LFL Monitoring - FPA · PDF file– NFPA 325 Guide to Fire Hazard Properties ......
Transcript of LFL Monitoring - FPA · PDF file– NFPA 325 Guide to Fire Hazard Properties ......
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LFL Monitoring
MEGTEC Systems
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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…
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LFL Monitoring
• LFL – Lower LFL increases safety of processes– Higher LFL reduces energy usage
• Lower process operating costs• Lower emission control operating costs
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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)
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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
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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)
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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)]
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LFL Detectors
• NFPA 86 Annex E lists 4 types– Catalytic– Flame Ionization– Flame Temperature– Infrared
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Catalytic
Detectors
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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.
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Flame Type
Detectors
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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
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Flame Type
• Flame ionization (FID) device good for low LFL ranges.– Typically used for area sources (like catalytic)
• Sample conditioning needed to keep sensing chamber clean.
Detector
Flame Source
Sample Flow
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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
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Flame Type LFL
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FTA Layout
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Infrared
Detectors
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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.
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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.
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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 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
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IR Layouts
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IR Layout
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Response Factors
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Group AGroup A for IR LFL
0 0.5 1 1.5 2 2.5
Me t hanol
Et ha ne
Ace t ic Ac id
n-He xa ne
P ent a ne
Group A
But a ne
D-Et hyl Et he r
n-P ropyl Ac e t a t e
R e l a t i v e S e n s i t i v i t y
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Group BGroup B
0 0.2 0.4 0.6 0.8 1 1.2 1.4
P e nt a ne
n-But yl Ace t a t e
Me t hyl Iso But yl Ke t one
Et hyl Ace t a t e
Et hoxy P ropa nol
Group B
Cyc lohe xa ne
Iso-P ropyl Ace t a t e
But a none
R e l a t i v e S e n s i t i v i t y
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Group CGroup C
0.67 0.68 0.69 0.7 0.71 0.72 0.73 0.74 0.75 0.76
P -Xyle ne
Group C
D-Me t hyl Forma mide
P rope ne
R e l a t i v e S e n s i t i v i t y
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Group DGroup D
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Me t ha ne
O-Xyle ne
Tolue ne
Group D
Ac e t one
Et he ne
Be nz e ne
R e l a t i v e S e n s i t i v i t y
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Typical Example for IR LFL
Client VOC Mixture:– Ethyl Alcohol 39.06%– N-Propyl acetate 14.76%– N-Propyl Alcohol 31.50%– Isopropyl Alcohol 5.53%– N-Heptane 0.29%– Heptane 0.43%
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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
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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
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Flame or IR?
• Traditional Flame Type LFL are widely used in flexible packaging in North America
• IR LFL systems popular in Europe.– Alcan, Amcor, and Printpack use these in Europe.
• W&H and PCMC now use IR as standard detector.
• IR LFL less costly• Jury is out on which system is best for your
application.
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Questions/Thank You
Source material provided by:
Jeff Sampson, Control Equipment CorporationChet Anderson, Honeywell Zellweger Analytics, Inc.