing the large fan in the engine (turbo-fan or high-bypassengine). When adapted for industrial applications,however, the thrust must be converted into mechanicaltorque, able to drive a generator, process compressor,blower, pump, propeller (when used in marine propul-sion), or other driven machine.

Channeling the hot gases into an aerodynamically coupled power turbine, rather than a nozzle, accom-plishes this. The hot gases spin the power turbine whichis coupled mechanically to the driven equipment.

The nomenclature used when referring to aeroderiva-tive gas turbines is shown in the simplified diagram ofFigure 1. It illustrates that most aeroderivative gas tur-bines consist of two fundamental components – the gasgenerator and the power (sometimes called ‘reaction’)turbine. Collectively, these are referred to as the “gasturbine.” While the gas generator does have an integralturbine section, it serves only to extract enough energyto drive the compressor section of the engine. The restof the energy produced by the gas generator is in theform of hot, high-pressure, high-velocity gas, whichdrives the power turbine. As shown in Figure 1, thepower turbine is not mechanically coupled to the gasgenerator – there is no shaft linking the two compo-nents.* They run at different rotational speeds and arecoupled aerodynamically by the hot gas produced inthe gas generator.

*While this is true for most aeroderivatives, there are someexceptions. For example, the GE LM6000 has a power turbinethat is an integral, direct-coupled part of the engine rather thana separate aerodynamically coupled element.

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[Vol.25 No.1 2005] ORBIT 51

A P P L I C A T I O N S

Industrial Gas Turbines

Unlike aeroderivative gas turbines, industrial gas tur-bines are not derived from a design with a different primary purpose. Instead, they are intended only forindustrial applications, and they employ design featuresthat reflect industrial needs, rather than the needs ofaircraft. They use fluid-film bearings for both radialand axial shaft support. They generally employ conven-tional lube oil and seal oil auxiliary systems, and theyfeature heavier construction, since the intense weight-driven concerns that govern aeroderivative designs arenot present. And, unlike the concept of a gas generatorsection and a separate reaction turbine present in mostaeroderivatives, many industrial gas turbines have adirect-coupled, integral power turbine that extracts allthe energy needed to both drive the compressor sectionof the turbine and transmit torque to the driven equip-ment through an output shaft.

While these are obviously not the only differencesbetween industrials and aeroderivatives, they do high-light some of the fundamental differences. As will be shown, the approaches to vibration monitoring used for these machines stem largely from the type ofbearings employed, causing distinctly different systemsto be used for aeroderivatives than for industrials inmost cases.

Variations

Categorizing gas turbines into one of two families,aeroderivative or industrial, does not fully reflect someof the nuances of design within the industrial family.Clearly, referring to a particular gas turbine as an“aeroderivative” is quite straightforward – it was eitheroriginally intended for use on aircraft (and has simplybeen adapted for industrial applications), or it was not.However, it is also typical for industrial gas turbines tobe further subcategorized into “packaged” and “heavy”classifications, and we will use this nomenclature whena distinction is necessary in this article.

By “packaged” industrial gas turbines, this article isreferring to those with power outputs generally less thanabout 20MW which are available as a completely pack-aged system on a single, movable skid – the gas turbine,auxiliaries, driven machine (often generator, compres-sor, or pump), and sometimes even the controls.Although not derived from an aircraft design, this

allows packaged industrials to enjoy one of the mainadvantages of aeroderivatives – namely, the ability torapidly “swap out” the entire gas turbine when majormaintenance is required. In contrast, larger gas turbines(sometimes called “heavy” or “heavy-duty”) are notpackaged in this single-skid fashion. Consequently, it ismore common to see maintenance performed in-situon these units, or with individual components or enginesections sent to a repair facility. The “engine swap”method is not generally a viable option for thesemachines.

Hybrids

It is also worth mentioning several new gas turbinesthat have emerged over the last few years, blendingdesign features that were traditionally the exclusivedomain of either aeroderivatives or industrials, but notboth. Although these ‘hybrid’ designs incorporateselected elements of both categories to a greater or lesserextent, they tend to align as either industrials oraeroderivatives in terms of the way they are monitored,generally a function of the types of bearings used.

Maintenance

Because aeroderivative gas turbines derive from enginesoriginally designed for aircraft, it is useful to considerthe difference in maintenance practices that have his-torically been employed in the aerospace industry, com-pared with the practices used in the power generationand hydrocarbon processing industries where industrialgas turbines are primarily focused.

Since aerospace applications of gas turbines, whethermilitary or civilian, involve transportation of humanbeings, both as passengers and as pilots, safety is of para-mount concern. An engine failure in an aircraft has amuch higher likelihood of jeopardizing human life thana gas turbine failure in most industrial settings. Not sur-prisingly, aerospace maintenance practices are prima-rily driven by safety concerns. Failures simply cannotbe tolerated because they impact much more than justbusiness interruption. Governing agencies mandate that engines be maintained and inspected at specifiedrunning-hour intervals.

