During a cold start, the temperature of the boiler should be raised gradually. Improper warm-up of the boiler severely harms the boiler health.
Usually after a shutdown, there is tremendous pressure to ensure that the boiler delivers steam as soon as possible so that production can commence. However a boiler should not be jump started to deliver steam as boiler need a definite warm up period. Improper warm-up of a steam boiler severely harms the boiler’s health.
Going through the cycle of start-up, operation, and shutdown for any boiler creates higher thermal stresses and, consequently, more maintenance related issues than continuous operation at maximum rated capacity.
Undergoing extreme transformation from ambient out of service conditions to operating conditions subjects the equipment to fatigue. Good design and the process of making a slow transition between these conditions are essential for prolonging boiler life and reducing the possibility of failure.
A typical boiler is constructed of different types of materials which operate in totally different environments. For example, Drums and headers and tubes are fabricated of metal and contain water and steam whereas refractory materials are exposed to high furnace temperatures on one side and cooling from water, steam, and air on the other side, etc.
By design, all of these materials heat up and cool down at different rates. Additionally, this situation is compounded when these components are exposed to different temperatures. For example, a steam drum that is operating at normal water level has the bottom half of the drum cooled by water and the top half by- air initially and steam eventually. If the boiler is fired from a cold start, the water will heat up very quickly in the drum and the bottom half of the drum will expand much more quickly than the top half which is not in contact with water. Consequently, the bottom of the drum will become longer than the top, causing the drum to warp and lead to stress fractures.
Thus as typically prescribed in the standard warm-up curve for boilers, the boiler water temperature should not be increased over 100°C per hour. The operating practice should therefore ensure the burner firing rate is optimized to enusre the boiler water temperature does not increase over 100°C per hour.