Engineering Chemistry

Lesson 12. Prevention methods of boiler corrosions

12.1.1 Intermittent stand by:

In general, the water level maintained in the boiler under these circumstances corresponds closely to that of operation, or reduced firing, the temperature is held closely to that of steaming temperature. Circulation, however, is very slight if at all.

During operation the boiler water is maintained uniformly in an alkaline state, and by its rapid circulation, segregation of any water containing oxygen is prevented. During the standby period, however, some loss of water occurs, slight leakage through the blow down valve, slight leakage which is replaced by feed water. If the feed water is sufficiently oxygen free and of suitable alkalinity (pH value), conditions leading to corrosion will not be developed. However, if the conditions are anything but ideal, delayed intermingling of boiler water and feed water due to lack of circulation or low alkalinity, oxygen rich water may form at the boiler surfaces and initiate corrosion. In the case of considerable length of time and of appreciable make up of feed water to replace losses, the boiler water alkalinity may completely disappear, and general corrosion will result.

No single rule can be given to assure correct conditions in the boiler. The regular boiler water tests must be made as carefully on these boilers as on the operating boilers. This is due to the fact that these boilers while in standby cannot be adjusted as easily as the operating boilers and as such, they can be severely damaged by water problems more easily. When testing multi-drum bent-tube boilers, it is very advantageous to take the samples from the rear drum when the boiler is in intermittent service.

If the alkalinity falls to low, it can be boosted by putting a small amount of alkali solution (preferably a caustic solution) directly into the boiler at the point of feed water entry with a pump or any other convenient manner.

In the cases where quantities of oxygen are dissolved in the feed water, a solution of sodium sulfite (Na₂SO₃) can be fed to the boiler by means of a pump either separately or in conjunction with the alkali solution. The minimum amount of residual sodium sulfate that should be maintained in the boiler is 100 ppm as a residual. The proper control of sodium sulfate as a residual in this type of storage should maintain the sodium sulfite from 100 ppm to 200 ppm. Remember that sodium sulfate is soluble in the boiler well above 1000 ppm, so over feed of sodium sulfate will not create a problem with deposition. Hence, it is better to have 150 ppm or 200 ppm of sodium sulfite residual than it is to try and stay close to the minimum level of 100 ppm.

If oxygen pitting and localized corrosion in separate sections of the boiler, then this is a display of typical corrosion due to feed water segregation. Should this occur, then the segregation of the feed water must be stopped. This can be done by intermingling feed water (either by boiler circulation, a circulating pump, or through an injection method). Additionally, the requirement for makeup water can be brought under control by stoppage of system leaks. Finally, if several boilers are subjected to intermittent use, they should be alternated in turn in operating service. This will stop the irregularities that may come about to one boiler that experiences extended standby conditions. This type of rotation prevents any one boiler from suffering from potential standby problems. On the other hand, do not alternate the boilers on a basis that is more frequent than every month, or you will experience problems from the heating and cooling of the boiler, which will result in tube leaks due to frequent expansion and contraction of the boiler tubes. The rules for treating a standby boiler are in large, the same as those for an operating boiler. The application of chemical treatment to a standby boiler must be as fully and more carefully performed.

 

12.1.2 Prolonged stand by

Two general procedures are available

1. The boiler may be emptied and dried out, and kept dry
2. The boiler may be filled completely with water

In this case the drums or the boiler body, whether a water tube or a fire tube boiler is being treated, are filled to the steam take off if the boiler is a water tube boiler or to the dry box if the boiler is a fire tube boiler.

 

12.2 DRAINING AND DRYING

This method will allow excellent protection from corrosion to the metal surfaces so long as there is no moisture present in the boiler. One method is to let the boiler open for free circulation of air after drying. Another method is to place a desiccant as silica gel in the boiler and the boiler is then closed up for drying. In either case, water leakage over, or sweating of the boiler metal surfaces must be protected against. Since this type of moisture is saturated with oxygen, its contact with metal surfaces will cause accelerated rates of corrosion. So long as the boiler metal surfaces remain free of moisture, no appreciable corrosion will occur.

Where a boiler has a super heater section, similar moisture free conditions will protect the drainable type of super heater. On the other hand, the lower bends of a non-drainable super heater may tend to collect moisture and as such, this condition will promote rapid attack on the metal surfaces. In order to protect the non-drainable type of super heater, you must blow, dry warm air trough each individual tube or element. To insure complete drying and elimination of any future condensation, a series of small heaters should be installed in the furnace at the lower extremities of the super heater section, equally spaced across its width. Maintenance of such a heating arrangement will insure internal and external dryness.

If the boiler's past history indicates that external sweating of the boiler tubes is a problem, then additional heater should be installed in the furnace at strategic locations in order to insure that the boiler tubes are maintained above the dew point. This concept must also include the super heater section if the boiler has a super heater. If you allow dew to form, then the purpose of layup is defeated, as the boiler tube metal will rapidly corrode both internally as well as externally. For extended dry standby, blanketing the super heater tubes with nitrogen should be considered. This method will dry out the metal in the tube and if maintained properly, it will help to maintain a moisture free atmosphere on the internal areas of the super heater tubes

 

12.3 BOILER FILLED WITH WATER

In this method, protection can be obtained for the boiler metal, if

1. Correct chemical conditions are maintained in the boiler water.
2. The boiler water is mixed adequately to maintain uniform Conditions throughout the boiler.
3. The boilers are completely filled with treated water as not to allow any boiler metal surfaces to come in contact with the air.

Sufficient caustic soda (or equivalent alkalinity builder) should be added to the boiler water in order to produce a hydroxyl (-OH) alkalinity  of 350 to 600 ppm. In addition, sufficient sodium sulfite must be added to the boiler water to establish a sodium sulfite residual of 100 ppm. The mixing of the water in the boiler must be thorough so that correct chemical conditions can exist in every section of the boiler. Mixing of the boiler water can be accomplished by circulating water from one section of the boiler to another with the use of a pump, as in prolonged standby, the boiler is not fired off, and as such, cannot mix the boiler water by means of circulation through heating up the boiler. If the pump method is practical, additional chemicals can be trickled into the boiler as they are needed during the circulation procedure.

Some boiler water sludge will form during standby conditions and this must be treated for. The cleaner the boiler surface metal, the lower the potential for corrosion. Either a Polymeric sulfonated copolymer, carboxylated copolymer, polyacrylate or a combination of these dispersing agents should be fed at a rate that would be considered the mid range for the sludge dispersant during normal operating conditions. The mixing of these dispersing agents into the boiler water in order to insure a clean boiler is as important as the required use of the previously discussed treatment chemicals.

If the circulating pump method is not practical, then if mixing is required, the boiler will have to be accomplished through light steaming. In this case, the proper levels of treatment chemicals will have to be built up during the week prior to the boiler going off the line. If this is the method

to be used, then increase the chemical treatment levels to; 500 - 800 ppm, -OH alkalinity upto 200 ppm.  Residual sodium sulfite and maximum mange meed for the sludge dispersant. This will supply the boiler water with sufficient treatment chemicals to maintain itself within the desired limits as the boiler sits and some boiler water is slowly lost.

Deaerated water should be used to fill the boilers that are going to be in standby service, whenever it is available.

A convenient method for keeping boiler full of water is to connect a small expansion tank to some connection at the top of the boiler. This tank is to be located above the boiler and is to be kept filled with chemically treated water. If the tank remains full, then the boiler will remain full. If the tank over flows, then the boiler is taking on water from some form of system leakage. It is convenient for maintaining the proper level of chemical treatment protection as well as acting as a level control indicator.