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Reliability improvements on high speed reciprocating compressors through bearing and process modifications
- Case Study
Jim Cencula GE O&GCarlo Mazzali GE O&G/ FLRVinh Do GE O&G/ HouRay Gallagher GE O&G/ HouGreg Bone Miba Bearings
291
Contents
• Brief overview to recip compressor bearing/ orbital needs
• Brief Issues history on EF bearing issues
• Discussion of Failure Modes and Root Cause Analysis (RCA)
• Use of Design for Reliability (DFR) process and corrective actions
• New EF bearing design options and testing
• Summary
292
Bearing analysis - Background
MOFT
POFP
SHAFT SHAFT
Bearing operating principle
The reciprocating motion of the shaft generates a variable lubrication film thickness: the pin changes its position both circumferentially and radially
The geometry of the bearing gives a pressure trend within its width
BEARING
LUBRICATION GAP
Fluid friction needed to minimize power losses and rapid wear → Hydrodynamic operation
Bearing performance
To avoid rapid bearing wear:• Adequate separation between moving
components (↑ MOFT)• Low stress level (↓ POFP)
Bearing design criteria
Wide bearing surface to ↓ POFP
↓ oil inlet T and ↑ cooling flux to ↑ oil viscosity and therefore ↑ MOFT
SAE40 Viscosity vs T
0
20
40
60
80
100
120
140
100 110 120 130 140 150 160 170 180 190 200 210 220
Oil Temperature [deg F]V
isc
os
ity
[c
st]
293
RCA Process – Key Parameters
Bearings• overloaded Babbitt• geometry out of cylinder/ parallel• improper clearance• oil delivery improper (flow/ temp)• contaminated lubricant
• anti-rotation tab deform• surface hardness mismatch• debris imbedded• poor fit / contact pressure
Crankshaft • overloaded journal• geometry out of cylinder/ parallel• oil delivery improper to bearing
Conn Rod • uneven /improper bearing housing • geometry out of tolerance/ assy fit• oil removal uneven or improper
Lubrication System • Packager– oil delivery (temp/ press variation)
294
The first major focus of this effort was to eliminate the early operation (infant mortality) issues by addressing the fundamental part/ unit quality issues.
• An extensive effort was made to work with the crankshaft, connecting rod, and bearing manufacturers to ensure the critical to quality component features were maintained (CS pin cylindricity, CS pin surface finish, bearing as-assembled roundness, tin-flashing consistency, etc.).
• The compressor team also focused on cleanliness processes and in-house inspections for increased confidence as well.
• At the same time, the high speed recip OEM had extensive P&ID reviewers with the packagers and were able to get lube oil supply temperatures and pressures to be delivered in a much more consistent way.
Within a late 2006 to early 2009 period, the two-year operational reliability increased from ~90% to 100% (that is, no new bearing failure for over 3 years of shipments now).
Approach
295
• Weibull analysis shows = 0.35.
• This indicates an Infant Mortality failure mechanism.
• The units produced since Mar-2007 demonstrate 14 times improvement in Reliability
Bearing Weibull AnalysisProbability - Weibull
before 3/19/2007: after 3/19/2007:
Service Hours
Unr
elia
bilit
y, F
(t)
10.0 1000000.0100.0 1000.0 10000.0 100000.00.1
0.5
1.0
5.0
10.0
50.0
90.0
99.0
0.1
Probability-WeibullCB@90% 2-Sided [T]
after 3/19/2007Weibull-1PMLE SRM MED FMF=0/S=123
Susp PointsProbability Line
before 3/19/2007Weibull-2PMLE SRM MED FMF=44/S=477
Data PointsSusp PointsProbability LineTop CB-IBottom CB-I
Grzegorz KubikWIA, EDC4/11/20082:54:06 PM
Before 3/19/2007 After 3/19/2007
No of Failures 44 0No of Suspensions 477 123Beta 0.35 0.35Eta 10,874,385 156,206,865MTTF 55,268,030 794,239,601B10 17,202 246,915
296
Weibull InterpretationBeta B10* Life (hrs)
Before improved Cleanliness .35 17,638After improved cleaning .38 26,683After New Bearing fit/ tab .27 141,168After New Factory Inspection .37 1,691,771
*B10 is the prediction of non-failure of this mode for 90% or more
B10 says ‘design life’ fall-out is very low; Beta supports failure occurs early in life
So what about beyond infancy?Detail: Crow AMSAA for 2006-ship Units…for units operating >5000 hrs, < 0.1% chance of failure in “design life” period (w/ 90% confidence)
1.00E-4
1.00
1.00E-3
0.01
0.10
1.00 100000.0010.00 100.00 1000.00 10000.00
ReliaSoft's RGA 6 PRO - RGA.ReliaSoft.com
Instantaneous Failure Intensity vs Time
Time
Inst
ant.
