Society of Petroleum Engineers

SPE 26157

Accelerated Fatigue Phenomena in High-HorsepowerTurboexpander WheelsT.G. Russell, * Canadian Hunter Exploration Ltd.; Colin Duncan, C&M Engineering Ltd.;Behrooz Ershaghi, Rotoflow Corp. Ltd.; G.J. Dyason, Noranda Technology Centre' andReza Agahi, Rotoflow Corp. Ltd. '

'SPE Member

Copyright 1993, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the SPE Gas Technology Symposium held in Calgary, Alberta, Canada, 28-30 June 1993.

This paper was selected for pre~entatlon by an S~E Program Committee following review of Information contained in an abstract submitted by the author(s). Contents of the paperas pres~~ted, have not been reViewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflecta~~ position of th~ Society of ~et~oleum En~lneers: Its officers, or members. Papers presented at SPE meetings are subject to pUblication review by Editorial Committees of the Societyo etroleum Engineers. Permission to copy IS restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledg­ment of where and by whom the paper is presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A. Telex, 163245 SPEUT.

ABSTRACT

Radial expansion turbines or "turboe~..panders" are widelyused in the gas industry in both process and powerrecovery applications. The increasing cost of plant inputssuch as power and feedstocks, coupled with abruptswings in plant throughput as a function of markets andpipeline system Iinepack, places a premium onturboexpander designs capable of operating efficientlyand reliably over a broad range of operating conditions.

High horsepower turboexpander wheels which operateover a wide speed band are subject to fatigue failures ifthe wheel's operating frequency window is too narrow.The fatigue failure mechanism is accelerated byresonance. Appropriate wheel designs, which will deliverextended operating life, may require modified wheel hubprofiles and the use of higher strength alloys. Strictcontrol of forging and heat treatments must bemaintained to ensure metallurgy of a uniform highquality.

This case study outlines the systematic methods adoptedin identifying the cause of turboexpander wheel failuresat the Karr Creek Gas Plant, and the engineering designrevisions required to eliminate them.

INTRODUCTION

The~ Creek Gas Plant located in section 10, township65, range 2 west of the 6th meridian in north-western

195

Alberta, Canada, first came on stream in 1981. This"shallow-eut" plant was initially designed as a dewpointcontrol facility utilizing a conventional refrigerationprocess. The products of the plant were natural gas andC3+ liquid. In 1990, a major addition was completed inthe form of a new "deep-eut" plant employing the OrtloffProcess. This addition boosted natural gas production~p~bility from 45 to 105 MMscflDay, improved C3+hqUld recovery from approximately 50% to 88% andallowed for the production of a stabilized C5+ liquidproduct.

Integral to the new "deep-cut" facility is the rotarye"'"pansion turbine or "turboexpander". Theturboexpander replaces the "JT valve" commonly foundin cryogenic processes and allows the recovery of "free"horsepower for other plant requirements by convertingthe pressure energy of the gas stream to mechanicalenergy. At Karr the turboexpander powers post-boostcompression into the sales compressors. Without boostcompression, the plant capacity is restricted by as muchas 35% depending on sales line pressures. Therefore, adependable turboexpander is essential for the mechanicaland economic efficiency of the plant.

Shortly after startup of the "deep-eut" facility, it becameapparent that the turboexpander system was notdependable. A catastrophic failure of the turboexpanderwheel occurred without warning after 1300 hours ofoperation. Subsequent wheel failures have occurred after4500, 2500, 750 and 9000 hours of operation (refer to

2 ACCELERATED FATIGUE PHENOMENA IN HIGH HORSEPOWER TURBOEXPANDER WHEELS SPE 26157

Table 1 for a summary of wheel failures). There havebeen no service problems with the compressor wheel.

Extensive mechanical analysis of the failed wheels andturboexpander operating conditions has been conductedin an effort to identifY and eliminate the cause of theserecurring short cycle failures. The specific failuremechanisms investigated were:

- overstress;- stress corrosion cracking (scc); and- fatigue.

To eliminate these suspected failure mechanisms. newwheel designs have been introduced which incorporatestress relieving, surface coating. higher strengthmetallurgy and improved wheel geometry.

