Boiler Tube Failure in Thermal Power Plant-Effects, Remedies by Mechwix Technology.

A Case Study on Boiler Tube Failure-Mechwix Technology.

Mechwix

Being a thermal power plant operation and maintenance engineer, I am interested to share my practical work experience. I joined in West Bengal power development corporation in the year 2008. Since then I am working in Sagardighi thermal power plant. This project has started with 2X300MW capacity Dongfeng Electric Corporation (DEC) supplied sub-critical boiler. Later this project is expanded to 2X500MW Bharat Heavy Engineering Limited (BHEL) make sub-critical boiler and later 1X660MW super-critical boiler is under construction. I joined this organisation as a Mechanical operation engineer. At this position I was operating Boiler as turbo generator for seven years. After mechanical operation I was transferred mechanical maintenance department. I looked after various aspect of Operation & Maintenance activities for the last eight years. So I am interested to share my work experience in this session. During boiler operation we consider the most critical issue as generating unit outage for boiler tube failure. This incurs a long term breakdown in power plant. Hence it impacts of huge financial loss for thermal power plant business. So in this discussion I will highlight some important topics regarding Boiler Tube Failure. What are different types & causes of boiler tube failure, What are the failure mechanism, What are the different detection techniques and what are best practices to minimize the boiler tube failure. 

Marginal Design aspects: Generally boiler tube design is classified into three categories. These are low temperature area, high temperature zone and highest temperature zone. The low temperature are accorded to water wall, economizer that are made of carbon steel, High temperature zone is consist of LTSH & wall Reheater that is made of low alloy ferrite steel and highest temperature zone is included the super heater and final reheater tube that is made of austenite stainless steel. Each designer consider the maximum permissible temperature, Pressure & allowable thermal stress. For the increasing competition in power facility manufacturing industries, designer are compelled to consider minimally safety tolerances. So it makes a constraint for regular boiler operation within a stringent operating domain. So it also contributes to tube failure proximity. 

Material flaws impacts: These are the defects in tube during manufacturing, fabrication and installation. While manufacturing of boiler tube some of metal flakes may be laminated. Sometimes deep tool mark while in rolling or extrusion process impinge in the tube surface enhances the reduction of strength. 

Construction Failure: During first commissioning of the unit, generally it is observed that there are several unit outage due to the boiler tube welding joint failure. A typical 300MW capacity boiler is constructed by three lakhs welding joints. Quality control department passed the protocol taking different tests & procedures. But there are always traces of some bad workmanship. Another important phenomenon is the dissimilar metal weld (DMW). It is due to stress developed in between two different materials such as butt weld between an austenitic(Stainless steel) and ferrite alloy like SA213, T22 material. Excessive stress developed due to the difference in expansion of two different materials. This incurs minor welding failure to major unit outage. In our 300MW capacity boiler we faced such initial boiler tube failure. Within two to three years this type of BTL failure can be controlled.

Poor Warehousing: During construction, poor warehousing of boiler tube metal introduces improper use of the specified metal. So the tube allocation improper tagging lead to wrong selection of metallurgy. In our station it was found that some low temperature carbon steel tube used in the superheater zone. It occurs several boiler tube failure.

Poor water quality: Water chemistry is the most crucial parts for boiler operation and control philosophy. Demineralized water is used in power plant cycle. DM water is a acid hungry in nature. So it tends to be acidic. For this reason pH of boiler water must be maintained at higher than 8. Fails to maintain this pH range enhances acid corrosion of boiler tube inner surface.

Thermal over stress: Boiler tube are stationed in high temperature and high pressure zone. So there is a chance of mechanical & thermal stress. most important phenomenon is the creep failure. This is the permanent deformation of tube surfaces for overheating at elevated temperature. This may be due to short term overheating or long term overheating. Fatigue is an another causes for tube failure. This is due to cyclic stress. Low volatile matter(VM) contaminated coal enhances the fire ball shifting to high  temperature zone to low temperature zone. It is also a cause of thermal over stress.

Corrosion & Erosion: Quality of coal is one of the major determining factor for boiler tube failure. High ash contaminated coal enhances tube failure due to high the erosion at outer surface of the boiler tube. Low air fuel ratio is prime cause of dislocation of flame within furnace leads to thermal stress in the boiler tube metal. Improper air distribution for poor secondary air damper control (SADC) operation and over fire air damper are also the leading factor for corrosion of the boiler tube. 

Team Member: 1) Er. Bhagirath Das, HOD & Senior Manager, Boiler Maintenance. 2) Er. Sudipto Das, HOD& Sr. Manager, Boiler Maintenance, 3) Er. Pradeep Kori, Sr. Manager, BMD. 4)Er. Sujit Biswas, Sr. Manager. 5) Er. Tuhin Ghosh, Manager, BMD. 6) Sri Prasenjit Datta(Dutta), Manager BMD. 7) Er. Somnath Kundu, Sr. Manager, BMD. 8) Sri. Gobindo Maity, AM, BMD. 8)Er. Prasenjit Mondal, Manager, 9) Sapath Dutta, Manager, BMD. SgTPP.

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