A practical guide to .....
Metal Detector Basics
for GFSI Audits
Simple Operation
Outstanding Reliability
Exceptional Performance
CONTENTS
1. INTRODUCTION
2. METALDETECTIONǦTHEBASICPRINCIPLES
2.1.
2.2.
2.3.
2.4.
2.5.
2.6.
TheoryofOperation
ProductEffectandPhasing
MetalFreeArea
Sensitivity
TypesofMetal
ShapesandOrientation
3. AUDITS
3.1. PreͲAuditGeneralConsiderations
3.2. Equipment
3.3. Documentation
3.4. Personnel
3.4.1. Operators
3.5. TestingMetalDetectors
3.5.1. TestSamples
3.5.2. TestFrequency
3.5.3. TestProcedure
3.5.4. TestRecords
3.5.5. FailsafeTesting
3.5.6. AutomaticTestSystems
3.6. PostͲAudit
4. SOURCESOFINTERFERENCE
5. APPLICATIONS
5.1. Conveyor(EndofLine)
5.2. Gravity
5.3. Pipeline
6. GOODMANUFACTURINGPRACTICEGUIDELINES
7. SAMPLEMETALDETECTORTESTLOG/REPORT
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Page1
1.
INTRODUCTION
Auditsareatimeconsuming,yetimperativepartoffoodprocessing.75%oftheauditsdonebyauditingbodies
encounteranonͲconformancewithCCPmetaldetectors.Often,theseareduetolackofpersonneltraining,record
keepingandtestingfailures.
Theseissuesareeasilyovercomewithpropereducation,trainingandstandardizedtestingprocedures.Thisbooklet
containsthebasicsofmetaldetectionandsimplifiedguidelinestopreparingaCCPmetaldetectorforaGFSIaudit.
Dependingontheauditscheme,orauditingcompany(privateretailer),actualrequirementsofproceduresand
documentationmayvary.Werecommendtheauditteamconsultdirectlywiththeauditschemeguidelines,audit
consultantormetaldetectormanufacturertoclarifyanyissuesnotaddressedinthefollowingdocument.
2.
METALDETECTIONǦTHEBASICPRINCIPLES
2.1.TheoryofOperation
BalancedCoil
Mostmodernmetaldetectorsoperateonthebalancedcoil,fullloopsystem.
Threecoilsarewrappedaroundtheaperturethroughwhichtheproductpasses.Inthecenteroftheenclosureisthe
transmittercoilthatbroadcastsaradiofrequencysignalandgeneratesanElectroͲmagneticfield.
Equallyspacedoneithersideofthetransmittercoilaretworeceivercoils(seefigures1and2).
Transmitter Coil
Receiver 1
Receiver 2
Receiver 2
Transmitter Coil
Receiver 1
Figure1
Figure2
Thefieldisgenerallytrappedinsidetheshieldedenclosureofthedetectorbutsomefieldescapesfromtheapertureon
bothsidesofthedetector.AnythingthatentersintothisfieldthatiseitherMagneticorElectricallyConductivewill
causeadisturbanceinthefieldstrengtharoundit.Allmetalshaveeitheroneorbothofthesecharacteristicsandwill
bedetectableifthesizeofthesignalislargeenough.
Thesignalsfromthereceivingcoilsareconnectedinoppositiontoeachotherandthereforewhennodisturbanceis
occurringtherewillbeanetsignalacrossthecoilsofzero–theyarebalanced.Thisformstheelectricalequivalentofa
balanceweighscale(figure3).
Asmetalpassesthroughthedetectorthebalancewillbeoffsetasthecontaminantenterstheapertureandagainasit
leavestheexitside.Thisdisturbanceisamplifiedandanalyzedbythecontrolelectronicsanddetectionwilloccurifthe
sensitivitythresholdhasbeenexceeded.
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FerrousInFoil
Ideally,productstobefoilͲpackedshouldpassthroughaconventionaldetectorsystembeforetheyarepackedinthe
foil.Wherethisisnotpossible,productspackedinaluminumtraysorwrappedinaluminumfoilmustgothrougha
‘FerrousͲinͲFoil’detector.Fortheseproducts,aconventionalmetaldetectorspecifiedcorrectly,canbeusedtodetect
ferrous,nonͲferrousandstainlesssteelmetals.
2.2. ProductEffectandPhasing
Thecontrolelectronicsactuallysplitthereceivedsignalintotwoseparatechannels:magnetic
andconductive.Thismeansthereareeffectivelytwobalancescalesinthedetector(seefigure
3).Thesescalescontinuouslymeasurethemagneticandconductivesignalcomponentofevery
disturbance.
Productsthatarebeinginspectedcanhaveoneorbothofthesecharacteristics.
Conductive Scale
ProductEffect
Metaldetectorsdetectmetalbasedonmeasuringelectricalconductivityandmagnetic
permeability.Manyproductstobeinspectedinherentlyhaveoneorbothofthese
characteristicswithintheirmakeup.Forexample,anyproductwhichisironenrichedsuchas
cereals,createalargemagneticsignalwhichthedetectormustovercomeinordertodetect
smallpiecesofmetal.Thesearereferredtoas“dry”products.Conversely,productswithhigh
Magnetic Scale
moistureandsaltcontentsuchasbread,meat,cheese,etc.areelectricallyconductiveand
Figure3
produceaconductiveerrorsignal.Thesearereferredtoas“wet”products.Thetablebelow
showstypicalproducterrorsignalsandcategorizesthemaswetordry.
