In large capacity boilers with high superheat temperature of the order of 540 degree C and above, it is practically not economical to use a single material. The main single reason for this is due to the fact that the steam from boiler drum enters the superheater around 350 deg C and leaves around 540 deg C. As the steam travels from the drum take-off pipes to the roof tubes to the low temperature tubes and high temperature superheater outlet, it picks up temperature at each unit length of tube.
This process changes the metal temperature of the tubes at each unit length of the tube. As the metal temperature of any tube increases there is a drop in the hoop strength of the tube material. This makes the requirement more stringent to the designers by forcing them to increase the thickness required to withstand the internal pressure in the tubes. However, because the thickness of the tubes cannot be increased beyond a certain range, this makes it necessary to upgrade the material of the tube to accept the increase in temperature. Designers use materials starting from carbon steel, to low alloy steel, to high alloy steel, and then to stainless steel to meet the requirements of the increase in metal temperature of the tubes as the steam picks up temperature in superheaters.
The location where such material transition requirement is fixed by the designers based on the margin between the allowable temperature for the material and the calculated metal temperature at the tube length. It is very critical to select this point correctly to avoid any tube failures in the superheaters. The point at which the difference between the allowable metal temperature and the predicted metal temperature can vary due to other reasons like low steam flow through the section of superheater due to high de-superheating water quantity, the upset in the radiation heat in the area, excess area for heat transfer provided, over loading of the unit, etc.
Correcting the transition point in any heat transfer surface like a superheater, reheater, etc. will be possible only with a long shutdown and a large volume of in-situ welding. The lower grade material will have to be upgraded suitably to have an adequate margin between the allowable temperature and the operating metal temperature. Any long term overheating failure near the transition point or at the transition point will need a very detailed review by the designers. The failed sample of the tube will have to be metallurgically evaluated for the reason of failure and the temperature to which the tube had been subjected.
About the Author
Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.
How Long Term Overheating Tube Failures Happen in Boilers - Long term overheating tube failures are due to operating metal temperature of the boiler tubes going above the allowable limit. These types of failure are seen in steam cooled tubes like superheaters and reheaters and in water cooled tubes of waterwalls.
Superheater Temperature Control in Boilers - Steam temperature control is carried out in all boilers to get a constant temperature within a range. Methods like desuperheating by water injection, flue gas flow diverting, and burner tilting are few systems adopted. Main steam and reheat outlet temperature are controlled by these methods in boiler.