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1 q05 ch_one-shotbalancing


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1 q05 ch_one-shotbalancing

  1. 1. CASE HISTORY CASE HISTORYPAY B A C K P R O F I L E :One-Shot Balancing For A Gas TurbineHow collaboration between OEM, End User, andthe Bently Nevada ® team saved over 1MM USD Editor’s Note: Mr. Foiles, a rotor dynamics specialist with GE Energy at the time the events in this story took place, now works for BP. This case history transpired in earlyYork LeeField Services Manager, Bently Nevada® Products 2001, nearly a year prior to the acquisition of Bently Nevada by GE. Although the caseGE Energy history describes the remarkable results achieved by cooperation between customer, the turbine manufacturer (in this case, GE), and Bently Nevada, this level of OEM collaboration was not then – or now – intended to be unique to GE. The BentlyWilliam C. Foiles Nevada team continues to work in the strictest confidence with all OEMs, both in theRotating Equipment Specialist supply of condition monitoring products and in the application of machineryBP diagnostics expertise. We welcome the opportunity to publish additional articles collaboration with any of our OEM customers produced favorable results for all parties. Introduction This case history chronicles a machinery balancing job at a large Asian chemical complex, demonstrating how collaboration between the customer, the machinery OEM, and Bently Nevada service personnel achieved sub- stantial improvements in the cost and time required to execute this service. The Setting The chemical complex is fully integrated with an adjacent refinery, pro- ducing a combined total of over one million tons per year of propylene, ethylene, and other petrochemical derivatives. The world-scale operation also contains an integrated cogeneration facility for powering both the refinery and chemical complex. Unlike many cogeneration installations where electricity is considered the primary “product” and steam is considered a byproduct, this petrochemi- cal complex’s use of cogeneration is just the opposite: steam is the essential product, required by the hydrocarbon cracking process; electricity, in this instance, is the byproduct. This explains why the economics and machinery objectives that drive a cogeneration process will often differ from the power generation sector to the petrochemical sector. When electric power is the primary product, machinery efficiency is paramount because competitive pressures focus on the lowest cost of generation. In contrast, the continuous processing indus- tries, such as this chemical complex, have processes with enormous down- time costs – often millions of dollars per day. When a cogeneration process cannot run, steam cannot be produced, and the multi-million-dollar-per- day petrochemical process that relies upon the steam must likewise stop. [Vol.25 No.1 2005] ORBIT 5
  2. 2. CASE HISTORY THIS END USER’S PRACTICE IS TO HAVE THEIR GAS TURBINES RUN WITH VERY LOW VIBRATION LEVELS. VIEW AS SEEN FROM DRIVER END 4YD 4XD (UCZ-710) 4 (UCZ-709) 3YD 3XD (UCZ-707) 3 (UCZ-708) LOW-SPEED SHAFT Kφ (UCZ-720) 7YD 7XD 8YD 8XD (UCZ-715) 7 (UCZ-716) (UCZ-701) 8 (UCZ-702) GAS TURBINE GEARBOX GENERATOR 1 HIGH-SPEED SHAFT Kφ (UCZ-721) 2YD 2XD 1XD 1YD 2 (UCZ-703) (UCZ-704) (UCZ-706) (UCZ-705) 5YD 5XD (UCZ-712) 5 (UCZ-711) HIGH SPEED GEAR SHAFT 6YD 6XD (UCZ-714) 6 (UCZ-713) LOW SPEED GEAR SHAFT GEAR END VIEW AS SEEN FROM DRIVER END MACHINE TRAIN DIAGRAM FOR GAS TURBINES SHOWING TRANSDUCER ARRANGEMENT | FIG. 1The cost of this lost production 3000 rpm generators through tical design. The gearbox inputfar eclipses machinery efficiency speed-reducing gearboxes. The shaft utilizes offset-halves radialconcerns. As a result, machinery machines are rated at 65MW and bearings.reliability – not efficiency – is para- operate at 5,230 rpm. There are All radial bearings are fitted withmount. eight radial bearings in each orthogonal (X-Y) proximity machine train. The gas turbines probes. Two Keyphasor® (Kφ)Machinery each utilize two tilting pad radial phase reference transducers areAt the heart of the cogeneration bearings of four-segment, load- installed on each train; one onfacility are two identical GE between-pad design. Radial bear- the gas turbine drive end and oneMS6001FA (“6FA”) single-shaft ings on the generator and gearbox on the generator drive end. Seeindustrial gas turbines, driving output shaft are sleeve-type, ellip- Figure 1.6 ORBIT [Vol.25 No.1 2005]