These efforts have lead us to conclude that the mode offailure was fatigue. The failures were caused byoperation of the wheel at speeds which generatedexcitation frequencies equal to one or more naturalfrequencies of the wheel. Excitation of the fourth nodenatural frequency by the third harmonic of the nozzlepassing frequency is the dominant factor in theseresonance induced fatigue failures. The wide range ofoperating conditions experienced by the equipment hascaused the wheel to operate at or near this frequency forextended periods. Operation of the wheel at or near thisharmonic will lead to fatigue failure.

The redesigned wheel now in use at Karr has higherfatigue strength due to the use of Aluminum alloy 7075aged to the T6 temper (AI 7075-T6) and has beendesigned to optimize the operating window such that theharmonic corresponding to the 3 times nozzle passingfrequency is above the nominal design speed of 30500rpm.

TURBOEXPANDER SYSTEM

Design. Manufacture and Testing

The turboexpander system is. in its simplest form. anenergy conversion system. The pressure energy of theinlet gas stream is converted to mechanical (rotational)energy and utilized to drive a booster compressor. towhich it is connected via a common drive shaft(Figure 1). The amount of mechanical energy extractedfrom the gas stream is dependent upon the design of theinlet guide vanes, or nozzles. the design of theturboe",;pander wheel, and their interaction. The expandersystem's geometry is designed such that approximately

50% of the total change in enthalpy occurs across thenozzles and 50% occurs across the wheel. While thedetail of turbomachinery design is a complex problem inaerodynamics and outside the scope of this paper. itshould be noted that the system design methodology(Figure 2) is not only concerned with energy efficiency.but also unit thrust balance. natural frequencies, stressconcentration (determined by finite element analysis) andvibration.

The most common material utilized in the construction ofexpander wheels is Aluminum alloy 6061 aged to the T6temper (AI 6061-T6). however. depending on theapplication, wheels can be fabricated from such alternatematerials as AI 7075-T6 & T73, Titanium or stainlesssteel. Each application is unique and there are no twoturboexpander wheels exactly the same.

Every wheel is subjected to vibration. overspeed (125%),performance and non-destructive mechanical tests oncemanufacture is complete. Wheels are mounted to a testshaft and excited to a range of frequencies. Naturalfrequencies of the wheel hub and disc. as well as theblades are identified and compared against the results ofthe computer simulations. Campbell diagrams, which areessentially vibration fingerprints for each wheel areprepared using actual test data (Figures 3 - 7).

Campbell diagrams plot the natural frequencies of thewheel against excitation frequencies generated by theinteraction of the blades and nozzles at various operatingspeeds. Particular attention is paid to excitation of thefourth node natural frequency by the lxNozzles.2xNozzles and lxBlades passing frequencies. The fourthnode is typically the strongest natural frequency. Theseresonance points are kept above the desired speed rangeof the wheel.

Harmonics of the nozzle passing frequency greater than 2have generally been considered too weak to cause seriousexcitation of the wheel's natural frequencies. In addition.these resonance points generally occur outside the designrange and are only encountered for short periods of timewhen the system is accelerating to design speed ordecelerating to shutdown. Although very few serviceproblems associated with 3xNozzles and 4xNozzlespassing frequencies have been reported. it is recognizedthat continuous operation at one of these resonance pointsover a period of hours or days may cause fatigue failures.Stress concentrations in the wheel and the microstructure of the alloy will greatly influence these failures,therefore. the quality of the wheel forgings must be keptvery high.

196

SPE 26157 T.G. Russell, C. Duncan, 8. Ershaghi, G. Dyason, and R. Agahi 3

General Characterization and Description of Alloys

The original 5 wheels were forged from AI 6061-T6.This is a ternary AI-Mg-Si alloy, in which there isformation of the strengthening inter metallic phase,Mg2Si system. The alloy makes use of the solubility ofMg2Si during precipitation hardening. Favorablecharacteristics of this alloy include moderately highstrength and good corrosion resistance.1,3,5

The redesigned wheel currently in service was forgedfrom high strength AI 7075·T6. This is a complex,quaternary AI-Cu-Mg-Zn alloy which has Zn as themajor alloying element. The strengthening of this alloyarises principally as a consequence of the magnesiumpresent in the matrix solid solution.1,3,5

Standard specifications for AI 6061-T6 and AI 7075-T6forgings include American Society for Testing andMaterials (ASTM) B247, Aerospace MaterialSpecifications (AMS) 4127 and 4139 and USAgovernment, QQ-A-367. ASlM B247 chemicalcomposition limits for these alloys are shown in Table 2.

