Vol 2High Frequency Fatigue Failures in Pulsation Dampers

7/21/2019 Vol 2High Frequency Fatigue Failures in Pulsation Dampers

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HIGH FEQUENCY IGUE F ILURE IN

SILENCEPULSATION DAMPERS FOR OIL-FREE SCREW COMPRESSRS

by

Jef ovlady

Design Engineer OS Engineering DevelopmentAC Compressor Corporation

Appleton, Wisconsin

andeter A BielskusReliability Engineer

BP-Amoco

Whiting Indiana

Je Lovelady is a Design Engineer withA -C Compressor Cootion, i leto,Wisconsin He is responsible for the designad developmet of oile screwcompessor products and has mo than

10years experience in design and analysis ofrotating equent Prior to joiig A-CCompresso he wored r Cetri asa Design Engineer and Aerojet as an

Aeroheodynamicist.M Lovelady has B.S . and S. des

(chaical gieerig) o Texas A&M Univei.

Peter A Bielskus is a ReliabiliEngieer with BP-Amoco, in iting,Indiana He has mo than 30 yearseerience with tating equment toinclude design, installation startup, admaitenance Prior to joining BPAmoco,

he worked with ANG Coal Gascatio,Consumers Powe and Uical 76 as aRotating quipmet Enginee

M Bielskus has a B.S. degee eroautical and Mechanical Egieerig)

m liois Istitute of Technology.

ABSTRAT

Oil-free screw (OFS) compressors are capable of producinghigh-pressure pulsations in the discharge gas stream over afrequency range of 200 to 3000 Hz. The ndamental pressurepulsation is due to the opening cycle of the comressor lobes at thedischarge porting and is reered to as the pocket passing equency(PPF). Fatigue failures have been observed in discharge

silencers/pulsation dampers as well as downstream pipingcomponents in oil-free screw compressor service where thefrequencies were greater tan 1000 Hz. A cel review of thepulsation damper design has shown that attenuation eectivenessis limited to equencies less han 1000 Hz. The principal purposeof the pulsation damper is to reduce the line pressure pulsations inorder to lessen the chances of downstrem piping failures. Tepulsation damper does very little to attenuate high equency noiselevels that are transmitted to the area suounding the compressor.

Te principal purpose of this paper is to document failures ofsilencer/pulsation dampers in OFS compressor service and providea root cause analysis of these failure modes. Pulsation dampers are

not used at the inlet ange in many applications due to the lowpressure pulsation in the inlet plenum of the compressor. It is noth intent of this paper to present any theoretical background in thedesign of acoustic dampers but to provide an overview of theacoustic phenomenon and how it is used in screw compressorapplications. The end result of this work wi be to providepractical guidelines r speciing silencer/pulsation dampers rOFS compressor applications.

INTRODUCTION

Pulsation damper (PD) design consists of a carelly selectedcollection of passive attenuation components that decrease thepulsation amplitude of a discrete set of equencies that areproduced by h compressor. Pulsation damers designed for APIservice ar designed for an acoustic cuto of approximately 1000z The pulsation pressure magnitude of equencies above 1000z is generally considered to be insucient to cuse pipingdamage. Measurements of pressure pulsation and piping vibrationhave shown that the higher order multiples over 1000Hz can excitemechanical vibration resulting in severe damage. Attenuation of

high order multiples results in lower noise levels in the pipingdownseam of the silencer and redces the source of excitationthat can prodce mechanical resonance.

COMPRESSOR PULSATIO

SOURCE THREAD PENING SEQUEN

Gas compression in the screw compressor takes place in threechambers within the compressor. One is the inlet plenum which isthe volme om the inlet ange to the inlet end-wall of the rotors.The second is thread volume along the rotors whic is bounded bythe casing bore walls and the rotor end-planes The third is thedischarge plenum that extends om the porting at the dischgeend-wll to the discharge ange. The pressure distribution duringthe discharge thread opening is the primary source of pressurepulsation at the dischage of the screw compressor. The volume

between the threads of each rotor is lled in the inlet plenum. Thethreads are exposed at the inlet end-wall aer the same volume isclosed at the discharge end-wall. Gas from the inlet plenum starts toll the threads as they rotate toward the convolution of the threadsat the inlet port cusp (Figure 1). The inrush of gas into the threadsproduces a pressure reduction in the inlet plenum. This pressurechange is one source of pressure pulsation at the inlet ange. Themagnitude of the inlet pressure pulsation is much lower than that atthe discharge port. e threads complete the inlet closing cyclewhen the pitch line of each rotor meets at the inlet-porting cusp(Figure 1). It is important to note that the discharge end-wall closesthe open area on the discharge end of the lobes that are being lle.


