Transcript
Abstract
Investigation of the failure of an alternator rotor shaft driven by a reciprocating gas engine An investigation of repeated failures of an alternator rotor drive shaft is reported. Among other things it is shown that torsional vibration was the major factor in the cause of these breakdowns and that comparison of analytical prediction of such vibration with its careful monitoring can be a valuable aid in deciding how to prevent recurrence of the problem.
TYPES OF DAMPERS There are two main types of engine torsional dampers (see Figure 1) Viscous and Tuned, each having its particular advantages. In brief; the Viscous Damper tunes itself to the system torsional activities with a seismic mass and specific silicone fluid, while the Tuned Damper, as the name implies, is tuned to the system;s fundamental torsional vibration activity. (It is outside the scope of this short study to try to explain the advantages and disadvantages of each)
UNDERSTANDING THE TYPE OF VIBRATION
Hamid R Malaki, VibraHiTec
[email protected]
INTRODUCTION Despite having worked for many years on analysing the noise and vibration problems of rotating machinery the author is still surprised to see that in the 21st century many industries still ignore vibration monitoring and its potential benefits. Above all, not many operators are willing to even acknowledge that it will help to reduce maintenance cost. Some operators believe that by religiously following a maintenance regime everything will be fine. They forget that some parts may have extended life but other parts may need replacing much sooner, perhaps because of an unusual or unforeseen incident. Constant vigilance, coupled with a willingness to contemplate a range of possible failure mechanisms rather than grasping at the first that comes to mind, may save a lot of time and expense in the long run. The following case study shows how the installation of a simple monitoring system could have given early warning of a first small incident and would have had aided the avoidance of two further catastrophic failures which then ensued because the first went unrecognised.
Unfortunately, vibration is not of one kind; there is linear vibration and also torsional vibration. The latter may not be noticed easily. However, most torsional vibration problems do ultimately show themselves as linear vibration, their indication and strength depending on the supporting bearing and structure. Few operators are familiar with torsional vibration and its effect on rotating parts, and many manufacturers marry up the drive and the driven systems with scant attention to linear vibration let alone torsional.
WHOSE RESPONSIBILITY IS IT TO ANALYSE TORSIONAL VIBRATION? Usually, most turbine and reciprocating engine manufacturers do carry out torsional analysis when designing the drive and the driven systems. It is the responsibility of the manufacturer of the drive system to ensure its compatibility with the driven system. In doing so, they may install a heavy or light flywheel and a specific torsional damper to tune the torsional vibration behaviour to ensure that the system is free from torsional vibration within the running range. Torsional analysis however is not easy, and must be carried out by specialists. Many manufacturers write their own software to carry out the torsional vibration analysis, while others consult specialised engineers in this field to help to ensure the shaft line is free from torsional vibration. Many manufacturers of rotating systems ignore the torsional compatibility of the drive and driven system and assume everything will be alright in the end. This wrong assumption will haunt them when torsional problems eventually occur.
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Figure 1 Torsional Dampers
INVESTIGATION OF A FAILURE OF AN ALTERNATOR SHAFT This case study report aims to – 1. Demonstrate the significant damage that can be caused by a small change in the shaft line. 2. Highlight the effect of inattentiveness to the mode of failure before any repair work is carried out. 3. Emphasise that torsional vibration can be a silent machine killer if ignored. 4. Recommend a full investigation and verification by measurement before putting the machine back in service. With this in mind we were recently consulted to investigate the failure of an alternator rotor shaft driven by a 3MW reciprocating V-form 16-cylinder gas engine installed in a power station alongside other similar machines. It had been reported that, on one of these sets, the alternator shaft had failed inexplicably after 64,000 hours and, to everyone’s surprise, the replacement shaft had also failed after only 800 hours operation – in exactly the same manner and in exactly
Investigation of the failure of an alternator rotor shaft driven by a reciprocating gas engine
stiffness value and/or damping are changed (by any means) the result could cause the stress in both crankshaft and alternator shaft to increase. The maximum stress in this situation would occur fairly close to the running range with the flank of the stress moving along into the running speed which could cause the alternator shaft to over-stress at the very point where Figure 2 Failure of the shaft at the flange connection
the same position as the previous failure, i.e. at the alternator shaft flange (see Figure 2). This suggested that some fundamental problem had developed, possibly after approximately 60,000 hours operation, to cause such failures, one after the other. The report for the first and second failures had been studied and found inconclusive; the failures being put down to the possibility of alternator abnormal load and shaft inferior material/process quality. It was surprising to see that the report only concentrated on the shaft material and a possible overload. The short running time before the second failure was put down to the shaft material having lower carbon content than the previous shaft. This completely ignored the fact that in this power station there were other generators of exactly the same build, seemingly without problems. But could they be on the verge of failure too? Other possible modes of failure did not seem to have been considered at all.