  3. 3. CASE HISTORY POINT: UCZ-704 55° Left 1X From 15FEB2001 20:00:00 To 14MAR2001 19:00 Steady State 21FEB2001 07MAR2001 19:00 19:00 90 PHASE LAG: 30 deg/div 270 90 150 10 µm pp/div AMPLITUDE: TREND OF 1X VIBRATION 100 AMPLITUDE AND PHASE AT 50 #2 BEARING Y-PROBE PRIOR 0 TO CORRECTIVE BALANCING 19:00 19:00 | FIG. 2 21FEB2001 07MAR2001 TIME: 1 Day/divMonitoring Systems A High Vibration Problem the end user’s 55 µm alert levelsThe end-user chose Bently Nevada The cogeneration units were in the (see Table 1 on page 10). Figure 2products and services for protect- process of being commissioned in shows a one-month trend of 1Xing and managing the critical late 2000 / early 2001. This end amplitude and phase from the #2machinery in their facility. Data user’s practice is to have their gas bearing Y-probe. Notice that theManager® 2000 (DM2000) soft- turbines run with very low vibra- phase is stable and the amplitude isware is used for mechanical condi- tion levels – generally well below approximately 110 µm, twice thetion monitoring and Bently the alarm levels discussed in ISO alert alarm value established by thePERFORMANCE™ software is Standard 7919-4. As such, radial end user.used for thermodynamic perform- vibration alarms for their proxim- Data obtained during test standance monitoring. The systems are ity probes were set at 55 µm (2.17 commissioning, as well as transienttightly integrated with the plant’s mils). While one unit (train B) was and steady-state data obtained atprocess control system, allowing running very smoothly, the other site, suggested unbalance as thecorrelation of machinery condition unit (train A) was exhibiting vibra- most likely source of the high vibra-data with process data. tion amplitudes that – while still tion. After conferring with GE’s on-The end-user also has a comprehen- within standard acceptance criteria site technical advisor, the end usersive services agreement covering – were higher than the end user’s enlisted the assistance of a Bentlyproduct repair and support, system preference. Specifically, by March Nevada machinery diagnosticsintegration support, and machin- 2001, three of the four proximity engineer to confirm unbalance asery diagnostics support via 24/7 probes installed on the #1 and #2 the source of the problem and tocall-out to the Bently Nevada serv- bearings were consistently showing assist in balancing the machine. Aice team at a nearby field office. vibration amplitudes well above decision was made to perform a [Vol.25 No.1 2005] ORBIT 7
  4. 4. CASE HISTORY2-plane balance on the gas turbine, dynamics engineers were brought regarding their machines. Theand the plant’s machinery engineer into the discussion, and their OEM possesses deep knowledgeinformed his management and involvement was pivotal. that may simply be unavailable any-operations personnel that correct- where else, and should be consulteding the unbalance would take 3-4 Consulting the Experts whenever possible. Fortunately, indays and multiple starts of the tur- While Bently Nevada field engi- this case, the relationship betweenbine. A location and weight for the neers are “OEM-neutral” in their the OEM (GE), the end user, andinitial trial balance was proposed approach to diagnosing and cor- Bently Nevada personnel was that(but not actually installed). recting machinery problems, they of a team tasked with solving aHowever, as we will discuss next, it also know that there is no substi- problem, and the parties were allwas at this juncture that GE’s rotor tute for consulting the OEM able to work cooperatively. Why Both Proximity and Velocity Transducers are Important It is noteworthy that the gas turbines in this case history had proximity probes and casing-mounted velocity transducers installed. The velocity transducers were mounted vertically on the #1 and #2 bearings of each gas turbine, while the proximity probes were arranged in an X-Y configuration as shown in Figure 1. It is customary for GE to install both types of transducers on many of their industrial gas turbines, and the authors strongly advocate this as a best practice for any gas turbine (such as the 6FA) that exhibits compliant casing and support structures [1,2]. The shaft-relative measurements provided by proximity probes are typically more sensitive to rotor- related vibration problems such as imbalance, rubs, misalignment, and bearing instabilities. In contrast, casing-mounted transducers are typically more sensitive to problems originating in the casing, sup- ports, and piping. Best practice for many