exposure evaluation and control风险评价与控制

Industrial Hygiene,Exposure Evaluation and Control,Industrial Hygiene -Exposure Evaluation and Control,Industrial hygiene is defined as the anticipation, identification, evaluation and control of occupational conditions which cause sickness and/or injury,Industrial Hygiene -Exposure Evaluation and Control,Exposure EvaluationMeasurement techniquesEstimating exposureOpen tanksFilling tanksControl TechniquesPersonal Protection,Evaluation of Chemical Hazards,Detector tubes - color change for specific speciesAdsorption tubes sample air drawn through adsorbent then released into GCFilters collects particulate dust and fibersPortable monitors hand held monitors to look for leaks or hot spotsReal time monitors used to determine average, maximum and minimum concentrations.Personal monitors used to determine exposure of worker,Air Monitoring Strategies,Determine worker exposureVariable concentrationContamination levelControl measuresBatch operationsAir circulations patternsSeasonal variations,Exposure Evaluation,Threshold Limit Value - Time Weighted Average, TLVTWAtw is the length of shifttn is 8 hours,Exposure Evaluation,Intermittent monitoringI = number of measurements during shift = is the time period over which measurement i is takenAssume concentration is “constant” during the time period,Exposure Evaluation,Multiple ToxicantsN is total number of toxicantCn is the concentration relative to other toxicantsHere we assume the effects are additive,Estimating Exposure from Open Tanks,Author derives relationships assuming no toxicants in ventilation air. I will present of more complete analysis.,Mass Balance on Room for Toxic Vapor,Estimating Exposure from Open Tanks (cont.),Assume Steady StateAssume Nonideal mixingCout = kCmax k=1 for perfect mixingTable 3-11 gives values of k, worst case scenario is k1/10,Estimating Exposure from Open Tanks (cont),Substituting,Estimating Exposure from Open Tanks (cont),Air mass balanceAssume steady state,Estimating Exposure from Open Tanks (cont),Assume ideal gas and that toxic vapor has negligible mass compared to mass of airSet equal, so,Estimating Exposure from Open Tanks (cont),SubstitutingQv,out 3000 ft3/min for out doors,Estimating Exposure from Open Tanks (cont),Now estimate evaporation rate diffusion away from the liquid surfaceM is molecular weightK is mass transfer coefficient (length/time)A is surface area over which driving force existsTL is absolute temperature of volatile liquid is partial pressure above surfaceWorst case Psat,Estimating Exposure from Open Tanks (cont),SubstitutingWith simplifying assumptions you get Eq 3-14,Estimating Exposure from Open Tanks (cont),Correlation for mass transfer coefficientsFor water M0=18 and K0=0.83cm/sec,Estimating Exposure from Filling Tank,Estimating Exposure from Filling Tank (cont),Assume vapor space above liquid is partially saturatedWith a heal left in vessel = 1,Estimating Exposure from Filling Tank (cont),displacementdiffusion out of tankAssume worst case Psat,Estimating Exposure from Filling Tank (cont),Similar to, but better, than Eq. 3-24,Textbook Error,Note that Example 3-9 on page 68 has error 7.481gal/ft3 is correct not 7.481 ft3/gal,Control of Chemical Hazards,Engineering ControlAdministrative ControlProtective Equipment,Engineering Controls,Inherent SafetyContainmentVentilation,Inherent Safety Aspects,SubstitutionUse chemicals and equipment which are less hazardousAttenuationUse chemicals under conditions which make them less hazardousIsolationIsolate equipment and/or sources of hazardIntensificationReduce quantity of chemical,Containment Principles,“Containment” refers to keeping the process materials contained within the processing equipmentDesign for internal deflagrationVent to containment or control equipmentUse rupture disks or safety valves to vent excessive pressure spikesVenting to containment vessel or flare, etc.,Containment PrinciplesSealing Points and Leak Protection,Static SealsWeldsFlangesCovers/HeadsWelds are better than flanges,Dynamic SealsRelative motion between seal partsRotating ShaftsValve stems,Containment PrinciplesRotating Shaft Sealing Methods,Stuffing Box and PackingMechanical SealDouble Mechanical SealAllows evacuation between sealsSeal