Although the terms LFL and UFL usually apply to a gas, there are some cases in which a suspended solid in a powdered form flour is one example or an atomized liquid gasoline, for example may also exhibit lower and upper flammability limits when disbursed in air. The ratio of fuel to air that will support combustion is different for different materials. Charts giving the limits for a wide range of materials are readily available.
In the case of atomized liquids and suspended powders, the LFL and UFL may, in part, be influenced by temperature as well as concentration. As discussed in the preceding blog, most solids and liquids must go through the transformation to a gas before a flame will occur due to the introduction of an ignition source.
More fuel was required to produce sufficient vapor to mix with the available air to produce a combustible mixture of vapor and air. Typical units are volume percent fuel as a percentage of fuel and air. Pressure has only a slight effect on LFL. Lower limits are essentially constant down to about 5 kPa generally, below which pressure flame does not propagate.
To ensure that the vapor concentration reaching the analyzer is exactly the same as the concentration at the point of origin, the sample must always be kept in a vapor state. The analyzer, the sample line and any other elements of the sample train may have to be heated to keep the sample above its dew point. If allowed to cool, the sample may condense, causing process components to drop-out, which will compromise reading accuracy and potentially result in clogging of the sample train, requiring maintenance and causing downtime.
When determining the operating temperature required for the analyzer and sample line, it is important to calculate the flash points or condensation temperatures of all substances in the sample atmosphere, even those that are not flammable. Any substance can clog the sample line or analyzer if it is allowed to condense. A sometimes overlooked aspect when selecting the right analyzer is if it is failsafe. What this means is if the analyzer were to go into a fault condition where the readings can no longer be relied upon, does the analyzer have the capability to alert the operator to this situation.
Processes that use flammable solvents can develop explosive concentrations due to failure of coating, metering, ventilation or heating systems, adverse chemical reactions, and human error.
LFL monitors are used for safety as well as process efficiency, protecting the system from fire and explosion while also allowing operating at higher vapor concentrations to save costs. Although several different types of sensors are employed as LFL monitors, each has an appropriate application to which it is best suited.
This is most often caused by a misunderstanding of the different available technologies. Catalytic-bead sensors are constructed of two small wire coils covered with a catalyst. A flow of electrical current through the internal coils heats the catalytic coating to a temperature at which the active coil will react with many flammable vapors and gases.
The resulting temperature change is converted into an LFL reading. The intensity of the catalytic reaction varies for different flammable substances. Therefore, response factors vary considerably, resulting in potentially inaccurate readings.
Calibration correction may be required for each different solvent or solvent mixture used in the process. Catalytic sensors have slow response times, as much as 15 seconds is not unusual.
That slow response alone may eliminate a catalytic sensor from consideration as a process monitoring device. Catalytic sensors are not failsafe. During operation they cannot detect and alarm all conditions that can cause the sensor to become non-functional. There are fault conditions that can only be detected by performing a calibration. This can lead to a false sense of safety, as it has no means of warning the operator that it is not functioning properly.
Because of these characteristics, catalytic sensors are typically used only as leak detectors. They are generally well suited to area-monitoring applications where response times of ten or more seconds are acceptable; where the atmosphere does not contain condensate, dirt or dust; is not heated and is normally free of flammable vapors.
Combustible gases absorb infrared radiation at certain characteristic wavelengths. A typical non-dispersive infrared NDIR detector passes a source of infrared energy through the sample and measures the energy received by one of two detectors.
The active detector responds to wavelengths in the same band as the sample gas, while the other detector measures a reference to compensate for changes within the instrument. When specific combustible gases are present, they absorb some of the infrared energy and produce a signal in the active detector relative to the reference detector.
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