Failure Investigation on Reheater Tube Due to Deposit and Wall Thinning
- PDF / 377,902 Bytes
- 5 Pages / 595.276 x 790.866 pts Page_size
- 54 Downloads / 212 Views
TECHNICAL ARTICLE—PEER-REVIEWED
Failure Investigation on Reheater Tube Due to Deposit and Wall Thinning J. Purbolaksono Æ F. Tarlochan Æ M. M. Rahman Æ N. F. Nordin Æ B. Ahmad
Submitted: 16 October 2008 / in revised form: 4 April 2009 / Published online: 29 April 2009 Ó ASM International 2009
Abstract Failure analysis of a reheater tube showed that the failure was caused by overheating and wall thinning. Finite element modeling, dimensional measurements, and microscopic examinations were performed to investigate the root cause of the failure. Wall thinning, due to excessive coal-ash corrosion, and overheating, due to scale buildup on the tube were measured, and estimations, the average corrosion and scale growth rates, were made. The combination of excessive scale formation on the inner tube surfaces and wall thinning due to coal-ash corrosion on the outer tube surfaces combined to cause the failure of the reheater tube. Keywords Boiler material Deposits Failure analysis Tube burst Thinning High temperature
Introduction The thermal efficiency of a boiler depends on the heat transfer process across the tube wall. With prolonged exposure at elevated temperature, boiler tubes may develop scales on the internal and/or external surfaces [1] which will retard heat transfer and reduce the thermal
J. Purbolaksono (&) F. Tarlochan M. M. Rahman Department of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, Malaysia e-mail: [email protected] N. F. Nordin B. Ahmad TNB Research Sdn Bhd, No. 1 Lorong Air Hitam, Kajang 43000, Selangor, Malaysia
efficiency. Additionally, the growing scales may cause the tube temperatures to be higher than those originally specified. High temperatures and scale formation may cause degradation of the tube material and eventually will lead to tube rupture. Tube rupture may also be caused by reduction in tube wall thickness as the alloy is consumed by scale formation and corrosion processes. It is estimated that 10% of all power-plant breakdowns are caused by creep fractures of boiler tubes due to the scale formation and 30% of all tube failures in boilers and reformers are caused by creep [2]. As reported in Ref. [3], coal-ash corrosion is caused by the formation of complex alkali sulfates such as K3Fe(SO4)3 and Na3Fe(SO4)3 at the scale/metal interface when the tube metal temperature is in the range of 566– 732 °C. Coal-ash corrosion may be accelerated when the fuel supply or fuel type is changed and the new heat source results in the production of an aggressive ash. Certain coals contain constituents which form ash deposits that are molten at typical operating temperatures and can cause extensive corrosion. The corrosion may lead to a significant concern if the wall thickness is reducing at rate of 8 mils/ year or greater [4]. In this study, dimensional measurements of the failed tube sample were made to obtain data of the wall thinning and scale thickness of the tube. Average rates of the scale formation on the inner surfac
Data Loading...
