BOILER PRESERVATION DURING SHUT DOWN OF BOILER
In this article we discussed about the procedure of boiler preservation . How to do boiler preservation during shut down of boiler ? Need of boiler preservation ? and What the chemicals are used in boiler preservation ? boiler is a very essentially part of any in industries for the purpose of steam generation so its care should be considered during off time , this time may be short or long according to the time period we preserve the boiler for long time or short time .
boiler preservation mainly two type first is wet preservation and second one is dry preservation . according the boiler situation we are use done of them for boiler preservation. lets discussed both of method in detailed way :-
Boiler preservation :-
Boiler preservation is process when putting the boiler ideal/stopped condition for non operational condition for short/long period . Boiler preservation is necessary beacuse if we do not preserve any boiler during short term or long term shutdown/ stoppage, then chances of water side corrosion which lead to scaling & finally tube failure may be occurred.
There are two method of boiler preservation for during boiler shutdown :-
1.Wet preservation :-
The wet preservation is used when boiler is to be placed on stand by for short period of time or when the unit may be required on short notice.
2. Dry preservation :-
The dry preservation is used when a unit is out of services for extended periods
1. Wet preservation :–
If the boiler 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 wet method ,the boiler is filled through the Economiser with Deaerator 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 maintain 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 Deaerator water ,until water spills over and fills the superheater using a high point air vent, such as a 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 compensation for volumetric changes due to temperature.
After firing the boiler, the setting should be closed and maintained in close condition. Periodically inspection may be carried out to observe if condensation has occurred on external surfaces , and if this happen ,heating devices may be placed at convenient points to keep metal surface 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 rotating once in a week and also electrical equipment attendant to. Oil lubrication bearings should also be attended to as suggested by the manufacturer for the equipment not in use. Generally gearbox and such application are drained and cleaned , and filled with medium viscosity oil excluding air.
Boiler connection should be checked for leakage and water samples analysis. 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 preservation of boiler must be cleaned properly and dried thoroughly, since the moisture may causes corrosion of metal surface. Precautions 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 or silica gel at the rate of 4.5 kg for 4546 liters capacity , should be placed on trays inside the boiler shell or drums to absorbed 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 judge through regular internal boiler inspection. Alternatively , dried air may be circulated through the boiler.
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