Fa
ilure
Int
ensi
ty
4/7/2008 05:50GEKasia Osowska
Crow-AMSAA (NHPP2006MLE
CB[C] 90%2-Sided-B [T2]
Beta=0.3268, Lambda=0.5493
… what else can cause infant failures?
297
Oil Cooling Flux (gallons/min)
0.0
0.5
1.0
1.5
2.0
2.5
HSRC 180F HSRC 160F HSRC 150F
Bearing analysis – HSRC oil cooling effect
= HSRC with upgraded oil cooling system (“plus” option, oil temp @ 150F)
= HSRC with upgraded oil cooling system (oil temp @ 160F)
= HSRC with actual cooling system (oil temp @ 180F)
Oil delivery system robustness is key impact to managing good bearing parameters… poor oil control impacts other factors (when MOFT is low)
MOFT [μm]
0.0
0.5
1.0
1.5
2.0
2.5
HSRC 180F HSRC 160F HSRC 150F
POFP [psi]
0
5000
10000
15000
20000
25000
30000
35000
40000
HSRC 180F HSRC 160F HSRC 150F
298
321
0.0010
0.0005
0.0000
Atlas (1) Lincoln (2) Ellwood (3)
All
Ven
dors
CS Parallel MainsCS Parallel MainsUSL=0.0010USL=0.0010
321
0.0010
0.0005
0.0000
Atlas (1) Lincoln (2) Ellwood (3)
All
Ven
dors
CS Parallel MainsCS Parallel MainsUSL=0.0010USL=0.0010
CS Parallel MainsCS Parallel MainsUSL=0.0010USL=0.0010
1 2 3
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
0.0014
Atlas (1) Lincoln (2) Ellwood (3)
All
Ven
dors
CS Parallel CranksCS Parallel Cranks
USL=0.0005USL=0.0005
1 2 3
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
0.0014
Atlas (1) Lincoln (2) Ellwood (3)
All
Ven
dors
CS Parallel CranksCS Parallel Cranks
USL=0.0005USL=0.0005
vendor vendor vendor
Bearing fit to Journals A Key …cylindricity/ parallelism
• Crankshaft geometry out of cylinder/ parallel … key manufacturing CTQ with suppliers
• Main CS journals ‘good’• Crank journals – vendor variation
‘devil is in the details’
• Also, bearing fit into Conn Rod in the final assembled condition a key to good journal
• Conn Rod cap torque level and machining are key
• Bearing ‘crush’ and deflection key
299
“DEF” Crankshaft/Bearing Rolling Failure Rate
• The last Crank end bearing failure occurred in units produced in Nov 2007.
• No failures since 3 years of shipment.
300
Time, (Hours)
Unre
liabi
lity,
F(t)
1.E-2 1000001.E-1 1 10 100 1000 100001.E-1
5.E-1
1
5
10
50
90
99
1.E-1
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
2.0
3.0
4.0
6.0
1/30/06 11/01/06 3/19/07 8/17/07
Cleanliness Improvement I
Bearings Improvement
Inspection Level Changed
Cleanliness & Test Load Improvement II
6/30/08
CS/ CR/ Brg re-certified process
1/30/06 11/01/06 3/19/07 8/17/07
Cleanliness Improvement I
Bearings Improvement
Inspection Level Changed
Cleanliness & Test Load Improvement II
6/30/08
CS/ CR/ Brg re-certified process
Test Inspection
Original
Bearings chng
Cleanliness I
Cleanliness II
DEF Crankshaft/ Bearing-only Weibull• Measurable Brg/ CS Reliability step
improvement from ‘05-’08 …. at three years (30 Khrs) : improved from >10% Failure Rate to < 0.5%
• No bearing failures on units shipped since November 2007… although some customers still limit bearings to ~ 2 years
Bearings /shaft Recertified
301
Bearing Design CalculationsHydrodynamic (HD) calculations have been preformed on a wide variety of
compressor connecting rod bearings.
