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58 ORBIT Vol .28 No.1 2008
CASE HISTORY
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Vol .28 No.1 2008 ORBIT59
CASE HISTORY
n ethylene plant experienced a vibration isse after start-p of a mlti-nit chare as train
followin a relatively short mitiation otae in Ast 2006. The chare as compressor
has five staes of compression via three different compressor bodies. All three are driven by
a 35,000 hp steam trbine with a desin speed of 5,300 rpm (Fire 1). The compressors were
oriinally installed in the late 1960s and still had the oriinal floatin oil rin desin and
lbricated ear coplins. The rotors had been praded with rb-tolerant internal seals and spherically
seated tilt-pad bearins.
The short dration of the otae limited the wor scope to overhals of the A and C compressor bodies
only. The C compressor had exhibited performance problems thoht to be cased by folin, while the
A compressor was overhaled as a reslt of recent vibration isses. The B compressor was not opened p
becase it had no mechanical or performance problems. The inboard bearin on the dischare end of the
B compressor (bearin 5) was chaned becase it was felt that the clearance had increased.
The train is monitored via a Bently Nevada* 3500 series machinery protection system and all critical trains
in the ethylene nit were connected to Bently Nevada Data Manaer* 2000 (DM2000) software to provide
continos condition monitorin and machinery dianostic capabilities. For increased
data collection resoltion, an ADRE* portable data acisition system was sed to captre
shtdown and startp transient data and ament the data available
from the online software.
Dianosin
Wallace E. (Ed) Wilcox, P.E. Staff Machinery Enineer Chevron Enery Technoloy Company [email protected]
a coplin problem on a chare as trainHow vibration data and a loical process of elimination saved millions in downtime
The eThylene unIT deTaIled In ThIs case
hIsTORy has suBsequenTly Been upgRaded
TO sysTem 1* sOfTwaRe whIch addResses
many Of The daTa ResOluTIOn lImITaTIOns
ThaT weRe InheRenT In dm2000 sOfTwaRe.
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60 ORBIT Vol .28 No.1 2008
CASE HISTORY
5Y 5X2Y 2X1Y 1X 6Y 6X
7Y 7X4Y 4X3Y 3X 8Y 8X
CompressorBody C1 2
3 4
4
5 6
7 8Compressor
Body B
CompressorBody A
35,000 hpSteam Turbine
View as seen from turbine lookingtowards driven machines
421 1T 53
Figure 1. Machine train diagram o charge gas compressor train showing A-B-C compressor bodies,fve compression stages, and proximity probe arrangements or all eight radial bearings.
Problem
Approximately 10 mintes after the chare as
compressor train was restarted followin the otae,
radial vibration on the B compressor increased rapidly.
Althoh the speed remained basically nchaned, the
radial vibration went from 0.5 to over 3 mils, as can be
noted in Fire 2. This increase in vibration occrred
immediately after the compressor train sred. Note
the lare increase in vibration amplitde even thoh
the speed remained relatively constant on the riht sideof Fire 2. The vibration amplitde contined to rise
after startp as the load on the compressor train was
increased in response to increasin nit rates.
Overall amplitdes had reached approximately 4.5
mils by early September with a short excrsion in
late September above 5.5 mils.
Figure 2. Bode plot o bearing #6 y-probe during
start-up on 8/27/06. Note substantial change
in vibration amplitude while speed remains
essentially constant at approximately 4,300 rpm(region enclosed by the red oval). The blue cursor
position shows the frst balance resonance or
this machine occurs at about 2400 rpm.
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Vol .28 No.1 2008 ORBIT61
CASE HISTORY
Vibration Analysis
Analysis of the problem was essentially a process of
elimination whereby the available data was examinedto systematically determine what was not casin the
vibration. Five separate malfnctions where considered.
1. Unbalance
An initial assmption by plant operations personnel
was that the hih radial vibration on the B compressor
was cased by nbalance. This assmption was not
withot basis since olefins chare as is considered
to be a folin service. In particlar, this compression
train had experienced heavy folin several years in
the past. Two other factors spported this assmption.
First, as previosly mentioned, the C compressor was
overhaled drin the otae and some folin was
fond. Second, the B compressor was the only case
not opened drin the otae, so its cleanliness was
nnown. While most of the vibration was indeed
occrrin at the rnnin speed (1X) of the compressor,
nbalance was not the major contribtin case of
the problem. This was ascertained by notin that the
lare increase in amplitde on the riht side of Fire 2
had occrred at relatively constant speed. While it was
possible that some folin material cold have been
thrown off one of the impellers resltin in a sdden
chane in balance, this wold have been accompanied
by a sdden phase shift. However, Fire 2 shows that
no sch phase shift occrredthe phase remained
constant at approximately 215, eliminatin nbalance
as the case of the lare observed increase in vibration
amplitde. To lend additional credence to this concl-
sion, an acceptance reion plot (i.e., trend of amplitde
and phase sin polar coordinates) of the bearin #6
y-probe was examined for a data rane encompassin
the two wees after startp. It showed that the synchro-
nos phase anle was almost constant (Fire 3).
