Common Boiler Problems, Part 4: Improper Warm-up
Improper boiler warm-up is a common problem because management and production often exert extreme pressure on utilities to complete forced or scheduled outages so that production can resume. As soon as the boiler is “capable’ of producing steam, they want it.
The improper warm-up of a steam boiler is one of the hardships a boiler must endure. Going through the cycle of start-up, operation and shutdown for any boiler creates higher equipment stresses and, consequently, much more maintenance type issues than continuous operation at maximum rated capacity. Any piece of equipment such as a boiler, airplane fuselage, or combustion engine that undergoes an extreme transformation from ambient to operating conditions is subject to fatigue and failure. 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, including:
- Drums and headers fabricated of thick metal which contain water and steam.
- Tubes fabricated of much thinner metal which contain water and steam.
- Refractory materials that are exposed to high furnace temperatures on one side and cooling from water, steam, and air on the other side.
- Insulation materials which are specially designed to operate at a much higher temperature on one side than on the other side.
- Thick cast-iron castings such as doors that are refractory-lined which are exposed to the full temperature of the furnace on one side and ambient air on the other side.
By design all of these materials heat up and cool down at a much different rate. This situation is made much worse when a component is exposed to different temperatures.
Refractory damage is most prevalent associated with quick warm-up of a boiler from a cold start. Refractory by design transfers heat very slowly and therefore heats up much more slowly than metal. Also, as the air inside the furnace and refractory cool, moisture is absorbed from the air in the refractory. A gradual warm-up is required to prevent refractory from cracking; this allows adequate time for the moisture to be driven from the refractory. Trapped moisture quickly becomes steam and causes the refractory to spall as the steam escapes.
The standard warm-up curve for a typical boiler does not increase the boiler water temperature over 100°F. per hour. It is not unusual for a continuous minimum fire to exceed this maximum warm-up rate. Consequently the burner must be intermittently fired to ensure that this rate is not exceeded.
Correct planning and education will allow a boiler to be started properly, which will prolong the life and eliminate costly maintenance repairs.
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