Preservation of Boiler
Preservation of boiler during shutdown
There are two methods of boiler preservation for during boiler shutdown :
1. Wet preservation
2. Dry preservation
1. Wet preservation:
If the boilers are required to be kept out of service, but are likely to be required as standby units, the wet method is a practical one, by which a unit could be made ready for service. In the wet method, the boiler is filled through the economiser with deaerated treated water to give the required alkalinity, and the boiler is fired with low sulphur fuel, and steamed in service so that the boiler water will be circulated to secure uniform concentration throughout the unit, and the oxygen eliminated from the water. The boiler load should then be reduced slowly and the water level raised as high in the gauge glass as its consistent with safe operation, while delivery some steam a concentration of 100 ppm of hydrazine should be maintained for preventing oxygen corrosion. When air cocks reveal no pressure in the boiler, and before a vacuum can be produced, the boiler should be filled in by deaerated water, until water spills over and fills the superheater using a high point air vent, such as pressure gauge connection as an indicator. The drum air cock should be connected to a surge tank located above the steam drum to ensure positive pressure and prevent ingress of air to the boiler, thus compensating for volumetric changes due to temperature.
After firing the boiler, the setting should be closed and maintained in a closed condition. Periodical inspections may be carried out to observe if condensation has occurred on external surfaces, and if this happens, heating devices may be placed at convenient points to keep metal surfaces above dew point. In wet storage, the boiler must be protected from frost. If the ambient temperature is likely to be dropped to freezing point, this type of storage must be avoided.
A protective coating should be applied to auxiliary equipment. All moving parts should be rotated once in a week and also electrical equipment attended to. Oil lubricated bearings should also be attended to as suggested by the manufacturers for the equipment not in use. Generally gearboxes and such applications are drained and cleaned, and filled with medium viscosity oil excluding air.
Boiler connections should be checked for leakages and water samples analyzed. If hydrazine concentration has dropped below 50 ppm, chemicals should be injected to bring the concentration to normal after lowering the level. The boiler should then be steamed sufficiently to circulate the water added with chemical and the procedure followed as stated above. In any case, if the boiler is to be stored for unduly long periods, it should be emptied and inspected for corrosion defects.
2. Dry preservation:
For dry storage the boiler must be cleaned properly and dried thoroughly, since the moisture may cause corrosion of metal surfaces. Precaution must be taken to prevent entry of moisture in any form, from steam, feed and air lines. For this intent moisture assimilating material such as quick lime at the rate of 0.9 kg (2 lb) or silica gel at the rate of 4.5 kg (10 lb) for 4546 litres (1,000 gallons) capacity, should be placed on trays inside the boiler shells or drums to absorb moisture from the air. The manholes should then be closed and all connections on boilers tightly banked. The serviceability of the materials for such purposes and their replacement may be judged through regular internal boiler inspections. Alternatively, dried air may be circulated through the boiler.