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NS 2 UMANY SYMSUM

Ma lobe

Figue 1 Roto Ple Coolutio at the Ilet Potig

The compression of the gas takes place as e threads continue toclose thus reducing e enclosed volume The shape of the discheporting determines the pressure ise wihin the enclosed threadsPressure in the dischae plenum begins to rise as the threads open atthe discharge port The pressure gradient prouced by this threadopening sequence determines the magnitude of the pma pressure pulsation The numer of thread opening cycles per seconddetemines the pmary pulsation equency which is known as thepockt passing equency (PPF) A screw compressor with a 46rotor conguration (four male and six male lobes) at is drivenby the male rotor at 3778 m has a ndental PPF of 51 z

Iteal Soue o Reace

There are several sources of pulsation excitation within thecompressor pulsation damper system at generate equenciesabove 1000 z The exposed volume of gas witin the rotorthreads chges as a function of time at the inlet and dischargeplenum This change in volume results in a change in impedance atthe inlet and dischge The thread volume that is exposed to thedownstream plenum causes an impedance mismatch that can initself be a source of higher order haonics of the undamentalChanges in impedance occur at changes in the crosssection alongthe ow path of an acoustic wave The opening at the end of theinlet tube to the P produces a very large change in impedancealong the process ow path The discharge plenum and inlet sideof the must be considered as a coupled acoustic system forevaluation of the resonant equencies that could occur at thecompressor dischge

The amplitude of the pure hamonics decays exponentially ase equency increases Measurements taken during operation of a

screw compressor in operation indicate that the equency contentof the pressure pulsation at the dischge includes high ordermultiples of the PPF up to 750 Hz, approximately 11 times thePPF (Figure )

The pulsation measurements (shown in Figure 2) were taken on acompressor operating on an open loop with a straight section of pipein place of the dischge silencer The PPF for these operatingconditions is 252 Hz There is a song pulsation peak at 1506 which is six times the ndamental PP The magnitude of thepulsation at high order multiples of e fundamental PPF indicatesat ese equencies may not be pure hmonics and e the resultof a resonance at the compressor discharge The acoustic system at

! 0.4j

l

2

' . - "

Fru (

gue 2 Dichage Peue Pulatio Fequecy Spectum.

the compessor discharge includes the plenum om the rotor endwall to e discharge ange and the inlet chamber of the silencerEvaluation of the acoustic resonance of the dischge pipingsystem should include analysis of the dischge plenum coupled tothe P

Many piping sections or pressure vessels have devies attachedto the wall that can act as a side branch resonator Side branchresonce can occur where there is a discrete volume that is opento the ow path of the gas The resonant frequency at wich a sidebranch responds is dependent on the length of the open volume Ifthe side branch is excited by a equency that is an odd multiple ofthe undamental resonance of the channel hen there is an acoustic

node at the opening of the side branch Side branch resonators canbe used as attenuation devices when tuned to odd multiples of anexcitation If the excitation occurs at an even multiple then anantinode is present An antinode that is present at the opening ofthe side branch results in amplication of the excitation eqencyInstrument connections inspection openings and drains can all actas sources of resonance

Iuece o Poce Coditio

The sound power at e dischage port can reach levels in excessof 150 dB under steadystate operating conditions Changes in theprocess conditions can have a signicant impact on the pulsationpressure level produced at the discharge port The pressure insidethe screw threads is xed by the port geometry at the dischrgeendwall of the compssor If the downstream pressure in the

process is set at a point that does not match that of the builtinvolume ratio then there is a pressure mismatch at the port openingresulting in an elevated pressure pulsation magnitude

The fundamental pocket passing frequency changes as aunction of compressor speed Attenuation chacteristics of thepulsation daer components are degraded when the comressoris operated o the optimum design speed esign length ofacoustic dampers such as choke tubes and side branches isdependen upon the wavelength of the equencies of interest Thepulsation damper design can be modied to accommodate odesign conditions such as speed vation by changing the endcondition of the tubes by tapeng or perforations but suchmodications reduce the attenuation at the center equency

The local speed of sound in the process gas is dependent upone molecular weight temerature and pressure

where

c Speed of sound s

Ratio of specic heatsR Universal gas constantmw Molec weightTk = Temperature 'Klvin

