Tech Talk

Steam Raising Plant Failures

Short Term Overheating

When metals operate at elevated temperatures the mechanical properties are significantly reduced. Therefore careful consideration must be given to the particular alloy selected for applications throughout steam raising plant.

When operating at elevated temperatures and stresses, much less than the high-temperature yield stress, metals undergo permanent plastic deformation called creep. Short term overheating is a type of creep that occurs when metal temperatures are well in excess of those typically encountered during stable operation. When the tube metal temperature rises so that the stress from the internal steam pressure equals the tensile strength at elevated temperature, rupture rapidly occurs. Short term overheating failures are characterised by a wide ‘fish mouth’ rupture of the tube in question.

Short term overheating failures can occur in tubes throughout the boiler. The following can be causes of this mechanism of failure:

1. Flame impingement from misaligned or worn burners that leads to the formation of a steam blanket, as the local heat flux is too great for the fluid flow through the tube.

2. Blockage of a superheater tube with condensate or foreign material that prevents steam flow. These problems are more frequent during start-up.

3. Reduced flow in either a water or steam circuit that leads to inadequate cooling.

4. Foreign objects, broken attemperation- spray nozzles, for example, in headers that partially block a superheater or reheater tube.

Green’s provides systematic boiler inspections, asset integrity assessment, failure analysis, chemical cleaning, repairs, spares and a variety of other services that can be tailored to your needs. Please contact Mike Cotton at MikeCotton@greenspower.co.uk for further advice.