1.9
Vibration
Vi
brat
i
on i
n equi
pmenti
st
heresul
tofunbal
anced forces. Thet
ransferenceofunbal
anced forcest
hrough equi
pmenti
nt
o
nei
ghbouri
ng st
ruct
urescausi
ng t
hem t
o shake i
svi
brat
i
on. The mot
i
on ofa body i
sl
i
mi
t
ed by whatconnect
si
tt
o a
machineand thewallsin which itmoves. Every timethereisachangeofdirection unbalanced forcesproduceashock.
Thi
sshockt
ravel
st
hroughoutt
hemachi
neandi
st
ransmi
t
t
edt
o al
lconnect
edi
t
ems.Out
-of-bal
ancei
scorrect
edbyaddi
ng
orremovi
ngmaterialso thatwhentheequi
pmentisoperatingtheunbalanceiscont
rolledto anacceptablelevel.
The rate ofvi
bration iscalled the frequency. Itismeasured in cyclespersecond and hastheunitsHertz. A four-pole
el
ect
ri
c mot
orrot
at
esatabout1500 RPM . Thi
si
s25 cycl
espersecond or25 Hert
z. Vi
brat
i
on caused by an ext
ernal
appliedforceisknownasaforcedvi
brationbecausethemassoscillatesatthefrequencyoftheexternalforce.Anexampl
e
istheshakeproducedbythemovi
ngpist
onsandcrankshaftinacarengi
ne.
Thefourmethodsofvi
brationcont
rolarelistedbelow.
Reduceorel
iminatetheexcitingforcebybal
anceorremoval
.
Usesufficientdampingto l
imitampl
itude.
Isol
atethevibrationsourcefrom thesurroundsbyusingspringmountsof
appropriatestiffness.
Introduceacounterbal
ancingforceoppositeinphaseto theexcitingforce.
M ostimportantlyeverymovi
ngmassmustbebalancedaboutitscenterofrotation. Everyrotatingmassmustbebalanced
to an acceptablest
andard. Out–of-balancerotorscausevi
bration becausethecenterofmassoftherotoriseccentric(not
runni
ng t
rue)t
oi
t
scent
erofrot
at
i
on. Thespi
nni
ng,off-cent
ermassi
scont
i
nual
l
y bei
ng fl
ung out
ward. Themachi
ne’s
bearingsholdthemassinplaceandreactagainstthedevelopedforces. Vibrationresultsasfi
rstthemassisononesi
deof
t
he beari
ngsand t
hen i
ti
son t
he ot
her. Bal
anci
ng ai
mst
o di
st
ri
but
et
he massevenl
y aboutt
he runni
ng cent
er. The
drawingaboveshowseccentricitybetweenthecenterofmassandthecenterofrotation.
M aterialssuch asrubberdampen shaking. The rubberfl
exesand absorbsthe movementwithin itself. Rubbermakesa
good vi
bration damper. Becauserubbercannotcompressmuchto accommodatemovement
,rubberdampersarenormally
usedforlow amplitude,hi
ghfrequencyvi
brationwherenoisetransferenceisaprobl
em.Shockabsorbersareusedforlarge
amplitude,low frequencysituationswherespringsalonewoul
dproducebouncing.Anexampl
eisincarsuspensions.
A vi
brat
i
ng mass can al
so be i
sol
at
ed from i
t
s surroundi
ngs by spri
ngs. The spri
ngs defl
ectundert
he shaki
ng body.
Inst
al
l
i
ngi
sol
at
i
onspri
ngsmaket
hespri
ng’snat
uralfrequencyt
hegoverni
ngfrequencyforforcedvi
brat
i
ont
ransfer.How
faraspringwillcompressunderloaddependsonitsst
iffness.Al
teringthespri
ngsti
ffnesscont
rolstheamountofvi
brat
i
on
transferred to theattachment
. Too stiffwilltransmitvi
bration,whileinsuffi
cientstiffnesswillcausebounce. Thecorrect
springstiffnesscanbefoundusingchartsavailablefrom specialistvi
brationcont
rolcompanies.
