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. When the metal heats up it expands proportional to the temperature rise. 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 occur causing a thermal stress gradient across the pressure vessel tube sheets. A simple but often times overlooked design feature that can minimize thermal stress is a low fire hold. The low fire hold is 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.
The Fulton Companies, www.fulton.com