In addition, an aircraft is not in the sky 100% of the time, running continuously. Thus, the ability to perform inspections and maintenance during these

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52 ORBIT [Vol.25 N o . 1 2 0 0 5 ]

relatively frequent windows of “down time” is morepractical than with industrial machines, which gener-ally run continuously. Consequently, gas turbine main-tenance in the aerospace industry employs the practiceof “swapping out” an engine or major sub-assembliesin need of inspection and repair with a fresh engine orsub-assembly, much as one might take a faulty starterout of an automobile, replace it with a refurbished unit,and send the old starter in for repair and refurbishment.The aerospace industry has become quite proficient atrapid change-out of parts and even entire engines. Thispractice allows aircraft to remain in service as much aspossible, but without jeopardizing safety.

In contrast, industrial gas turbines are often involved in continuous service that does not have frequent “normal” machine downtime of sufficient duration toperform maintenance and inspections. Instead, main-tenance outages must be scheduled, often only once peryear or even less frequently. While industrial gas tur-bines rely in part upon set intervals for determiningwhen maintenance is required, such as a fixed numberof operating hours for a set of blades, they also rely heav-ily upon continuous condition-based information fordetermining maintenance. Industrial gas turbines (withnotable exception of some packaged units as mentionedearlier) are generally not removed from service andreplaced with a refurbished unit, as is often the case foraeroderivatives. This is partly due to the historical main-tenance practices in the process industries versus theaerospace industry, but it is also due to the design ofthe machines themselves. Heavy industrial gas turbinesare simply not designed to be lifted out of place as a sin-gle unit and replaced with a new unit. In contrast,aeroderivatives (and some packaged industrials) arealmost always designed to facilitate such a “drop in”replacement approach.

The aerospace heritage of aeroderivatives helps explainwhy the industrial usage of aircraft-derived gas turbineshas resulted in maintenance practices and contractualscenarios that are often closer to those of the aerospacesector than the industrial sector. Many aeroderivativeusers will even contract with an Original EquipmentManufacturer (OEM) to have entire spare engines on-hand, or will purchase spare engines and keep them intheir own warehouse for rapid change-outs should a fail-ure occur. In some instances, a crew can change out anentire engine in as little as eight hours.

Before turning to a discussion of condition monitoringdifferences between these two types of gas turbines, it isimportant to note that differences in maintenancephilosophies should not be construed as being inferioror superior to one another, or that the aerospace indus-try is “safe” while others are not. Aircraft engines simplyfunction under very different operating and business con-ditions than do gas turbines in an industrial setting. Themaintenance practices and philosophies that have evolvedto address these machines reflect those differences.

Condition Monitoring and Machinery Protection

Understanding the background behind the two basiccategories of gas turbines – aeroderivative and indus-trial – is helpful because it provides a framework forbetter understanding why two approaches to conditionmonitoring for gas turbines have arisen over the years.Basically, aeroderivatives use one approach while indus-trials use a different approach, as discussed next.

A P P L I C A T I O N S

GAS TURBINE MAINTENANCE IN

THE AEROSPACE INDUSTRY EMPLOYS

THE PRACTICE OF “SWAPPING

OUT” AN ENGINE OR MAJOR SUB-

ASSEMBLIES.

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[Vol.25 No.1 2005] ORBIT 53

A P P L I C A T I O N S

ME A S U R E M E N T DE S C R I P T I O N

TABLE 1 | RECOMMENDED VIBRATION MONITORING FOR INDUSTRIAL GAS TURBINES

Two proximity probes should be mounted in an X-Y (i.e., orthogonal) configuration at or near each bearing, directlyobserving the relative motion between bearing and shaft. Recommended as a shutdown parameter.

Two proximity probes should be used, observing the axial position of the shaft within the thrust bearing clearance, andvoting1 these two probes. Recommended as a shutdown parameter.

A Keyphasor® transducer should be supplied on each independent (non-mechanically coupled) shaft to provide a once-per-turn reference signal for determining vibration phase and shaft rotative speed. It is generally recommended that theKeyphasor transducer be installed on the driver in a machine train, so that if the unit is run uncoupled, a phase referencesignal will still be available. This is not possible when a Keyphasor transducer is only installed on the driven machine.

Radial Bearings

For machines that are extremely compliant (i.e., exhibit significant motion between the bearing housing and free space),seismic vibration transducers should be mounted on the machine’s casing (typically, the radial bearing housings), andthese signals should be used along with the radial bearing proximity probes for assessing machinery malfunctions and foruse as shutdown protection when vibration becomes excessive. If a gas turbine does not exhibit significant compliance inits casing or mounting structures, these measurements may be considered optional, as the radial proximity probes willgenerally provide superior machinery protection and diagnostic capabilities.

Gravity sag causing extremely massive rotors to bow from their own weight after sitting idle for an extended period oftime is a well-known phenomenon on large steam turbines. This can also affect many of today’s largest gas turbines.Allowing a machine to reach speed when too much bow is present can cause extensive damage and a direct or indirectmeasurement of shaft bow is often used as a control system permissive, ensuring the machine is not started whenresidual bow is outside of permissible limits. An eccentricity monitor is designed to measure this residual shaft bow duringthe slow speeds encountered as part of start-up and while on turning gear. It is measured using a single radial proximityprobe. On gas turbines, the most accessible location for this measurement will often be near a coupling, several shaftdiameters away from the nearest bearing. This ensures the probes are not located too near a nodal point.