An out
-of-phase massi
sa met
hod notoft
en used t
o cont
rolvi
brat
i
on. Iti
spossi
bl
et
o use a wei
ghtwi
t
h an opposi
t
e
vi
brationpatternto negatetheout-of-balanceforces.Thismethodhasbeenusedinmotorcarengi
neswhereashaftwithan
eccentricmassisspunintheoppositedirectionto thecrankshaft
.
Causesofout-of-balance
Thetablebelow listssomecommoncausesforunbalance.
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
14
Bentorbowedbetweensupportbearings
Overhungweightbendsshaftundergravity
Unevenl
ydistributedsol
idorl
iquidinsiderotor
Loosepartsontherotor
Eccentrical
l
ymanufactureddiametersontherotor
M isal
ignmentofthedrivetrainto therotoraxis
Loosedrivecoupl
ingsfl
opabout
Loosetol
erancesbetweenassembl
edpartsonrotor
Shoul
dersonrotoraremadeout-of-squareto axis
Voidsorcavitieswithintherotor
M isal
ignmentofbearingsforceshaftto bow
The frictionallosses along the pipe cause a pressure loss. If the gas is now ata lower pressure itmustbe ata
correspondi
ngl
ylesserdensity. (Itislesssquashed togetherthan itwasatthestart.
) Thismeansthedensityofafl
owing
gasvariesalongthelengt
hofthepi
pe.Theeffectisgreaterathi
ghervelocities.
Foramassofgasto enterapi
pean equalmassmustleavethepi
pe. W eknow thedensity iscontinuallythinni
ng asthe
pressure dropsalong the pipe. Onekilogram oflessdense gasrequiresmore space (volume)than thesameweightofa
morecompressed gas. To getoneki
l
ogram ofexpandi
ng gas,whi
ch i
st
aki
ng up morevol
ume,outfrom t
heend oft
he
pi
pei
tmustgo fast
ert
hanwheni
tent
eredt
hepi
pe.Gasfl
owi
ngt
hroughapi
peexpandsast
hepressurefal
l
sandspeedsup
thefurtherittravelsalongthepipe.
Expandinggascool
s. Thisprincipl
eisused inrefrigeratorsand airconditioners. A gasfl
owinginapi
peisexpandingas
the density falls. Thisiswhy compressed airlinesarecoolto touch and why waterdropl
etscollectin pneumaticvalve
act
uat
ors.Thet
emperat
urehasfal
l
enl
ow enought
o condenset
hewat
ervapour.
W HERE PRESSURE LOSSES COM E FROM IN PIPES
Li
qui
dmovi
ngi
napi
pehast
o pushi
t
swayal
ongpi
pewal
l
s,aroundbends,t
hroughval
ves,pastproject
i
onsandencl
osed
i
t
ems.Throughouti
t
sprogressfri
ct
i
onrobsenergy.Threefact
orsaffectpressurel
ossi
npi
pes.
SurfaceFriction
Al
ong wal
l
st
hel
i
qui
d hast
o overcomefri
ct
i
on. W al
lfri
ct
i
on dependson surfaceroughness. Thehi
ghert
heproject
i
ons
i
nt
ot
hel
i
qui
dt
hemoret
hefri
ct
i
on.Atl
ow vel
oci
t
i
esal
ami
narsubl
ayerofsl
ow fl
owi
ngl
i
qui
dcoverst
heroughness.But
athi
ghvelocitiesthesublayerthinsaseddi
esintheturbul
entcenterfl
ow extendto theprojections.
Di
rect
i
onChanges
Li
quid thatisforced to change direction losesenergy. W hen a liqui
d goesaround a 90o bend i
t
smoment
um hast
o be
redi
rected.From goingatvelocityinonedi
rectionitnow hasto go atthesamevelocityinacompletelydi
fferentdi
rection.
To do thisitconvert
spressureinto theenergyitneedsto changedi
recti
on. A pressurelossresult
s. Themoresuddent
he
changeindirectionthegreatertheenergyitneedsandthehi
gherthecorrespondingpressurelosses.
