CODE OF PRACTICE FOR ATMOSPHERIC OIL MIST DETECTORS
1 Inanidealworld,thesimplesolutionforpreventingoilmistfiresistoensurenoleaksoccur inthefirstinstance,buttheharshrealityisthatoilmistfiresdooccurandtheproblemneedstobe addressed. Onepracticalansweristoinstallanoilmistdetectionsystemthatwilldetectanoilmist beforeitcanreachlevelswhereitsaturatestheatmosphere to such an extent that there is a risk of fire.
2 It is generally accepted that oil mist can be formed in one of two ways:
.1 minuteleaksinoillineswhich,underpressure,produceaveryfineatomisedspray; or
.2 oil,whenallowedtocomeintocontactwithahotsurface,willboiloffproducinga fine vapour.
3 In the first instance, the danger occurs when the particle size formed is between 3 to 10 micronsandisallowedtoformamistintheatmosphere. Whenoilvapourreachestherangeof flammability,theconditioncanbeclassedastruly hazardous and, if no action is taken, a fire may result. Theignitiontemperatureforthistypeofoilmistcanbeextremelylowdependingonthetype
of oil being atomised.
4 Oilmistsgeneratedbybeingboiledoffcanproduceparticlesbetween3to10microns. This mistisvisibleandisknownasabluesmoke. Temperatureandareaofsurfacecontactaffecttherate of oil mist generation. At this stage, a temperature as low as 150°C could result in ignition.
5 Sourcesofoilmistincludepumpseals,leakinginjectors;looseorincorrectlyfittedpipe fittings, weld fractures and poor maintenance of machinery.
6 Possible heat sources causing the ignition include heat exchangers, exhaust pipes, turbocharger,electricalcontacts,staticelectricity,faultywiringandhigh-andlow-pressureturbines.
7 Types of detection systems:
.1 single sampling units; and
.2 multiple sampling systems.
8 Location of detectors and sampling lines
8.1 Ineachcase,thenumberofdetectorsorsamplingpointstobeusedisdependentonthesize andlayoutoftheparticularapplication.Forasinglepointapplication,theunitmaybemountednext totheapplicationorconnectedviaasamplingline. Multiplesamplingsystemsarefittedina suitablelocationawayfromtheapplication. Samplelinesarefedtoacommonmanifoldwitha suitablecontrolunittoallowalternativesamplestobetakenfromcontinuouslyflowingsample streams,thusoneunitcanbeutilisedtomonitorseveralpoints. Theoilmistmaybedrawnintothe unit by its own built in fan or by an independent blower.
8.2 Todeterminesuitablepositionsformountingdetectorsorforfittingsamplinglines,asmoke testwouldberequiredtoverifyairmovementsinrelationtoapplication. Ingeneral,airwillmove towards ventilation extractors and turbo chargers, so any detector or sampling line should be positionedascloseaspossibletothemachinery.Likewise,detectorsorsamplinglinesshouldnotbe sightednexttoventilationblowersasthesewillpreventmist formation frombeingdrawnintothe unit. Oninstallation,asmoketestshouldbecarriedoutwithallengines,ventilationandmachinery fully operational to ensure that detectors/sampling lines are correctly positioned.
8.3 Ifdetectorunitsaretobelocatedclosetothesourceofapplication,careshouldbetakento avoidlocatingtheunitinplaceswhere:vibrationisexcessive;extremesoftemperaturemaybe experienced;itwouldbedifficultformaintenancepersonneltogainaccess;highlevelsofhumidity and water may occur; and there is a risk of electromagnetic interference.
8.4 Locatingofanydetectorinanexplosiveatmosphereshouldnotbeundertakenunlesstheunit is certified intrinsically safe for the hazard area.
9 Setting alarmlevels
9.1 Thismaybedeterminedbytherequirementsoftheenduser.Ingeneral,therearelikelytobe twostages: firstanearlywarningthatsomethingiswrong;andthenasecondaryalarmindicatinga fullalarm. Theseshouldbeadvisoryandthemonitor should be able to define the areas where oil misthasbeendetected. Incertaincircumstances,alarmsmaybeusedtoshutdownindividual sections of the plant if deemed necessary, but there should be an overriding control.
9.2 Thealarmlevelsetinitiallyshouldtakeintoaccounttheatmosphericconditionwhenthereis noproblem,forexamplethereisalwaysasmallamountofmistgeneratedwithinanoilpurifier room. Theinitiallevelsetwillchangewitheachapplication,butshouldnotexceedconcentrations ofgreaterthan2ppm atmosphericoilcontent. Alarm levelswillbeindicatedasthelevelrisesto certain preset percentages of this set value.
10 Test procedure
10.1 Aswithmostelectronicequipment,unitsshouldbefixedbythecomponentsusedinthe design stage and have built-in calibration routines to correct slight deviations.
10.2 Themanufacturershouldcalibratethedetectoragainstaknownoilmistmeasurement. No adjustmenttothecalibrationshouldbepossiblebytheuser. Itisnotsatisfactorytosetupasystem against an electronic procedure or a piece of filter glass. If deemed necessary, a calibration certificate should be issued.
11 Maintenance
Thereisverylittlemaintenancewhichcanbecarriedoutontheunitsinrelationtotheelectronic components. Themainareaswhichwillrequireattentionareinlinefilterseitherinthesampleline orwithintheunititself. Thesewillrequireeithercleaningorreplacementdependingonthetype, and recommendations of the detector manufacturer. Sensor faces will also require cleaning periodicallywithapprovedcleanertocleananyoilfilmbuildup. Somedetectorsmayhavebuilt-in faultdiagnosticcircuitrywhichwillgiveanindicationthatcleaningisrequiredorthatfiltersneed attention due to fall off in flow.