The Use of Weibull Analysis Methods in Assessing Field Failure Problems

Transcription

1 Prde University Prde epbs nternational Compressor Engineering Conference School of Mechanical Engineering 1994 The Use of Weibll Analysis Methods in Assessing Field Failre Problems F. M. Simpson Copeland Corporation Follow this and additional works at: Simpson, F. M., "The Use of Weibll Analysis Methods in Assessing Field Failre Problems" (1994). nternational Compressor Engineering Conference. Paper This docment has been made available throgh Prde epbs, a service of the Prde University Libraries. Please contact epbs@prde.ed for additional information. Complete proceedings may be acqired in print and on CDROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 The Use of Weibll Analysis Methods in Assessing Field Failre Problems F. M. Simpson Copeland Corporation ABSTRACT Unacceptable sction reed failre rates were occrring in a critical field application. Compressor failre reslted in excessive downtime costs, and extreme reliability was reqired in this application. A modified reed was placed into operation in a portion of the poplation. After a year's time failres were also occrring in the modified reed, althogh they were fewer in nmber. Sfficient field information with this new poplation was available to carry ot a Weibll analysis. The analysis indicated an infant mortality problem was present, which was later fond to be cased by the normal sbsrface microscopic inclsions in the reed steel. The analysis also indicated that completing the field campaign of the initial poplation wold increase the probability of failre, and that no frther changeots shold be made. t also indicated how mch more improvement wold have to made in a second reed modification to reslt in fewer failres compared to leaving the modified reed poplation in the field. BACKGROUND Particlar installations of some semihermetic compressors reqire extremely high reliability of the compressor to prevent downtime of a major system. Down time of some systems can reslt in very high financial penalties to a cstomer, at times as high as $50,000 per hor. n sitations like this the compressor manfactrer can be certain that he will be informed of any failres, and reqested to prevent ftre failres of the same type. One benefit of an occrrence of this natre is that fairly reliable data is available concerning the field failres and statistical analysis can be carried ot to predict what ftre action shold be taken to minimize any frther cstomer disrptions. A sitation of this natre occrred in the early 1990's, and Weibll techniqes proved invalable in the decision process involved in solving the problem. Abot 250 compressors of a given type had been placed in the field. After some time in service it was determined that the sction reed failre rates were too high and the cstomer demanded that improvements be made. Strain gage testing indicated that the sction reed stresses were indeed too high nder the particlar operating conditions for that cstomer, and the reed was increased in thickness to redce the stress levels, to what was thoght to have been a conservative vale. Dring 1990, 150 compressors were field fitted with the new reed configration. One hndred compressors remained nchanged, with the original reed configration. 157

3 By the end of 1991 three of the new, redced stress level reeds had failed, and the cstomer was adamant in demanding a frther change. Record keeping had improved by that time, and reliable "time to failre" data was available. The three times to failre were 8 hors, 500 hors and hors. This data was plotted on Weibll paper sing standard Weibll techniqes, and the reslts can be seen in Figre 1. t was immediately apparent that the Weibll slope was well below one, which indicates an infant mortality problem. At the same time, WE BULL ANALYSS :; LL. c 5. Q) (.) L (.; lj lj 1 1 v metallographic examinations of the failed reeds indicated that the failres all started at sbsrface microscopic inclsions, in high tensile stress areas, located near the srface of the reed. 10 l..""""" v Slope= Hors to Failre Figre 1 Weibll plot of compressor field failres after modification These inclsions were of a size that can be normally expected, even in a very clean valve steel, and were within the specifications for the material. t became apparent that the failres were a reslt of the stress concentrations cased by the presence of these inclsions, and that the inclsions had to be present in a high tensile stress area of the reed, in order for a crack to be initiated and propagated. The combination of a rare inclsion in a highly stressed volme of the reed (which is also a relatively small percentage of the overall reed volme) cased a valve failre. The particlarly severe reqirements for high reliability of this particlar application reslted in the discovery of these low probability circmstances. At the time the analysis was started there were 150 compressors in the field with the new reed. t was estimated that the time on those compressors was abot 7000 hors. There were also 1 00 compressors in the field with the old, more highly stressed reed, and 158

4 these were estimated to have abot 16,000 hors. The expected life of the total system was arond 24,000 hors, at which time it wold be replaced. n order to predict what wold happen to the entire poplation of 150 a "Daser Shift" (Reference 1) was applied to the original Weibllline, sing the following techniqe: *The crrent failre rate was estimated at 2% (3 failres divided by 150 operating compressors) *The mean time to failre was estimated by adding the hors to failre of each failed compressor and dividing by three *This point was then plotted on Wei bll paper, and a line passed throgh that point with a slope of that of the original three failres. This plot then becomes a means of predicting what will happen to the remaining compressors in the field. The slope of the original failres is, of corse, critical to the accrate analysis and the more points one has to determine that slope, the higher the confidence level in the analysis. However, in sitations of this natre, very limit,ed amonts of data are likely to be available. Figre 2 is the reslting shifted Weibll plot. One comforting fact was immediately apparent. At the time the analysis was carried ot, the compressors had arond 7000 hors o. 'ffi10. LL c 5. \) _... 1.'"" v,.,......: Q) 2% crrent failre rate (.) r v,..,.,... 0 J:... l,l) 0. 1.: 0 o:i._: ::!: v: v P1 v Q)l il ol OJ f=l.til Hors to Failre Figre 2 Weibll plot with "Daser shift" to determine failre rates for the entire poplation At the 7000 hor mark, it can be seen that a 5% to 6% failre rate shold have occrred. This wold indicate that B failres shold have occrred by that time. 159