Thedetectormustremoveorreducethis"producteffect"inordertoidentifyametalcontaminant.
MostmoderndetectorswillhavesomeformofautomaticcalibrationtodothisͲitisoftenreferred
toaphasing.
Typical‘Wet’Products
Typical‘Dry’Products
Food:Meat,Cheese,BreadandBakery
Products,Fish,DairyProducts,Salads
Packaging:MetalizedFilms
Other:PlasticandRubberproductswith
highcarbonblackcontent
Food: Cereal,Crackers,Flourandpowders,Biscuits,FrozenFood
Products(<Ͳ1.0DegreesC),PeanutButterandMargarine
(Vegetableoilisnotconductive)
Other:WoodProducts,PlasticsandRubber(Productswithhigh
carbonblackcontentmaybeconsidered‘wet’),Textiles,Paper
Products
2.3. MetalFreeArea
TheElectroͲmagneticfieldistrappedinsidethedetector'senclosure
(shield).
However,somefieldescapesoutoftheapertureonbothsidesandforms
themetalfreeareaorMFA.
Generally,thesizeofthepracticalleakageisabout1.5timesthe(smaller)
aperturedimensionandnometalshouldbeallowedin
thisarea.
Largemovingmetalshouldbekept2xaway.
WhereapplicationsdemandasmallerMFA,specialdetectorsare
availablewhichcansubstantiallyreducethetotalarearequired.
Total Metal Free Area
Figure4
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Page3
Thetheoreticalsensitivityofagivenmetaldetectorisdeterminedbythe aperturesize;thesmallertheaperture,thesmallerthepieceofmetalthat
canbedetected.Thesmallerdimensionofrectangularaperturesisused
tocalculatethesensitivity,althoughthelengthalsocontributes.
Tomaximizesensitivityadetectorthesmallestsizeapertureshouldbe
selected.Howevertherearesomeexceptions:
x
MetalizedFilm
x
OxygenScavengers
x
HighlyConductiveproduct(largeblocksofcheese)
Metal Size
2.4. Sensitivity
Aperture Size
Figure5
Producteffect,metalfreearea,typeandorientationofcontaminantand
otherfactorscanaffectthepracticalsensitivityinanyapplication.
Thepositionintheaperturealsoaffectsthesensitivity(seefigure6).
Thecenterlineaxisistheleastsensitivepointandthereforethisiswhere
performancetestingshouldtakeplace.Asmetalgetsclosertothesides
(andcoils),thesignalitgeneratesgetslarger,makingiteasiertodetect.
3
1
Regulartestingofthedetectorshouldbedonesothatthetestsphere
passesclosetothecenteroftheaperture.Ifthisisnoteasilydone,thena
consistentpositionshouldbeusedsothatthetestresultswillbeconsistent
(seefigure7).
Figure6
Signal Size
2
1
2
3
Position in Aperture
Figure7
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2.5. TypesofMetal
2.00mm
Thesensitivityofametaldetectorisnotthesameforalltypesofmetal.For
simplicity,wetendtocategorizeallmetalsintothreetypes:
x
Fe
Ferrous:
Anymetalthatcaneasilybeattractedtoamagnet(steel,iron,etc.).Typically,
theeasiestandusuallythemostcommoncontaminantmetaltodetect.
x
NFe
3.00mm
NonǦFerrous:
HighlyconductivenonͲmagneticmetals(copper,aluminum,brass,etc.)When
inspectingdryproductsthesemetalsproducealmostthesamesignalsizeas
ferrousduetothefactthattheyaregoodconductors.
Increasethespheresizebyatleast50%wheninspecting
wetproducts.
x
2.00mm
"Dry Product Mode
SS
2.00mm
NonǦMagneticStainlessSteel:
Highquality300seriesstainlesssteels(Type304,316).
Thesearealwaysthemostdifficultmetalstodetectduetotheirpoorelectrical
conductivequalities.Bydefinition,theyhavelowmagneticpermeability.
Wheninspectingdryproductsastainlessspherewillhavetobe50%largerthan
aferrousspheretoproducethesamesignalsize.
Wheninspectingwetproductsastainlessspherewouldhavetobe200to300%
largerthanaferrousspheretoproducethesamesignalsize.
3.00mm
Fe
NFe
4.00mm
"Wet Product Mode
SS
ExampleofRatiosONLY
Figure8
2.6. Shapes&OrientationofMetal
Metaldetectionstandardsaremeasuredinspheresbecauseasphereis
thesameshapefromanyaspect.Realcontaminantsarerarelyspherical
andmayproduceadifferentsignaldependingonitsorientationwhenit
passesthroughthedetector.Themostdramaticexampleofthisisshown
bywirecontaminants.