The following general guidelines have been established based on these calculations and actual performance:
• MOFT greater than 2.0 µm (≈ .00008”)• MOFP less than 3000 bar (≈ 43000 PSI)
Market Study of Crankshaft Diameter and MOFT at 85ºC
0
0.5
1
1.5
2
2.5
3
3.5
100 120 140 160 180 200 220 240
Crankshaft Diameter (mm)
MO
FT (m
icro
n)
. FS
Market Study of Crankshaft Diameter and MOFP at 85ºC
200
220
240
260
280
300
320
340
360
380
400
100 120 140 160 180 200 220 240
Crankshaft Diameter (mm)
MO
FP (M
Pa)
.FS
302
Bearing Materials• Steel Backed Leaded Bronze Tri-metal bearings are the default material choice for
compressor rod bearings. Given that MOFT range from roughly 2.0-3.0 and MOFP averages around 3000 bar on the rod bearings.
• Bi-metal Aluminum lined bearings are sometimes used. However, these are mainly for sour gas (H2S) applications that will destroy the leaded bronze bearings.
• The main bearings see much less loading than the rods (only about 25-30%).
HD Calculation range for Compressors
303
Improved oil feed - NP vs. current style
• leverages experience from other GE recip products• Double oil feed / bearing• Removed circumferential groove at high load• increased pin width• improved manufacturing local geometry at crank wallcurrent New - NP
304
Proposed Changes – NP Style Bearing Comparison of Standard Bearing vs. NP Style HD Plots.
Standard Crank Drilling will pass through these high press zones.
Note: ~2x reduction in POFP (on axis)
305
Instrumentation
6.25*
9.5
1
3
2
4
14.5
14.5*
* Includes Block I & II upgrades
Skid Package
• Engine Power / Torque
• Crankshaft Angle, RPM
• Gas Mass Flow Rate
• Suction, Interstage & Discharge Press
• Frame Vibrations
Lubrication System
• Oil Sump Temp
• Oil Cooler Oil Inlet & Outlet Temp
• Oil Cooler Coolant Inlet & Outlet Temp
• Main Saddle Oil Temp, Press, Flow Rate
• Main Bearing Temp, Orbits
Throw 1
• ConRod Rifle Hole Oil Press, Temp, Flow Rate
Throw 1 vs. 3
• Crank Pin Bearing Temp, Orbit
• Wrist Pin Oil Press, Orbit
• Bushing Temp
• Crosshead Vibration
• Rod Strain
• Cylinder Head & Crank End Press
• VVCP Backside Press
• VVCP Rod Load
Throw 2 & 4
• Cylinder Head & Crank End Press
DEF Block Change TestScope of design change
• Widen crankpin to big-end bearing contact surface to enhance the lubrication behaviour and achieve a higher degree of reliability
Crankgear components to be modified
• Crankshaft
• Crank pin bearing
• Connecting rod
306
Summary
OEM/ Field issues have been resolved for high bearing reliability (99.5+% reliable) for 2+ yrs. operation; the
– Key RC manufacturer improvements (CS, cleanliness, and bearing re-certifications; test process improved)…. ‘the devil’s in the details!’
– Packager > oil viscosity/ temperature
These and other Lessons Learned have been fed back to OEM and formally incorporated into Design Specification, vendor qualifications and Mfg work instructions
For further end user variation and confidence-– New Babbitt (Synthec) bearing option for increased
margins– New NP-style dual feed for significant margin
improvement to new units
307