Last week we discussed the 6 essentials of meeting NFPA 86. Let’s take a closer look at the first monitoring requirement: Sample delivery system (section 11.6.10.11).
It is recommended that process applications employ active sample draw systems to continuously deliver a sample to the solvent vapor monitor.
Why?
Static sensors placed inside the process are not recommended because, among other drawbacks, it is not possible to prove that a representative sample is being delivered to the sensor.
Annex E of NFPA 86, states that “the oven atmosphere should be sampled at a point that best represents the average concentration of solvent vapor in the oven or oven zone. This is usually at the oven exhaust point. The volume of the sample system should be as small as possible and the sample flow rate maximized for fast response of the system. Special precautions, such as heating the sample lines and analyzer to prevent condensation of volatiles in the sample, might be required. The length of the sample line should be minimized by locating the analyzer close to the sample point.”
Therefore, the best sensor site is on or adjacent to the oven zone’s exhaust ductwork so that the sample line is kept short. Rack-mounted sensors (often found in FID designs) that employ long sample lines should be avoided.
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