Successful completion of boiler remaining life assessment

Successful completion of boiler remaining life assessment (RLA)

Introduction

Petromaster Ltd. has been engaged to perform Inspection and Remaining Life Assessment study for the waste heat boiler at Industrial Waste Recycling Company. The company used to provide industrial waste management solutions for hazardous & non-hazardous wastes generated by the industries and operates its Treatment, Storage and Disposal Facility . The subjected waste heat boiler was classified as horizontal smoke tube shell type boiler with water wall Tubes, Screen Tubes, super heater coils, Water Headers and Steam Drum. It received flue gases from the incinerator and uses these hot flue gases to convert feed water into high temperature steam. Subsequently, the saturated steam is used to drive turbine for power generation.

Figure 1.General arrangement of the boiler

The scope of study is summarized in the table below with respect to the different sections of the boiler.

Figure 2.0 Summary of the Inspection Scope

TECHNICAL INVESTIGATION

Visual Examination

The observations during the internal visual examination are listed below.

1. The flue gas duct from the incinerator to the boiler section was full covered light-brown coloured deposits.
2. The water wall tubes and screen tubes were covered with dark-brown oxide layer.
3. Significant amount of loosely adhered deposits similar to flue gas duct were found on the water-wall and screen tubes.
4. The upper section of the screen tubes revealed comparatively more deposits residing.
5. The deposits were not uniform in appearance and apparently followed the trajectory of the flue gases.
6. No significant tube damage was observed in visual examination.

Figure3.0 Visual Examination of wall and Screen Tubes

Replica Metallography

In-Situ Replica Metallography and hardness testing was performed in the waste heat boiler as a part of remaining life assessment study to investigate the microstructural integrity of the boiler components. The replication was performed on randomly selected locations from the water wall tubes, screen tubes, super-heater tubes, steam drum and headers to assess the level of materials microstructural degradation in these sections. Hardness testing was performed on all the replicated locations to provide the quantitive insight of the mechanical properties. Standard procedure of replication as per ASTM E1351 was followed. All the locations were etched with 2% Nital solution after polishing and replicated onto the cellulose acetate films.

The microstructural analysis from the water wall and screen tubes revealed varying degree of microstructural degradation. The material grade of these tubes was informed to be BS 3059 PT1 which contains ferrite and pearlite phases in the as received condition. However under the high temperature condition, the pearlite phase starts degenerating into globular carbides. This phenomenon is called spherodization which reduces the creep strength of the component.

Figure 4 .0 illustrates the step-by-step process of ageing and its effect on the microstructure of low alloy ferritic steels. Stage A is the normal microstructure of low alloy ferritic steels containing ferrite and lamellar pearlite. With time in service, spherodization begins and carbides precipitate on the grain boundaries (stage B). Stage C represents the intermediate stage of the spherodization process, and then in stage D the spherodization has completed. Stage E represents evenly dispersed carbides with few traces of prior ferritic/pearlite structures and coarsening of carbide particles. Precipitation of carbides and coarsening of carbide precipitates are the main contributors to microstructural deterioration in this steel as a result of an increase in the severity of thermal ageing. Ageing adversely affects creep strength at the various stages of spherodization and carbide coarsening. Microstructural instability of this steel type in terms of cavity formation and cavity linkage under stress also leads to a generation of microcracks.

The observed microstructures in the water wall & screen tubes show varying degree of microstructural ageing from stage C to Stage E. The hardness values show similar trend. The tubes having high level of spherodization had comparatively lower hardness. The In-situ spherodization is the precursor for the creep void formation in the ferritic steels. The observed locations did not reveal any signs of creep damage but significant level of ageing was observed which requires continuous monitoring in future. Figure 9 below summarizes the extent of damage observed in the water wall and screen tubes.

The microstructural analysis of most of the locations from superheater tubes revealed significant microstructural degradation in terms of full spherodization of carbides and presence of isolated and in few locations oriented creep voids.

Figure 4.0 Life Exhaustion based on Creep Damage

Figure 5.0 Micrographs for wall tube

Figure 6.0 Micrographs for wall tube

Figure 7.0 Micrographs for Superheater Tube

Figure 8.0 Micrographs for Superheater Tube

Figure 9.0 Level of Spherodization in Wall Tubes

Figure 10.0 Creep Damage Level and Creep based remaining Life

Steam Drum and Water Headers

In case of steam drum top and water headers the observed thickness values were significantly higher than required twin, hence they were considered to be in safe operating envelop. Figure 11.0 RFT results for maximum wall loss

Conclusions

The above inspection and testing results provided the following concluding remarks.

1. The external examination of the boiler components did not provide any substantial indications apart from some localised coating damages.
2. Significant deposits and scaling was observed on the boiler internal components during visual examination.

III. Low hardness along with microstructural ageing was observed in few water wall and screen tubes.

1. The super heater tubes showed significant microstructural degradation in terms of spherodization and creep voids.
2. No signs of damages or cracks were found during the PT and MPT testing.
3. The thickness survey showed varying degree of metal loss in the different boiler components. The maximum metal loss was observed in boiler smoke tubes (97%) followed by screen tubes and water wall tubes. The remaining life for each section was calculated based on creep damage and thickness loss.

REFERENCES

1. ECCC RECOMMENDATIONS – VOLUME 6 [Issue 1], Residual Life Assessment and Microstructure.
2. Components. Int. Conf. on Plant Life Management & Extension, Helsinki, 1992. Cane B. J., Townsend R. D. – Prediction of Remaining Life in Low Alloy Steels. CEGB-CERL Report no TPRD/L/2674/N, 1984.

III. Viswanathan R. and others – Life Assessment of Superheater / Reheater Tubes in Fossil Boilers. Journal of Pressure Vessel Technology – Vol. 116, February 1994.