Casing Vibration2,3

Eccentricity

1. A discussion of voting logic and philosophies is beyond the scope of this article, but suffice to say that a thrust bearing failure, unlike high vibrationat a radial bearing, can actually cause a probe to go beyond its linear range and can be indistinguishable from an instrument failure under somecircumstances. To assist in preventing false shutdowns, two probes are used in redundant fashion and the signals are voted. Only when both probesshow excessive axial movement is a high thrust excursion annunciated. For additional information, refer to the article “Voting Thrust Measurementswith Other Parameters” in the First Quarter 2001 issue of ORBIT, pp. 51-54.

2. For additional information on this measurement, refer to the article “Vibration Characteristics of Industrial Gas Turbines” in the Third Quarter 2000issue of ORBIT, pp. 18-21.

3. Occasionally, some manufacturers will include only seismic transducers on smaller industrial gas turbines. While these afford a degree of machineryprotection capabilities, proximity probes will generally also be available from the OEM as a standard option. We recommend the installation ofthese additional proximity transducers because they allow superior machinery diagnostic information – particularly important when the machinewill be in unstaffed or inaccessible locations such as offshore platforms. They also allow a greater degree of machinery protection because they aresensitive to machinery problems that originate at the rotor (such as bearing preloads, bearing wear, and other conditions) that may not transmitfaithfully to the machine’s casing and/or be observable with casing-mounted seismic transducers.

NO T E S :

Thrust Bearings

Shaft Speed / Phase Reference

RE C O M M E N D E D F O R A L L D E S I G N S

SU P P L E M E N TA L M E A S U R E M E N T S R E C O M M E N D E D F O R S E L E C T E D L A R G E-F R A M E M A C H I N E S

Industrial Gas Turbines

Vibration monitoring for industrial gas turbines is much the same as most other types of high-speed turbomachinery that employ fluid-film bearings. Thisis summarized in Table 1. Similar recommendationsoccur in widely used industry standards pertaining togas turbines .

Almost every manufacturer of industrial gas turbineswill provide provisions (i.e., holes drilled and tapped)for the radial vibration, axial position, and Keyphasor®

probes noted in the first three entries of Table 1. Often,these transducers are used as part of acceptance testingand commissioning. For the oil and gas industries, thisrequirement is typically driven by American Petroleum

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Institute (API) Standard 616, covering industrial gas tur-bines, and calls for not only the use of proximity probesduring acceptance testing of the machine, but also theirinclusion as part of a permanent machinery protection(monitoring) system. The details of this permanentmonitoring system are covered in a separate standard,API 670. It is common for end users to use this standardin industries beyond just petroleum, such as power gen-eration. Thus, gas turbines used across many differentindustries will often be supplied with transducers andmonitoring systems in accordance with API 670.

Regardless of whether API 670 is specified or not, inmany cases the manufacturer’s standard offering formachinery protection will include a Bently Nevadamonitoring system, such as 3500, 3300, or 1701, con-nected to the transducers noted in Table 1 and inter-connected to the turbine control system for alarmingand auto-shutdown. In other cases, the manufacturerwill leave the transducers on the machine, but they willnot be connected to a permanent monitoring systemunless specified by the end-user, in which case the man-ufacturer generally provides a standard option for aBently Nevada monitoring system. In still other cases,the manufacturer will not provide either the transduc-ers or the monitoring system, unless the customer spec-ifies otherwise. However, holes will generally be drilled

and tapped to allow for the installation of trans-ducers for connection to a monitoring system ata later time. It is strongly recommended that endusers ensure that all machines they purchase areat least supplied with provisions for proximityprobes, as this is generally far less expensive andinvasive than a project to drill and tap holes in amachine in the field as part of a later transducerretrofit.

It is important to note that considerable engineer-ing often must be applied to accommodate vibra-tion transducers, particularly when they areembedded in the machine, as is frequently the casewith proximity probes. Gas turbines can present special challenges, such as routing cables throughareas with high temperatures and/or areas in directcontact with lubricating oil. Machinery OEMs haveoften worked collaboratively with us to develop spe-cial solutions to such issues. For example, one well-known manufacturer routes the cables for certainbearings on its industrial gas turbines through oilreturn lines, ensuring that the cables remain well below their upper temperature ratings, but presentingspecial challenges for sealing against oil leakage.Bently Nevada high-pressure feed-through cableassemblies are available in special lengths to preventpressurized oil migration on the outside of the cable,as well as the incorporation of FluidLoc® technologyin many of our newest transducer cables that preventsoil migration through the inside of the cable. Othermanufacturers have worked with us to develop specialprobe configurations, unique to their machines, thataddress tight or difficult-to-access mounting locations.

Obviously, we advocate not just installation of thetransducers noted in Table 1, but also their connectionto a permanent monitoring system. And, the vast major-ity of gas turbine manufacturers go beyond just making‘provisions for’ these transducers and instead provideboth permanently installed transducers and monitor-ing systems as standard. They, and their customers,understand the valuable role a monitoring system pro-vides in protecting the gas turbine and in helping todiagnose problems that may arise during the machine’sservice. Indeed, the emergence of long-term service con-tracts have helped OEMs partner more closely withtheir customers, allowing the OEM to play a more

54 ORBIT [Vol.25 N o . 1 2 0 0 5 ]

A P P L I C A T I O N S

API 616 is one of the most

widely used standards for

industrial gas turbines.

