Boiler Shutdown Procedure

When a boiler is taken out of service, take special precautions to prevent mechanical damage. These precautions will also help minimize water-side deposition as a result of suspended solids baking on to internal surfaces during the cooling process. To avoid mechanical damage as a result of thermal stresses, follow the manufacturer’s instructions for the rate of boiler cooling as well as any other recommended procedures.

In operation, boiler water contains solids suspended by water circulation and the action of dispersant treatment chemicals. Unless suitable procedures are observed during the entire shutdown process, these suspended solids will settle on boiler surfaces to form an adherent deposit that may require special removal methods. In addition, unless such deposition is examined carefully, it may be incorrectly assumed to be scale that was formed during operation. Proper care is imperative during shutdown, both for correct evaluation of treatment programs and to eliminate unnecessary boiler cleaning.

Water-side Precautions

Manual blowdown should be increased for a period of 5 to 7 days before shutdown. During this period, observe the lower conductivity or chloride limit as a maximum. Increase the feed of internal treatment to maintain treatment concentrations/residuals at or near the maximum recommended levels.

Wash-down

Difficulties may arise if the boiler is not drained shortly after it is cooled. If draining is delayed for even a short period of time, sludge accumulations may become difficult to remove. If draining is delayed for an extended period of time, corrosion and pitting can also occur if proper storage procedures are not initiated.

Immediately after draining the boiler, open the manholes and conduct a quick, preliminary inspection. Note any unusual conditions. As soon as possible after the inspection, wash the waterside surfaces with a high-pressure hose. This will remove the sludge that accumulated as the boiler was drained while it is still in a fluid form.

For a watertube boiler, the washing procedure should start in the steam drum, continue with the individual tubes, and proceed on to the mud drum(s) and bottom headers.

For a firetube boiler, first wash from the top manhole, followed by washing the side and bottom sections through the hand-holes. Check all bottom blowdown lines to be sure they are free of deposit before the washing procedure is completed.

NOTE: Any delay in washing the boiler after it is drained may result in soft sludge baking into adherent, difficult-to-remove deposits as a result of residual furnace heat.

Shut-down

It is strongly recommended that you adhere to the boiler manufacturers shutdown procedure. However, when a shutdown procedure has not been provided by the boiler manufacturer, consider the following procedure:

1. Reduce load gradually. Be sure the rate of cooling does not exceed the boiler manufacturer’s recommendations. The rate of cooling suggested by a major boiler manufacturer is not more than 150°F (66°C) per hour.
2. Cut fuel off gradually.
3. Maintain normal water level until the boiler is ready to be drained.
4. When the steaming rate has reached 20% of normal load, go to full manual operation of fuel and feedwater controls.
5. Before cutting off the last burner, open drain valves at steam, non-return, and superheater outlet header valves. Be sure the bypass around the non-return valve is closed. Operate drain valves as necessary to maintain prescribed rate of cooling.
6. Operate draft fans until all fuel has been purged.
7. Shut down fans.
8. Close all dampers.
9. When boiler pressure starts to drop, close stream-line stop and non-return valves.
10. When the non-return valve is closed, open valve in economizer recirculation connection (if present).
11. When drum pressure is less than 25 psig (1.75 kg/cm², open drum vent valves.
12. Empty the boiler only after boiler water temperature is below 200°F (93°C).
13. Follow manufacturer’s/insurance carrier’s instructions regarding fireside inspection and cleaning.