Flow
Withwireshapes,thesignalproducedwillvarygreatlydependingonthe
typeofmetalanditsorientationasitpassesthroughtheaperture(see
figure9).Intheworstcaseawiremayproduceasignalnobiggerthana
sphereofthesamesizeasthediameterofthewire.
InFigure9:
FerrousWires:
x A–Easiestposition,biggestsignal.
x B,C–HardestPosition,smallestsignal.
NonͲFerrousandStainlessSteelWires:
x B,C–Easiestposition,biggestsignal.
x A–Hardestposition,smallestsignal.
A
B
C
Figure9
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3.
AUDITS
3.1. PreǦAuditGeneralConsiderations
TheentireauditteamshouldbefamiliarwiththetestinganddocumentationproceduresforyourCCPmetaldetectors;
itishowever,advisabletorefertoyourproceduraldocumentationwhenaskedspecificquestionsbytheauditorto
avoidspontaneousandpossiblyincorrectresponses.
Priortotheaudit,ensureyourmetaldetectorsystemsmeettheGFSIschemerequirementsforrejectsystem,
sensitivityandcontaminatedproductsisolation,forwhichyouarebeingaudited.
HACCP:Reviewthecurrentplanandmakenoteofanychangesthatmayhavehappenedinthelastyearthatcould
affectfoodsafety(newequipment,newproduct,andnewprocesses).Makesureanychangeshavebeenproperly
addressedandvalidated.TheMetalDetectorCCPprocessesanddocumentationproceduresshouldbeconfirmedas
effectiveandaccurate.
Recall:TheAuditteamshouldbewellͲversedonthenecessaryprocedurestoeffectivelyandsuccessfullydoametal
contaminatedproductrecall.Thiscouldincludetherequireddocumentationofanyproductalreadyshipped,andthe
testingrecords,theverbalcommunicationamongsttheteammembersinvolvedinarecallcouldalsobeevaluated.If
yourcompanyhadametalcontaminaterecallinthelastyear,alldocumentationmustbeonhandforreviewand
discussionwiththeauditor.
3.2. Equipment
MetalDetectorEquipmentshouldbeprotectedfromdamage,deteriorationormisuse.Priortoanaudit,itshouldbe
carefullyexaminedforanypotentialcomplianceissues,suchas:
x
x
x
Overallwearandtear
Beltmaterialdegradation
Overallcleanliness
Itisrecommendedthattheannualcalibrationbedoneapproximately2Ͳ6monthsbeforeanaudit,soanyrequired
maintenancecanbedonepriortotheaudit.
Aswithanypieceofvitalprecisionmachinery,highperformancecanonlybeassuredifthemetaldetectorisregularly
andproperlymaintained.Thiscanbeachievedbyimplementingaplannedprogramofpreventativemaintenanceat
regularintervals,inaccordancewiththemanufacturer’srecommendations.Maintenancecanbecarriedoutbythe
equipmentmanufacturersorbyincompanyengineers,providedthattheyhavebeentrainedbythemanufacturers.
Afteranyrepairs,maintenanceoradjustments,afullmetaldetectortestmustbecarriedoutbeforethesystemisused
again.
SpareParts:Itisadvisabletoensurethatacompletesetofspareparts(includingtestpieces)beonhandforeach
detectorpriortoanaudit.
3.3. Documentation
Havingproperandaccuratedocumentationforanauditisjustasimportantasthephysicalinspection.Ifyourteamhas
concernsthatarenotaddressedinthisdocument,thenitisrecommendedtocontactthemanufacturerofyourmetal
detectorequipmentandscheduleaninͲhousetrainingseminar.
Ifyourmetaldetectorisusedformorethanoneproduct,thenyouwillneedtohaveonhand,thenecessary
documentationforeach.
Becausepaperrecordscaneasilybealtered,manyauditingbodiesprefertoseesoftwarederivedreportsforroutine
testingandmonitoring.AlldocumentationgeneratedfromtestingthemetaldetectorshouldbeclearlytimeͲstamped
atthepropertestingintervals.
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Whenmetalcontaminationisdetected,thesourceofthecontaminationshouldbeinvestigatedanddocumented.Ifa
patterndevelopsthenaprocedureshouldbedevelopedtopreventfuturecontamination.
Ensurethesedocuments(orsimilar)areonhandfortheauditandreviewedwiththeteam,prior:
x
x
x
x
x
x
x
x
x
x
x
x
x
ListofallMetalDetectors(ProcessandCCP)andlocation
IDcodeandcalibrationduedatepermetaldetector
RoutineTestingLog/Reportspermetaldetector
Correctiveactionreports
CalibrationCertificates(perproduct)permetaldetector
StartͲup/Commissioningdocumentspermetaldetector
Achievablesensitivitydocuments,perproduct,permetaldetector
Maintenanceserviceagreements
Recordsofallmaintenancedoneandplanned
Validationdocumentspermetaldetector
InternalAuditsrecords
PersonnelTrainingReports
Testsamplecertificates
**RoutineTestingLog/Reportexampleonlastpage
3.4. Personnel
ManagementplaysaveryimportantroleinallFoodSafetyplans.Allmanagementshouldbetrainedonthemetal
detector’sprocessesandprotocolspriortoanaudit.Allrelevantstaffshouldbeproperlytrainedintheprinciplesand
useofthemetaldetectionsystemandtheuseoftestingroutinesandshouldbeonsitethedayoftheaudit.
