case study - seal oil piping 2012

8/2/2019 Case Study - Seal Oil Piping 2012

1/20

Seal Oil Piping Vibration on an Oil Free

Screw Compressor in an FPSO

Application

RayD.Kelm,P.E.(*presenter)KelmEngineering

DeanJefferyGEOil&GasWilliamEganGEOil&Gas

AbstractAdryscrewcompressorskidwasfabricatedforoffshoreservicethatincludedoilbufferedmechanicalsealsonthecompressor. Duringshoptesting,excessivesealoilpipingvibrationwasobservedthatresultedinpipecomponentfailures.Theoilsystemwasdesignedtoincludeacommonlubeandsealoilsystemwithapumpdesignpressureof590psi. Thereweresixoilcontrolvalvesonthesystem,includingalubeoilpressurecontrolvalve,oneflowcontrolvalveontheinletendofthecompressor,anotherflowcontrolvalveonthedischargeend,twodifferentialpressurecontrollerstomaintaindifferentialpressureonthesealsforeachendofthecompressor,aswellasabackpressurecontrolvalvefortheoilsystem.Vibrationandpressurepulsationmeasurementsindicatedthattheresponsewasatthepocketpassfrequencyof206Hz. Thesourcewasdeterminedtobeatthecompressor. Arrangementofthecontrolvalvesandcontrolsensinglineswerelikelycontributorstotheseverityoftheresponse.Modificationtothesealoilsystemincludedadditionofseveralvolumesatstrategiclocationstochangetheacousticalcharacteristicsoftheoilsystemtoreducetheshakingforcesinthepipesystem. Thesechangesalongwithpipesupportmodificationsandchangestothecontrolvalvesensinglinesweresuccessfulatreducingthepipepulsationsandvibrationresponsetoacceptablelevels.


2/20

SealOilPipingVibrationonanOil

FreeScrewCompressorinan

Ray

D.

Kelm,

P.E.KelmEngineering

DeanJeffery

WilliamEgan

as


3/20

E ui mentDescri tion

acombined

lube

and

seal

oil

system.

o or r venw gear ncreaser.

Oil

buffered

mechanical

seals.Verycompacts i require orFPSOapp ication

includingtheentiremachinetrainaswellasthe

.Skidispointsupportedon3AVMs(antivibration

.


4/20

Com ressor

Details

2950HP4poleinductionmotor(50Hzpower)

2.13:1gearincreaser

3175RPM

Input

Speed

to

Compressor

211.7HzPocketPassFrequency


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GeneralSkidLayout

Compressor

r ver

Baseplate


6/20

SealOilSupplySystem


7/20

Vibration

Test

Results Generalpipingvibrationamplitudeswerein

the

0.2

1.0

in/sec

peak

range. Somelocationsonvalves(handles,instrument

connections,drains,etc.)wereashighas10

in/secpk.

Vibrationwasallat206Hz(1Xpocketpass

Theentireskidhadelevatedvibrationat206.


8/20

InitialSolutionPath

Investigateforpipingsystemmechanical naturalfrequencies.

Numerous

resonances

near

pocket

pass

frequency

identified.

usingelastomericclamps.

Replaced

anumber

of

U

bolt

clamps

with

lined

clamshell

typetoimproverigidityandpossiblyadddamping.

theaddedcompliancewould reducethe forcedresponse.

Someim rovementbutacce tablelimitswerenotmet.


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AdditionalTestResults


10/20

TestforAcousticalResonance

Pressurepulsationsensor

installednearoneflow

controlvalvesuggested

thatan

acoustical

resonanceexistednear1X

poc etpass requency

whenpipeimpactedwith

softmallet.


11/20

AcousticalAnalysis

Acoustical analysis of the seal oil supply and return piping was doneusing a pulsation source at the connections to the compressor withvariable input frequency. An acoustical resonance was suspected in the

compressor for the seal supply piping.


12/20

SealDescription

Thesealsusewereanoilbuffered mechanical

.

assemblyatapproximately30 PSIabove

reference as ressure usin back ressure

controlvalves.


13/20

PossibleCauses

Controlvalvesamplifyingpulsationsdue

inthe

system

possibly

cross

talking.

Initialpulsationreadingsindicated

pulsationswerehigherbetweenthetwo

.

feedingbackintothesealoilsupply

.


14/20

FinalSolution

Installedacousticalisolationvolumesinthe

acousticalresonance.

Similar volumes

added to both

return piping

near the

compressor.


15/20

AcousticalVolumes

The size of the volumeswas selected based on theconce t that addin

compliance would shift thenatural frequency as well asprovide some low pass filtereffect.

selected was 0.1 at 206 Hz based on standard acousticallow pass filter theory (Kinsler, et. al.)

S12

2

121

1

+

kLSS

T

L

1


16/20

PredictedResultswithVolumes

The predicted results were shown to almost eliminatepulsations upstream of the volumes as well as

connections at the compressor.


17/20

FinalTestSummaryLocation Original Hoses Volume Prediction Final Test

LP Supply downstream

of FCV

75 psi pk-pk 40 psi pk-pk 1.0 psi pk-pk 1.2 psi pk-pk

LP Return upstream ofBPV

5 psi pk-pk 0.02 psi pk-pk

HP Supply downstream 51 psi pk-pk 24 psi pk-pk 1.3 psi pk-pk 4.6 psi pk-pk

of FCV

HP Return upstream of

BPV

32 psi pk-pk 0.9 psi pk-pk

Seal Oil Header 13 psi pk-pk 11 psi pk-pk 0.5 psi pk-pk 1.5 psi pk-pk

LP upstream of FCV 90 psi pk-pk

HP upstream of FCV 66 psi pk-pk

Pressure pulsation levels were significantly reduced

along with corresponding vibration response.


18/20

LessonsLearned

Acousticalresponseofsealoilpipingcanproducehighpressurepulsationsandvibrationwithinsealoilsupply

sys emg ven er g con ons.

Previoussystemsofsimilardesigndidnothavethisproblem.

producedpulsingflowattheseals.

Thepresence

of

multiple

control

valves

may

have

contributedtotheseverityoncetheresponsestarted.

Inletsideflowcontrolvalves.

.

Acousticalresonancegreatlyamplifiedthepulsations.


19/20

LessonsLearned,cont.

Correctingtheacousticalforcedresponsebyaddingvolumeswasmuchmoreeffectivethanstiffeningthe

p pean or e un ngmec an ca na ura requenc es.

Althoughmechanicalresonanceswereidentifiedonthe,

significantlyreducepipevibration.

Manyof

the

measured

natural

frequencies

were

not

easy

to

calculate

,

makingthemverydifficulttopredict.

Variationofvalvebrands(differenthandwheeldesigns,etc.)changed.

Theforcedresponsefromthepressurepulsationsproducedexcessivevibrationevenwhenresonanceswerecorrected.


20/20

LessonsLearned,cont.

Pulsationlevelswerereducedbyacousticalmodification(addedvolumes)toreducetheforced

response

energy.Thepipesystemresponsedroppeddramaticallyoncethe

pu sat onenergywasre uce nt ep p ng.

Futuredesignsshouldconsiderreviewofthepossible.

Additionofpulsationdampinghardware(compliantvolumesand/ororifices similartoconce tsusedforreci rocatin

pumpsshould

be

considered

to

reduce

the

potential

for

resonantresponse.

case study - seal oil piping 2012

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