Meeting NFPA 86: Failsafe Malfunction Logic

We've discussed the 6 essentials of meeting NFPA 86, this week let's take a closer look at the fifth monitoring requirement: failsafe malfunction logic.

According to section 11.6.10.8 of NFPA 86, "alarms shall be provided to indicate any sample, flow, circuit or controller power failures."

The best analyzer design should be failsafe: it will provide malfunction alarm for any and all faults. For greatest safety, the malfunction alarms should shut down the process.

• Catalytic sensors cannot provide failsafe malfunction logic because their design requires calibration to prove proper function. Catalytic sensors can be poisoned by coating and corrosive agents, including silicones and plasticizers, compounds often found in solvent vapor oven atmospheres. The only way to know if a catalytic sensor is working correctly is to calibrate it with a known concentration of test gas.
• Infrared sensors optics can become fouled by the process environment.
• The flame temperature sensor is unaffected by catalytic poisons such as silicone, halogenated hydrocarbons and plasticizers, and has no optics that can be fouled.

Only the flame temperature sensor offers malfunction indication for all faults. If the sensor is not functioning correctly, the operator is notified.