Inaddition,itisimportantthatcompanymaintenanceandcleaningstaffreceivetrainingonthepreventionofmetal
contaminationandonthecorrectprocedurestoadoptduringcleaningandmaintenancework.
3.4.1. Operators:
AsmanymetaldetectornonͲconformancesareoperatorrelated,itisprudenttoensurealloperatorshaverecently
beenproperlytrainedandtestedonthefollowing:
x
x
x
x
x
x
Testingproceduresanddocumentation
CriticalLimit(perproduct)
CorrectiveActionsanddocumentation
Proceduresforprocessingcontaminatedproduct
Generaloperationofthedetector
Managementcontactpershift
Becauseoperatorscanrotatethroughtheplant,wesuggestthataquickreferenceor“cheatsheet”beplacednearthe
detector.Thisdocumentshouldincludetestingfrequencyandcriticallimit,alongwithanoverviewoftesting
procedures,correctiveactionsandmanagementcontactbyshift.
Thedetectorshouldhavepreventivecontrolstoensureonlyauthorizedstaffareabletoadjustthedetectorsettings.
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3.5. TestingMetalDetectors
Regardlessofhowsophisticatedorreliableametaldetectionsystemmaybe,itisessentialthatafrequentand
thoroughtestandrecordingprogrambeestablished.ThisisanessentialcomponentofanyqualityorHACCP(Hazard
Analysis&CriticalControlPoints)system.Intheabsenceofanyindustrystandardsformetaldetectortestingeach
companymustestablishtheirowntestcriteria.Todatemanyofthestandardsadoptedhavebeendefinedbythemajor
retailersand/ormanufacturers.
Thefollowingfivepointsshouldbeconsideredwhendeveloping,changingyourprogramorpreparingforanaudit.
3.5.1. TestSample
Historically,metaldetectorshavebeentestedwithaferrous,anonͲferrous,andastainlesssteeltestsample.Foraudit
compliance,thesetestsamplesshouldbecolorͲcodedandcertified.
Thesizeofthetestsamplemustbeestablishedsothatitcanbereliablydetectedinsidetheproductpassingthrough
thecenterlineofthedetectorͲleastsensitivepoint.
Everyapplicationwillbedifferentandthereforethesamplesshouldbetailoredtoeachdetector.Ifthesampleistoo
smallfortheapplication,itwillcauseunnecessarytestfailuresandcreateahighfrustrationlevelwiththetest
operators.Ifthesampleistoolarge,itwillnotaccuratelytesttheperformanceofthedetector.Usingaselectionof
testsamplesizes,establisharealisticandrepeatableoperatingperformancelevel.Thenadetectabletestsample(s)
canbechosen.
Typicalguidelinesforsensitivity:
Aperture
Height
Upto50mm
Upto125mm
Upto200mm
Ferrous
1mm
1.5mm
2mm
DryProduct
NonͲFerrous StainlessSteel
1mm
1.5mm
1.5mm
2.25mm
2mm
3mm
Ferrous
1.5mm
2mm
2.5mm
WetProduct
NonͲFerrous StainlessSteel
2mm
3mm
2.5mm
3.5mm
3mm
5mm
3.5.2. TestFrequency
Thecompanymanagementmustdecidethefrequencyoftestingthedetector.Typically,detectorsaretested:
x ShiftChange
x ProductChange
x Hourly
Thereisanobvioustradeoffbetweenthecostsoftestingversustheriskofpotentialdetectorfailure.Theuseofan
automatictestsystemcanincreasethefrequencyofdetectorperformancetestingatnoadditionalcost.
3.5.3. TestProcedure
Theprocedureitselfshouldbekeptassimpleaspossible,butmusttakeintoaccountthefollowing:
a) Thetestsampleshouldtravelthroughtheapproximatecenterlineoftheaperturewhichistheleast
sensitivepoint.
b) Thetestsamplesshouldbeplacedwithintheproductifpossible.
c) Thetestproceduremustallowtherejectdevicetoactivatesothattheentiresystemistested.
d) Thiscaninclude:
x Testingwithcontaminantatleading,centerandtrailingedgeoftestpack
x Testingsuccessivepacks
x Testingalternatepacks
e) Theresultsofthetestmustberecorded.
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3.5.4.TestRecords
Asampleofatestrecordisincludedonthelastpageofthisdocumentforyourinformation.
Theformatisnotimportant,howeveritshouldinclude:
a) Lineordetectoridentification
b) Adateandtime
c) Thesamplesused
d) Identificationoftheoperator
e) Apassorfailresult
f) Correctiveactiontakenifresultwasafailure
3.5.5.FailsafeTesting
DependingontheGFSIaudit,FailSafetestingmayberequiredalongwiththeroutinetesting.Thiscouldincludetesting
thefollowing:
a)
b)
c)
d)
e)
f)
g)
RejectConfirmSensor
BinFullSensors
BinDoorSensors
AirPressureFailure
InͲfeedPhotoEye
RejectTiming(EncoderorInverter)
BackUpSensor(Exit)
3.5.6. AutomaticTestSystems
Thesesystemsaredesignedtoautomaticallytestferrous,nonͲferrousandstainlesssteelsamplesonmetaldetectors.