5 Since there were no frther indications of failres, it was conclded that we were conservative in or estimates, and that the actal slope was lower than that predicted by the original three failres. Figre 2 also shows that by the end of 24,000 hors we cold expect a failre rate of arond 8% to gok, or a total of 13 compressors. n other words we cold expect at least 6 more failres in the next 17,000 hors operating time. Figre 3, which is a cmlative distribtion plot, illstrates the same information more clearly. One of the more important qestions to be resolved was shold we contine the field campaign of the remaining 1 00 compressors with the original higher stressed valve. Strain gage tests had indicated that we had redced the stress levels by aoot 15% when making the reed modification. With this information, a Weibllline cold be constrcted for the old reeds, if one can assme that the Weibll slope for the old reeds is the same as the modified 22r 'i 10 LL. 12 b : 8 z "'"'"" 4 6 i r.l '" reeds. The mechanism casing the infant mortality problem was considered to be the same for the original reed is the modified reed, and it was conclded that the slope shold also remain the same....!... 1,,.....:. L..1= """......:.... ' '0 l Hors n Operation (Thosands) Figre 3 Cmlative failre distribtion for entire poplation Since the difference in stress levels is known for the two different reeds, the time to failre for the old reeds can also be estimated, by sing information available from the material SN crve and finding the eqivalent times to failre. f this is done the Weibll plot shown in Figre 4 can be established and the cmlative failre plot (Figre 5) drawn. Since the compressors with the old reed had 16,000 hors it can be seen that the failre rate shold be abot 18%, or 18 failres cold have been expected. This was indeed close to the nmber that was thoght to have failed (accrate data was not available). 160

6 More importantly, the failre rate after 24,000 hors is 200.k, or 20 nits. Ths, only two more nits cold be expected to fail in the remaining time that the compressors wold be tilized. However, if the compressors were campaigned with the new reed, an 8% failre rate (8 compressors) cold occr in the remaining 8000 hors of compressor life, since we wold be starting ot with a new poplation of reeds. Ths, it is nwise to campaign the 1 00 compressors that remain in the field. The compressors that had inclsions in the highly stressed volmes have already failed, and those that remain are safe and need not be changed ot, since they have been "screened" by this time. The next qestion to be answered was how mch does the reed stress have to be redced from the modified reed to warrant a field campaign of the 150 compressors with the modified reed. Since in the remaining 17,000 hors of sefl compressor life less than 6 failres are desired (only 6 more ::J 'ii LL 't5 J E z ::J :;20.0 'lii10.0. c: 5.0 Q)... () , v.. r f l :6... _....fl Q)l!t:l _J 'fl.. 1$1" g1 Ql 11 L L Hors to Failre Figre 4 Estimated Weibll plot of the poplation remaining with original reeds r. '.. + r ;... : ; i... : """".... +"i i ;.. o... r. ;! " i.! t. + ""f _j!.... _...,L + ',._, Hors n Operation (Thosands) Figre 5 Cmlative failre distribtion of compressors remaining with original reeds 161

7 compressors will fail withot a campaign) a new line can be established (Figre 6). Again, sing material properties from the SN crve it can be dedced that at least a 1 0% redction in stress had to be realized. t was determined that this cold not be done withot seriosly compromising the performance of the compressor, so the decision was made to not make any frther modifications or field campaigns. Since the time this analysis was carried ot, two additional failres have occrred in the modified reed design, _ % Decrease in Stress c 5.0 C/..r Q) (.) _ one at 3600 hors and another at 8000 hors. This information was added to the initial failre information and reinforced those decisions already made. As of this time there have been no frther failres of either the modified reeds or the original designs.... CONCLUSON _... _v Hors to Failre Figre 6 Stress redction reqired to reslt in less than six failres in remaining lifetime 510,.C.!: ol.!: Oct! E e Weibll analysis methods have been sccessflly tilized to predict the nmber of failres that wold occr in compressors in the field. Wei bll techniqes avoided a costly field retrofit program which wold have introdced additional failres in a critical field operation. These techniqes can be applied with a minimm of data, and conservative approaches can betaken. References: "lhe New Weibll Handbook", Dr. Robert B. Abernethy, Athor and Pblisher, 536 Oyster Road, North Palm Beach, Fla