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BOILER PRESERVATION PROCEDURE
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 practical.
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 water.
DRY STORAGE PRESERVATION
When it is known that a boiler is to be idle for a considerable length of time and that a brief period will be allowed for preparation to return it to service, the dry storage method is recommended. In this method the unit is emptied, thoroughly cleaned internally and externally dried, and then closed up tight to exclude both moisture and air. Trays of lime, silica gel, or other moisture absorbent may be placed in the drums to draw off the moisture in the air trapped by the closing up of the boiler.
The following general procedure is recommended when placing a unit into dry storage.
Step 1
Fire the boiler according to the normal start-up procedure and establish upto 3.5-kg/cm2G-drum pressure. Stop firing. Secure the boiler and when the pressure decays to 1.3 kg/cm2G, immediately drain the boiler and headers under air. As soon as possible, open the drums to allow air to circulate for drying of all internal surfaces.
This step is included for a unit that has been in service and is to be placed into storage. For a unit that has never been in service, start with Step 2.
Step 2
If the unit is full of water and cold, drain the unit under air. All non-drainable boiler tubes should be blown with compressed air. If an external source of heat is available such as a steam coil air heater, portable heaters, etc., operate these heaters to assist in drying the internal boiler surfaces.
Install trays (of non-porous construction and capable of passing through the drum manhole) containing the moisture absorbent (silica gel is preferred) into the drums. Insert the trays into the drum being certain that none of the absorbent comes into contact with the metal surface of the drum. To insure against an overflow of corrosive liquid after the moisture has been absorbed, the trays should not be more than ½ full of dry absorbent. The amount of absorbent can vary but the recommended minimum is one pound of absorbent per 1000 pounds per hour steam flow capacity of the unit.
Step 3
Open the isolation valve for nitrogen connection, on the steam drum, close all other vents and drains and pressurize the boiler to 0.3 to 0.6 kg/cm2G with nitrogen. The amount of nitrogen required will vary according to the volume of the unit.
Step 4
With the boiler pressurized, alternately open all boiler drains to purge air from the unit until pressure decays to zero. It may be necessary to repeat this process several times to reduce the amount of oxygen left in the unit to a minimum.
The unit should now be stored under 0.3 to 0.6-kg/cm2G nitrogen pressure maintained at the steam drum. To maintain the nitrogen pressure, all connections and valves should be blanked or tightly closed. Check gas pressure daily to ensure protection.
We would recommend that periodic inspection of the unit be performed every 3 months to assure that no corrosive action is taking place and to replenish the absorbent as required. Since air will enter the unit during this inspection, it will be necessary to repeat Steps 3 & 4 to expel the air.
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 %.
The above procedure is intended to include the economizer.
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.
NITROGEN BLANKET
Nitrogen can be introduced at the following locations
Through the steam drum
Through the fuel gas line
The nitrogen required to seal the drainable components may be supplied from a permanent nitrogen system or portable tanks located near the vent elevations. Due to differences in plant layout, the owner should choose his own method of piping the nitrogen, either from their permanent system or from portable tanks, to the vent (or drain) locations listed.
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 %.
BOILER LAY UP PROCEDURES
| TYPE OF SHUTDOWN | PROCEDURE |
| SHORT OUTAGES
4 DAYS OR LESS. UNIT NOT DRAINED |
Maintain the same hydrazine and ammonia concentration as present during normal operation.
Establish 0.3 to 0.6 kg/cm2G nitrogen cap on the steam drum |
| TYPE OF SHUTDOWN | PROCEDURE |
| SHORT OUTAGES
4 DAYS OR LESS. UNIT IS DRAINED |
Drain and open only those sections require repair.
Isolate remainder of the unit under 0.3 to 0.6 BarG nitrogen pressure where possible. Maintain the same nitrogen and ammonia concentration for water remaining in the cycle |
| LONG OUTAGES
LONGER THAN 4 DAYS UPTO 15 DAYS. UNIT IS DRAINED |
Fill the boiler with Polish water having 200 ppm of hydrazine and 10 ppm of ammonia to maintain pH 10.
Establish nitrogen cap of 0.3 to 0.6 kg/cm2G over the steam drum. |
| LONG OUTAGES
MORE THAN 15 DAYS – UNIT IS DRAINED.
|
Dry storage of boiler with nitrogen alone is preferred procedure. Nitrogen cap of 0.3 to 0.6 kg/cm2G to be maintained on the steam drum.
Installed silica gel tray in the steam drum to soak moisture if any present in the drum atmosphere. |
