Boilers used for industrial steam generation and power generation have kilometers of tubes that carry water and steam in circulation system and superheaters, respectively. These tubes are of various sizes and thicknesses depending upon the pressure and the mid-wall metal temperature. The tubes selected are boiler quality tubes manufactured under various standards like ASME, BS, DIN, JIS, etc. While selecting the tube there is a requirement to select the correct material for withstanding the metal temperature. This will depend upon the location where the heat transfer surface is located. Normally the water cooled areas like economizer and waterwalls are made of carbon steel of boiler quality. Superheaters and reheaters will have combination of low alloy tubes to stainless steel tubes selected to withstand the metal temperature.
Mid-wall metal temperature
When heat is transferred from the burning fuel or hot gases by radiation or convection, the outside tube temperature is very high; the heat is then transferred to the medium inside the tube which is water or steam. There is a temperature gradient that exits from the outermost layer of the tube to the inner most layer of the tube. The temperature at the middle of the tube thickness is called the mid-wall metal temperature. The waterwalls in the boiler furnace face the highest temperature, namely the flame temperature, and since the water flowing though the tube is so designed to carry the heat away and the cool the tube by proper circulation, the mid-wall temperature is maintained within allowable limit of the material. The mid-wall metal temperature can be accurately determined, however there are many rules of thumb used by the operating engineers. In the water wall area the metal temperature will be saturation temperature of water at the drum operating pressure plus 30 degree centigrade. In the superheater area it is steam temperature inside the tube at the location plus 50 degree centigrade for radiant heat transfer surface and 40 degree for convective heat transfer area.
Overheating of tubes in boiler
When the mid-wall metal temperature exceeds the allowable metal temperature of the tube material, overheating sets in. This can happen due to many reasons like internal deposit, low flow though the tube due to partial choking of the tube internal diameter, due to sudden load raise, due to sudden fuel input, etc. When the metal temperature of the tube exceeds the allowable limit the material strength reduces drastically, depending upon the material composition.
Long term overheating of the tube happens over a period of time, as the tubes are subjected to temperatures which are not so high to cause instantaneous bulging of the tube to a failure point. During this period of long term overheating the tube outer surface develops bulging, creeping elongate fissures along the axis of the tube. There will be little or no wall thickness reduction in the non-blistered area. These are the typical identifiable signs of long term overheating. These failures are also called as high temperature creep failures. (Please see the photos below.) Areas in boilers prone for long term overheating Waterwalls and superheaters are more prone to long term overheating. Waterwalls normally, due to internal deposits and partial choking of the tube internally, are subjected to long term overheating. Superheaters are subjected to long term overheating over and above the said two reasons due to high desuperheating, higher radiant heat fluxes in the region, and lower grade material at transition points. Reheaters are also prone for long term overheating, but not so much like superheaters. How to avoid long term overheating failure in boilers
- Operating the boiler within the specified range of parameters and regime can almost eliminate this type of failure in boilers
- Flushing the tubes to ensure any blockages are removed
- Keeping the water quality at the recommended level
- Making sure drum internally are set and maintained properly
- Ensuring the right material transition point, higher grade material in the lower temperature portion of the tube for some length can take care of operational variation.
- Ensuring the right quality material selection during design stage.