HRSG Burners High Spread

HRSG gas burners are experiencing high spread reaching to 120 Deg C spread on partial load (4 out of 8 burners in operation) as compared to 60 Deg C spread on base load (8 out of 8 burners in operation). Partial load operation also have changed the stack smoke from clear to yellowish.

What is the effect of high spread on boiler and what is the safe limit of operation?