Failure Investigation of a High-Temperature Cast Soot Blower Lance Tube Nozzle
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Investigation of a High-Temperature Cast Soot Blower Lance Tube Nozzle M. C. Fatah . D. T. Putra . B. A. Kurniawan
Submitted: 14 March 2020 / Accepted: 17 May 2020 ASM International 2020
Abstract Failure in the form of stress corrosion cracking has occurred on the SS304 lance tube soot blower. The hardness test showed high value for the area close to the failed region. The SEM test showed formation of heterogonous and porosity film on the surface. XRD indicated that the oxides determined were Fe2O3 and Fe3O4. Metallographic of the lance tube revealed pitting as the stress concentrator. Pitting is caused by chloride (as chlorine) and SO2 from the flue gas. SO2 reacts with water to form acidic sodium bisulfate (NaHSO4). Stress is generated from exposure of lance tube parts to high and low temperature during its service. Keywords Stress corrosion cracking Hardness Pitting Sodium bisulfate
Introduction Soot blowers were developed to perform in situ cleaning of heat recovery area of superheater, reheater, economizer and air heater from accumulation of soot over time. Proper cleaning is important to maintain heat transfer efficiency and lifetime of the boiler [1]. If soot blowers in the boiler are out of service for an extended period, massive deposits build up and lead to an unscheduled shutdown of the boiler.
M. C. Fatah (&) D. T. Putra Mechanical Engineering Department, Institut Teknologi PLN (IT PLN), Jakarta, Indonesia e-mail: [email protected] B. A. Kurniawan Materials and Metallurgical Engineering Department, Institut Teknologi Sepuluh November (ITS), Surabaya, Indonesia
In the case of retractable soot blower, a lance tube is inserted into the operating boiler and the nozzle sprays a jet of steam through a venturi to the ash-covered boiler tubes. A constant flow of steam through the nozzle is necessary to keep the component cooled below its maximum specified operating temperature of 515 C. Once the cleaning is completed, the lance tube nozzle is retracted to a cooler location of the boiler prior to stopping the flow of steam.
Materials and Methods The failed lance tube was analyzed visually for the type of failure, and a photograph of the same was recorded. The hardness of the failed tube at the location closer to and away from the failed region was determined using a portable hardness tester (General Electric, MIC 10). A small piece of sample was cut from the failed region and un-failed region, polished using standard metallographic techniques, etched using Kalling reagent and analyzed for its microstructural features, using an optical microscope (Olympus, BX41M—LED). A scanning electron microscope (Inspect, F50) attached with energy x-ray analysis (EDAX) and XRD (Shimadzu, type 6000) were used to analyze the morphology and chemical composition of the deposit, which would help in ascertaining the nature of failure.
Results The operational parameters of the failed lance tube are given in Table 1. Water and flue gas analysis results are shown in Tables 2
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