industrial gas turbines – particularly those that have very large frames – is to use both shaft-relative (i.e., proximity probe) and casing-mounted (i.e., seismic velocity) transducers. In this particular case history, the data from the proximity probes suggested that the unbalance was more pronounced than would have been concluded by looking merely at the velocity transducers. Also, the value of velocity transducers is in understanding the relationship between casing motion and shaft-relative motion. Recall that proximity probes measure the relative motion between their mounting location (often, the bearing housing) and the shaft. If the housing is quite stiff, casing motion will be negligible and does not generally factor into the diagnostics of the machine for activities such as bal- ancing and malfunction detection. However, if this motion is appreciable – as is the case on a 6FA – it cannot be ignored. The authors are familiar with the use of Bently Nevada dual probe monitors on large steam turbines, which allow the signals from proximity probes and bearing-mounted seismic trans- ducers to be vectorially combined for shaft relative, casing absolute, and shaft absolute signals. In our opinion, these monitors – while historically used only on large steam turbines – should also be considered on a case-by-case basis for use on certain industrial gas turbines as they provide many diagnostic and machinery protection benefits.8 ORBIT [Vol.25 No.1 2005]
  5. 5. CASE HISTORY GE’S ROTOR DYNAMICS ENGINEERS WERE BROUGHT INTO THE DISCUSSION, AND THEIR INVOLVEMENT WAS PIVOTAL.As GE’s rotor dynamics experts in ginal unbalance state. Then, the try and balance the machine on theAtlanta became aware that the end- machine is stopped, a test mass is first run, it is merely to quantifyuser was proposing to field balance added at a known location, the how the machine responds to thethe machine, they were confident machine is restarted, and the result- addition of a known balance mass.this could be accomplished success- ing vibration is measured. This isfully with their assistance, but there known as the trial run. With this Balancing With Prior Datawere several important issues that information, the machine’s re- Influence Coefficients are a charac-had to be considered: sponse to the addition of mass (i.e., teristic of the machine. They do not its Influence Coefficients) can be change unless something otherr 6FA machines are balanced computed, and the amount and than unbalance is wrong – such as during factory tests; however, location for the final corrective bal- a shaft crack – altering the relation- GE had never performed an ancing weight is determined. The ship between rotor excitation and in-situ (i.e., field) balance of the machine is stopped once again, the rotor response (i.e., the Transfer 6FA design. balance mass is added, and the Function). When a rotor’s Influ-r The unit was still under machine is restarted. This is known ence Coefficients are already warranty. as the “correction run.” known, balancing can theoretically Two-plane balancing is a similar be accomplished in a single “shot”r In order to meet the end user’s process, but requires two trial runs, since the rotor’s response to the objectives of minimal starts/stops since weight must be added inde- addition of weight has already been and the fewest possible runs to pendently at each balance plane to established. balance the machine, prior see the response, allowing compu- Unfortunately, while GE had knowledge of the Influence tation of two Influence Coefficients access to a database of Influence Coefficients would be helpful. instead of one. Ideally, a minimum Coefficients for similar 6FA units, Without this information, data of three runs (two trial and one they did not have the Influence would have to gathered empir- correction) will be required; how- Coefficients for the particular unit ically at the site by starting ever, machines do not always in question. Nor did the database and stopping the machine mul- respond exactly as anticipated to contain field data from a unit that tiple times and installing trial the addition of balance masses. was coupled to a gear. However, all balance masses. Consequently, more runs are some- was not lost. It was reasoned that by times required to obtain the desired consulting this database of rotorBalancing Without Prior Data results. dynamic response data from a pop-Single-plane balancing normally Also, without prior knowledge ulation of similar 6FA machines,takes a minimum of two “runs” – of the machine’s Influence Co- it may be possible to statisticallyone “trial run” and one “correction efficients, the initial amount of the determine the approximate Influ-run.” First, the vibration is meas- required correction is – at best – an ence Coefficients for the rotorured with the machine in its ori- educated guess. The goal is not to under consideration [3]. [Vol.25 No.1 2005] ORBIT 9
  6. 6. CASE HISTORYTABLE 1 | SUMMARY OF VIBRATION AMPLITUDESAT GAS TURBINE BEARINGS BEFORE AND AFTER 2-PLANE BALANCING OVERALL UNFILTERED 1X FILTERED AMPLITUDE AMPLITUDE AND PHASE MEASUREMENT LOCATION (µm, pk-pk) (µm, pk-pk – degrees) Alert Before After Before After Level Balancing Balancing Balancing Balancing Bearing #1 Y-axis (Gas Turbine Non-Drive End) 55 55.5 14.7 49.6 ∠ 63° 6.71 ∠ n/a Bearing #1 X-axis (Gas Turbine Non-Drive End) 55 41.1 9.7 36.0 ∠ 153° 2.12 ∠ n/a Bearing #2 Y-axis (Gas Turbine Drive End) 55 116 29.9 111 ∠ 181° 21.9 ∠ 268° Bearing #2 X-axis (Gas Turbine Drive End) 55 97.7 27.1 92.4 ∠ 266° 19.6 ∠ 11°A linear regression of this data was other methods of selecting initial machine had been allowed to runperformed, and the expected Influ- trial weights. In contrast, the best- under steady-state conditions at aence Coefficients were statistically case scenario would be to balance 50MW load for a minimum of ninedetermined. While it was antici- the rotor in a single run by placing hours before data was collected,pated that these Influence exactly the right weights in exactly allowing any thermal transients toCoefficients would be close, they the right locations on the first settle out and to help ensure thatwere not expected to be exact, since attempt. Realistically, it was data was collected under similarthey were based on a population of expected that the actual result operating conditions.similar rotors – not the particular would fall somewhere between The results exceeded everyone’srotor in question. these two extremes, particularly expectations, substantially reducingArmed with these “expected” since the population of rotors in the the vibration amplitudes measuredInfluence Coefficients, calculation GE database for which Influence at each proximity probe on the #1of the corresponding “expected” Coefficient data was available was and #2 bearings as summarizedbalance correction weights for each very limited. in Table 1.of the two planes was straight for-ward. The calculated weights would Results Figure 3 provides another view ofbe used for the initial run. The The balance masses were placed as the before/after results, using orbitworst-case scenario was simply that predicted by the model and the plots obtained from DM2000.the theoretically determined machine was restarted. Data was Another important result of the bal-weights – if not exactly correct – obtained immediately prior to and ancing job is conveyed in Figure 4,would at least be less arbitrary than after balancing, and in each case the a continuation of the trend plot of IT MAY BE POSSIBLE TO STATISTICALLY DETERMINE THE APPROXIMATE INFLUENCE COEFFICIENTS FOR THE ROTOR.10 ORBIT [Vol.25 No.1 2005]
  7. 7. CASE HISTORY BEARING #1 BEARING #2 Y: UCZ-705 135° Right VECTOR: 49.6 µm pp 63° Y: UCZ-704 55° Left VECTOR: 111 µm pp 181° X: UCZ-706 135° Left VECTOR: 36.0 µm pp 153° X: UCZ-703 35° Right VECTOR: 92.4 µm pp 266° 02MAR2001 20:37:23 Delta Time 1X COMP 02MAR2001 20:37:23 Delta Time 1X COMP UP UPBEFORE BALANCING 10 µm/div ROTATION: Y TO X (CW) 5233 rpm 10 µm/div ROTATION: Y TO X (CW) 5233 rpm Y: UCZ-705 135° Right VECTOR: 6.71 µm pp NA Y: UCZ-704 55° Left VECTOR: 21.9 µm pp 268° X: UCZ-706 135° Left VECTOR: 2.12 µm pp NA X: UCZ-703 35° Right VECTOR: 19.6 µm pp 11° 16MAY2001 05:20:00 Delta Time 1X COMP 16MAY2001 05:20:00 Delta Time 1X COMP UP UP BELOW MIN AMPLITUDEAFTER BALANCING 10 µm/div ROTATION: Y TO X (CW) 5237 rpm 10 µm/div ROTATION: Y TO X (CW) 5237 rpm 1X FILTERED, COMPENSATED ORBIT PLOTS CONTRASTING VIBRATION AMPLITUDES BEFORE AND AFTER 2-PLANE BALANCING | FIG. 3 [Vol.25 No.1 2005] ORBIT 11
  8. 8. CASE HISTORYTHE SAVINGS ACHIEVED BY BALANCING IN A SINGLE-SHOT VERSUS MULTIPLE STARTS AND STOPS WERE WORTH “…WELL OVER A MILLION DOLLARS.”Figure 2. Here, we see not only the state value of approximately 20 µm. turbine, the effect of correctivedramatic reduction in 1X ampli- The balancing calculations consid- weights be observed only after thetude achieved after balancing, but ered this thermal transient effect, unit has been given enough time towe can also observe the effect that and the corrective weights were cho- stabilize thermally and only whenthe corrective balancing had regard- sen to optimize the results expected all other pre- and post-balancinging the machine’s thermal transient. during both steady state and ther- operating conditions have beenNotice that prior to balancing, the mal transient operating regimes. made as consistent as possible. Thisthermal transient (see reference 2 A final note regarding Figure 4: The helps ensure that changes in vibra-for additional information) caused large (approximately one month) tion are truly due to balancing –the 1X vibration amplitude to gap occurring between March 13 not other factors. For a large gasclimb to approximately 140 µm, an and April 4 is due to other work turbine, this can take many hoursundesirably high level for the end being done in the plant that and explains why multiple balanceuser. During the re-start immedi- required the unit to be off-line. It runs are undesirable. Not only doately after balancing, the thermal was not possible to run the unit at multiple starts detract from the lifetransient can again be observed, but speed and load until this other of the hot gas path components,now the vibration never exceeds 50 work had been completed. It is they can also add several days to theµm and settles rapidly to a steady- essential that when balancing a gas balancing job. POINT: UCZ-704 55° Left 1X From 15FEB2001 20:00:00 To 07APR2001 04:40:00 Steady State 21FEB2001 07MAR2001 21MAR2001 04APR2001 19:00 19:00 19:00 19:00 90 PHASE LAG: 30 deg/div 270 Thermal transient during 90 startup before balancing. Note 1X amplitude of approximately 140 µm. 150 10 µm pp/div AMPLITUDE: Thermal transient during 100 startup after balancing. Note 1X amplitude does 50 not exceed 50 µm. 0 19:00 19:00 19:00 19:00 21FEB2001 07MAR2001 21MAR2001 04APR2001 TIME: 1 Day/div TREND PLOT OF 1X AMPLITUDE AND PHASE SHOWING DRAMATIC REDUCTION IN BOTH STEADY-STATE AND THERMAL TRANSIENT VIBRATION AT #2 BEARING FOLLOWING CORRECTIVE BALANCING | FIG. 412 ORBIT [Vol.25 No.1 2005]
  9. 9. CASE HISTORY THE RESULTS EXCEEDED EVERYONE’S EXPECTATIONS. A Satisfied Customer alert operators to machinery abnor- Commenting on these results, the malities that, if left unchecked, can customer’s on-site machinery engi- lead to expensive failures of equip- neer was extremely pleased with the ment and process interruptions. It outcome, noting that the savings also underscores the use of these achieved by balancing in a single- systems in correcting problems and shot versus multiple starts and stops in verifying that corrective actions were worth “…well over a million produced the intended results. dollars.” OEMs possess deep knowledge of their machines and including them He was quick to reiterate the in discussions – as shown by this importance of the cogeneration case history – made the difference process for reliably producing between the ability to balance in a steam 24/7, 365 days a year. “The single attempt and a more tradi- number of starts on our gas tur- tional balancing exercise requiring bines are very, very low because several more days, several more they are base loaded, and our machine starts, and many, many emphasis on reliability. Certainly dollars. less starts equals better hot compo- nent life, but it also equals fewer References: interruptions to our petrochemical [1] M. DIMOND, Vibration Charac- process and that translates to mil- teristics of Industrial Gas lions of dollars for us.” He concluded Turbines, ORBIT magazine, Vol. 21 No. 3, September 1998, pp. by noting another important out- 18-21. come. “It was also a great boost to [2] A.W. VON RAPPARD and A.T. our credibility with management. HECKMAN, Best Vibration We did what we said we could do, Monitoring Practice for Large in much less time than we prom- ABB Gas Turbine Protection and Machinery Management, ORBIT ised, not by being lucky, but by magazine, Vol. 19 No. 3, Third being smart and using all of the best Quarter 2000, pp. 10-13. resources at our disposal.” [3] L.-O. LARSSON, On the Deter- mination of the Influence Coefficients in Rotor Balancing Summary Using Linear Regression This case history demonstrates the Analysis, in Proceedings of Conference on Vibrations in value achieved when all parties – Rotating Machinery, Cambridge, customer, machinery OEM, and England, 1976, Institute of Mechanical Engineers, pp. 93-97. Bently Nevada – work coopera- tively to solve problems. It under- scores the value of continuous condition monitoring systems to [Vol.25 No.1 2005] ORBIT 13