What may lead to excessive soot deposits on tubes in a boiler?

Excessive soot deposits on tubes in a boiler can often be traced to a change in fuel flow. When the fuel flow to the burner is not consistent or changes unexpectedly, it can disrupt the combustion process. This disruption can lead to incomplete combustion, which produces more particulate matter, including soot.

Soot deposits occur when the carbon in the fuel does not burn completely, often as a result of improper combustion conditions. For instance, when the fuel flow is too low, it may lead to a lean mixture that can burn cooler and incomplete, while an overly rich mixture might not allow for adequate oxygen supply, leading to a similar issue. Therefore, maintaining a controlled and stable fuel flow is essential for optimal combustion and minimizing soot production.

The other options do not contribute to soot buildup in the same way. A high water level can actually cause issues like water carryover but not soot. Regular maintenance is typically aimed at preventing such issues and ensuring efficient operation. Proper combustion is desired and should reduce the chances of soot deposits rather than increase them.