Values in Table 3 illustrate the significantly highertensile strength of AI 7075-T6. While ASlM B247 doesnot specify fatigue strength or toughness values, thefatigue strength for Al 7075-T6 is significantly higherthan that of AI 6061-T6 and its fracture toughness isadequate. Typical.fatigue strengths of 23 ksi and 14 ksi,respectively, are reported for 5 x 108 cycles of completelyreversed stress.3

Acceptable Forging and Heat Treatment Procedures

ASlM B247 provides detailed requirements formanufacture, heat treatment, inspection and testing offorgings. When specifying forgings which conform toASlM B247, the purchaser must state whether additionalspecific requirements are to be satisfied. These includetension test specimen orientation, certification, ultrasonicinspection and liquid penetrant inspection.4,5,6

Corrosion Resistance

The general corrosion resistance of AI 6061-T6 is goodcompared to other aluminum alloys since most of theMg2Si is present in solid solution or as a microscopicprecipitate. Suseeptibility to intergranular corrosion islow, since the MglSi in the alloy is well balanced.During initial failures of Al 6061-T6 wheels, thepossibility of stress corrosion cracking (sec) was raised,although AI 6061-T6 is not susceptible. There are norecords of service problems - this also applies to forgingsstressed in the most susceptible, short, transversedirection. Resistance to sec exists despite the fact that

this alloy may be subject to intergranular corrosion.While sec has been induced in the laboratory withnaturally aged 6061-T4, exposed to abnormally highsolution heat treatment temperatures and slowquenching, this sensitivity can be eliminated by aging toT6 temper. Consequently, sec has been excluded as afactor in the failure of the original wheels.

The general corrosion resistance of AI 7075-T6 is lowerthan that of the Cu free alloys of this group and all otheralloy groups except 2XXX. It is fortunate that operationof the wheel does not promote sec, since 7070-T6 issusceptible to this failure mechanism. Research hasindicated that water or water vapor is the keyenvironmental factor required to produce sec. Halideions, and most importantly Chlorine (CI) accelerateattack.7,s

PROCESS AND OPERATING ENVIRONMENT

Plant Design Conditions

After passing through an inlet separator, where freeliquids are removed, inlet vapors pass through the inletcompressor and are boosted from 2410 kPa to 6895 kPa.The gas is then sent through the molecular sieve dryingtrain where it is dehydrated to approximately 2 ppm(equivalent to a dew point of -100 0q, prior to enteringthe cryogenic process. The gas flows through theexpander inlet scrubber, where condensed liquids dropout, and on to the turboexpander where it is expandedfrom 6650 kPa at 12°C. to 1580 kPa at -47 °C. Designspecifications for the turboexpander are outlined inTables 4 and 5. A IT valve is provided for backup incase the turboexpander is not available. Discharge fromthe turboexpander is directed to a mid-point feed on thede~thanizer column. The de~thanizer is a fractionationcolumn which will reject C2 and Cl from the LPG's.Propane recovery in the C3+ mode is approximately88%. Gas from the top of the column is fed through thebooster compressor where it is compressed from 1450kPa to 2275 kPa. The booster compressor is a singleimpeller centrifugal compressor which is on a commonshaft with the turboexpander. After booster compressionthe gas flows through sales compression to sales. If theturboexpander is not operating, the sales compressors areunable to overcome the extra compression ratio caused bythe lack of booster compression. Therefore, whenoperating with the backup IT valve, pressure in thefractionation column must be raised from 1580 kPa to2170 kPa. This mode of operation results in lower liquidrecovery.

