Boiler Corrision

            

COMMON DAMAGES THAT CAN ARISE IN THE OPERATION OF BOILER

CORROSION:

There are two principal forms of corrosion. One is direct chemical attack and mainly occurs in super heaters due to the high metal temperatures involved  result in pitting or cracking in tube bores, or in scaling or flaking on the gas side of tubes. This form of corrosion also occurs when loss of water circulation causes the metal to overheat in the presence of steam.

The more common form of corrosion found in boilers is the result of electro chemical attack usually involving acidic water conditions in the presence of dissolved oxygen. General wastage of the boiler metal due to this form of attack has eliminated by the use of chemical feed water treatment, but isolated pitting can occur if the treatment is not operated within the correct limits. It may be found along the water level in the drum, generally as the result of poor shutdown and storage procedures where the boiler is left partly filled with cold water. Pitting along the roof of the drum can result from condensation. In the lower parts of the boiler it can be due to poor drainage, pools of water remaining in drums and headers. However isolated pitting can also result from operating with the boiler water allowed to remain at low a pH value and in this case will also tend to occur in the bores of tubes subjected to the highest rates of heat exchange, such as screen and water wall tubes.

ERROSION: This is a mechanical wearing away of the boiler metal due to water, steam or gas flowing over the metal surface, Thus the tubes can wear thin in the region of bends, due water impingement, or wear externally as they stand in the flow of hot abrasive gases leaving the furnace. Serious local erosion can result from the direct impingement on to the tube surface of a steam jet from a badly aligned soot blower.

OVER HEATING: In-service boiler metal subjected to the heat of combustion must be continuously cooled by water or steam. If for any reason this cooling affect is lost, or greatly reduced, the boiler metal over heats loses strength and distorts. This can result expanded tubes pulling out of tube plates, local bulging of tube surfaces with eventual rupture, the sagging of super heater tubes between their supports, etc.

Loss of water brings about the most immediate and serious damage, but loss of re circulation through tubes will also quickly result in damage. A build-up of deposits on the waterside acts as an insulating layer, reducing the rate of heat transfer through the metal so causing it to overheat and leading to eventual distortion. Oil entering the boiler only forms a thin, but efficient, insulating layer upon heated surfaces, but also encourages a further build-up of scale deposits.

Super heaters at their operational metal temperatures are very vulnerable to over heating and a circulation 0f steam through them must be provided at all times when the boiler is steaming. Priming and carry over must be avoided, as impurities passing over with the water will result in a build up of deposits in the super heater tubes, again causing eventual overheating.

Direct flame impingement onto water walls will lead to overheating and distortion. Fire in uptakes or super heaters due to a build up of deposits in the gas passages can also result in serious overheating and damage even to pressure parts of the boiler.

CRACKING: Welded boilers are especially vulnerable lo fatigue cracking resulting from bad design poor workmanship or both. Cracks of this nature, even if starting in a minor weld can continue to propagate even into the main shell plate.

Fatigue cracking is also associated thermal cycle stressing, which can result from poor steam raising procedures lack of expansion or even from a continual carry over of water droplets causing cracking inside super heater headers. Over expansion of tubes into tube plates can lead to cracking of the bell mouthing and in some cases to cracks forming in the tube plate between the holes.

Cracking due to caustic embrittlement take place in boilers of riveted construction due to slight leakage allowed to continue over a period of time in a riveted seam.

MECHANICAL DAMAGE: This can result from poor workmanship, such as damage to tube plates by over-expanding during tube attachment, scoring of joint faces, distortion of doors by over tightening.

Another source of damage are explosions in the furnace due to bad flashing-up procedures. These can be especially serious in roof-fired radiant heat boilers with gas tight water wall panels; the force of the explosion acting directly upon these can cause them to suffer severe distortion. It is normal practice to warm this type of furnace with heated air.