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.
However, a process oven is allowed to operate with only a 2:1 safety margin (up to 50% of the LFL), when continuous flammability analyzers are used. Solvent vapor monitoring in ovens and dryers has special requirements, set out in the NFPA-86 code, stipulating real-time, fast-response, triggering corrective action at predetermined alarm points:
Speed - Experience has shown that explosions in ovens and dryers can happen very quickly. The code explains that an abnormally high solvent concentration should be detected and corrected within five seconds or less. Some approved sensors, having response times up to 12 seconds, clearly are not suitable.
- Sampling systems should be optimized for speed, with short tubing lengths, high flow rates, and very low filter volumes. During an accident, speed and accuracy are identical. When the solvent concentration is increasing at +10% LFL per second, a sensor that lags behind by five seconds displays a reading 5s. x 10 % LFL/s = 50% LFL below the actual concentration in the dryer. A 50% LFL alarm from such a sensor occurs when the dryer is already at 100% LFL!
Accuracy - Most dryers are used for more than one solvent. Accuracy depends completely on how the sensor responds to each solvent. The measuring principle of the sensor is important. Typically, catalytic bead response factors are within +/-35%, flame temperature within +/-10%, and infrared can easily vary from -50% to +150% or more, however some process infrared sensors with multiple detectors can provide more precise readings.
- The code requires calibration for the solvent to which the sensor is least sensitive. If the response to one solvent is several times lower than another, as for point infrared, the reading can be several times higher than the actual concentration, making it impossible to operate the dryer at its intended speed due to false alarms.