It references API 670

for vibration

monitoring.

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[Vol.25 No.1 2005] ORBIT 55

active role in operating and maintenance decisions overthe life of the machine. This has led to more prevalentutilization of not just transducers and permanent monitoring systems, but also the diagnostic and condi-tion monitoring software connected to such systems,allowing end users and OEMs alike to more easily incor-porate detailed condition information into their assetmanagement decisions.

These are just a few examples, intended to underscorethe importance of working with the OEM when speci-fying vibration monitoring for new or retrofit applica-tions. We are very proud of the working relationshipsthat have been established with virtually every gas turbine manufacturer worldwide. These relationshipsallow us to supply pre-engineered monitoring solutionsthat have been approved by the OEM for use on theirmachines, and which reflect both the needs of the OEMand the end user. Because of the familiarity they havewith our quality solutions, you can confidently workwith your machinery OEM and ask for Bently Nevadaproducts and services by name.

Aeroderivative Gas Turbines

As has already been discussed, aeroderivative enginesdiffer from their industrial counterparts in at leastthree very important respects pertaining to conditionmonitoring:

r They were originally designed for use as aircraftpropulsion, and are usually adapted for industrialapplications by the use of an aerodynamically coupledpower turbine which converts engine thrust intomechanical torque;

r They use rolling element, rather than fluid-film,bearings (with exception of the aerodynamicallycoupled power turbine section, which sometimesuses fluid-film bearings);

r The maintenance practices surrounding aeroderiv-atives rely predominantly on changing the engineout in its entirety, rather than performing in-siturepairs.

Next, the implications of each of these differences areexamined.

Aircraft Heritage

An aircraft engine does not run continuously. While it must be extremely dependable in flight, once the aircraft is on the ground a failure may be inconvenient– and perhaps expensive – but it is not life threatening.Conversely, when the engine is running during a flight,one is not likely to stop it unless it sustains truly catastrophic damage (such as foreign object damage),which keeps it from operating. Although modern commercial aircraft are designed to be able to operatesafely even with the loss of one engine, an engine is notturned off in-flight unless warranted by the mostextreme circumstances.

With this in mind, it is easy to understand that as air-craft engines evolved and vibration monitoring wasintroduced, the purpose of vibration monitoring wasnot so much to provide detailed mechanical diagnos-tics as it was to provide very basic protective functionsfor failures that might be encountered during a flight –namely, foreign object damage as mentioned above.Such damage tends to result in loss of blades and grossimbalance, which generally can be detected by examin-ing the vibration amplitude of frequencies primarilycentered about the running speed (1X) of the engine.Consequently, the vibration monitoring for in-flightengines is generally designed to filter out everythingexcept the basic shaft rotational speeds. On someengines, this is accomplished by processing the vibra-tion signal through a narrow band-pass filter that atten-uates everything except a very narrow band offrequencies centered about the running speed of theengine. The center frequency of the filter automatically“tracks” up and down as the engine changes speed byuse of a speed signal from the engine shaft. On otherengines, the filter does not track; it simply uses a broadenough pass-band to encompass the 1X vibration com-ponent for the envisioned range of running speeds.Also, many aeroderivatives use concentric shafts whichpermit compressor and turbine sections to operate atdifferent speeds, so there are often multiple shaft speedsthat must be monitored.

Regardless of whether a tracking filter or fixed-band fil-ter is used, by looking at only the 1X aspects of thevibration signal, the pilot is alerted to catastrophic

A P P L I C A T I O N S

GAS TURBINES USED ACROSS MANY DIFFERENT INDUSTRIES WILL OFTEN BE SUPPLIED

WITH TRANSDUCERS AND MONITORING SYSTEMS IN ACCORDANCE WITH API 670.

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engine damage, but certainly not subtle changes in the turbine such as bearing wear. This is by design.There is no point concerning a pilot with such infor-mation when what is needed is merely a go/no-go type of signal that indicates whether the engine has sustained damage that is too serious to allow continuedoperation.

As aircraft engines began to be adapted for industrialapplications, this approach to monitoring that hadevolved for use on “flying” engines remained largelyunchanged. Hence, the monitoring systems on mostaeroderivatives today suppress most aspects of the vibra-tion signal with the exception of 1X by using band-passfiltering as described above. While monitoring thesefrequencies is useful for detecting gross problems suchas loss of blades and severe imbalance, it is not reallycapable of permitting advanced condition monitoringto the extent possible on industrial gas turbines. This ispartly due to the signal processing performed on theraw vibration signal, which suppresses many frequencycomponents as just explained. However, it is also dueto the types of transducers and mounting locations, as discussed next.

