Investigation into the Probable Cause of Failure of Economizer Tube of a Thermal Power Plant
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CASE HISTORY—PEER-REVIEWED
Investigation into the Probable Cause of Failure of Economizer Tube of a Thermal Power Plant Atanu Saha • H. Roy • A. K. Shukla
Submitted: 25 November 2009 / in revised form: 4 March 2010 / Published online: 18 March 2010 Ó ASM International 2010
Abstract In this investigation, we examined the probable cause of failure of a welded joint of a economizer tube of a 210 MW thermal power plant. X-ray radiography and chemical analysis along with detailed optical and scanning electron microscopic (SEM) examination of the economizer tube were carried out. It was concluded that severe blowholes, pipings, and porosity along with brittle fracture were responsible for the failure. Keywords
Weld Economizer tube HAZ Failure
Introduction The fundamental sources of failure include many aspects of design, material selection, material imperfections, fabrication and processing, reworking, assembly, inspection, testing, quality control, service conditions, maintenance, and unanticipated exposure to overload or mechanical or chemical damage in service. Often, more than one source contributes to the occurrence of a given failure. In some cases, failure may lead to serious consequences such as huge financial loss, environmental contamination, and even loss of life. Therefore, it is often essential to investigate the cause of failure in terms of design, quality of material, and fabrication/service procedures. Few available reports [1–4] have shown that the majority of forced outages of power plant boilers are due to premature failure of vital components such as boiler tubes, headers, steam drums, and main steam line and that failure of boiler tubes is the leading A. Saha (&) H. Roy A. K. Shukla Central Mechanical Engineering Research Institute, CSIR, Durgapur 713209, India e-mail: [email protected]; [email protected]
cause of forced shutdown [5]. Failure of boiler tubes may occur due to creep, fatigue, corrosion, erosion, embrittlement, and other degradation processes [6].The economizer may be defined as a heat recovery device designed to transfer heat from the products of combustion to a fluid, usually feed water. Its use is justified only when it can absorb/transfer heat more economically than other types of heating surfaces. Ordinarily, no steam is produced in the economizer. The maximum inlet temperature normally should not exceed 220 °F. In the case under investigation, a welded joint of an economizer tube of lower coil in a coalbased 210 MW power plant failed. One piece of the failed tube was received for conducting the failure analysis. The welded joint was produced by gas tungsten arc welding processes using carbon steel filler wires following the standard welding parameters. The original outside diameter (OD) and thickness of the failed tube were 44.5 and 4.5 mm, respectively. The working temperature of the tube was 372 °C and operating pressure was 180 kg/cm2. The tube is made of SA210 Gr. A1 grade steel, which is basically plain carbon steel. The tube was in service for about 60,000
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