Li
qui
dvi
scosi
t
y
Al
i
qui
dt
hateasi
l
ysl
i
desrequi
resl
essenergyt
o movet
han onet
hatdoesnotwantt
o fl
ow. Honeyi
st
housandsoft
i
mes
morevi
scoust
han wat
er. Pushi
ng honeyt
hrough pi
pes,around cornersand pastval
vesrequi
resfarmoreenergyt
han for
water.Ifthehoneywaswarmeditwoul
dthenfl
ow easierandthepressurelosswoul
dbeless.
CONSTRUCTION
DESIGN
M INIM ISE FRICTION IN PIPELINE CONSTRUCTION
Duringdesign,procurementandinstallationofapi
pelinefollow therecommendationsinTabl
e1.
Sel
ectsmoot
hbore
100metersof100mm (4”)plasticpipecont
aini
ngwatermovi
ngat1 m/
s(3.
25ft/s)will
pi
pes.
lose900mm (35”)ofpressure.Thesamefl
ow ina100mm st
eelseamlesspipewilllose
1000mm (39”)pressure.
M ini
mi
sefl
ow
Usel
ongradi
usel
bows;usepi
pereducers;mi
ni
mi
set
henumberoffl
anges;useful
lbore
di
srupt
i
ons.
st
rai
ghtt
hroughval
ves;usepi
pesi
zebrancht
eesandreducedownont
hebranch(amust
onsuctionlines);useinstrument
ationwhichdoesnotprojectinto thefl
ow.
Keepfl
ow velocities Selectpi
pesizesthatarepracticablewhilekeepingfl
ow velocitiesreasonable.
l
ow.
Cutgaskethol
est
o
theborediameter.
Cl
eanoffweld
splatter.
Keepbuttweldpeaks
l
ow.
M akethegasketboreholeto thepipeborediameter.Do notletthegasketprojectinto the
l
i
qui
dfl
ow pat
h,asi
twi
l
lcauseunnecessaryfri
ct
i
onandt
urbul
ence.
W eldsplattersitsonthepipesurfaceanddisruptstheliqui
dfl
ow pattern.Thesplattercan
comeloseanddamagedownst
ream inst
rument
sandequi
pment.
Li
kegasketscutwiththediameterlessthanthepipebore,weldswithhi
ghpeaksproject
into theliqui
dfl
ow.Trykeepweldsbelow theheightofthesub-layerthickness.
M akemultiple
segmentmitered
bends.
‘Lobst
erback’90o bendsareusedwhenlargediameterlongradiusbendsarenotavailable.
M akethem outofequalsi
zed15o segmentsinsteadoftheusual22.
5o segment
s
TABLE 1 Pract
i
cest
hatmi
ni
mi
sepressurel
ossi
npi
pes.
21
Inast
eam jacketedtanktheinsi
dewallismadeasthinaspossi
bleto reducethethermalresi
st
ance.Theoutsidewallofthe
jacketisinsulated to preventheatlossto thecold surroundings. Ifitwerenotinsulated theouterwallofthesteam jacket
woul
d radiate heatinto the cold surroundings. An uni
nsul
ated,jacketed tank woul
d wast
e a lotofst
eam to heatthe
cont
ents. M uchheatwoul
dbelostasradiationfrom thetank’souterwallandasconvectionoftheairtouchi
ngtheoutside
wall.Insulationsavesagreatdealofotherwisewast
edenergy.
HeatLossM axi
mi
sat
i
on
Insituationswheremaxi
mum transferofheatisrequireditbecomesnecessaryto fi
ndwaysto:
€ ma
kethefl
ui
dcont
actthehotsurfacesasmuchaspossi
blebyusingturbul
ence,
€ k
eepthetemperaturedifferencebetweenthesurfaceandthefl
ui
dasgreataspossi
ble,
€ k
eepthethermalresi
st
anceofmaterialsseparatingcoldfl
ui
dsfrom hotfl
ui
dstheaslow aspossi
ble,
€ g
ettheheattransfercoeffi
cientofthefl
ui
dsup,
€ r
emoveanysurfacescaling,
€ u
sematerialsofhi
ghthermalconductivity(i.
e.low thermalresistance).