(1)

If the process molecul weight varies om the design point ina sort cycle then the change in te attenuation eectiveness of te


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QUNY AU LU N SLNULSAN AMS L SW MSSS

PD is temporary and these conditions will not signicntly alterthe design of the PD. If the molecul weight vies continuouslythen the resulting uctuation in wavelength may require amodication to the PD design. The temperature of the gas willchange the attenuation characteristics of a passive acousticelement. The transmission coecient of a side branch in a pipedecreases as the temperature of the process gas increases Thedashed line in Figure 3 indicates the transmission coecient of aside branch where the temperature is increased by 10.Processes that have uctuations in the gas compsition by designshould be evaluated by he compressor and PD manufacturers forresonance and attenuation analysis.

120

100

080

060

040

020

-

000

100

I

1000 10000 100000

Fqy H

Figu 3 Teerature Dpedece miio CocietCoarion i a ide Bch

ULSION DAMER COMONENT CONSTCTION

Pulsation dampers that e pplied in process gas service areconscted from a combination of passive acoustic elements.Passive acoustic elements are those that have a xed acoustic

resonance characteistic. Examples of passive acoustic elementsinclude open choke tubes open and closed peforated tubes oriceplates baes and acoustic packing material as shown in Figure4. The tubes oce plates and baes have discrete acousticcuto equencies that can be tuned for the PPF Reduction of thepulsation amplitude for equencies below 1000 Hz is not dicultto achieve using passive pulsation damper designs. The acousticpacking materials are broad band absotive devices that areespecially eective at high frequency attenuation. Absorptivepacking materials such as berglass or glass beads must becontained in a mesh or prforated material in order to be ective.The perforated retention material may il in fatigue and willgenerally not meet the three ye service life expected in API 619equipment. If the perated material could be designed with asuitable fatigue life then e packing material may settle break

apart or become blocked due to solids cayover in the process.The packed mateials e not considered as acceptable designcomponents for most AI applications due to these designlimitations. The pulsation dampers used in API renery processequipment are designed with passive acoustic elements which aretuned to attenuate discrete equencies.

ide Brach Reoace i ecoda Coectio

Pulsation dampers are constructed with a number of secondaryconnections for pressure and temperature measurement as well asinspection pors. The process connection anges ad the adjoiningtube section are considered as ctical components i the acoustic

Opn Pfte Tube

Choke e

couc Pa It

Closed Prted e

ce Pte

gu4

Paive Acoutic Elemet Deig

design of the pulsation damper; however the mechanical oracoustic inuence of secondary connections is not alwaysthoroughly evaluated.

Any opening in the wall of the pulsation damper or compressorplenum is a potential source of acoustic resonance. This resonanceoccurs at a discrete equency that is a unction of the openinggeometry and the process conditions in the chamer. If the openingin he wall is closed at one end then a standing wave is set up wian antinode at the closed end-wall. A tube that is closed at one endand designed to a length that is an odd muliple of the wavelength of interest will have a node at the opening along the silencerwall. Tubes at are designed with nodes at the wall opening willdissipate some of the energy in the acoustic wave at the tunedequency. If the tube length is coincident with a even multiple of

the / waveength then an antinode is present at the wall and thetube will act as an amplier at a multiple of the PPF. The closedend tube is created where here is a connection for pressure ortemperature measurement. If a thermal well is extended into heprocess stream a resonant cavity may still exist in the annuluscreated by the temperature probe and the piping connectionthrough which it is inserted.

Pipe connections that are designed for a specic wavelength willoperate well under conditions that prouce pulsation at a multipleof the PP. If he process conditions change the side branchacoustic resonant frequency then the tube opening at e vesselside-wall may become excited.

Mechaical Itegri o Atchmet to the hell

The mechanical resonant frequency of each pulsation damper

component should be considered as the are coupled to the shell atthe attachment point. Components attached to the shell act asconstraints and produce sress risers in the shell wall. f there is ashell vibration mode that is coincident with the PPF or anymultiple up to approximately 4000 Hz, then the fatigue life of theshell at these stress risers can be reduced signicantly. Most tubesthat connect the chambers in a PD are long enough to require somerestraint to prevent large cantilever deections at the bae. Thesetubes are usually braced to the oter vessel shell wall by weldedplate as shown in Figure 5 The PD shown in Figures 5 and 6 wasused in a service with uctuation of molecular weight om 50percent to 00 percent of the rated design value. A nite element