Rolling Element Bearings

Because aeroderivative engines use rolling element bearings, bearing vibration cannot be addressed withconventional proximity probes – the rotor is essentiallyrigidly fixed to the bearing providing no relative vibratory movement between bearing and shaft. Thus,mounting a conventional proximity probe to observethe shaft vibration relative to the bearing (as is donewith industrial gas turbines) is not an option. Oneapproach that we pioneered is to install special high-gain proximity probes in aeroderivatives that observethe outer ring of the bearing. As the elements of thebearing pass over a fixed location on the outer ring, tinydeflections occur. These deflections are observable bythe probe and provide both rotor-related and bearing-related information; however, this practice is not yetwidespread.

[Editor’s Note: For additional information on this monitoringmethod, refer to the article “Aeroderivative Internal TransducerDesign” in the Second Quarter 2001 issue of ORBIT]

A P P L I C A T I O N S

A WORD ABOUT POWER TURBINES…Aeroderivative gas turbines, as previously discussed, often incorporate an aerodynamically

coupled power turbine, which in turn drives a load such as a generator or pipeline com-

pressor. Often, the power turbine, since it is used only in industrial applications, will use

fluid-film bearings rather than the rolling element bearings typical of aeroderivative gas

generators. In this case, it is recommended that the power turbine be monitored with con-

ventional radial proximity probes in the familiar X-Y configuration. In other cases, the power

turbine uses rolling element bearings, in which case it is monitored with casing-mounted

transducers, just as the gas generator portion of the machine. Internally mounted bearing

housing accelerometers, as discussed above, are also a viable option for these designs.

Bently Nevada does not recommend the exclusive reliance upon casing-mounted seismic

transducers for power turbines that employ fluid-film bearings. Fluid-film bearings serve

to dampen rotor-related vibrations and simply do not allow sufficient transmission of these

vibrations to the machine casing in many instances.

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[Vol.25 No.1 2005] ORBIT 57

Another approach that is gaining interest among usersis to mount accelerometers on the bearing housings,inside the machine. This approach does not require sig-nificant modifications to the bearing support structureand puts the transducer much closer to the source ofvibration. Unlike fluid-film bearings, a rolling elementbearing very stiffly couples shaft and bearing vibrationto the bearing support structure. Thus, seismic trans-ducers can be a good choice for these machines, pro-vided they are mounted in a location to which shaft,bearing, and other vibration is faithfully transmitted.Because the transducers are “closer to the action” whenmounted internally in this fashion, they are better ableto observe bearing degradation and rotor-related prob-lems (such as imbalance and misalignment) than trans-ducers mounted on the outside of the engine. Also, the temperatures near the bearings – although insidethe machine – are, ironically, less extreme than whentransducers are mounted on the outside of the machine.OEMs and end users are encouraged to contact us ifthey would like to participate in this approach. Thistechnique is presently being tested at several locationsand has received support during dialogs with customersat several industry conferences.

While internally mounted transducers show potentialfor providing superior machinery condition informa-tion and machinery protection capabilities, mostaeroderivatives are not currently monitored with inter-nal vibration transducers. Instead, these machines havehistorically been monitored by mounting casingaccelerometers on the outside of the engine. The trans-ducer mounting locations are generally chosen by theOEM to coincide with engine frame points that repre-sent the most direct bearing-to-structure load path thatis externally accessible.

The signal processing applied to these transducers gen-erally incorporates band-pass filtering, designed to cap-ture little more than the 1X amplitude. In addition tothe reasons already noted for such filtering, anotherpractical reason is to ensure that the large amplitudesin the acceleration signals do not “swamp” the smallerrotor-related signals that are present. Most often, thesehigh-frequency, high-amplitude signals are generatedby blade/nozzle pass, by acoustic airflow through thegas generator and power turbine sections, and by com-bustor noise. Although the amplitudes can be high, the

actual energy and destructive capabilities may be small.Even though acceleration units are the intrinsic outputof an accelerometer, most aeroderivatives do not useacceleration units for monitoring. Instead, it is custom-ary to integrate or double-integrate the signal, in addi-tion to the band-pass filtering mentioned above,resulting in either velocity units (integrated accelera-tion) or displacement units (double-integrated acceler-ation). Although some OEMs recommend that bothvelocity and displacement units be used as part of a“dual path” monitoring scheme with alarm levels estab-lished for both, it is more typical to see machines thatuse velocity or displacement units, rather than both.Depending on aeroderivative engine size, make, andmodel, anywhere from one to three casing-mountedseismic transducers are typically supplied on the gasgenerator. As examples of this variation in the numberof sensors and the signal conditioning applied to each,one popular two-spool machine (concentric shafts) usestwo casing accelerometers, with tracking filters, to pro-vide the 1X amplitude at both shaft speeds. Anothermanufacturer supplies one of its larger engines withthree accelerometers with a fixed band-pass filter in eachaccelerometer circuit. The filters pass frequenciesbetween 40 Hz and 350 Hz to the vibration monitor.Still another popular engine has historically been supplied with a single seismic transducer and a filterthat suppresses all frequencies below 110 Hz. Thus,although the general approach is to use casing-mountedtransducers exclusively and some form of signal condi-tioning that intentionally limits the frequency contentseen by the monitoring system to a rather narrow band,there are very few other generalizations that can be maderegarding the monitoring systems used on aeroderiva-tive engines. The sensor scale factor, the sensor mount-ing footprint, the sensor mounting locations, the filterroll-offs and band-pass ranges, integration or double-integration – all tend to differ from one manufacturerand one engine to the next, and are normally detailedin a monitoring specification generated by the OEM.Bently Nevada monitoring systems are available toaddress these OEM specifications.Another aspect of monitoring aeroderivatives relates to the frequent requirement for a setpoint multiplier(often called “trip multiply”) capability in the monitoringsystem. This is a feature of the monitoring system