Forexample,boiling apotofwateroveran open fi
rein theforesttakeslongerthan boiling thesamepotoverafi
replace
hearthinahouseandbot
htakelongerthatboilingthesameamountofwaterinanelectrickettle.
Outintheopenair,thepotgetsonlyasmallamountofthefi
re’sheat. M ostisradi
ated inalldi
rections. Someradi
ation
hitsthepotand someconvectiveheatfrom thehotrisingairtouchthepot
. Inthehearththepotgetsradi
ationoffthehot
surroundingbricksandthefi
reandalso convectionheatfrom thehotrisingair.Inthekettlethewatergetsheatfrom di
rect
cont
actwiththeheatingelements. Alltheheatinthekettle’sheatingelementsisinjectedinto thewaterandlittleislostt
o
t
hesurroundi
ngs.
W hentryi
ngto heat-upasubstanceitisnecessaryto i
nsurethatthemaxi
mum heatpossi
bl
eisputinto i
tandnotl
ostt
o the
surroundi
ngs.
27
almostacertaintyovertheyears. Ifyoumustconnectplasticto metaluseafl
angedjointofsuitablepressurerating.Even
withafl
exiblesealantbetweenthetwo materialsitislikelythatoveryearsofservicemanyconnectionswillleak.
Pl
asticthreadedfittings
Pl
asticthreadsareusableonlyinlow-pressureservicewheretheywillremainataconstanttemperature.Ifyouarelooking
for50yearsoftroublefreelifefrom abovegroundreticul
atedgasplasticpipingbesureallplasticconnectionsarewelded
orfl
anged,notthreaded.
M ixi
ngpi
pefittingsandcompatibility
Usingpi
pefittingsmadeofdi
fferentmetalscanleadto galvaniccorrosionofoneoftheparts.Oneitem willeventuallyfail
andleak.Checkthegalvanicseriesforbothpartsandonlyusedifferentmetalsiftheyareverycloseintheseries.
Itisalso necessaryto checkthechemicalcompatibilityofmi
xed threaded fittingsto insuretheywillhandlethechemicals
and conditionsin cont
actwith them. M ixi
ng brassfit
tingswith st
ainl
esssteelorst
eelfi
tt
ingswil
llead t
o dangerand
failureinthewringchemicalenvi
ronment.
Differentcoeffi
cientsofexpansion between threaded partscanproducehi
ghst
ressinoneortheotherathi
ghtemperature
andagapbetweenthetwo atlow temperature.
Don’tmi
x thread st
andards. BSP and NPT are totally differentthreadsand mustneverbe forced together. They have
differentfl
ankangl
esand exceptforthe1/2”and 3/4”si
zes,theyhaveadifferentpitch. Theyarenotcompatible. Ifyou
haveajobwithmi
xedthreadsmakesurethatyougo andgettherightfittingsortherightadapterfittings. Goodpractices
alwayssavealotofprobl
emsinthefuture.
Pressuret
estpi
pi
ng
Al
wayspressuretestthreadedpi
peworkto 1.
25timesitsworkingpressurebeforeputtingitinto service. W hilethepi
pi
ng
isunderpressure,go around to allthethreaded jointsand ‘soapywater’testthem forleaks. Sprayon ami
xt
ureofwater
andl
i
qui
ddet
ergentandl
ookforbubbl
esformi
ngast
heai
rl
eaksout
.Ifbubbl
esappearremaket
hejoi
nt
.
Onceinst
alled repeatthe‘soapywater’testevery36 mont
hsasaregul
arpreventativemaintenanceroutineonallthreaded
gasandai
rpi
pel
i
nes.
39
2.10
Process controlvalves
Cont
rolval
vesregul
at
et
hefl
ow ofal
i
qui
dorgasbyopeni
ngorcl
osi
ngi
nt
ernalpassages.Theyform partofacont
roll
oop
usedto cont
rolaprocess. Thecont
rolvalvesrespondsto inst
ructionsfrom thecont
rollerandadjust
stheinternalopeni
ngs
accordingl
y.
