LFL Monitoring MEGTEC Systems
Flammability For each volatile flammable substance there is a concentration in air (usually expressed as % of volume, known as Lower Flammable Limit LFL). Below LFL concentration is too lean to support combustion. ASTM E681 Test Method for Flammable Limits NFPA 325 Guide to Fire Hazard Properties 2
LFL Monitoring LFL Lower LFL increases safety of processes Higher LFL reduces energy usage Lower process operating costs Lower emission control operating costs 3
LFL & Safety Margins Safety authorities required a 4:1 margin of safety below the LFL. NFPA 86 requires enough dilution air to always maintain less than 25% of LFL (for systems w/o monitors) 4
Safety Margins Drying systems or oxidizers are allowed to operate with a 2:1 safety margin (50% LFL) when a continuous flammability monitor is used. (NFPA 86, 9.2.6.1 and 9.2.8) Must be real-time Must provide fast-response Must be connected to trigger corrective action at predetermined alarm points 5
Safety Margins Processes without monitors typically run at 10% - 12% LFL to avoid reaching 25% in case of accidental upset. Adding a monitor allows much higher concentrations to be run. Result is cost savings in operation of process (as long as higher LEL does not impact product retained VOC) 6
Impact of Temperature LFL published values are typically calculated at room temperature. When mixture is heated, it s flammability increases and the concentration to achieve 100% LFL is less. Therefore a safe level at 75 degrees may become dangerous at elevated temperatures. NFPA 86 9.2.5.2 LEL F = LEL77 F[1-0.000436(T F-77 F)] 7
LFL Detectors NFPA 86 Annex E lists 4 types Catalytic Flame Ionization Flame Temperature Infrared 8
Catalytic Detectors 9
Catalytic Low cost Small size VOC response specific Poisons and Masking Slower response time 15-20 sec Typical for area monitoring not process Sample passes over catalyst. Temp rise from oxidation converted to LFL. 10
Flame Type Detectors 11
Flame Type LFL Higher initial cost Requires fuel source for operation Requires regular calibration Auto calibration often available Senses wide range of hydrocarbons Proven technology Response times down to 1 second High accuracy 12
Flame Type Detector Sample Flow Flame Source Flame ionization (FID) device good for low LFL ranges. Typically used for area sources (like catalytic) Sample conditioning needed to keep sensing chamber clean. 13
Flame Type Pass metered sample across a flame. Oxidation of VOC produces measured signal. Flame temperature device good for wide LFL ranges Thermocouple Sample Flow Flame Source 14
Flame Type LFL 15
FTA Layout 16
Infrared Detectors 17
Infrared LFL Monitors Lower initial cost Does not require fuel source Does not require O2 for operation VOC specific responses Calibration needs to be done at least once per year. 2-5 second response time Sample conditioning needed to keep sensing chamber clean. 18
Infrared LFL Monitors IR LFL monitors are sensitive to what VOCs are to be monitored and at what levels. This requires client inputs into what will be used in the process to insure the proper monitors are supplied. Not only are types of VOCs required, but ratios of each are also required to minimize nuisance alarms. 19
Infrared LFL Monitors If mixtures are used in the process, IR LFL monitors need to be calibrated for worse case VOC and could limit maximum VOC load for process One IR LFL manufacturer has develop their product to use 4 different parameters to change (automatically) according product code This may be risky as the inputs are manual and may be missed by operator 20
IR Layouts 21
IR Layout 22
Response Factors 23
Group A Group A for IR LFL n-propyl Acetate D-Ethyl Ether Butane Group A Pentane n-hexane Acetic Acid Ethane Methanol 0 0.5 1 1.5 2 2.5 Relative Sensitivity 24
Group B Group B Butanone Iso-Propyl Acetate Cyclohexane Group B Ethoxy Propanol Ethyl Acetate Methyl Iso Butyl Ketone n-butyl Acetate Pentane 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Relative Sensitivity 25
Group C Group C Propene D-Methyl Formamide Group C P-Xylene 0.67 0.68 0.69 0.7 0.71 0.72 0.73 0.74 0.75 0.76 Relative Sensitivity 26
Group D Group D Benzene Ethene Acetone Group D Toluene O-Xylene Methane 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Relative Sensitivity 27
Typical Example for IR LFL Client VOC Mixture: Ethyl Alcohol N-Propyl acetate N-Propyl Alcohol Isopropyl Alcohol N-Heptane Heptane 39.06% 14.76% 31.50% 5.53% 0.29% 0.43% 28
IR LFL Manufacturer Recommended System Relatively tight bundle of responses Calibration "Band B" with 50% Propane/bal Nitrogen cal check mixture Presuming that the gases in your mixture have a relatively constant concentration ratio between them 50% LEL propane cal check mixture will read about 75%LEL with our analyzer 29
HHe exx aann ee HHe epp tatan nee B Buu ttyyl l AA ccee tata tete AAcc eet t oonn ee MM EEK K MM eet thha no ll Et hy yll AAcc eet t aat t ee Et thh aann ool l DD MM EE BBe enn zze enn ee B ut yl AAl lcco ohh ool l C yc lloo hhe exx aan nee Response Factors Relative response factors 1.6 1.6 1.4 1.4 1.2 1.2 11 Factor 0.8 0.8 Infrared FTA: 0.6 0.6 0.4 0.4 0.2 0.2 00 Substance Substance 30
Installation Locations Duct mount Protect sensor from Condensation Dirt Heat Vibration Faster response time Limited access for maintenance Remote Mount Add sample transport time to safety calculations Easy access May need heated sample line 31
Flame or IR? Traditional Flame Type LFL are widely used in coating and laminating facilities in North America IR LFL systems popular in Europe. IR LFL less costly Jury is out on which system is best for your application. 32
Questions/Thank You Source material provided by: Jeff Sampson, Control Equipment Corporation Chet Anderson, Honeywell Zellweger Analytics, Inc. 33