Although several different types of sensors are employed as LFL monitors, each has an appropriate application to which it is best suited.
Fires and explosions in what was thought to be “protected” equipment can occur without warning when a sensor is not capable of doing the job that had been assigned to it.
This is most often caused by a misunderstanding of the different available technologies.
Typical Flexographic Application Examples Include:
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.
There are important considerations when choosing an LFL monitor: Response Accuracy, Response Speed, Condensation Issues and Failsafe.
There are important considerations when choosing an LFL monitor: Response Accuracy, Response Speed, Condensation Issues and Failsafe. We've discussed response accuracy and response speed, let's look at condensation issues:
There are important considerations when choosing an LFL monitor:
There are important considerations when choosing an LFL monitor:
First let's look at response accuracy.
There are several technologies used to measure flammability, and few react the same way to all substances.
A sensor or analyzer calibrated to accurately read one substance may be significantly inaccurate for another process component. This variance from perfect accuracy (error) is known as the Response Factor.
Most published LFL values are calculated at room temperature.
However, as a given mixture is heated, its flammability increases and thus the concentration required to achieve 100% LFL is less. This source of increased danger is often overlooked.
Almost all safety authorities require a 4:1 margin of safety below the LFL, based on worst-case conditions. This means that enough dilution air must be used to always maintain a concentration of less than 25% of the LFL, according to the National Fire Protection Association standard NFPA 86.
For each flammable substance there is a level of concentration in air, usually expressed as a percent by volume, that is known as its Lower Flammable Limit, LFL, or Lower Explosive Limit, LEL. Below the LFL, the mixture of fuel and air is too lean to support combustion. For example, a mixture of 1.1 percent Hexane in air is equal to 100% of its LFL - just rich enough to propagate a flame.
Industrial fires and explosions happen more frequently than most people think. They cause downtime, property damage, injury and sometimes death. These fires and explosions result from a dangerous mixture of flammable vapors with air and a source of ignition.
