What guidance is available for addressing issues with LEL sensor reliability when commissioning a regenerative thermal oxidizer? The project in question uses hydrogen gas along with some volatile organic compounds mixed at approximately 1-2% by volume in air. The flow is under slight vacuum at -8 water column in an 8” pipe. The sensor needs to be able to detect at least two levels of hydrogen concentration (1.5% and 3.5%) to ensure the LEL is not exceeded. The project also wants to attach an alarm to the output from the sensor. The service is continuous monitoring, and a sensor that can last at least 6 months is desired. The project may consider a second system to enable change-out of sensors while running.
The answer will depend on if it is only H2 measured or if VOCs are also included. The type of gas sensor may change since many detectors are limited to flammable gases. To provide feedback, the Panel would require more details on the sensors being used on the project and the failure modes. There are concerns about allowing hydrogen concentrations as large as 3.5% for the high level alarm. Since VOCs are mixed with the hydrogen, the mixture lower flammability limit is probably slightly less than 4%. A typical practice would use 25% of the LFL, or 1% hydrogen), for alarm, not 1.5% to 3.5%. The vacuum may be an issue, as normally these are operated at atmospheric pressure. Engineering discussions with vendors and research would be needed to assure VOCs are not an issue and the vacuum is suitable. Staff at the National Renewable Energy Laboratory have experience with a range of sensors and may be able to help, particularly if the failure mode with the current sensors is known. As always, experienced sensor manufacturers should also be consulted for assistance.