Figure 3. Bearing #6 y-probe acceptance region plot or 2-week period between 8/25/06 and 9/7/06 showing
an essentially constant phase angle o 215.
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62 ORBIT Vol .28 No.1 2008
CASE HISTORY
2. Locked oil seals
The possibility also existed that the floatin rin oil
seals cold have loced in an eccentric position, actin
as additional rotor spports instead of jst the jornal
bearins. However, this type of problem wold have
ndobtedly exhibited some sb-synchronos vibration
peas in the spectrm, becase of the oil whirl that
wold be present in the seals at this speed. Examination
of the spectral data showed that the vibration was
almost entirely at 1X and had no sb-synchronos
components. Conseently, loced seals were extremely
nliely. Additionally, loced seals typically affect the
rotors natral freencies becase they effectively
redce the rotors span between spports and ths
increase its stiffness. Had a chane in the compressors
1st balance resonance been noted (particlarly an
increase), this wold have pointed toward the oil seals.
However, comparison of the Bode plot collected prior
to the shtdown (Fire 4) with that taen drin the
post-otae startp (Fire 2) showed that the rotors
first balance resonance had not chaned. Combined,
the lac of sb-synchronos freency components
and no observed chane in the first balance resonance
eliminated loced oil seals from frther considerationas a liely root case.
3. Cracked shat
A craced shaft will normally be indicated by a decrease
in shaft stiffness that reslts in a noticeable (and
sometimes sbstantial) redction in the rotors critical
speed. As noted previosly by comparin Fires 2 and
4, the rotors critical speed measred prior to the otae
was essentially nchaned from that measred after
the otae, main a craced shaft very nliely.
4. Internal rub
In contrast to a craced shaft, a hard rb on internal
seals will normally increase the critical speed becase
of the redced span between rotor spports, since the
seals are now actin as asi-bearins. For the same
reasons listed above, the lac of chane in the first
balance resonance between the pre-otae coastdown
and post-otae startp data did not spport a rb
hypothesis. Additionally, a rb wold normally be
observable as flat spots or other anomalies in the
shaft orbits. However, the shaft orbits (Fire 5)
exhibited no sch evidence. Combined, this made
a rb extremely nliely.
Figure 4. Bode plot o bearing #6 y-probe on shutdown, 8/3/06, prior to the outage. The blue cursor positionshows the frst balance resonance or this machine occurs at about 2400 rpm, essentially unchanged rom
the post-outage resonance o Figure 2.
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Vol .28 No.1 2008 ORBIT63
CASE HISTORY
Figure 5. Bearing #6 orbit ater start-up
showing no evidence o seal rub.
5. Locked Coupling
Althoh the chare compressor train had a history
of loced coplin problems, these had not manifest
for several years. After a maintenance trnarond
several years prior, the radial vibration on the A and B
compressors increased to approximately 3.5 mils, and
at the time, it was assmed the problem was cased by
coplin loc-p. However, the vibration had trended
down to normal levels over time. Additionally, the
coplins removed drin the Ast 2006 otae did
show indication of wear between the end of the hbs
and spacer, and for this reason were replaced with
identical new components. The fact that the coplins
were brand new seemed to rle ot the lielihood
of coplin loc-p. Liewise, the alinment of the
compressor had been checed and fond to be in ood
condition. Measrements had been taen in the past
that showed the thermal rowth assmptions in the
cold alinment tarets were ood.
For all of these reasons, the coplin was not
immediately sspect.
IN THIS SPECIFIC INCIDENT,
THE ABILITY TO ACCuRATELY
DIAgNOSE THE ROOT CAuSE
ALLOWED MACHINERY
PERSONNEL TO RECOMMEND
A REDuCED WORk SCOPE.
THIS SAVED APPROXIMATELY
1011 DAYS OF LOST
PRODuCTION WHEN
COMPARED WITH A COMPLETE
OVERHAuL THAT WOuLD
HAVE BEEN REquIRED IN THE
ABSENCE OF SuCH DATA.