A P P L I C A T I O N S

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whereby the alarm setpoints are temporarily elevatedduring start-up to allow the machine to pass throughits first, and sometimes second, balance resonance(s)without generating spurious vibration alarms. Becauseaeroderivatives employ rolling element bearings, theyexhibit considerably less damping than machines usingfluid-film bearings or squeeze-film dampers. Thisresults in higher synchronous amplification factors (i.e.,the ratio of vibration amplitude as the machine passesthrough a resonance to the vibration amplitude whenoperating well away from a resonance is quite large).Although the use of setpoint multiplier functionalityis not strictly limited to aeroderivative turbines, its useis more predominant on these machines due to therotordynamic response characteristics just discussed.Also, some OEMs use an inhibit function that bypassesalarms altogether, rather than setpoint multiplication.However, when feasible, it is advisable to use setpointmultiplication rather than complete bypass, as thisensures that the machine will always have some type ofvibration alarms enabled, regardless of its operatingstate.

Aeroderivative Maintenance Practices

The third difference between aeroderivatives and indus-trial gas turbines with respect to how they are moni-tored lies in the way that they are maintained. As hasalready been discussed, aeroderivatives rely more onmaintenance practices that simply “swap out” anengine, rather than attempt in-situ repairs. The ten-dency for industrial units to incorporate more compre-hensive condition monitoring is due, in part, to theinability to easily swap out the gas turbine. While thisis feasible on aeroderivatives, and many industrial“packaged” units, it is not possible on so-called heavyindustrial units. Hence, the need to diagnose themachine in-situ drives a greater reliance on conditionmonitoring achievable from vibration data. This is notto say that aeroderivatives and packaged industrials can-not benefit from more capable condition monitoring.It is merely to point out that the time to swap out anengine is often measured in hours, rather than days,and the relative speed with which an engine can simplybe replaced when there are problems allow aeroderiva-tive (and some packaged industrial) users the luxury ofmaintenance options that are not practical for heavyindustrial units.

A P P L I C A T I O N S

ANOTHER ASPECT OF MONITORING

AERODERIVATIVES RELATES TO THE

FREQUENT REQUIREMENT FOR A

SETPOINT MULTIPLIER (OFTEN CALLED “TRIP

MULTIPLY”) CAPABILITY IN THE

MONITORING SYSTEM.

THE SETPOINT MULTIPLIER FUNCTIONALITY OF

A MONITORING SYSTEM, AND THE REASONS

FOR ITS USE, ARE DISCUSSED IN MUCH

GREATER DETAIL IN API 670, APPENDIX I.

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Recommendations for Vibration Monitoring on Aeroderivatives

The vibration monitoring supplied with most aero-derivative packages is designed to provide a measure ofbasic machinery protection, but does not generallydeliver the same level of diagnostic information for in-depth condition assessment that can be achieved onindustrial gas turbines fitted with proximity probes.This is not to say that condition monitoring onaeroderivatives is impossible, or does not provide value.It is merely to point out that aeroderivatives, comparedwith their industrial counterparts, present a differentand more challenging set of constraints to the diagnos-tician. There are several things that can be done toimprove the condition monitoring capabilities onaeroderivatives. The first, and often the simplest, is tobring not only the band-pass filtered vibration signalinto the monitoring system (which is used for machin-ery protection purposes), but to also bring the unfil-tered acceleration or velocity signal into the monitoringsystem. By doing this, the “raw” signal can be used bycondition monitoring software to look at the fullfrequency content, providing additional useful signalcontent rather than merely the 1X amplitude which isgenerally all that is available within the band-passfiltered signal. Although the broadband accelerationsignals are often difficult to interpret due to the issuespreviously discussed, improved methods are beingdeveloped to extract meaningful condition monitoringinformation from these signals. Another strategy forimprovement, as previously discussed, is to mount seis-mic transducers internally, on or near the embeddedbearings, close to the source of vibration.

Regardless of whether a user seeks to increase the con-dition monitoring capabilities on their turbines or merely ensure that the existing instrumentation con-tinues to operate trouble-free, the importance ofobserving proper seismic transducer installation prac-tices on aeroderivatives cannot be over-emphasized, asdiscussed next.

