The pressure vessel designs found in most horizontal firetube type boilers can be subjected to excessive stresses if the pressure vessel is heated up unevenly. To avoid subjecting the boiler to undue stresses, Pulaski, N.Y.-based Fulton recommends boiler users take advantage of low fire hold.
"When the metal heats up, it expands proportional to the temperature rise," explains Fulton's Carey Merritt. "If a boiler filled with cool water is subjected to burner operation at high fire, the metal close to the furnace will heat up at a higher rate than the bank of tubes located on the last flue gas pass. Consequently, uneven tube metal expansion can occurm causing a thermal stress gradient across the pressure vessel tubesheets."
While the problem is simple to understand, fortunately, so is the fix. "A simple but oftentimes overlooked design feature that can minimize thermal stress is a low fire hold," Merritt says. "The low fire hold is a temperature or pressure sensor, in the boiler, that limits the burner to low fire until the boiler is at a warmed up state. Heating the boiler up at low fire will prevent excessive thermal stress."