.
Purposeofcont
rolvalves
A cont
rolval
vevari
est
herat
eoffl
ow passi
ng t
hrough i
t
sel
f. Theval
vest
em moves,al
t
eri
ng t
hesi
zeoft
hepassageand
thisincreasesdecreasesorholdsst
eady the fl
ow. The cont
rolvalve openi
ng isaltered wheneverthe processparameter
beingcont
rolleddoesnotequalthevalueitismeantto be(thesetpoi
nt
).
Operationofacont
rolvalve
A cont
rolvalveconsistsofthreekeycomponents-thevalve,theactuatorandthecont
roller. Thevalvecanbeoneoftwo
basi
ctypes. A plugandseatdesi
gn,inwhichaplugisclosedagainstaseatoraquarterturnvalveinwhichadisc,ballor
coneisturnedagainstaseat.Thedrawingsbelow show thetwo typesofcont
rolvalve.
Theactuatorisused to movethevalvest
em. They areusually air(pneumatically)orelectrically driven. Thecont
roller
calculatesthesizeofthechangeto thevalveinternalpassageto bringthefl
ow to thedesiredrate.
Therat
eoffl
ow t
hrough t
heval
vedependson t
hepressuredi
fferencebet
ween t
hei
nl
etand out
l
et
. To sl
ow t
hefl
ow t
he
valve is closed which causes more back-pressure and a greaterdifference between the inletand outletpressures. To
increasethefl
ow thevalveisopenedwhichreducestheback-pressureandthepressuredifferenceacrossthevalve.
The portion ofthe valve thatregul
atesthe fl
ow isknown asthe valve trim. Itconsistsofa fi
xed-in-place seatand the
movableplug,disc,ballorcone.Thetrim canbeselectedto createavarietyofpassageshapesthatcont
rolthefl
ow.
Cont
rolval
vet
uni
ng
Thecont
rollertuni
ngdictatesthewaythevalverespondsto thechangi
ngprocessparameter.Tuni
ngofacont
rolvalvesets
thespeedandintensityofthevalveresponsewhentheneedforacorrectionisdetected.
The cont
rollercont
ainsinternallogi
c thatproducesan outputto theactuatorto movethest
em apredetermi
ned amount
.
Thi
sl
ogi
cl
ooksat
:
a) t
hesi
zeoft
hedi
screpancybet
weent
hesetpoi
ntandt
hecurrentval
ueoft
hecont
rol
l
edparamet
er(Proport
i
onal
)
b) thelengt
hoftimethediscrepancyhasbeenpresent(Integral)
c) thespeedatwhichthediscrepancyhasbeenchangi
ng(Derivative)
anddetermineshow fastandhow farto movethest
em.
Oncethepositionofthetrim isalteredthecont
rollerwaitsforthenextsamplesignalfrom thesensingelementto checkthe
di
fferenceremaining to thesetpoi
nt
. Thevalvestem ismoved and thefl
ow altered untilthedi
fferencebetween theset
pointand theactualvalueofthecont
rolled parameteriswithin tolerance. An exampleoftuni
ng acont
rolvalvecan be
likenedto apersonunderashoweradjustingthetapsto getthewatertemperatureright
. Thesketchbelow showsthelogi
c
involvedincont
rollingthewatertemperature.
Ift
he hotwat
eri
sfi
rstputon,t
he cont
rolval
ve becomest
he col
d wat
ert
ap. Yourbody sensest
he t
emperat
ure oft
he
water. Ifthewateristoo hotyouopenthecoldwatertap. Youwaitawhile(thetimelag)to sensetheeffectofincreasing
the cold waterfl
ow.The cold waterisadjusted untilthe temperature from the combined fl
owsisjustright
. Once the
temperatureisrightthevalvesareleftaloneand thetemperatureisst
able. Ifachangeoccurs,suchassomeonedoingthe
laundrywithcold water,adrop incold waterfl
ow to theshoweroccursand thewatertemperaturegetshotter. Youagain
sensethechangi
ng temperatureand makethenecessary adjust
ment
s. In responsethecold watertap isopened furtheror
thehotwatertapisclosed.