Automatictestingremovestheriskofhumanerrorandworkplaceinjuryatcriticalcontrolpointsonamanufacturing
linewhilecomplyingwithindustrystandards.Thisisespeciallyusefulinapplicationswherethetestingofametal
detectorismadedifficultduetoaccess,position,accesstotheproductflow,environmentalconditions,etc.
Thesystemwedeveloped,asanextensiontoourmetaldetectortestprograms,enablescompaniestodecreasethe
frequencyofmanualtestingdramatically(althoughnoteliminateitcompletely).Thetheoryistoharnessthesignal
generatedbyanexternalsource.Atthepressofabutton,achangeinthesignalflowingthroughtheloopcausesa
disturbanceonthedetector’sreceivingcoil.Thisisthesameprocessthathappenswhenmetalpassesthroughthe
detector’saperture,thusprovidingconsistenttestresultsatthepushofabuttonorautomaticallytriggeredwitha
timer.
3.6. PostǦAudit
Theauditteamshouldconveneshortlyafteranaudittoreviewtheoverallsuccessesandfailuresencounteredduring
theprocess.Atthistime,theteamshoulddeterminethecourseofactiontopreventfuturenonͲconformancesand
decideprocessestorectifythosefound.
Overalltheauditprocessisexhaustiveandyetnecessaryforindustryandcustomercompliance.Besuretocelebrateall
successeswiththeteamandenjoyabreakuntilthenextone.
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4. SOURCESOFINTERFERENCE
EnvironmentalconditionsmayaffecttheperformanceoftheMetalDetector,particularlywherehighlevelsof
sensitivityaretobeachieved.Whereverpossiblethedetectorshouldbepositionedtoavoidorminimizetheeffect
fromsuchconditions.
Thesecanbegeneratedbyanumberofsources:
x AirborneelectricalinterferenceͲstatic,radio,earthloops
x VibrationͲmovingmetal
x TemperaturefluctuationͲovens,freezingtunnels
Whilethedetectormaybecapableoffilteringsomeofthisinterferenceout,throughsuchfeaturesas“Automatic
Balance",inmanycasestheonlyoptionistoreducethesensitivitylevel.
Thisisanimportantconsiderationtobetakenintoaccountwhencomparingthecapabilitiesofdifferentmetal
detectors.
5. APPLICATIONS
5.1. ConveyorSystemsǦEndofLine
Endoflinemetaldetectionhasbeen,generallythepreferredoption,asatthispointproducthasbeenpackagedand
thereisnofurtherriskofcontaminants.
Foranumberofreasons,asdefinedabove,thisisnotalwaysfeasible.
x PhysicalrestrictionsͲnospaceavailable
x Packagetype/materialͲfoillidsortrays
x PackageSizeͲtoolargefordetectionstandard
x CriticalpointsͲmachineryprotectionorrawingredients
5.1.1.DesignConsiderations
Thebasicpurposeofaconveyorsystemistomoveproductthroughthedetectorandsuccessfullyrejectcontaminated
product.Inorderthattheoptimumperformanceisachievedfromthesystemthereareanumberofdesignissues
whichshouldbetakenintoconsideration.
DetectorPerformance
ManufacturerswillgenerallysupplyrecommendationsforachievingthebestperformancefromthemetaldetectorͲ
theyshouldalwaysbeapplied.
x IsolatedRollersͲpreventgroundloops
x HighqualitybeltͲmetalfree,interlockedfingerjoin,plasticmodularbelt
x Lowvibrationandstatic
x Adequatemetalfreearea
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Page10
2 Non-Metallic Slider Bed
Detector
1 Metal Free Area
4 Foot Isolation
3 Clearance
6 Roller Isolation
5 Metal Free Belt
Figure10
ReliableReject
Theselectedrejectdevicemustbesuitablefor:
x PackͲsize,weight&shape
x PresentationͲpitch,linespeedandbeltwidth
Thefollowingtablegivesageneralguidelinetowhichdesignwillbebestsuitedtoyourapplication.
Thefiguresmayvaryslightlywithmanufacturer.
Type
AirBlast
DiverterArm
Pusher/Ram
Suitedfor
Lightconsumerpacksi.ebiscuits,
chocolatebars.
Highthroughput
Mediumtolightpacks.
Mediumthroughput
Max.Wt
1kg
Mediumpacks
Highthroughput
7kg
Unsuitableforlooseorfragileproduct
25kg
Requiresanoperatortoremove
contaminatedproduct
2kg
Dimensionsareforwholelineor
batchofproducts
StoponDetect Largebagsorboxes,handfedorbulk
material.
Slowthroughput
Retracting
Smallproductinlinesorofirregular
Band/Carriage shape.
Mediumthroughput.
