The firing rates of burners could be determined by using individual fuel flowmeters. However, this is usually prohibitively expensive in plants that have many process heaters and where each heater has multiple burners. Instead, the most common method used to determine the firing rate of individual process burners is with fuel capacity curves, which are graphs of firing rate vs. fuel pressure.These curves are generated specifically for a given burner design and fuel composition. In many cases, multiple curves are needed for a particular process heater as multiple fuel compositions are used. The curves are generated based on the number of tips used in the burner, the sizes of the holes in each tip, and the flow coefficient for each tip.
The total fuel flow to a heater is usually measured with a flowmeter. Because all the burners in a heater are typically designed to fire at the same rate, the firing rate of an individual burner can be calculated by dividing the total firing rate of the heater by the number of burners. The actual measured fuel pressure required to produce a given firing rate then can be compared to the predicted fuel pressure for that firing rate from the fuel capacity curve. Assuming that the correct burners tips are installed in the burners, if the actual fuel pressure is higher than predicted by the capacity curves, then this usually indicates the tips are plugged. If the measured fuel pressure is lower than predicted, it usually means the holes in the tips are larger than designed which can occur if they have been eroded or corroded from contaminants in the fuel or from repeated cleaning.