External tube failure

From the graph above for carbon steel, it can be seen that there is a rapid drop in strength above 430^oC.
Long term overheating is a condition where the metal temperature exceeds the design limit for a long period. The mechanical strength is reduced as a function of the increase in temperature.
Deposits on the external surface and thin gas film layer aid in reducing the metal temperature. Deposits on the inside increase tube metal temperatures.

Temperature drop across the thin film gas layer

Bulging of many different forms tend to precede bursting.

Thermal oxidation

If the metal temperature exceeds a certain value dependant on the material rapid excessive oxidation can occur This oxide layer can often form with faults, and can be exfoliated due to thermal stressing or vibration. The result is a thinning of the tube due to this cyclic thermal oxidation and spalling A failed tube suffering from this will have the appearance of tree bark.

Creep rupture

Plastic deformation due to metal overheating may occur. Microvoids form eventually leading to failure. Can be distinguished by a thick ragged edged fish mouth with small ruptures and fissures leading off.

Chain graphitization

Uncommon. Damage begins when iron carbide particles (present in plain carbon or low alloy steels) decomposes into graphite nodules after prolonged overheating ( metal temperatures > 427^oC ). If the nodules are evenly distributed then this not cause a problem. However, some tomes the nodules can chain together and failure occurs along the length of the chain ( as in ripping a postage stamp along the perforations) Normally found adjacent to welds and determination as cause of failure requires examination under a microscope to observe nodules.

Short term overheating

Metal temperatures of at least 454^oC and often exceed 730^oC; failure may be very rapid. Not normally associated with a water chemistry problem rather than maloperation or poor design. In very rapid overheating little bulging occurs and the tube diameters are unchanged in way of the fish mouthed failure ( normally thick walled edge)
Under less arduous conditions some bulging occurs and the failure may have a finely chiselled edge
Multiple ruptures are uncommon. care must be taken not to confuse a thick walled short term overheating failure with the many other possibilities such as creep failure, hydrogen embrittlement and tube defects.

Erosion

One of the most common causes of erosion within a boiler is sootblowing erosion . Especially those tubes adjacent to a misdirected blower. Should the blower stream contain water then the erosion is much more severe. Ash picked up by the steam also acts as an abrasive. This is why proper warming through and drainage is essential
Other causes may be failure of an adjacent tube or to a much lesser extent by particles entrained in the combustion products

Internal water chemical causes

For a listing of the failures caused by water chemistry see relevant document 'Corrosion and failures in boiler tubes due to water chemistry'

Oil Ash Corrosion

High temperature liquid phase corrosion phenomenon where metal temperatures are in the range 593'C to 816'C. hence normally restricted to superheater and reheater sections.
It can effect both the tubes and their supports.
May arise after a change of fule with the formation of aggressive slags. Oil Ash corrosion occurs when molten slag containing vanadium compoundsform on the tube wall according to the following sequence

• Vanadium and sodium compounds present in the fuel are oxidised to Vn₂O₅ and Na₂O.
• Na₂O acts as a binding agent for ash particles
• Vn₂O₅ and Na₂O react to form a liquid (eutectic)
• Liquid fluxes the magnetite exposing metal to rapid oxidation

Catalytic oxidation of the metal surface by Vn₂O₅ occurs. The tube outer surfaces are thinned, stress increases in the inner layers and failure by creep rupture occurs
Corrosion of superheater by slag with a fusion temperature of 593 to 704'C requires all utility boilers to have a steam temperature not exceeding 538 to 551'C
Scale formation in the tubes leading to high metal temperatures can lead to this type of corrosion.
Elimiation may require the chemical analysis of both the fuel and the slag to determine the corrosive constituents. The fusion temperature of the ash can be determined. Fuel additives may be used. Magnessium compounds have been used successfully to mitigate problems by forming a complex with Vn₂O₅ and Na₂O with a very high fusion temperature.
Low excess air retard the oxide formation
Chemical cleaning

Water wall fire side corrosion.

may occur where incomplete combustion occurs. Volatile sulphur compounds are released which can form sodium and potassium pyrosulfates
Thesechemically active compounds can flux the magnetite layer. This is more commonly found in coal fired boilers