197

4 ACCELERATED FATIGUE PHENOMENA IN HIGH HORSEPOWER TUR80EX~ANDERWHEELS SPE 26157

Actual Conditions

The attached graphs indicate the actual operatingconditions encountered during the run life of each wheel.There are large swings in plant throughput (Figures 8 ­12) and sales line pressures (Figures 13 -17) on a dailybasis. Plant throughput varies as a function of bothgathering system deliverability and markets, however thesharp daily fluctuations seen here represent primarily theeffects of markets. Sales line pressures never approachdesign pressure of 8619 kPa and rarely exceed 8000 kPa.

The effect of these swings on the operating condition ofthe turboexpander can be seen as wide daily variations inturboexpander speed. (Figures 18 - 22) Due to the lowerthan design sales line pressures, the turboexpander israrely required to run at its design speed of 30500 rpm.The turboexpander is conservatively loaded and generallyruns between 26000 and 29500 rpm. Operating pressures(Figures 23 - 27) across the expander and the compressorremain fairly constant near the design levels as expected.Although not indicated in the graphs, temperaturesacross the unit are also fairly constant at or near designlevels.

The actual gas analysis shown in Table 6 are indicativeof the benign, sweet gas environment anticipated in thesystem design. There are only trace amounts of sulfidesand chlorides.

DESCRIPTION OF FAILURES

The failures in wheels 1,2,3 & 4 appear to have initiatedin the keyways via an intergranular fracturing mode. Theinitiating cracks propagate in a pseudo fatigue fashion asintergranular and also as a mixed inter/transgranularfracture mode under the apparent influence of theoperating stresses, however, characteristic fatiguestriations were not identified. Asymmetrical keywayfracture initiations and propagation have beenexperienced consistently. Fracture initiation andpropagation is identical in wheels with square and roundkeyway configurations (Figures 28 and 29). Final failureof the wheels (1,4,5) occurs by ductile overload shearingand tensile failure. Intergranular and transgranularcracking has been identified by all investigators.Compositional and hardness tests conform to Al 6061-T6standards in all investigations.

Wheels 1 & 2 contain tramp element impurities,specifically Lead (Pb). Pb was identified by SEM andatomic adsorption analysis. In the failure of wheel 1, thefracture surfaces were shown to have a finely depositedintergranular phase at many locations. Pb is associated

198

exclusively at these deposits in intergranular fracturefacets. This intergranular precipitation was also noted tobe in a very large grain structure.

In the failure of wheel 2, large grains are visible in the .fractUre origin. Nodular grain boundary deposits areagain present. Prominent precipitates are visible in theprior cast grain boundary structures and there is anexcessively large grain structure, unexpected in a forgedproduct. The crack origin reveals high copper (Cu) andPb concentrations. There are a large number of ratchet­like steps at the fracture origin, similar to ratchet stepsdeveloped in some fatigue failures where coincidentalfatigue sites are initiated.

In the failure of wheel 4 there is no detectable lead and afar reduced incidence ofgrain boundary features. '

The failure of wheel 5 is clearly fatigue and differs fromthe other failures in that it suffered a blade failure.Distinct striations originating in the keyway confirmedthe nature of fatigue failure. No material or heat treatanomalies were noted. The blade exhibited two distinctclamshell marks suggesting that failure had originated onthe concave as well as convex side of the blade, as twoseparate fatigue fronts propagating towards each other.There was clear indication of fatigue striations andratchet marks indicative of fatigue failure.

DISCUSSION

After reviewing the operating data for the turboexpanderit is clear that the nominal design conditions for the gasplant are not the normal operating conditions of theplant. The plant swings through a wide range ofoperating throughputs and sales line pressures, causingthe turboexpander speed to fluctuate markedly from dayto day. The plots of critical speeds derived from theCampbell diagrams against actual operating speeds foreach wheel (Figures 18 - 22) show many hours andlordays of operation at or very close to resonance points.Clearly the operating frequency windows for wheels 1 to5 were too narrow for the actual operating conditionsencountered in service.

After the failures in wheels 1 and 2, the keywayconfiguration was changed from square to round in aneffort to reduce the effects of stress concentrations at thekeyways. The subsequent failures of wheels 4 and 5 withidentical failure characteristics is clear evidence thatoverstress at the wheel hub was not a contributor to thefailure of the wheels.