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64 ORBIT Vol .28 No.1 2008
CASE HISTORY
The first stron indication that the coplin miht be the
problem afterall came approximately one wee after
the startp. Becase of a overnor control oil problem,
the trbine speed dropped rapidly (from approximately
5000 to 3500 rpm in a few seconds). When this occrred,
the overall radial vibration at bearin #6 dropped from
approximately 4.5 mils to 0.7 mils. However, the speed
retrned almost as icly as it had dropped, and the
hih vibration retrned (Fire 6). Initially, this event was
not that enlihtenin becase the 1X trend (see Fire 7)
seemed to indicate that the synchronos amplitde and
phase anle had not chaned maredly, in star con-
trast to the chanes observed in the overall nfiltered
vibration of Fire 6. However, after contactin gEs
Bently Nevada Machinery Dianostic Services (MDS) per-
sonnel, it was learned that the overall vale displayed in
the DM2000 software is received directly from the 3500
series machinery protection system and merely passed
throh, while the 1X vale is calclated in the software
from the raw vibration waveform. Since the chane in
speed and vibration happened so rapidly, the DM2000
softwares tracin filter was not fast enoh to captre
this rapid excrsion. Ths, it seemed liely that the 1X
amplitde and phase had in fact chaned alon with
the overall nfiltered amplitde drin this excrsion.
Figure 6. Rapid drop in bearing #6 y-probe unfltered overall vibration amplitude during speed upset.
Note that ollowing the event, the amplitude returns to nearly the same level as prior to the speed upset.
Figure 7. Same data as in Figure 6, but fltered to compressor running speed (1X).
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Vol .28 No.1 2008 ORBIT65
CASE HISTORY
In an attempt to re-create the drop in vibration
mentioned above, approximately one month after the
otae, the compressor speed and load were decreased
drin a redction in nit rates. Drin this test, an
ADRE portable data acisition system was connected
to the 3500 machinery protection rac as it provided
hiher data resoltion than the DM2000 software.
Becase the speed decrease was radal, there was
no rapid decrease in vibration as seen earlier. However,
drin the redction in nit rates, the compressor was
accidentally sred and becase the ADRE system
was connected, hih resoltion data was acired.
Examination of this data showed that the synchronos
amplitde had dropped rapidly drin the two sre
events. Liewise, the synchronos phase anle had
chaned as well (Fire 8). These facts pointed stronly
to the lielihood that the coplin between the B and
C compressors was loced in a riid condition, bt
had momentarily broen free drin the sre event
becase of the rapid flow reversal and sbseent
relaxation of torsional loadin on the coplin.
Figure 8. Bearing #6 y-probe 1X fltered amplitude and phase data collected during surge events on 9/28/06.
Note high-resolution time span on horizontal axis (1 second per division) indicating that the back-to-back
surge events occurred within approximately 10 seconds o one another.
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66 ORBIT Vol .28 No.1 2008
CASE HISTORY
Examination of the shaft centerline plots of the #6
bearin revealed that the jornal had dropped approxi-
mately 2.0 mils drin the coast-down before the
otae, bt had raised 4.0 mils drin the sbseent
start-p (see Fires 9 and 10 respectively). This meant
that the sction end jornal was ridin in the center of
the bearin, instead of the lower half, since the desin
bearin clearance was approximately 8 mils. The indica-
tion was that somethin was liftin the B compressor
sction end jornal off of its bearin. This was frther
confirmed by examination of the bearin metal tem-
peratres for bearin #6 (see Fire 11). As can be seen,
the bearin #6 temperatre is approximately 15 coolerafter the mitiation otae, even thoh the vibration
was obviosly hiher. In contrast, the bearin #7 tem-
peratre was approximately the same before and after
the otae, indicatin that no other external factors,
sch as oil temperatre or ambient temperatres, were
affectin the reslts.
Figure 9. Shat centerline plot or bearing #6during pre-outage shutdown on 8/3/2006.
Figure 10. Shat centerline plot or bearing #6during post-outage startup on 8/27/2006.
THIS TEST WAS A FuRTHER
CONFIRMATION THAT THE B
COMPRESSOR ROTOR WAS
BEINg LIFTED OFF BEARINg
#6 BY A RIgID COuPLINg
BETWEEN THE B AND C
COMPRESSORS.
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Vol .28 No.1 2008 ORBIT67
CASE HISTORY
Figure 11. Trend rom the plants process historian showing bearing #6 (red) and bearing #7 (blue)
temperatures beore and ater outage. Note that the temperature o bearing #6 drops approximately 15
degrees ater the outage, while the temperature o bearing #7 stays approximately the same. This helped
corroborate that bearing #6 was more lightly loaded and riding more towards the center o its clearance
(i.e., lower eccentricity ratio) ater the outage.