5kg
Notes
Unsuitableforlooseproduct,boxes
curvedsurfacesandsomebagged
product
Productgenerallyentersbin
diagonallyͲmustensureitwillfit!
Thefollowingadditionsarealsorecommendedtoensurereliablerejectionofcontaminatedproduct.
x RegistrationphotoeyeͲtoensurecorrecttiming
x EnclosedareafromdetectortorejectͲtoavoidproductremoval
x LockablebinͲensurecontaminatedproductsarequarantined
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5.1.2.TestingConveyorDetector
Inadditiontoensuringthemetaldetectorisfunctioningcorrectly,thecompletesystemfunctionalityshouldbechecked
aspartofthetestingprocedure.
Thisbasicallyinvolvesensuringthattherejectdeviceisoperatingcorrectlyandthecontaminatedproductisproperly
handled.
Thisisgenerallyprovedbythefollowingmethods:
x Placingthetestsampleattheleadingedgeoftheproduct
x Placingthetestsampleatthecentreoftheproduct
x Placingthetestsampleatthetrailingedgeoftheproduct
x Passingsuccessivetestpacks
x Passingalternatetestpacks
Foralltheabove,thesystemshouldbeobservedtoensurethattherejectsystemoperatescorrectly.
5.1.3.OtherConsiderations
Inadditiontoregularmanualtestingadditionalfailsafemethodscanbeincorporatedintothesystemdesign.
x Rejectconfirmation/BinFullsensors
x AirPressureFailure
x Fault/Shutdown
5.2 DropThrough/GravityMetalDetector
Onthesurface,thedropthroughapplicationofmetaldetectorisverysimple.However,caremustbetakeninthe
initialdesigntoavoidhavingtomakemajormodificationsafterinstallation.Pleaseconsiderallthefollowingsteps
whendesigningadropthroughdetectionsystem.
Product Freefall Start Point ( Test Piece Access )
Drop Height
Distance
D1
Plastic Pipe
Selection
Detector
Detector To
D2
Valve Distance
Overall Length
Reject Output
Test Retreival Drawer
Good Product Output
Figure11
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5.2.1.DesignIssues
Agravitydetectionsystemisideallysuitedtoinspectingdry,freeflowingproductssuchas:
x Grains,flours,cereals
x Rice,nuts,sugar
x Plasticpelletsandflakes
Theproductmustremainfreeflowingandneverbackupintoanypartofthesystem.
DeterminePipeInternalDimension
Oftentheexistingpipingwilldeterminethepipeanddetectorsize,orusethefollowingformulabasedonknowingthe
peakexpectedflowrateoftheproductanditsbulkdensity.
Areaofthroatrequired(inchessquare)=0.024XFLOWRATE(#/hr)
BULKDENSITY(#cu.ft.)
Example:Aproductwithaflowrateof30,000pounds/hourandabulkdensityof40pounds/cubicfootwillrequirea
calculatedpipeareaof0.024x30,000/40=18squareinches.
RoundversusRectangularPipes
Aroundpipewillutilizethepipearearequiredforproductflowmoreefficientlythanasquareorrectangularpipe,and
thereforetheflowcapacityofrectangularpipesshouldbeincreasedbyatleast20%.
Arectangularsystemmaysometimeshaveanadvantageofallowingashorteroveralllengthduetotheshorterstroke
andreactiontimeofthevalve(seebelow).
SystemOverallLength
Oncetheproperpipesizehasbeenestablished,theoverallsystemlengthcanbeconsidered.ThebiggerthepipeI.D.
(orsmallerdimensionofarectangularsystem)thelongerthesystemmustbecome.Thedetectorthroughdimension
mustincreaseasaperturesizeincreases,thevalveheightwillalsoincreaseduetotheincreasedstroke,andthe
requireddistancebetweenthevalveandthedetectormustincrease.Thelatterisduetothelargervalvetakingmore
timetoreachthefulldivertpositionandthereforeitmustbelocatedfurtherfromthedetector.
Itisveryimportanttoconsidertherelationshipbetweenvalveresponsetimeand,productfreefallheight,andsystem
length.
Inordertoproperlydesignthesystem,thefollowingmustbeknown:
x Productpipesize(fromabove)
x Valveresponsetime(frommanufacturerͲbiggervalvesrequiremoretime)
x Productfreefalldistancefrominitialdroptothecenterlineofthedetector(SeeFigure11–D1)
Ifthefreefallheightisincreased,thedistancebetweenthedetectorandvalvemustbeincreasedinordertomaintain
adequatetimeforthevalvetorespond.
Duetothefactthattheproductisacceleratingasitfallsat32ft/secsquared,smallchangesinthedetectortovalve
distancewillhaveadrasticeffectonthemaximumallowablefreeͲfalldistance.
Forexample,givenavalveresponsetimeof50msandadetectortovalvedistanceof8”,thecalculatedmaximumfreeͲ
fallwouldbe29”.Ifthevalvewasmoved2”closertothedetector(to6”),thenthemaximumfreeͲfallwouldnowbe
only16”.