SPE 26157 T.G. Russell, C. Duncan, B. Ershaghi, G. Dyason, and R. Agahi 5

There is strong evidence that metallurgical defects,namely tramp elements such as Pb and Cu introduced inthe meltshop, have played a role in the failures of wheels1 and 2. The presence of these elements set up plains ofweakness at the grain boundaries and facilitated theintergranular fracture mode. However, stringentmetallurgical controls utilized during the construction ofwheels 4 and 5 apparently did little to prevent identicalfailures from occurring in these wheels. Therefore,metallurgical defects are not the principal cause of wheelfailure. The failure of wheel 5, which is clearly the resultoffatigue, has no signs of intergranular cracking.

There is also evidence to suggest that keyway stresses arenot uniform. Failure initiation is more strongly developedon one side of the wheel due to variable keyway stressdistribution. The cracks do not propagate diagonally aswould be expected if stresses were uniform. Theasymmetrical nature of the fracture orientation is due tothe assembly stresses and not imbalance, therefore, crackinitiation may be the result offretting.

Initial studies indicated that a scc failure mechanism wasoperative. However, as indicated by the gas analysis nomercury and only trace sulfides and chlorides are presentin the process stream. Most importantly due to thedehydration process there is very little moisture presentduring normal operation, startup or shutdown. Therefore,there is no support for an scc failure mechanism. There isno record of any service problems with scc in AI 6061­T6. In addition, there have been no service problems withthe compressor wheels, which are made from the samealloy and operate in a process stream of comparablesulfide and chloride composition. Application of ananodized surface coating to wheel 5 did nothing toprevent the rapid and obviously fatigue related failure inthe wheel. This clearly indicates that scc is not playing arole in the wheel failures.

The wheel which is currently in service has been tuned toprovide a more appropriate operating range for the wheel(Figures 6 and 7) given the normal plant conditions. TheAl 7075-T6 alloy used for the construction of this wheelhas significantly higher fatigue strength. Stringentmetallurgical controls were utilized to ensure metallurgyof the highest possible quality. Therefore, the new wheelappears to have the qualities necessary for a prolongedrunlife.

CONCLUSIONS

1. The failures of the turboexpander wheels at the KarrCreek Gas Plant have been caused by fatigue. The fatiguefailure mechanism has been accelerated by prolonged

199

operation at speeds which create excitation frequenciesequal to one or more natural frequencies of the wheel.

2. Metallurgical problems, specifically the presence oftramp elements Lead and Copper at the grain boundaries,contributed to the failure of wheels 1 and 2.

3. Overstress and stress corrosion cracking are not causesof turboexpander wheel failure.

4. The wheel which is currently in service has anoperational window which is wide enough toaccommodate the actual operating range of theturboexpander system.

5. In the high cycle fatigue regime of wheel operation, selectionof Aluminum alloy 7075-T6 significantly improves the wheelfatigue resistance over the original Aluminum alloy 6061-T6.While this choice makes sacrifices in terms ofcorrosionresistance and specifically resistance to scc, corrosion does notappear to have played a role in the wheel failures.

REFERENCES

1. Aluminum. Properties and Physical Metallurgy. JohnE. Hatch, Editor, American Society for Metals, 1984.Pages 50-52 & 64-82.

2. Standard Specification for Aluminum and Aluminum­Alloy Die Forgings, Hand Forgings, and Rolled RingForgings. Designation B247-88. Annual Book ofASTM. Standards. Volume 2.02,1989.

3. Metals Handbook. 9th Edition, Volume 2. Propertiesand Selection: Non-ferrous Alloys and Pure Metals.American Society for Metals, 1979. Pages 61-62 & 132.