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PROCEEDINGS OF THE 28TH TOMACNERY SYMPOSIUM

analysis (FEA) of the shell identied natural frequencies of theshell which were resonant at several multiples of the PPF Ringtesting of the vessel was conducted by placement ofaccelerometers along the shell wall and exciting the wall with animpact hammer at several places along the shell. The weldattachment is a point of high stress concentration along the shelland tube walls The tube supports failed at the weld joint in six ofthe eight plates used for bracing Investigtion of the fracturesurface revealed that the cracks wee initiated at the suce of theweld and propagated into the prent metal of the shell and choketubes The tubes shown failed in less han 700 hours of serviceSide branch connections are also stress risers that can result inilure of the pressure containment vessel The PD shown in Figure6 had a crack around a drain connection Liquid seepage oundthis weld was one of the rst indications of filure of the PDInstrument connections usually consist of a weldalet attached tothe vessel wall with a threaded pipe and instrument valve forisolation. Fatigue lure of these connections can occur in lessthan 100 hours of operation and frequently results in theconnection breaking o the vessel The pressure vessel should postweld heat treatment attachment of any device including nameplatesand instrument connections in order to reduce residual stresses inthe heat acted zone Design changes can be made to reduce thestress concentration at component attachment points Modalanalysis of the PD assembly is citical in determining the bestattachment points The shell mode shapes of the D unit can be

complexand

ring testing is a very good method of veriing themodal analysis model If process conditions change to the pointthat an acoustic resonance is set up in an opening to the shell th na mechanical resonance at th same frequency cn be verydestructive

Figure Choke be Suppos

MPRESSR PERFORMANCE

An increase in dischge pressure has a negligible impact on the

inlet ow to the compressor The compressor performance ismeasured by the inlet volume ow rate and power consumption.Predicted perforance for a screw compressor with 20 inchdiameter rotors indicates that an increase in the pressure at thedischarge ange will increase the power requirement and decreasethe ow as shown in Figure 7 A 1 percent increase in thedischarge pressure results in an increase in power of 1 percent forthe case shown The reduction in ow for a simil pressure riseis less than 01 percent. The pressure drop at the inlet ange to thecompressor has a greater impact on the inlet volume ow rate thane pressure at the discharge. A plot showing the inuence of inletpressure on the ow rate is shown in Figure 8 A limit of 1 percent

Figure 6 Choke Tube Sort Failure

pressue drop across the PD (as recommended by API) is intendedto minimize the impact that the PD has on the compressorperfomance. The 1 percent limit at the inlet is a god specication

to follow and in practice the actual pressure drop across e inletPD is usually less than 1 percent. The attenuation cuto in manypulsation damper designs is limited to proximately 1000 z inorder to meet the 1 percent pressure drop specication. Passiveattenuion of higher frequencies requires silencer componentswith reduced open areas resulting in higher pressure drop. ighequency pulsation at the dischage of the compressor requiresthat the design attenuation cuto equency should be greater than3500 z. The inlet pressue pulsation amplitude that is seenupstream of the PD is not high enough to be a conce. Increasingte discharge pressure drop will increase the power requirementsand operating costs but these drawbacks are usually oset byreduced downtime and repair costs associated with piping ilures.

7000

6500

l 6000I

5500

e .o

J 4500I 4000

3500

3000

1

6750 6850 6950 7000 7050 7100 7150 7200 7250

Inlt Vlum mh

Figue 7 Compressor ow Versus Discharge Pressure

8000

7500

' 7000i 6500! 6000 5500

a 5000

f 450 :05 4000

3500

3000

-

r

-

.

.

.

6750 6800 6850 6900 6950 7000 7050 7100 7150 7200 7250

Inlt Vlum mhr

Figure 8 Compressor Flow Versu let Pressure


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FEQUENCY FATUE FALUE N SLENCEULSAT N AMES F L-FEE SCEW CMESSS

WNTE M E

EQUIMENT EIGN NIETINS

Te principal pose of A esign reqirements regingplsation rection is to protect te piping ownstream of thecompressor iscarge Failre of components in piping systemsoes not always reslt in actre or rupre of te piping wallElectronic insmentation, sigt gages, an loose bolts or ttingscan be case by excessive piping plsation Hig freqencyplstion may be strong enog to rive piping attacments intoresonance iping attacments sc as teal wells, pressre taps,

an brackets can easily be excite by an acostic plsation Thepiping or sell can ave a resonance at a mltiple of tepocketing eqency an te ynamic isplacement of te sell cnbe excessive wen riven by a oincient acostc eqenc Tepiping attacment elament is a point of ig stressconcentration