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5.2.2.TestingGravityMetalDetectors
Oneofthenotsoobviousdrawbackswithagravitydetectorsystemisthatitcanbedifficulttotest.However,Fortress
typicallydesignstestingaccessandrecoveryintothesystem,soroutinetestingcanbedonequicklyandreliably.Itis
importanttorecognizethatthetestingproceduremustconfirmthedetectors'performanceaswellastheresponseof
therejectvalve.
Toachievethis,thedesignmustincorporate:
TestAccessPort
Anaccessporttointroduceatestsample(plasticballwithmetalsampleimbedded)mustbeprovidedattheproduct
freeͲfallorigin.Thetestportshouldallowthesampletofallfromthesameplacethattheproductbeginsitsfall,sothat
thetestsamplespeedwillbethesameastheproduct.
TestSampleSafetyRetrievalGate
Asafetycatchgateshouldbeinsertedintothenormalproductflowbelowthevalve“good”productoutputwhen
testingiscarriedout,sothatthetestsamplecanbesafelyrecoveredifthedetectorfailstodetectthesample,orthe
valvefailstoreactproperly.
Inagooddesign,thetestgatecanbequicklyinsertedintotheproductflowduringatest,andremovedfromtheflow
afterwards.
AutomaticTesting
Amanualtestmustbeperformeduponinitialinstallationandatreasonabletimeintervals.However,inthis
application,anAutomaticTestsystemcanofferconsiderablebenefits.Ifcorrectlydesignedthistestsystemcanensure
thattestingisbothconsistentandrelevant.RefertoSection3.5.6.formoreinformation.
5.2.3.OtherConsiderations
Static
Withallfallingdrypowdersandgranules,staticelectricityisgenerated.Someproductsaremorepronetodothismore
thanothers,andenvironmentalconditionslikehumiditywillalsocontributetotheequation.
Tohelpreducestaticdamageandinterference,thefollowingmeasuresshouldbeconsidered:
x Allmetalnearthedetectorsystem(pipes,flanges,structuralsupports)shouldbeproperlygroundedsothat
largechargescannotaccumulate.
x Plasticparts(producttube,etc.)mayneedtohaveconductiveshieldswrappedaroundthemtohelpdissipate
largecharges.However,groundingstandard(nonͲconductive)plasticswillnoteliminatestatic.Some
conductiveplasticsareavailableforfooduse,butmayinterferewiththedetector.
x Thedetectoritselfshouldalsohaveamajorsinglepointground(consultthemanufacturerfortheir
recommendations).Detectorsthenuseremotepowersuppliesmaybemoresusceptibletodamage.
x Asalastresort,anionizingantiͲstaticdevicemaybeconsidered.
Withcarefuldesignandaccurateinformation,thedropthroughgravityapplicationofmetaldetectorscanprovide
excellentsensitivityandearlywarningofproductcontaminationconcerns,butthecriticalparametersmustconsidered
intheearlydesignstages.
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5.3 PipelineMetalDetector
Pipe System Drawing
Detector
D1
Reject Valve
Non Metallic Pipe Section
Good
Product
Flow
Figure12
Rejected
Product
ThepipelineapplicationinvolvestheinstallationofanonͲmetallicpipesectionaroundwhichametaldetectorinspects
productstravellingthroughthepipe.Thistypeofapplicationshouldbeconsideredwheretheextrasensitivity
capabilityofarelativelysmallapertureoutweighsthebenefitoffinalpackageinspection.Thisisespeciallytrueifthe
finalpackagingmaterialcontainsmetal,suchasacanningline.
Apipelineinspectionsystemisideallysuitedtoinspectingliquid,slurriesorpasteproductsthatcanbepumped
throughapipe.Typicalpipeproductswouldinclude:
x sauces
x dairyproducts
x meatslurries
x Juicesetc.
5.3.1. DesignIssues
Thefollowingcriticalfactorsmustbeknownwhendesigningasuccessfulpipelinedetectionsystem:
x PipeI.D.
x Pipeclampconnectionstyle(triͲclamp,IͲlineetc)
x Productflowrate(GMP)
x Productviscosity
x Producttemperaturerange
x Productpressure
x Expectedcleanupprocedures(washdown,pipepig,etc.)
DetermineSystemLength
Theproductspeedinthepipewilldeterminethepositionoftherejectvalverelativetothedetector.Sincethevalve
hasaminimumdivertresponsetime,thedistancebetweenthevalveandthedetectormustbeincreaseddirectly
proportionaltotheproductspeedandvalveresponsetime.Productspeedcanbeestimatedgiventheflowratein
gallonsorlitersperminute.Duetolaminarflowcharacteristicsofliquidsinapipe,anappropriatesafetymarginmust
beaddedtothespeedcalculation,andthesystemmustberigorouslytestedatfullproductionratestoensurethevalve
canrespondintime.
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Forexample,givenasystemusinga2”pipewithamaximumflowrateof80gpm,theaveragespeedoftheproductwill
be10feet/second.Giventheresponsetimefora2”valveis0.25seconds,thevalvemustbeatleast2.5feetfromthe
detector.Toallowforlaminareffectsa3’spacingisrecommended.
