Some questions on flame detector reliability came my way the other day from a blog reader in the refining industry. I ran the questions by Emerson’s Gary Hawkins, a refining industry consultant.
Gary’s first response was to point to the authoritative source for fired heater protection in the refining and petrochemical industries, API Recommended Practice RP 556, Instrumentation, Control, and Protective Systems for Fired Heaters. The second edition is nearing completion and should be available toward the end of 2009 or early 2010. Gary is a member of the RP 556 committee and is providing comments back on this American Petroleum Institute recommended practice.
He notes that there are two categories of flame detectors: flame ionization rod and optical flame scanners (including both ultraviolet and infrared). You must consider numerous factors when selecting a flame monitoring system, including types of fuels, heater geometry, number of burners, and the type of process safety layers of protection.
Flame ionization rods are generally considered consumable and require periodic replacement. The flame ionization rod is generally only used to detect the pilot flame, as it is not suitable for long-term use in the main flame.
The reliability of flame scanners is a more complex topic with a number of considerations:
- Generally require a purged sight tube to keep the lens clean
- Difficulty to detect both the flame at the primary tips and secondary tips
- Distinguishing between the monitored flame and background radiation in the heater
Gary notes that reliability is a function of the quality, the installation, and the maintenance of the instruments involved in flame detection. Quantification requires an analysis of the installation to collect the data required for the reliability calculations. This is usually performed as part of the hazard and operability (HAZOP) analysis.
On a question about whether or not a flame detector is required when there are other indicators such as low combustion air or low fuel pressure, Gary cautions that there are many factors to consider. Each site requires compliance with their company’s standards, local codes and the safety requirements specification (SRS) if the flame detectors are part of a safety instrumented system (SIS) or only used to alert the operators of abnormal conditions.