4. Metals Handbook. 9th Edition, Volume 14. Formingand Forging. American Society for Metals, 1988.

5. Aluminum. Properties and Physical Metallurgy. JohnE. Hatch, Editor. American Society for Metals, 1984,.

6. ASM Handbook. Volume 4. Heat Treating.American Society for Metals, 1991.

7. Metals Handbook. 9th Edition, Volume 13.Corrosion. American Society for Metals, 1987.

8. Aluminum. Properties and Physical Metallurgy. lE.Hatch, Editor. American Society for Metals, 1984.

6 ACCELERATED FATIGUE PHENOMENA IN HIGH HORSEPOWER TURBOEXPANDER WHEELS

Table 1SUMMARY OF WHEEL FAILURES

WHEEL Haun of Operation No. of Normal Starts Wiled Condition atPrior to Failure Prior to Failure Diseovery of Failure

1 1300 9S Split in half,2 4S00 69 Severely craela:d

3 9000 74 Severely c:rac:kcd

" 2500 9 Split in half

5 7S0 14 Severely crackcdII blade missinl!

Table 2OIEMICAL COMPOSmON LIMITS (from AS1M B247-88)

ELEMENT AI 6061 (Wt.%) AI 7.075 (Wt.%)

Si 0.40-0.8 0.40 max.Fe 0.7 max O.SOmax

Cu 0.IS-o.4O 1.1-2.0

Mn 0.15 max '0.30 max

Mg 0.8-1.1 2.1-2.9

Cr 0.04-0.35 0.18-0.28

Zn 0.25 max 5.1~.1

'11 0.5 max 0.20 max

Othcn, Each 0.05 max 0.05 max

Othcn, Total 0.15 max 0.15 max

AI Remainder Remainder

. Table 3MECHANICAL PROPERTYLIMlTS FOR DIE FORGINGS (from AS1MB247-88)

PROPERTY 6061-T6 707S-TISUP to " In. nick 3.001-4 In. nick

TcnsUe strength:

parallel" ksi 38.0 min 73.0 min

non-oar:alIel· ksi 38.0 min 70.0 min

Yield strength #I:

paralIeI" ksi 35.0 min 62.0 min

non-oarallel· ksi 35.0 min 60.0 min

Elongation:

paralIel'f% 7 min 7 min

non-oarallel* % 5 min 2 min

Hardness:

Brinell 80 min 135 min

SPE 26157

# 0.2% Offset 'fspecimen axis parallel todirection of grain flow

200

*specimcn axis non parallclto direction of grain flow

SPE 26157 T.G. Russell, C. Duncan, B. Ershaghi, G. Dyason, and R. A'bahi 7

__e-

FIGURE #1

TURBOEXPANDER SYSTEM SCHEMATIC

CllIIlIS

CFIIII( BS£llrlM.'ISl51

lID!

FIGURE #2 EXPANDER WHEEL DESIGN METHODOLOGY

201

8 ACCELERATED FATIGUE PHENOMENA IN HIGH HORSEPOWER TUR80EXE'ANDER WHEELS SPE 26157

Campbell DiagramWheels 1,2 & 3

Campbell DiagramWheel6a

......... ,...I

15

10

54

2

O.....,=L...--'----'----'----'---.la---'---'----'---Oo 5 ro ~ ~ ~ ~ " ~ ~ ~

RPM (Thousands)

CPS (Thousanda)1<4 ;:.:....:.....:.;......:..:=,:.:.:..:=-=-----,.------r--12 I10 :,

8 J..""",•.""..,."., "---,..."....,.j.""--:,,".',,,,,.--

8 F----...,;..---:---=---::~.::::::=---:..-.~=:,:;::.::l

4

2O~~.::.---L_-L._..J...._.J.-----'LI---'-_-'-_....l--...JO

o 5 ro ~ ~ ~ ~ " ~ ~ ~

RPM (Thousands)

14 ~C:::.PS~(T:.:h::o::ua::a~n::da:::)~~~1-----.r---.:.....:::.;.:=~=~

i 1512 I

10 ! I : i I.' i

: ..:.: l L L ; :.,~:.Z;.,=:.~.-:~.:.~.::::: :::~,.:........;.

15

.. x Nozzl•• - 41th NodI _. Ith Nod.

Blad•• .-.... B.ad.' .•••.• 2 x Nozzl•• - 3 x Nozzl.. ._ •• 8Iod••

- 3 x Nozzle.

81.d..

