Operating Burner Outside the Specific Gravity Range
Don’t expect satisfactory operation outside the design specific gravity range. This is undoubtedly the most common mistake when firing gas. Natural draft burners depend on the momentum of the fuel jets to give good fuel-air mixing. For hydrocarbon gases, the heating value is directly proportional to specific gravity. When the heating value (specific gravity) of the gas is too high, the burner pressure at design heat release may be too low to produce good mixing. Long, billowy, yellow, smoky flames result.
Below the design specific gravity range, the burner pressure must be increased to get the necessary fuel into the box. Usually this happens when the hydrogen content in the fuel gas is high. Even though it has low heating value, hydrogen is a fast burner and moderately higher pressures just lead to better mixing and even smaller lames. However, operation far above the design pressure range (due to low specific gravity gas or overfiring) is unwise because any gas burner can blow itself out at a high enough gas pressure.
Premix burners are much more sensitive to changes in fuel specific gravity than are raw gas burners. The premix burner has less turndown than raw gas, and high hydrogen contents cause backfiring to occur at higher turndown.