Figure 3 Fracture faces
Looking at the fracture faces (see Figure 3) it was fairly obvious that the failure mode had resulted from torsional vibration and torsional fatigue stress, which the report had indeed acknowledged, but, clearly, there had been no investigation of the primary cause of this before re-assembling and running the generator set after the first failure. Knowing the mode of failure, the investigation was quite straight forward. It was decided to first focus on the torsional vibration behaviour of the drive and driven system in order to obtain ‘analytically’ the shaft stress level during normal and misfire (cylinder out) operation. The torsional stress can rise significantly during a misfire condition, and on a gas engine
misfire is common. Upon determination of the normal and misfire condition, other possibilities that could change the system torsional vibration behaviour characteristics could be investigated.
TORSIONAL VIBRATION ANALYSIS The results of torsional investigation showed that the torsional vibration (TV) stress for both drive and driven system were well within (Standard running with misfire) the allowable stress level under both normal Figure 4 Analysed torsional vibratory stress at the alternator shaft running condition and cylinder out (misfire). fracture actually occurred – just before the flange connecting it to the flywheel and crankshaft. Bear in mind that the crankshaft is of stronger material and the maximum alternator shaft vibratory stress occurs at system 2nd, 3rd and 4th modes which are located between the alternator shaft and flywheel.
Also, from the database it was possible to find a set of TV measurements carried out during the Factory Acceptance Test (FAT) at full load and speed. The measured values were lower than the predicted values, so the system was torsionally sound before leaving the factory. Similar analysis was carried out for firing order changes (see Figure 4) due to possibly incorrect camshaft assembly at some stage during maintenance work. This also did not reveal the possibility of causing any levels of stress in the shaft line that could have been so high as to cause the observed failure. Finally, the damper sensitivity was checked. The torsional analysis (see Figures 5 and 6) showed that if the damper
The torsional vibratory stress can reach its maximum at the points of twist which are called nodes. The node location can change if the rotating inertia and or shaft/ system stiffness changes. This change would change fundamental torsional natural frequencies and in turn the natural mode. It should be noted that the mode is the shape of the twist and the node is the point at which the shaft will twist. Therefore, the damper’s fundamental characteristic is to reduce the amount of twist in the shaft line hence the amount of shaft stress at the nodes. Based on this it was decided to focus on the torsional damper as a primary suspect. As it was a tuned damper, it had to be maintained by the specialised engineer in this field. The maintenance report revealed that internal bolts had failed and also that there was evidence of wear on the dynamic
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a torsional failure would normally be far greater than just the failure of the shaft.. Although the drive system did not see any damage, all bearings and the shaft line had to be checked for cracks and signs of consequential damage. (It is important to note that condition monitoring would have helped the situation here – limiting the extent of damage if not preventing it completely.) Figure 5 Analysed torsional vibratory stress at the alternator shaft (Damper malfunction and misfire)
Unfortunately, many operators incorrectly conceive the damper to
Linear vibration measurement Electrical excitation check/ measurement
TORSIONAL VIBRATION MEASUREMENT The results of a set of torsional vibration measurements taken at a suitable location (see Figure 7) were analysed and the performance compared with those of similar measurements on one of the other sets at the same station. In general, the results for the re-assembled engine were slightly lower, in all frequencies, than those from the other generator set. Also, when the measurement results were compared with the torsional analysis results (see Figures 8 and 9), the measured values were slightly lower than the calculated values at full load (hence the resulting stress will also be lower). At no load up to 90% running speed the measured results for the ½ and 1st orders were higher than the calculated values. The ½ order higher values are usually due to imbalance within the engine cylinder pressure at no load. The 1st order is usually due to the encoder fixing to the crankshaft and tolerance on eccentricity between the encoder and the crankshaft. Although anything below ½ order is ignored one needs to be aware of the associated frequencies and their effect.
