Figure 1-36. - Acid cleaning by fill-and-soak method.

solution. In multiple-drum boilers, divide the charge and put equal amounts in each of the lower drums.

2. Fill the boiler with hot feedwater to the level of the bottom of the steam drum. Turn the steam into the boiler through the usual boiling out connections, or bottom blow, and allow the boiler to fill gradually to the top of the gauge glass.

3. Steam pressure in the boiler should be kept between 5 and 10 pounds. The boiling out should continue for 48 hours. Immediately after boiling out, give a series of bottom blows to remove the bulk of the sludge. The boiler should be cooled, washed out immediately, and given the usual mechanical cleaning.

You may not always want to use the above method for boiling out. The steps for a second satisfactory method for boiling out are as follows:

1. Clean out all loose scale and any scale adhering to the boiler that can be removed manually.

2. Place about 15 pounds of caustic soda or soda ash and 10 pounds of metaphosphate for each 100-boiler horsepower (hp) of the boiler.

3. Seal the boiler openings but OPEN ALL VENTS. Fill the boiler about three-quarters full with water.

4. Start the burner and raise the temperature of the water in the boiler to about 200F. Maintain this temperature for about 24 to 48 hours. Add makeup water as required during this period to fill the boiler to the base of the safety valve.

5. Analyze the boiler water during the boiling out period and add enough caustic soda and metaphosphate to maintain the following concentrations:

Causticity as ppm OH 300 to 500

Phosphate as ppm PO₃ 100 to 150

6. Open the boiler at the end of the boiling-out period and clean out the sludge and loose scale. Pay particular attention to removing scale and sludge from water legs in fire-tube boilers.

7. Flush the boiler thoroughly.

8. If a lot of corrosion is exposed when the scale is removed, notify your superior so a boiler inspection can be made.

When the boiler is operated, any residual scale may cause faulty operation. The boiler should be taken out of service at frequent intervals to remove sludge formed from disintegrated scale. As soon as personnel can work in the boilers, wire brush the drums and ends of all tubes.