This whol
e process involved sensing the temperature and movi
ng valve positions untilthe parameter under cont
rol
(temperature)stabilised.A cont
rolvalveworksthesameway.
Probl
emswithcont
rolvalves
• Cont
rolvalves may notproperly regul
ate the parameterthey are cont
rolling. Ifthe process cannotbe cont
rolled
productqualityissuesarise.
50
2.15
Gear box drives
A gearbox isused to cont
rolthe operating speed ofindust
rialequi
pment.Properselection,care and maintenance of
gearboxesiscritical.Failureofthegearboxcausestheequi
pmentandassociatedplantto st
op.Keywords:serviceduty,
drive,breather,lubricant,oilanalysis.
Gearboxselection
Loadtype
Environment
Forces
Robustness
-
Drive
-
Geartype
-
M aintenance
Operations
Steady,consistent
Intermittent,on-off,fl
uctuating
Impact,hammering,surging
Temperatureofl
ocation
Cool
ingandheatdissipation
Dustiness
Humidity,dampness
How aretheforcescreated?
W herearetheforcesl
ocated?
W herearetheforcestransmitted?
Isthemountingsturdy?
W il
ltheshafthandl
ethetorque?
Isthecasingsol
idandrigid?
Is there sufficient excess service duty to
handl
eunexpectedl
oads?
Directcoupl
ed,chain,bel
tdriven
M inimum bearingl
oadsrequired
Orientation ofgearbox in rel
ation to forces
generated
Loadsandforcesappl
iedto gears
Heatgeneratedbythemeshing
Lubricationtype
Partsavail
abil
ity
Standardisationacrossthesite
Skil
ll
evelofmaintainers
Presenceofprocedures
Productiondemands
Operatorcare
Overl
oadpotential
A gearboxisselectedforthedutyitmustperform.Itconvertsahi
ghinputspeedto aloweroutputspeedandso permits
one driverelement,such asan electric motor,to do numerousduties. Each selection musttakeconsideration ofthe
fol
l
owi
ngpoi
nt
s.
Forexample,in theselection ofagearbox to bedirectcoupled to an agitatorforastirred tank,consideration mustbe
gi
vento thedi
stancetheoutputshaftbearingsareapart. Bearingspositionedfurtherapartcopebetterwiththebending
forcesgeneratedbythepaddleslocatedattheendoftheagitatorshaft.
Theload typewillaffecttheselectionofthegeartype. Steady,continuousloadscanbeaccommodatedbyspurgears
butimpactloadscanbreakspurgearteeth.A morerobustgearto useinimpactloadsituationswoul
dbeahelicalgear.
Thisgearshape offersmore tooth cross-sectionalarea and because ofthe helix dissi
patesimpactforcesaxially and
tangentially.
Anotherst
yl
e ofgearing often seen on sl
ow movi
ng,hi
gh load applicationsisthe worm and worm wheel. Pl
anetary
gearboxesareavailablewherespaceislimitedforst
andarddesi
gngearboxes.
Gearboxcare
A well-selected gearbox willhavealong servicelife. Long term maintenancewillinvolvecheckingoillevelsand for
cri
t
i
calorexpensi
ve gearboxeschecki
ng oi
lqual
i
t
y. Oi
lqual
i
t
y oughtt
o be t
est
ed atl
eastevery yearforst
at
i
onary
pl
antandeverysicmont
hsformobilepl
ant.M oreofteniftheloadingorservicedutyisarduous.A simplesitetestfor
gearbox oilisto take a sample and look atthe color,note the smell,look and feelforpresence ofgritand feelthe
slipperinesscomparedto freshoil.Ifyouareconcernedthensendasampleto alubricationtestlab.