Figure 6 Torsional mode shapes (16 Vee 3MW generator shaft line)
seal face of the inner component of the damper, although the damper unit was still intact and showed no external signs of problems. The fractured bolts together with the seal wear had most likely caused the change in damper stiffness values and damping level. This triggered a change in the damper and its designed characteristic which was tuned to overcome the fundamental excitation frequencies within the shaft line. Torsional vibration resonance and, ultimately, shaft failure resulted. The fact that the broken bolt had not been found pointed the finger of suspicion towards a failure in the damper inspection regime – and irregular firing due to spark plug problems probably exacerbated the condition (misfire usually increases torsional stresses). It can also be appreciated that consequential damage as a result of such
be similar to a flywheel (a lump of mass but at the other end of the engine). Dampers are a fundamental part of the engine, and with engines working harder than ever, are considered as critical components. Advances in design and technology have enabled many engine manufacturers to increase power by changing the cylinder pressure, material and a few associated parts to deal with the power increase. But one of the ways to reduce shaft line stress is to employ a correctly designed torsional damper tuned to the exact disturbing excitation frequency. Finally, after all repair work was complete, the engine was started and - to ensure that the machine was running as expected and running similar to the other machines – three tests, (which should have been carried out after the first failure) were carried out, viz. Torsional vibration measurement
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Figure 7 Measurement location and TV encoder
If they are significantly high then further investigation would be needed. In this case the identified frequencies were not associated with the engine main torsional excitation but mainly mechanical behaviour.
Investigation of the failure of an alternator rotor shaft driven by a reciprocating gas engine
Figure 8 Analysed vibratory angle at engine free end
Figure 10 Linear vibration measurements
Figure 9 Measured vibratory angle at engine free end
From the plotted results it can be seen that the generator is operating as expected where the measured values are almost in line with the calculated values and better than those from the comparison generator set. When measuring torsional vibration on a reciprocating engine attention should be paid to a few areas such as – The levels of engine misfire and engine imbalance. Generator sets are usually tuned and balanced to run at full load within 0.95 to 1.1 of the nominal speed. It is known that at no load and low speeds there is considerable variation in cylinder pressures, especially in a gas engine, hence the level of ½ order activity is usually higher than calculated. The TV pick-up (encoder) and adaptor piece. When connecting the encoder adaptor to the free end of the crankshaft care must be taken to ensure that the adaptor is made with a high tolerance and connected to the crankshaft free end with high accuracy
in order to reduce imbalance activities and movement. Controlling eccentricity is paramount otherwise the measured results will show high levels of first order activities. The engine overall movement. Torsional frequencies at low engine speed can coincide with a number of engine natural frequencies, such as that of the antivibration mount, hence some of these movements might cause an indirect engine torsional imbalance.
LINEAR VIBRATION Due to time constraints only one set of linear vibration measurements, in order to establish the level of linear vibration at the bearing, were carried out at the alternator none-drive end. The vibration level for all the generator sets was found to be within an acceptable level. This measurement (see Figure 10) was not intended to substitute for a full vibration survey, but to provide an indication, for future reference, of vibration level at the alternator free-end bearing. It should be remembered that linear vibration measurements at only one location can only be used as a general guide, and cannot lead to a conclusion that the machine is healthy. Iinstalling a low cost vibration monitoring system to monitor vibration level at a carefully
selected position at the alternator non-drive end could give warning of catastrophic failure. Such monitoring systems can be obtained for as little as £2K to £3K.
ELECTRICAL HARMONIC CHECK Harmonic values and waveforms showed that there were no electrical current issues with the load.
CONCLUSIONS AND RECOMMENDATIONS Shortcoming in the material is only one of a number of possible reasons for generator shaft failure. When two consecutive failures of a similar nature occur, everything must be scrutinised, from design to operation. When the mode of failure has been determined look for the primary cause. If unexpected failure has occurred on one generator and not on the others, keep looking for the difference – it is surely there. Don’t put the machine back into full operation until that difference has been isolated and its effect quantified. Healthy operation of the other generator sets and their engine dampers must also be confirmed. Dampers must be regularly checked at the manufacturer’s recommended interval. Outward physical appearance can be deceptive. Vibration monitoring will in most cases help to spot failure before it actually occurs.
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