Seismic Transducers

The seismic transducers used on aeroderivatives (andindustrial gas turbines) are specially engineered to with-stand the extreme temperatures and g-forces encoun-tered. While the high surface temperatures of gas

turbines are readily apparent and are rarely overlooked,many users are not similarly aware of the high g-forcesencountered on these machines, requiring special instal-lation considerations for sensors, cables, and connec-tors. It is very typical for aeroderivatives to havesteady-state vibration of 20 g’s or more, with peaks ofup to 100 g’s regularly occurring. This introduces anumber of mechanical fatigue concerns in the instru-mentation system which have been responsible for morethan their fair share of false alarms and trips.Fortunately, these can generally be overcome by carefulinstallation practices, summarized in the five recom-mendations below:

t Use a solid-state transducer without movingparts. Velocity transducers using moving-coiltechnology are sometimes used for gas turbineseismic measurements. Due to the very aggres-sive g-forces and surface temperatures encoun-tered on gas turbines, the moving parts in suchsensors degrade and have a limited useful life.Unfortunately, they do not degrade linearly, andit is difficult to predict when they will fail. Forthis reason, we advocate the use of solid-stateaccelerometers which are not susceptible to thistype of degradation since they do not employmoving parts. Also, moving-coil sensors aredesigned to measure vibration in one axis only.However, gas turbines sustain considerable vibra-tion in all three axes, and this cross-axis vibrationat very high g-levels is particularly damaging tomoving-coil sensors, accelerating their fatigue.While industrial gas turbines will sometimesemploy moving coil sensors with better results,this is largely because such turbines use fluid-filmbearings which tend to dampen the vibration seenat the machine casing. Aeroderivatives do nothave the “benefit” of this damping, so the use ofmoving coil transducers on these machines isstrongly discouraged because they will generallyfatigue much too rapidly.

t Where possible, use a transducer with an integralcable. The connection between the accelerometerand its extension cable is a very common sourceof failures. As a high-impedance device, it is

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4particularly sensitive to dirt, oil, or other debristhat can invade this connection. Also, when a con-nector is located on the engine, the high g-forcescan tend to loosen the connection. Both of theselead to intermittent connection problems whichresult in noise and “spiking” in the signal, oftencausing false alarms and even false trips. In fact,this is the single most common problem encoun-tered with aeroderivative gas turbine vibrationinstrumentation systems. Ideally, a transducerwith an integral cable is recommended. Thisallows the connection between the sensor and theremovable cable to be located off the engine,where vibration and contaminants that serve tocompromise this connection are less of a problem.When this is not possible, it is very important tokeep this on-engine connection tight and to pro-tect it from contaminants.

t Anchor the cable securely. Regardless of whetheran integral cable is used, it is extremely importantto anchor the cable securely. This is particularlytrue for so-called “hardline” cable that is quitestiff. When not anchored adequately and at fre-quently short intervals, the cable can vibrate –behaving like a miniature piece of unsupportedpiping – leading to high-cycle fatigue at the con-nector or anywhere along the cable. This can leadto a number of problems, the most common beinghigh-cycle fatigue at the junction between cableand sensor, even when this is a permanent moldedor welded connection. Another common prob-lem is that the cable will chafe against another partof the machine, eventually wearing out the cableand causing noise or other intermittent problems.When clamping the cable, be certain to use clampinserts that will not degrade and can sustain thesurface temperatures encountered. Otherwise, theclamp inserts will deteriorate, allowing metal-to-metal contact and the reintroduction of high-cyclefatigue and chafing problems. Another reason foranchoring the cable is that many aeroderivativeaccelerometers are charge-coupled devices andcable movement can result in a triboelectric effect,generating noise that interferes with the actualvibration signal.

t Be sure that brackets are properly designed. As istrue for any transducer installation, it is veryimportant to ensure that transducer mountingbrackets will faithfully transmit the vibration ofthe machine to the sensor without introducingtheir own vibration characteristics. The resonanceof the bracket must be well above the vibrationresponse of the transducer, and the bracket mustbe rigidly affixed. A bracket that may seem “stiff”may in fact be stiff in only one axis, and attentionmust be paid to stiffness not only in the measure-ment axis, but all other axes as well. In addition, abracket that may be tight today may not be tighttomorrow due to the high g-forces previously dis-cussed. Be sure to make certain that any boltedconnections are tight – and stay tight. Surface fin-ish is also an important consideration withaccelerometers. Small imperfections on themounting surface, whether directly on themachine or on an intermediate bracket, can intro-duce strain (force) into the accelerometer, distort-ing its signal since it is inherently a forcemeasuring transducer.

t Mount interface modules in a vibration-free environment. Some acceleration sensors (such asthe new High-Temperature Velocity AccelerationSensor – see page 76) integrate the charge ampli-fier and other signal conditioning electronics aspart of the sensor and cable assembly. In thesecases, the sensor, cable, and signal conditioningelectronics are designed to sustain the vibrationnormally encountered in their mounting loca-tions. However, sensors using external chargeamplifiers and/or integration and band-pass fil-tering electronics must take care to mount anyexternal signal conditioning (often called anInterface Module) in an environment with littleor no vibration. If these practices are not observed,internal components and the field wiring connec-tions at the Interface Module can vibrate looseover time, causing intermittent problems and pos-sibly even false alarms.

