Oxygen/Combustible Safeguards in Low Nox Fired Heaters

Refineries have been installing low NOx equipment on their fired heaters in response to tighter environmental regulation. Low NOx burners minimize air ingress in locations other than directly at the burner, and internally recirculate flue gas in the heater firebox. They operate on much narrower fuel-to-air ratio tolerances, which result in inherently riskier design. Refineries have had recent incidents related to low NOx heater design, where a malfunction in firing control caused a highly fuel-rich condition in the firebox. In some cases an improper control action rapidly introduced air into the firebox, which then caused uncontrolled combustion or an explosion. Incidents such as these can have severe commercial and safety consequences. Some refineries have elected to implement automatic low-O2 or high-combustible shutdown functions to improve the safety of low NOx heater operation. However, when these systems are considered they must be carefully designed to account for not only the safety reliability (Safety Integrity Level), but also the spurious trip rate. This is because a disproportionate number of heater incidents occur during heater startups and shutdowns. These functions are challenging to implement for other reasons – the analyzers operate in severe service, and physical constraints may limit their effectiveness. This paper discusses design alternatives in implementing low-O2 and high-combustible safety functions in heater service. Practical techniques to increasing safety reliability and reducing spurious shutdowns will be discussed. The authors will use Fault Tree Analysis to support their conclusions. The proper use and selection of O2/Combustibles analyzers will also be reviewed.