14-06-2014, 10:46 AM
BOILER PRESERVATION PROCEDURE
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INTRODUCTION
Both the gas and waterside of a boiler should be protected against corrosion during out of service periods. It is known that many of the corrosion problems of boiler and auxiliary equipment have their inception during storage. Rusting of tube surfaces, as indicated by the formation of the red hematite (Fe2O3), not only cause a roughened tube surface but also results in attack of parent metal.
The advantages of efficient feedwater and boiler water treatment during operation may be lost if the same diligence is not applied to protect heat Transfer surfaces during idle periods. Protection from corrosion during storage becomes vitally important considering the number of times during the life of a boiler when it and its auxiliary equipment are idle
To minimize the possibility of corrosion, boiler to be placed into storage must be carefully prepared for the idle period and closely watched during the outage. There are two methods available for storing the unit dry storage and wet storage. Although the wet storage procedures is preferred such factors as availability of good quality water, ambient weather conditions, length of storage period, auxiliary supply of heat, etc may dictate that the dry storage procedure is more practica
DEFINITIONS OF WATER QUALITY
Some cleaning procedures, hydrostatic testing and storage require water of higher quality than others. For the purpose of economy and convenience the lowest water quality consistent with requirements is specified in these various procedures. The terms that identify the different water qualities along with their definitions are list below:
Station service water - Water normally used for drinking, fire protection, etc.
Softened water - Filtered, sodium zeolite softened water with total hardness less than 1 ppm.
Two- bed demineralised water - Water then has been passed through cation and anion ion exchanges in series.
Mixed bed demineralised water - Water that has been passed through a mixed bed demineraliser. Water from an evaporator is considered to be of equal quality.
Treated demineralised water - Mixed bed demineralised water that has 200 ppm of hydrazine and enough ammonia added to give final concentration of 10 ppm (or a pH of 10.0). In this procedure, condensate is considered to be treated demineralised wate
WET STORAGE
The advantage of employing the wet storage procedure is that the unit is stored completely wet with the recommended levels of chemicals to eliminate a wet-dry interface where possible corrosion can occur. It is suggested that volatile chemicals be used to avoid increasing the level of dissolved solids in the water to be used for storage.
In preparing a unit for wet storage, the following procedure is recommended.
The unit should be filled with deaerated, Demineralised water treated with 200 ppm hydrazine (N2H4) for oxygen removal and sufficient ammonia (NH3) in order to attain a pH of 10 (for demineralised water, this will require approximately 10 ppm ammonia).
We strongly recommend pre-mixing of the chemicals with the water to insure a uniform mixture entering the boiler. This can be accomplished by the blend-fill method. The blend-fill method consists of blending the chemicals with the demineralised water at a continuous rate such that a uniform mixture is entering the boiler. Simply introducing the chemicals through the drum after establishing water level will not insure adequate dispersion of chemicals to all internal surfaces, unless sufficient heat is delivered to the furnace (i.e. firing the boiler) to induce natural circulation throughout the boiler.
Fill the unit with the treated demineralised water to the normal centerline of the steam drum. Stop filling further.
Back-fill the with treated Demineralised water until a rise in steam drum level is noted. Continue filling until water exits from the steam drum vents. After filling, all connections should be blanked or tightly closed.
A source of low-pressure nitrogen should be connected at the steam drum to maintain 0.3 to 0.6 Bar G to prevent air from entering the unit during the storage period.
CAUTION
The unit should be properly tagged and the appropriate warning signs attached noting that the boiler is stored under nitrogen pressure and that complete exhaustion of the nitrogen must occur before anyone enters the drum. Before entering drums test to prove that the oxygen concentration is at least 19.5%.
If storage continues into winter, ambient temperatures below the freezing point of water create a real hazard to the boiler pressure parts and it will be necessary to provide a means of keeping the unit warm to avoid damage.
At some later date when the unit is to be placed into service, the boiler can be drained to normal start-up water level and placed into operation.
In some cases, an expansion tank or surge tank (such as a 55-gallon drum) above the steam drum elevation may be required to accommodate volume changes due to temperature changes. This tank is equipped with a tight cover and sight glass and contains properly treated water. The tank should be connected to an available opening, such as a vent line at the top of the steam drum in order to create a hydrostatic head. This tank will provide a ready, visual check of water level or in leakage during lay up.
A source of low-pressure nitrogen should be connected to the surge tank to maintain 0.3 to 0.6 Bar G to prevent air from entering the unit during the storage period.
The treated demineralised water should be analyzed weekly, and when necessary, sufficient chemicals should be added through the chemical feed line, to establish the proper levels recommended. Samples of the treated water can be taken at the continuous blowdown line or any suitable drain connection.
No unit should be stored wet when there is any possibility of a temperature drop to the freezing point unless sufficient heat can be provided to the unit to eliminate the danger of water freezing and subsequent damage to pressure parts