Validation
In an attempt to redce the radial vibration amplitde atbearin #6, and confirm that that the low jornal eccen-
tricity was associated with the problem, an online test
was condcted to chane the relative vertical alinment
between the B and C compressors. The basic concept
was to pll the B sction end jornal down frther in
its bearin, thereby increasin its eccentricity and bear-
in stiffness and redcin the radial vibration. This was
accomplished by sin an air mover with water spray
on the dischare foot of the C compressor, and a steam
hose on the sction pedestal of the B compressor. The
objective was to chane the alinment, lowerin the C
case dischare end and raisin the B case sction end
(Fire 12). Becase heatin and coolin effects were
localized to the compressor spports, it did not chane
the bearin hosin temperatre, which potentially
cold have sewed the shaft centerline plots de to the
probes rowin away from the shaft.
The chanes had exactly the effect predicted. The shaft
centerline plots of bearins #6 and #7 are shown in
B Case C Case
Cooling applied to C
case dischargewobble foot
Heat applied to
B case suctionpedestal
Figure 12. Graphical depiction o online alignment
adjustments used to confrm the locked couplingpostulate.
Fire 13. Liewise, the redction in radial vibration is
shown in Fire 14. This test was a frther confirmation
that the B compressor rotor was bein lifted off bearin
#6 by a riid coplin between the B and C compressors.
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68 ORBIT Vol .28 No.1 2008
CASE HISTORY
Figure 13. Steady-state shat centerline trends o bearings 6 (let) and 7 (right) showing eects o online
alignment adjustments. The rotor moves downward in its bearing clearances or bearing #6 and upward or
bearing #7.
Figure 14. Drop in bearing #6 y-probe vibration during online alignment adjustment occurred exactly
as predicted.
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Vol .28 No.1 2008 ORBIT69
CASE HISTORY
The hih-resoltion data (Fire 8) taen drin the
compressor sre showin the rapid synchronos
phase chane, and the drop in vibration (Fire 14)
observed drin the online alinment chane all
indicated that coplin loc-p was the problem.
Approximately two months after the otae, the overall
vibration at bearin #6 was still trendin above 4 mils
with occasional excrsions above 5 mils.
Corrective Actions
Another otae was planned in the winter months
to address the vibration isse. De to the isolation
reirements of the compressor, the coplins cold
be chaned withot completely clearin the compres-
sors. This wold allow the compressor to be ready for
maintenance approximately one day after the nit was
sht down. In contrast, if the case had to be opened,
this wold reire a minimm of 4 days jst to clear
the compressor cases before maintenance wor cold
bein. The end reslt was that a coplin chane
cold be accomplished with 3 to 4 days of interrpted
prodction, while a complete overhal wold reire at
least 2 wees, incrrin 10 or more days of additional
downtime. While this made the coplin wor loo very
attractive, the prospect of chanin only the coplin,
restartin the compressors, and then discoverin that
the hih vibration was still present was hihly ndesir-
able. For this reason, some advocated the safe rote
of a complete overhal. However, based on the clear
indication of the coplin loc-p from the test data,
the site machinery personnel were able to alleviate
manaements concerns and redce the wor scope
to that of jst replacin the coplins.
Reslts
The chare as compressor train was broht down
and the last two ear coplins in the train were
replaced with dry, flex-element style coplins. As
expected, the vibration on bearin #6 retrned to
normal levels of approximately 0.5 mils. Fire 15 shows
the ear coplin removed from between the B and C
compressor bodies. It clearly exhibits an abnormal wear
pattern considerin it had only been in operation for
approximately two months.
Conclsions
Chevron is increasinly sin condition monitorin data
as an effective method of enhancin decision-main
confidence and accracy, and the willinness of plant
manaement to rely on this data when main operat-
in and maintenance decisions is increasin as well. This
case history was merely one example of more than half
a dozen that the athor has compiled drin the last
several years.
In this specific incident, the ability to accrately dia-
nose the root case allowed machinery personnel to
recommend a redced wor scope. This saved approxi-
mately 10-11 days of lost prodction when compared
with a complete overhal that wold have been reiredin the absence of sch data. Each day of downtime typi-
cally costs in excess of 200k uSD for this plant, meanin
that several million dollars in savins were realized de
to accrate and timely machinery data.
* denotes the trademars or reistered trademars of Bently Nevada,
LLC, a general Electric company.
Figure 15. November 2006 photo showing
replaced gear coupling between B and Ccompressor bodies. Abnormal wear is clearly
evident ater only 2 months o operation.