Tube failures in boiler tubes happen when the metal temperature crosses the allowable limit. These long-term failures due to overheating can be seen in the steam cooled tubes like superheaters, reheaters, and in water-cooled tubes of waterwalls.
Boilers operated for industrial power generation and steam generation have kilometers long tubes that transport water and steam in the circulation system and superheaters, respectively. These tubes come in different sizes and thicknesses depending upon the mid-wall metal temperature and the pressure it will bear. The boiler tubes are manufactured under various industry standards like ASME, BS, DIN, JIS, etc. To select the perfect tube for your boiler system you need to choose the correct material that can withstand 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 a combination of low alloy tubes to stainless steel tubes selected to withstand the metal temperature.
Mid-Wall Metal Temperature
When heat gets transferred from hot gasses or burning fuel by convection or radiation, the temperature of the outside tube is very high. The heat is then transferred to a medium temperature inside the tube which is either steam or water. There is a temperature gradient that exists from the innermost layer of the tube to the outermost layer of the tube. The temperature in the middle, is known as mid-wall metal temperature. The waterfalls inside the boiler furnace faces the highest amount of temperature, namely the temperature of the rising flame. Since, the water flowing through the tube is so well- designed to carry the heat away and cool down the tube by appropriate circulation, the mid-wall temperature is properly maintained within a stipulated limit of the material. The temperature of the mid-wall can be accurately determined, however there are multiple thumb rules utilized by the operating engineers. In the water wall arena, the metal temperature will be saturated temperature of water in the drum operating in plus 30 degrees centigrade. In the superheater sphere, it is the steam temperature within the tube at the location 50 degrees centigrade, for radiant heat transfer surface and 40 degrees for the convective heat transfer area. Global Seamless is the best boiler tube manufacturer in India.
Overheating of the tubes
Whenever the temperature of the mid-wall area exceeds the allowable temperature of the tube material, overheating begins. This can take place due to multiple reasons like internal deposit and low flow through the tube because of partial choking of the internal diameter of the tube. This can happen due to sudden rise in load, due to sudden fuel input, etc. When the temperature of the metal inside the exceeds the allowable limit, the material strength falls down drastically, completely depending upon the material composition.
Usually overheating of the tube take place over a period of time, as the tubes are subjected to temperatures that are not high enough to cause immediate bulging of the tube to a point of failure. During this period of extended overheating the outer surface of the tube begins bulging, creeping elongate fissures along the axis of the tube. There will be minimum or no wall thickness reduction in the area that is non-blistered. These are the typical identifiable signals of long-term overheating. These failures are also known as high temperature creep failures. Areas in boilers are prone to long term overheating waterwalls and superheaters are majorly prone to long term overheating. However, waterfalls, due to partial choking and internal deposits of the tube, are subjected to long term overheating. Superheaters are usually subjected to long-term overheating over and above the said two reasons due to high heating, higher heat fluxes in the region, and lower grade material at the transition points. Also, reheaters are prone to long term overheating, but not so much like superheaters.
Ways to avoid long term overheating of tubes inside boilers.
In depth understanding Tube Failures under High Pressure Boilers – Failures of boilers tubes and pipes are inevitable. There are twenty-two main reasons for tube failures in a boiler. It is very true that being forewarned is being forearmed. Good operating and maintenance practice helps in reducing tube failures.
System of Steam Soot Blower Erosion – There are several mechanisms that can result in steam soot blower erosion of the boiler tubes at various heat transfer sections. Through knowing the method these mechanisms contribute to erosion will automatically help to prevent loss of availability of boiler.
Reasons leading to High Temperature Corrosion in Boilers – The corrosion due to high temperature in boilers occurs mainly in two spheres- they are waterwalls and superheaters. The inorganic impurities in fuel like Sulphur, sodium, vanadium, etc., are largely responsible.
Operating the boilers within the subjected range of parameters and regime can almost remove this type of failure in boilers.
- By keeping the water quality within the recommended level.
- Flushing the tubes to make sure that any blockages are removed.
- Ensuring that internally the drums are set and maintained properly.
- Ensuring the correct material transition point, higher grade material in the lower temperature area of the tube to some extent can take care of operational variation.
- Ensuring the right quality selection of material during the stage of design.