SelectingtheValveStyle
Thechoiceofvalvewillbeinfluencedbytheproducttemperatureandviscosity.Somevalvesarebestsuitedforlow
viscosityproductssuchasjuices,etc.Ifthepipecleanoutprocedureincludestheuseofa“pig”flusheddownthepipe,
thevalvechosenmusthaveastraightthroughnonͲrestrictivedesign.
SelectingtheNonͲMetallicPipe
Thechoiceofthepipewillbeinfluencedbythestyleofpipeconnectionrequired,theproducttemperatureand
especiallythepipepressureexpected.Caremustbetakentodesigntheinstallationsothattheplasticpipewillnotbe
loadedinanywaybytheincomingstainlesssteelpiping.
5.3.2.TestingPipelineMetalDetectors
Oneofthenotsoobviousdrawbackswithapipedetectorsystemisthatitisverydifficulttotest.However,iftesting
accessandrecoveryisdesignedintothesystem,thentestingcanbedonequicklyandreliably.Itisimportantto
recognizethatthetestingproceduremustconfirmthedetectors'performanceaswellastheresponseofthereject
valve.
Toachievethis,thedesignmustincorporate:
x TestAccessPort
Anaccessporttointroduceatestsample(plasticballwithmetalsampleimbedded)mustbeprovidedupstreamof
thedetectorsystem.Thetestportlocationshouldallowthesampletotravelatnormalspeedthroughthedetector
system.
x TestSampleSafetyRetrievalGates
Asafetycatchgateshouldbeinsertedintothenormalproductflowafterthevalve“good”productoutputwhen
testingiscarriedout,sothatthetestsamplecanbesafelyrecoveredifthedetectorfailstodetectthesampleorthe
valvefailstoreactproperly.Itisalsorecommendedthatasimilarcatchgatebeusedontherejectoutputtoease
therecoveryofthetestsamplewhenitisrejected.
Inagooddesign,thetestgatescanbequicklyinsertedintotheproductflowduringatest,andremovedfromthe
flowafterwards.
AutomaticTesting
AswithGravityapplications,thebiggestdrawbackofmanualtestingisthatitisimpossibletopredictwherethetest
piecewillberelativetotheapertureofthedetector,andforthesamereasontestresultswillbeinconsistent.Forthis
reasonanAutomaticTestsystemcanofferadvantages.Seesection3.5.6formoreinformation.
Detectorscapableofautomatictestingcanbeusedtoperformshortintervaltestingofthedetectorandvalvereaction
withoutanyoperatorinvolvement.Theuseofanautomaticrejectresponsechecksystemisalsorecommended.This
involveshavingavalvepositionswitchfeedaconfirmationsignalbacktothedetectorwhichcanthenmonitorthe
responsetimeoftherejectdeviceduringanyrejectoccurrence.
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Page16
6. GOODMANUFACTURINGPRACTICE–TYPICALGUIDELINES
Prevention
x
x
x
x
Trainingformaintenanceandcleaningstaffinmetaldetectorbasics
Planned&controlledmaintenanceofproductionlineduringnonproductionhours
RegularinspectionofproductionlineͲforidentificationofpotentialcontaminants
Goodhousekeeping
Sensitivity
x
x
x
x
x
Identify"standards"withFortressTechnology
ReͲevaluatestandardswhenconditionschange
Maintainrecords
Maximizesensitivitywithoutcompromisingperformance
Implementsecuritylevels–passwords
Testing
x
x
x
Document&communicateͲwho&why
Establishfrequency
Createtestpackswhererelevant
RejectedProductHandling
x
x
x
x
Isolate&reͲscreenpotentiallycontaminatedproductontestfailure
InvestigatesourceofcontaminantͲtrainedpersonnel,offline&withinreasonabletime
Onrepeatdetection'sidentifysourceofcontaminant
Onmultipledetection'sstopproduction
RecordKeeping
x
x
x
x
x
Commissioning&sensitivitydetails
Testresults
ShiftresultsͲnumberofrejects
Maintenanceschedules
Training
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Page18
Fortress Technology began in 1996; believing that through superior product design and engineering, the
production of higher quality equipment with better sensitivities could be achieved. The phenomenal market
response to our Fortress Phantom line of metal detectors validated that belief and the philosophy on which it is
based; simple operation, outstanding reliability, and exceptional performance.
We continue to set the trend with technological advancements. Our Stealth Metal Detector is an evolutionary
design that offers full backwards compatibility for existing Fortress systems. Thus ensuring our customers aren’t
left behind when new technology is developed, enabling them to stay up to date with food safety initiatives.
We custom design and manufacture our equipment to suit your needs. If you are running a high speed product
line, you need the detector to reject test samples properly; without false rejects. We design our equipment to do
that for you; without the operator requiring an engineering degree.
We have absolute confidence in our products; if they do not perform as anticipated, we will work with you to
rectify the situation or provide a full refund.
For more information on our products and service, please visit our website:
WWW. FORTRESSTECHNOLOGY. COM
Fortress Technology Inc.
Fortress Technology (Europe) Ltd.
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Toronto, Ontario
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Tel: 1-416-754-2898
sales@fortresstechnology.com
The Phantom Building
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Fone: 55-11-3641-6153
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