--- • x N011I••

...... 2 x Nozzle•

FIGURE #3 FIGURE #6

Campbell DiagramWheel 4

Campbell DiagramWheel 6 Final

15

454015 20 25 30 35RPM (Thousands)

105

.:. ~.::_.!._.:~_·:·:·t······:·:···· __ ··_···_·:····_~~]" __---_.-'-_-'-_-'-----..1:50

Cps (Thousands)14

12 ~ I .'

1: .==~~.:~T=j:~::::~::::::~;:;:::t:.::~:~.~:.~-::::~.~j:o::0:0::0: 10

8 .. --

4r2

oo

10

5

;

, - 15...... -!"

4

2o.....=.-'--_-'-_.L-_-'-_-'-_......_ .....- .. J. .• --'-- 0

o 5 ro ~ ~ ~ ~ " ~ ~ ~

RPM (Thousands)

:~[~-=~'--------~~-o.! -8 !::::::::::::::::::.::::::.:::_:.:::::::.:,,;::::~~::::::: ....~::::::::::::..

Ilad•• ...... Blad•• .__..• 2 x Houl•• - 3 x Nozzl•• Blad•• ---... Blad•• ...... 2 x Noz.z••• - 3 x NOZz.I••

-- .. x Noul•• - 4th Nod. -- Ith Nod. - .. x Nozzl•• - "'h Nodi - 8th Nodi - Blad'.

FIGURE #4 FIGURE #7

Campbell DiagramWheel 5

C~P~S~(T~h~OU~a~a~nd~a~),...__-""T-....,--:--_--II--:114 il_~ 15

12 ;. II

10 iii .i l.. _.iJ.<..L.... L............. 10:~....=......~.+..=.....~....,~.....=.....±....~::.::$=;;F~=t~~

W:,,! 5.. ~ \2 : ~o!,.'<:::::::=:,.. ....i.._-'-_.~-'-- .......--'-- . 0

o 5 ro ~ ~ ~ ~ " ~ ~ ~RPM (Thousands)

....... Blade. .-., Blad••

-_. .. x Nozzl•• - 4th Nod.

...... 8Iad.. - 3 x Nozzl••

._. 11th Nod. - 81.d••

FIGURE #5

202FIGURES #3-7 CAMPBELL DIAGRAMS

SPE 26157 T.G. Russell, C. Duncan, B. Ershaghi, G. Dyason, and R. A~ahi 9

TURBOEXPANDER DESIGN SPECIFICATIONSTable 4

UNIT EXPANDER COMPRESSOR

Molecular Wt. 20.10 18.00

Plnlet, kPa 6778 1551

Toudet, °C 12 41

Plnlet, kPa 1668 2354

Toudet, °C -45 78

Flow, t03m3/hr 55.929 72.848

Power,kW 1336 1318

Design Speed, rpm 30500 30500

Shutdown Speed, rpm 33550 33550

Efficiency, % 82 77

Weight % Liquid 13.44 0

TableSEXPANDER WHEEL DESIGN DETAIL

ITEM DESCRIPTION

# Long Blades 7

# Short Blades 7

# Nozzles 4

Wheel Diameter, inches 7.5 (Wheels 1,2,3,4, 6); 7.375 (WheelS)

Tip Sileed U, ftlsec. 999

Weight, Ibs. 3.16 (Wheels 1,2,3); 3.140 (Wheels 4,5); 3.620 (WheeI6)

# Keyways 4

Keyway Configuration Square (Wheels 1,2,3); Round (Wheels 4,5,6)

203

10 ACCELERATED FATIGUE PHENOMENA IN HIGH HORSEPOWER TUR80EJ<P.ANDER WHEELS SPE26157

PLANT THROUGHPUT !WJwI "1PLANT THROYGHPUT IWhnl '41

...",.2100 -.-------------------------

2250 .

-- ....... (.03m31 -5-..FIlMtl03nt3l· - ·I,,*~

ft '100 ~~;;,.:;_:::::::~~_l_------_tJ.H-I-.!::L:....--!f_--__+:

: 1210

~ ,.... L-----i----------I,.f----+----

. . . - . . . . . .....~ .., II \

"""~.

