วันเสาร์ที่ 13 กันยายน พ.ศ. 2557

Preservation treatments for Boilers chemical during Stoppage

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The preservation treatments of boilers are classified into the wet preservation and the dry preservation. An appropriate preservation method should be chosen considering the term of preservation, the structure of boiler, air temperature and so on.

(1) Boiler chemical Preservation during short term


When a boiler is stopped and preserved in a short period, the same chemicals using for the boiler operation are applied for the preservation. When the boiler pressure becomes approximately 1 kgf/cm2 after the stoppage, water and chemicals are injected into the boiler to fill the boiler with water and to adjust the water quality to an appropriate one for the preservation.


When the boiler operation is started again, the water level is reduced to the normal level of the operation by blow down and the operation starts up.


(2) Boiler Chemcial Preservation during long term


(a) Wet preservation


The wet preservation of boilers have to be filled with water, is widely applied. This method is suitable for boilers which the perfect blow down of the holding water is impossible or an urgent restart-up is expected. However, in the case that the freezing of boiler water is anticipated, the dry preservation has to be applied.

Hydrazine or nitrite based chemicals are generally applied for the wet preservation. For boilers installed in hospitals, food factories and so on, non-toxic chemicals, such as organic acids, are applied preservation.

The chemicals for the preservation are continuously injected into the feed water so as to be kept the specified concentration in the boiler. After the chemical dosing, it is desirable to mix the chemical sufficiently by the water convection caused by the residual heat of boiler water or by the water circulation using a temporary circulation pump.

For the monitoring of the preservation condition, the pH, total iron and the chemical concentration are analyzed once or twice a month to keep the water quality in the control range. When the water quality is out of the range, the chemicals should be dosed to keep the water quality within the specified range.

In case that the mixing of chemicals with the boiler water is difficult or the total iron of boiler water tends to increase, all the water is discharged and the boiler should be filled with new feed water including the specified concentration of the chemical.

Before the start-up of the boiler operation, all the water is discharged and the inside of boiler should be inspected.

(b) Dry preservation


Dry preservation is suitable for the boilers that the freezing of boiler water is anticipated or have to be preserved during long term. Two kinds of dry preservation using desiccants and sealing with nitrogen gas are usually used.


For the application of dry preservation, all the boiler water is blew down when the water temperature drops around 90 to 95°C, and the boiler inside is dried with the residual heat.


The desiccant should be inspected periodically (once or twice a month) and the desiccant losing the moisture absorbing capacity should be replaced with a new one at once.


In the case of preservation sealing with nitrogen gas, the gas is introduced into the boiler to replace the water with the gas before the pressure in the boiler reaches the atmospheric pressure.


The pressure of the nitrogen gas in the boiler should be maintained at about 0.5 kdf/ cm2 and the gas should be injected if the pressure reduces around 0.3 kgf/ cm2.




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