Failure Analysis of an Aqua Tubular Boiler Tube
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of an Aqua Tubular Boiler Tube Cassio Barbosa • Simone Kessler de Barros • Ibrahim de Cerqueira Abud • Joneo Lopes do Nascimento Sheyla Santana de Carvalho
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Submitted: 20 October 2011 / in revised form: 17 May 2012 / Published online: 22 August 2012 ASM International 2012
Abstract In general, boilers are subjected to aggressive environmental conditions and, for this reason, there is a real necessity of controlling some parameters such as quality of input water, amount and types of addition agents used in the water, and operational conditions of the equipment (pressure and temperature). The lack of control of these and other parameters can cause loss of performance of the boiler by higher amount of fuel consumption, loss of efficiency, or even rupture of components, causing severe accidents. This study has as purpose the failure analysis in a pipe of the lateral wall of a boiler, known as aqua tubular, of an aircraft carrier flat top boat during a test in sea. The material of the pipes, according to preliminary information, is ASTM 192 steel, which is adequate for this kind of application. Through microstructural analysis in important regions for the study of this failure and earlier information about equipment operation, the cause of rupture of the pipe, which was changed only 50 h before the accident, was analyzed. It was observed that incrustations, corrosion pits in the walls of the pipe, and high temperatures together with microstructural changes were the probable causes of the pipe rupture. It can also be concluded that the analyzed failure had no relation with the quality of the material or the fabrication process of the component. Keywords Failure analysis Boiler Steels Microstructure
C. Barbosa (&) S. K. de Barros I. de Cerqueira Abud J. L. do Nascimento S. S. de Carvalho Instituto Nacional de Tecnologia (INT), Avenida Venezuela, 82, sala 620, Rio de Janeiro CEP 20.081-312, Brazil e-mail: [email protected]
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Introduction Aqua Tubular Boilers Aqua tubular boilers were projected to provide elements to follow the evolution of industrial processes and their demands, such as better performance, lower consumption, rapid generation, and larger amounts of vapor. This kind of boiler is characterized by the circulation of water inside tubes, which are placed outside the large tubes of the boiler, forming with them a tubular structure. All types of gas which come from the fuel that is burnt in the furnace circulate in the outer parts of the tubes [1] and operate at medium and high pressure, resulting in high production of vapor. The aqua tubular boilers are the most common concerning thermoelectrical plants or generation of electric energy in general, except for small scale units. The work pressure of such boilers can reach up to 26 MPa, i.e., higher than the critical point [2]. Accidents in Boilers When feeding of vapor generators is carried out with nontreated water, i.e., water which contains soluble salts, it can cause two main types
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