BOILER SHUTDOWN PROCEDURE
During the normal operation of a boiler, suspended matter is maintained in a fluid state, using 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 extensive 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 a procedure to use when shutting down a boiler.
1. 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 (this will mean an increase in feed of the chemical treatment, as blowdown will increase markedly, and the chemical feed will have to be increased accordingly).
- If it is being fed separately, the level of sludge conditioners should be increased by 50-100%.
- 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 to 10 seconds every 1 to 2 hours) rather than fewer longer blows.
- When the load is dropped from the boiler, include header blowdowns, along with mud drum blows.
3. Cooling should be accomplished at the rate specified by the boiler manufacturer (this is usually anywhere from 3-5 days, depending on the size of the boiler).
- 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. Special attention must be paid to cooling the boiler down and not draining it too soon, if the tubes are hot enough to bake on any remaining sludge, it can be just as hard and difficult to remove as scale, and will inhibit heat transfer in the same way.
4. Open the boiler as soon as it is drained and immediately wash out any remaining sludge with a high-pressure hose (pressure washers work well).
- Washing at this 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.
5. Boiler water chemicals maintain a relatively high pH and alkalinity in the boiler, to prevent conditions such as acid corrosion due to low pH and silica deposition. If a high pH water discharge due to draining the boiler for inspection would be a problem, then the boiler chemical feed (except for the sulfite/oxygen scavenger) can be reduced or even discontinued for up to a week before the boiler is to be drained, which will reduce the pH of the water significantly, usually down to acceptable levels for discharge. Remember, if this is done, please do not discontinue the sulfite/oxygen scavenger, or, if a single boiler treatment is used, substitute sulfite/oxygen scavenger for the single treatment, to prevent oxygen corrosion and pitting during this time.