Anoi
lanal
ysi
si
ncl
udest
est
sfort
heoi
l
’sl
ubri
ci
t
y,changesi
nt
heoi
l
’scondi
t
i
on,condi
t
i
onofaddi
t
i
ves,sol
i
dspart
i
cl
e
count
sforlargerthan2,5and15mi
cronparticles,moisturepi
ck-up,dust/dirtcont
aminationandthemetalsintheoil.
The choice oflubricantis critical. The oilmustretain its properties atthe operating temperature. The gearbox
manufacturerisbestableto advi
setheoilto use. W herethemanufacturercannotbecont
acted,oneofthemajoroil
compani
es can offerpract
i
calsuggest
i
ons on t
he oi
lt
o use. M i
xi
ng ofdi
fferentoi
l
si
nt
he same gearbox i
spoor
practice. Unless you can tellthe oilsupplierwith certainty whatoilis already in the gearbox so a match can be
specifi
ed,do notcrossmi
xoils.M ineralandsynt
heticoilsarenotcompatible.Itisbetterto draintheol
doilout,fl
ush
t
hegearboxt
hroughwi
t
ht
henew oi
l
,puti
nt
henew oi
lt
ot
herequi
redl
evelandrunt
hegearboxfort
wo t
ot
hreehours
andfi
nallydump,fl
ushandrefillthegearboxagain.
Breathersarefitted to preventshaftsealsbl
owingoutastheinternalpressureriseswhenthegearboxiswarmingupto
operating temperature. A breatherallows moisture in the airand dirtfrom the surroundings to enterthe gearbox.
Locateand protectthebreatherto reducetherisk ofcont
amination. Check with thegearbox manufacturerto seeifa
58
3.4
5 W hys
This topic is provided by –
http://www.isixsigm a.com /library/
It
'
si
mport
antwhenresol
vi
ngaprobl
em t
hatyouaddresst
heprobl
em andnotasympt
om oft
heprobl
em.Det
ermi
ni
ngt
he
realproblem iscalledfi
ndingtherootcause.Therearespecialtechni
quesfordeterminingtherootcauseofaproblem.
W e'
llrevi
ew onetechni
queknownas“The5W hys.
”
W hatIsIt?
Thefi
vewhysisaform ofrootcauseanalysis.Youst
artwithast
atementofthesituationandaskyourselfwhyitis
happeni
ng.Thenyoulookatyouranswerandask"W hy"againandagainuntilyouhavedoneso fi
vetimes.Byrefusingto
besatisfi
edwithjustoneexpl
anation,youincreasethepossibilityofidentifyi
ngtherootcauseofthesituation.
How doesi
twork?
Afterdescribi
ngthesituation.Youaskyourselfwhythatsituationisoccurringandenteryourresponseintheappropriate
spaceinthetool.Youthenlookatyourfi
rstanswerandaskyourselfwhythatsituationisoccurring.Youdo thisagainand
againuntilyouhaveaskedwhyfi
vetimesoruntilyoucanno longeranswerthewhyquestion.
W hen woul
d Iusei
t?
Veryoften,therealcauseofaprobl
em orsituationisnotobvious.Theobviousexpl
anationoftenrevealsyetanot
her
probl
em.Thefi
vewhysisveryhelpfulinsuchsituations.
Byrepeat
edl
yaski
ngt
hequest
i
on"W hy"(fi
vei
sagoodrul
eoft
humb),youcanpeelawayt
hel
ayersofsympt
omswhi
ch
canleadto therootcauseofaprobl
em.Veryoftentheostensiblereasonforaprobl
em willleadyouto anot
herquestion.
Al
t
hought
hi
st
echni
quei
scal
l
ed"5W hys,
"youmayfi
ndt
hatyouwi
l
lneedt
o askt
hequest
i
onfewerormoret
i
mest
han
fi
vebeforeyoufi
ndtheissuerelatedto aprobl
em.
BenefitsOfThe5W hys
Hel
pi
dent
i
fyt
herootcauseofaprobl
em.Det
ermi
net
herel
at
i
onshi
pbet
weendi
fferentrootcausesofaprobl
em.Oneof
thesimplesttool
s;easyto completewithoutstatisticalanalysis.
W henIs5W hysM ostUseful
?