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Additional Solutions

Today, a much larger selection of measurements for protecting and managing gas turbines is available, complementing the vibration monitoring traditionallysupplied. Many of these can be integrated into aBently Nevada® 3500 Series Machinery ProtectionSystem. Others act as independent systems that can beintegrated within System 1® optimization and diagnos-tic software.

t Overspeed DetectionOne particularly important aspect of machineryprotection for gas turbines is overspeed detection.The 3500 Series is a very capable platform for pro-viding this important measurement. There arevery strong economic and safety incentives toreplace the mechanical overspeed bolt, andupgrading to an electronic overspeed detectionsystem can often pay for itself immediately byeliminating the process interruptions that nor-mally occur as part of testing an overspeed bolt.

[Editor’s Note: You can read more about overspeed inthe feature-length article on page 16.]

t Combustion MonitoringToday’s lean-burning gas turbines must operateon the verge of flame-out in order to achieve theextremely low emissions that are being demanded.Combustion instabilities are a consequence of this lean-burn operation, resulting in pressure pul-sations in the combustors. If left unchecked, thesepulsations can quickly damage components, andthe ability to detect incipient and actual combus-tion instabilities are thus important. The 3500Series can be used not only for vibration moni-toring and overspeed detection, but also for mon-itoring pressure pulsations in gas turbinecombustors. The 3500/64M Dynamic PressureMonitor provides this capability and is designedto be used with a variety of industry-standarddynamic pressure transducers. It is already widelyused on several aeroderivative and industrial gasturbines and is designed for inclusion by OEMson new units and for retrofit to field units by endusers or OEMs.

t Flame SensingGE Energy’s Flame Tracker™ sensor is an innova-tive solution for monitoring the presence of aflame, as needed for permissives during start up,and to ensure that fuel does not continue to flowand build up to potentially explosive levels if aflame-out occurs during start-up or steady-stateoperation. The inability to reliably detect a flamemanifests itself in either falsely detecting thepresence of a flame when one is not present (asafety hazard), or the failure to detect a flame whenone is actually present (entailing an expensiveprocess interruption and restart with subsequentimpact on hot gas path component life). TheFlame Tracker sensor uses Silicon Carbide (SiC)technology to provide a reliable and robust measurement that is ten times more sensitive than Geiger-Mueller sensors and much less sus-ceptible to the conditions (such as steam injec-tion) that often cause flame sensors to give falsereadings. A retrofit of these sensors often has avery rapid payback by eliminating false flame-out indications.

[Editor’s Note: You can read more about Flame Trackersensors in the feature-length article beginning on page 77 in this issue.]

t Exhaust Gas Temperature (EGT) profilesSystem 1® optimization and diagnostic softwarenow features the ability to monitor this impor-tant parameter, indicative of hot gas path compo-nent life and proper combustion. Temperaturedeviations, differentials, and a variety of OEM-and user-defined variables can be calculated,allowing advanced monitoring of the hot gas path.

[Editor’s Note: You can read more about this new plottype beginning on page 88.]

t Performance MonitoringThermodynamic performance information is particularly relevant to gas turbines, providinginsight into efficiency degradation and allowingoptimal maintenance planning, such as identify-ing the ideal time to conduct a compressor wash.Because gas turbines are often used as part of a

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larger thermodynamic system, such as combinedcycle or cogeneration plants, end-to-end heat bal-ance monitoring of all parts of the plant (not justthe gas turbine) are important as well.

[Editor’s Note: The article on page 64 discusses ouronline software solutions for both single machine trainperformance monitoring as well as plant-wide heatbalance monitoring.]

t Hazardous Gas DetectionIt is very common to find control panels for gasturbines (particularly those in the oil & gas sectorsuch as pipeline compressor stations) fitted withboth vibration monitors and hazardous gas detec-tion monitors. Combustible gases are often usedas the fuel for gas turbines and/or in the processesfor which gas turbines drive pumps or compres-sors. Leaks of such gases must be detected to pre-vent the buildup of explosive levels, particularlyin enclosed spaces, and dedicated monitoring systems are used for this important measurement.Today, the 3500 Series can make this measure-ment by way of the 3500/63 Gas DetectionMonitor. The ability to achieve hazardous gasdetection by using only a single slot in a 3500 rackcan result in substantial installation savings, andalso allows easier integration to plant control andautomation systems.

[Editor’s Note: You can read more about the 3500/63in the article in the Second Quarter 2004 issue of ORBIT,page 28.]

Summary

Aeroderivative and industrial gas turbines come fromvery different backgrounds and design criteria, result-ing in different bearings, different maintenance prac-tices, and different construction. In turn, this has led todistinctly different vibration monitoring practices. Thisarticle has explored some of these differences, alongwith some of the most common approaches to moni-toring both types of gas turbines. In today’s competi-tive climate, a well-instrumented gas turbine shouldinclude more than just rudimentary vibration monitor-ing for basic machinery protection; it should alsoinclude the capabilities for more advanced conditionmonitoring, thermodynamic performance monitoring,flame detection, overspeed detection, combustor insta-bility monitoring, EGT profiling, and (in some appli-cations) hazardous gas detection.

Additional Reading

API Standard 616 – Gas Turbines for the Petroleum, Chemicaland Gas Industry Services, Fourth Edition, August 1998,American Petroleum Institute, Washington, D.C.

API Standard 670 – Machinery Protection Systems, FourthEdition, December 2000, American Petroleum Institute,Washington, D.C.

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