~

I I ! I ~ ~ ~ ~ ~ ~ ~ ! ~ ~ ~ ~ ~ I I ~ ~ ~ ~ ~ ~ ~ ! ! !- ~ - - a a a a a a S 0 S Ss _ ___ ~ _ ~ ~ ~ __ ___- HftL t~~ - -=-= s:..:':nc:...: : . .INit DeIiIn - - :: .-,:.:..:. - -

710

100

'10

'IOO~..50

: "-1ft'lOO..~ 1250

~,....

FIGURE #8FIGURE #11

ptANT THROUGHPUT !WJwI '21"'-ANT THRQUGHPUT !WJwI161

'100 .,.-,----- _

-. ...... Row 1103m31 - s.a- Row (103m31 - - • In6M DUgn- • s..DuIgn

__~I~_. .. • i:':..l •• _ • -.~~•~~ ...\1,---... I 1'1" v -.::::::::0I~·· \1 1\ il.,

~

,--..... RowIIQ3m31 -s-..RowlfD3nl31· ...... DrIIign

210

100

750

ft'lOO'.R 1250

! ,....

-._-

.- ....,jo~ J.. ••• . .. ..t"_'-_ ..j.l'l._ : ~fI~k·.:!'~ ~t~tLa~:

," - . . .

i • I ~ ~ 1 P -t- ~-.r"<!Jw .... "':;..- .~... r "'i

. -I ._--

" I210

"50

710

'710

.....

100

ft'lOO

: 1250..-,....

FIGURE #9FIGURE #12

PLANT THRQUGHPUT IWhnl '31

2500 r---'------------

'100 -HIII.:..-----1250 -i-:~C__'t_---__i_iP~~,----f___f_f_1t_---1t--

,.... -I+--I-I----+1ll~_1_----+-+_II_+----~-7IO-I+--+-ll----l-iL-+----+-~_f__+-----

100 ++--t-!l----~---------t_-----

210 -II----!-#-- -+-!-----------if-------

FIGURES #8-12 PLANT THROUGHPUT - ACTUAL VS. DESIGN

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FIGURE #10 204

SPE 26157 T.G. Russell, C. Duncan, B. Ershaghi, G. Dyason, and R. A'gahi 11

SALES LINE PRESSURE mill". SALES UNE PRESSURE !Wblll '41

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FIGURE #13 FIGURE #16

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FIGURE #14 FIGURE #17

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FIGURES #13-17 NOVA UNE PRESSURES - ACTUAL VS. DESIGN

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SPE 26157 T.G. Russell, C. Duncan, B. Ershaghi, G. Dyason, and R. Agahi 13

n OPERATING PRESSURES IWIwI III IE opERATING I'REl!SURES IWIwI '41

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• 410011

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FIGURES #23-27

207

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FIGURE #25

14 ACCELERATED FATIGUE PHENOMENA IN HIGH HORSEPOWER TURBOEXPANDER WHEELS

ACTUAL GAS ANALYSIS

SPE 26157

Table 6

COMPONENT MOLE FRACTIONS

He 0.0001 0.0001

N2 0.0036 0.0039

CO2 0.0116 0.0126

H2S Trace Trace

C1 0.8337 0.8840

C2 0.0905 0.0965

C3 0.0386 0.0027

iC4 0.0058 0.0001

C4 0.0096 0.0001

iCs 0.0024 0.0000

Cs 0.0022 0.0000

C6 0.0011 0.0000

C+ 0.0008 0.00007

Molecular Wt. 19.8 17.9

H2S 0.8 ppm 1.1 ppm

Carbonyl Sulphide 0.3 ppm 1.1 ppm

Methyl Mercaptan 0.1 ppm 0.0 ppm

Chlorides <0.1 mg/m3 <0.1 mg/m3

208

SPE 26157 T.G. Russell, C. Duncan, B. Ershaghi, G. Dyason, and R. Agahi 15

FIGURE #28

TURBOFAILURE OF WHEEL 1 (SQUARE KEYWAYS)

209

16,

ACCELERATED FATIGUE PHENOMENA IN HIGH HORSEPOWER TURBOEXPANDER WHEELS

FIGURE #29

TURBOFAILURE OF WHEEL 4 (ROUND KEYWAYS)

210

SPE 26157