W henprobl
emsinvolvehumanfactorsorinteractions.Inday-to-daybusinesslife;canbeusedwithinorwithoutaSix
Sigmaproject.
How ToCompl
eteThe5W hys
1.W ritedownthespecifi
cproblem.W ritingtheissuehelpsyouformalizetheproblem anddescribeitcompletely.Italso
helpsateam focusonthesameproblem.
2.AskW hyt
heprobl
em happensandwri
t
et
heanswerdownbel
ow t
heprobl
em.
3.Iftheansweryoujustprovideddoesn'
tidentifytherootcauseoftheproblem thatyouwrotedowninst
ep1,askW hy
againandwritethatanswerdown.
4.Loopbackto st
ep3untiltheteam isinagreementthattheproblem'
srootcauseisidentified.Again,thismaytakefewer
ormoretimesthanfi
veW hys.
Probl
em Statement:Youareonyourwayhomefrom workandyourcarst
opsinthemi
ddleoftheroad.
1.W hydi
dyourcarst
op?
-Becauseitranoutofgas.
2.W hydi
di
trunoutofgas?
-BecauseIdidn'
tbuyanygasonmywayto work.
3.W hydi
dn'
tyoubuyanygast
hi
smorni
ng?
-BecauseIdidn'
thaveanymoney.
4.W hydi
dn'
tyouhaveanymoney?
-BecauseIlostitalllastni
ghtinapokergame.
5.W hydi
dyoul
oseyourmoneyi
nl
astni
ght
'
spokergame?
-BecauseI'
m notverygoodat"bluffi
ng"whenIdon'
thaveagoodhand.
Asyoucansee,inbothexamplesthefi
nalW hyleadstheteam to ast
atement(rootcause)thattheteam cantakeaction
upon.Itismuchquickerto comeupwithasyst
em thatkeepsthesalesdirectorupdatedonrecentsalesorteachapersonto
"bluff"ahandthanitisto tryto di
rectlysolvethestatedprobl
emsabovewithoutfurtherinvestigation.
69
M ACHINE
Hazardous,Safety,
Envi
ronment
aldangersfrom
process
Breakdown,st
opsproduct
i
on,
affect
squal
i
t
y
R
A
Breakdown,st
opsproduct
i
on,
affect
squal
i
t
y
Affectsdownstream plant
Affectsdownstream plant
Canbefi
xedon-l
i
ne
Canbefi
xedon-l
i
ne
B
C
77
5.4
Flanges leaking
Aswellasusingthechecklist,readthenotesinthesectionsonChemicalandM aterialCompatibility,BoltingandGaskets
inFlangedConnectionsandThreadedConnections.
Observati
on
Leakingfl
ange
Possi
bl
eCause
Gasketmaterialincompatible.
Gasketi
st
oo t
hi
ck.
Boltsareloose.
€
€
€
€
€
€
Pi
peworkwasover-pressured.
Corrosi
onattack.
€
Flangesareoutofsquare.
€
€
€
Flangefacesaredirty,dist
orted.
€
€
€
Remedy
Checkgasketmaterialselection.
Uset
hi
nnestgasketpossi
bl
e.
Usehi
ght
ensi
l
ebol
t
sandnut
s.
Tensi
onbol
tt
o 75% ofyi
el
dst
ressi
nproper
sequence.
Usesamebol
tsi
zeandspeci
fi
cat
i
ont
hroughout
.
Ifnut
sbi
ndonboltthreadsgetanew boltand
nutt
ot
heri
ghtspeci
fi
cat
i
on.
Inst
allpressurereliefto asafeplace.
Protectboltsfrom chemicalandenvi
ronmental
corrosi
on.
Cutofffl
angesandreplacethem squareto pipe
andeachother.
Removeanypi
pemi
sal
i
gnmentbyremovi
ng
andinst
allingthepipeagain.
Cl
eanoffalloldgasketsso faceislikenew.
Fileburrsoffl
angefacesso facesarefl
at.
Replaceorre-machinefacesthataredeeply
scoredorcorroded.
85