Most instrmentation connections can be isolated om teprocess replace wit no own time; owever, the stbotconnection at te piping wall is te most ssceptible point offailre roper spport of instrment line connections reces temass spporte at te pipe connection an the displacement of teinstrment line Treae connections ave ig stressconcentrations at the root of te tread Redcing te nmber oftreae connections will sbstantially increase the fatige life ofinstrment lines

Pipe sppos are another point of stress oncenration in thepocess piping A ringing moe in te pipe may reslt in igstresses at an attacment point Man strctral weleconnections are not stress relieve an reinforcement or redesignof these connections may be neessary

Piping ilres have occe in processes downstream of aplsation ampe, evn togh e amper was esigne to theproper instr standar Plsation levels that exist in OFSappliations cn proce plsating pressre levels capable ofexciting ig orer piping moes Plstion dampers designed inaccorance to AP 619 may not be aeqate in recing teplsation (at eqencies greater tan 1000 Hz) to an acceptablelevel Canges in te PD esign shold be te rst step inimprovement of piping relibilit, bt cefl analysis an esignof attchment etails will provie a mc improved sstem design

LUTIN

Rection of pressre plsation in any process sold start at tesorce Tere e several metos tat are i se for edcing tepressre plsation level at te comressor iscarge One metoinvolves bypass of some of te compresse gas witin te rotortreas, to te lowpressre sie of te trea jst before openingat the iscarge po Tis meto of iscarge bypass is not veryeective an reslts in a loss in volmetric eciency Recingte plsation level exiting te rotor treas lowers te plsatingpressre levels seen at te Anoter metho involves passivefeeback of iscarge plsation to te inlet plenm ot of pasewit te inlet plsation Te inlet plsation is of mc lower

magnite, ming te pase ccellation marginally eectiveOptimization of te iscge prt to release te gas om tetreas into te plenm is te mst eective means of recingpressre plsation at te iscge ort optimization also as teae benet of improve eciency

Commnication of te process operating conitions to eeqipment manfactrers is essential in procrement processPoint conitions are sel for etermining steaystate performance, bt o esign conitions can ave a sbstantial impact onte compressor an P peormance A escription of processoperating n stp proceres wil go a long way towar atorog esign ait of te conitins at te eqipment will see

Simple esign canges in e or process piping can reslt insignicant improvement i n fatige life Welalets that e se forinstrment connections sol be gssete or replce witnipolets Te nipolet as a male connection, wc can beconnected to the instrment valve an oes not reqire anintermeiate reae pipe Te rection in lengt reces emoment applied by the insrment valve to te wele connectioStress relief of all process piping connections between tecompressor an te shol be manatory

Thorogh mechanical analysis of te P an te ownstrepiping is necessary in orer to avoi mechanical resonance over ell range of operating conitins Acostic analsis of teownstream piping system is anoter improvement in te systemesign tat will identi possible srces of ostic resonance

e AP 619 speciction for silencers an plsation ersreires tat te sign attenate eqencies i te aible eingrge witot exceeing the pressre rop limit An increase in epressre rop limit to percent to 3 percent can increase e ctoeqenc to approximately 2000 z withot an appreciable loss incompressor permac

IBLIOG PHY

Benk, L L, 1960 Nis Rdutio New rk, New York:McGraw-Hill Book Coman, nc

Cmmins, J R. an Golen, B G, 1993 Silr AliatioHadbook Universal Silener, Stoghton, Wsonsin

arris, C M, , 1957 dbook o Nis Cotl New York,New rk McGraw-Hill Book Compan, nc

Jngbaer, E an Blogett, L , 1998 ostic Fatigenvolving Lrge bocompressors an Presre RectionSystems," Proedigs o th wSth urboahiSyosiu Trbomachinery Laboratory, Texas A&MUniversity, College Station, Texas, pp 111118

nsler, L E, rey, A R, Coppens, A B, an Saners, J V., 198Fuatals o Aoustis i ditio New Yok, NewYor: Jon Wiley an Sons

KNWLEGEMNT

r Jay Stell, BrgessMannig, for is valable assistance iniscssin of silence/plsation amper esign an analsis


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PROCEEDINGS OF THE 28TH TRBOMACHINERY SYMPOSIUM

Documents

Vol 2High Frequency Fatigue Failures in Pulsation Dampers