Mechanical deterioration of boiler parts

Mechanical deterioration of boiler parts can result from a number of causes:

a. Fatigue from repeated expansion and contraction and corrosion-fatigue from the combined action of fatigue and corrosion.
b. Abnormal stresses created by rapid changes in temperature and pressure, especially in the case of thick-walled drums.
c. Improper use of cleaning tools.
d. Improper use of tube rollers.
e. Settlement of foundations.
f. Excessive external loading from connected piping, wind, earthquake, and similar sources.
g. Breakage and wear of mechanical parts.
h. Firebox explosion.
i. Vibration due to improper design or support failure.
j. Improper gaskets that allow steam leaks to score the seating surface.
k. Non-weather-tight casing that allows external tube corrosion during extended shutdowns.

If metal is cyclically stressed in operation repeatedly, it can eventually fatigue and may crack under a stress far below its normal breaking load, as discussed in API RP 571. The metal in boiler parts may experience expansion and contraction because of temperature changes involved in taking a boiler out of service, and putting it back into service. Also, expansion and contraction can be caused by the normal temperature fluctuations during operation. Tubes may also become fatigued as a result of alternate wetting by steam and water, which causes fluctuating conditions. If corrosion acts concurrent with fatigue, the fatigue resistance of the metal is reduced because of the corrosive medium, and corrosion fatigue cracks will result. When very rapid temperature changes occur in metal parts (especially thick metal parts), they may be overstressed by the expansion or contraction of the portions of the metal that have changed in temperature against the portions of the metal that have not changed in temperature. A similar situation exists when a cold glass tumbler is only partly filled with hot liquid and shatters.

Improperly employed tube cleaners—allowed to operate too long in one position, for example—may cause damage by cutting grooves inside the tube. Incorrect or excessive cleaning operations especially utilizing acid based products can lead to excessive removal of protective oxide films in boiler tubes and may lead to accelerated corrosion upon re-commissioning. Improper use of tube-rolling tools by under-rolling or over-rolling may cause tube-roll leaks or damage to the tube ends or tube seats.

Breakage and wear of mechanical parts are probably the most common forms of deterioration of the various parts and auxiliaries of boilers. This is especially true for burners and equipment that handle solid fuel and ash. The associated services are very severe, involving high temperatures, almost continuous operation, and extremely abrasive operating conditions when solid fuels are used.

No comments: