During the normal operation of a boiler, suspended matter is maintained in a fluid state, using a chemical treatment and the circulation pattern. If proper care is not taken during the shutdown stage, these solids can settle out in drums and tubes and dry to a hard, tightly held deposit that could require chemical or severe mechanical cleaning. It is also possible to mistake the deposits formed at this stage for a problem that developed during the normal boiler operation. Following is the procedure recommended when shutting down your boiler:
1. Three to five days before a scheduled shutdown, increase the blowdown rate by at least 50%
- The lower blowdown limit should be considered the maximum during this phase
- Chemical residuals must be maintained at normal levels
- If it is being fed separately, the level of sludge conditioners should be increased by 50 to 100%
- As much of the additional blowdown as possible should be accomplished by the manual rather than the continuous blowdown system
2. During the last 24 hours before shutdown, decrease the continuous blowdown and increase the manual blowdown
- Employ frequent short bottom blows (open 5-10 seconds every 1-2 hours) rather than fewer longer blows
- When the load is dropped from the boiler, include header blowdowns, along with mud drum blow
3. Cooling should be accomplished at the rate specified by the boiler manufacturer
- Blow down the water column, gauge glass, and feedwater regulator while there is still pressure in the boiler
- Continue heavy manual blowdowns, and refill with deaerated feedwater
- Maintain all chemical residuals
- Continue to blow down and refill until the furnace is cool enough to enter. At this point, the boiler is cool enough to drain
- If the boiler is drained too soon, it will contain enough heat to bake on remaining sludge
4. Open the boiler as soon as it is drained and immediately wash out any remaining sludge with a high-pressure hose
- Washing at the point will ensure that remaining sludge will be in a fluid form that is easily removed
- Never leave the boiler filled with water unless measures are taken to prevent corrosion