Maintenance procedure required,Avoiding Dynamic Seals,“Seal-less” pumpMagnetic couplingCanned rotorDiaphragmBellows-Seal Valve,Potential Leakage Locations/Occasions,Sight glassesGage glassesSampling pointsAddition pointsBatch processing vesselsLoading/UnloadingPackagingMaintenance,Ventilation for Control,Outdoor constructionLocal VentilationDilution Ventilation,Local Exhaust Ventilation,Removes contaminants at sourcePrevents toxic material from entering the workplace airRequires less airflow than dilution ventilation,Components of a Local Exhaust Ventilation System,Hood or “Elephant Trunk”Duct systemAir cleaning systemAir moverOutlet,Hood Ventilation,Totally EnclosedEnclosed structure around processing equipment with limited (No) access. Emissions taken to be treatedExterior HoodAlso called “Elephant Trunk”. Duct inlets located close to source. Often flexible duct that can be moved some, i.e. elephant trunk.,Hood Ventilation - Booth,Booth HoodStandard “fume hood” seen in laboratoriesNeed to keep the window always slightly opened to ensure there is some are flow,Hood Ventilation - Booth,Booth HoodBypass laboratory hood ensures that there is always a positive flow through the hood and minimizes the circulation patterns that might allow fumes to be released,Negative Ventilation Systems,Need to keep exhaust system under negative pressure so that any leakage will be from the rooms into the exhaust system and not vice versa.,Duct System Design,Basic fluid mechanicsPublications/RecommendationsCapture velocityEntrainment velocityPressure losses,Dilution Ventilation,Air flow throughout buildingHigh air flow requiredBest used in conjunction with localized hoodingIntegrated with local HVAC system,Ventilation Exhaust May Require Cleaning,AbsorptionAdsorptionFlare or IncinerationStack to prevent re-entryBest to treat localized exhaust system, prohibitive to treat a dilution ventilation system.,Administrative Control Techniques,Work Rules to Limit Exposure Time and/or limit accessibility to areas with high concentrations.Good HousekeepingFunctional Operating and Maintenance ProceduresEducation and Training of all personnel,Good Housekeeping,Keeps toxics and dusts containedUse dikes around tanks and pumpsProvide water and steam connections for area washingProvide lines for flushing and cleaningProvide well-designed sewer system with emergency containment,Elements of PPE Training Program,Standard and regulatory requirementsHazard characterization in the workplaceImplementation of engineering and management controlsDescription of need, capabilities and limitations of PPEDemonstration of proper use, fit, care, maintenance and repair of PPEExplanation of PPE written policy, regulations and enforcementDiscussion of record-keeping requirements,Personal Protective Equipment,Engineering and Management controls can reduce or even eliminate many occupational safety hazards. However, it may be impractical or impossible to keep the work area completely free of contaminants or to keep all workers away from dangerous locations. PPE is the last line of defense,Personal Protective Equipment,Routine EquipmentEmergency,Protection of the Head,Hard hats should be able to withstand the impact of a 8 lb iron ball dropped from 5 feetShould be non conducting,Eye Protection,Unvented gogglesImpact resistant lenses and side shieldsChemical splash goggles,Hearing Protection,Ear plugsRange from 17 - 25 dBHearing bands allow on-off useEarmuffsProvide wide range of protection from 19 to 30 dB,Respirators,Dust and mist respiratorsFilter out particulateNeed to have ambient oxygenDoes not stop vapors or gases,Respirators,Air-Purifying RespiratorsAdsorbent removes gas, vapor, or particulateDifferent cartridges for different types of vaporNeeds tohave ambientoxygen,Respirators,Supplied Air“Unlimited” air supply from remote siteRequires compressorDisadvantage of possible damage to hose, limited mobility and contamination from compressor,Respirators,Self Contained (SCBA)Avoids problems of supplied airLimited supplyTypically used for emergency operation,Respirators,All respirators need to be fit properly and tested routinely to ensure that they function.Emergency respirators need to be serviced routinely to ensure that they function when needed.,