Maintaining stable combustion is critical to the safe operation of fired heaters. When the ratio of air flow to fuel flow to a burner transitions from a point inside the burner’s operating envelope to a point outside the burner’s operating envelope, the combustion process will become unstable and then stop. If this loss of flame is not recognized and acted upon properly, an explosion may occur.
The transition of a burner’s air/fuel ratio from inside to outside the burner’s operating envelope may be caused by a number of different initiating events including a significant change in fuel gas composition, a sudden decrease in air flow, a sudden increase in fuel flow or a shift in the boundaries of the burner operating envelope. Some oil refinery process fired heaters with only a few burners are equipped with flame scanners that are used to detect loss of flame. However, most do not as it is impractical to install flame scanners on multiple burner heaters. For these heaters, current industry practice is to train operators to manually recognize and respond to loss of flame. While this approach has proven acceptable, Valero is interested in finding better ways to detect loss of flame.
One possible way to detect loss of flame may be to monitor pulsations in the flue gas pressure inside a fired heater. During stable combustion, the flue gas pressure inside a fired heater will pulsate slightly. This phenomena is commonly referred to as combustion rumble. As a burner’s air/fuel ratio transitions from inside to outside the burner’s operating envelope, the pressure pulsations may exhibit a sharp increase in amplitude and frequency. Valero has worked with Rosemount to detect this sudden change in pressure pulsation using pressure transmitters fitted with advanced pressure diagnostic capability. Recent testing of this technology was performed on five different burners at three different burner vendors. The technology was also evaluated on a heater at the Valero Texas City refinery. Initial results indicate that a standard deviation of 30 thousandths inches of water column may be associated with a transition of a burner’s air/fuel ratio from inside to outside the burner’s operating envelope. Additional work is required to validate this observation but these initial results indicate the use of advanced pressure diagnostics may be an effective way to detect loss of flame.
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