Pressure part components operating at high temperature and pressure are designed for a finite life. These components are subjected to various degradation processes like creep, fatigue, erosion, corrosion, etc., which will result in unforeseen outages. In this context residual life assessment (RLA) assumes greater significance [1].

Most of the available NDT techniques are employed for detection of defects. However, estimation of remaining life calls for application of crack growth analysis using finite element technique for thick walled components like headers and pipes. For thin walled components application of non-destructive evaluation (NDE) technique in correlation with evaluation from destructive sampling meets the requirements.

M/s. BHEL has carried out RLA studies of several boilers in India and abroad. In addition to the conventional NDT techniques, M/s. BHEL has developed and established special NDT techniques for performance evaluation and life prediction of boiler components.

Detection of hydrogen damage in case of water wall tubes, measurement of oxide scale thickness of the tubes, detection and monitoring of ligament cracks in headers are some of the areas where special NDT techniques are adopted by M/s. BHEL during RLA study.

For life prediction of Super heater/Reheater coils, estimation of metal temperature by in-situ measurement of steam side oxide scale is done by measuring time of flight of high frequency ultrasonic waves [2]. The data obtained is used in the soft ware programme developed by BHEL for life prediction.

Detection of hydrogen damage in water wall tube is carried out by a specially developed ultrasonic technique based on the attenuation principles. The micro fissures produced due to hydrogen damage attenuate the ultrasound during its propagation, severely affecting signal amplitude. A specially developed high frequency transducer with a digital ultrasonic flaw detector was used for this purpose.

Assessments carried out in some of the units revealed cracking of ligaments in headers [3]. Detection of ligament cracks required inspection of tube hole penetrations with a fibroscope and for sizing of cracks the pulse echo angle beam technique was developed.

The information gained from NDE is used in conjunction with data on material aging, stress and failure analysis before making any decision on continued operation of the boiler and also for estimating the remaining life of the components.

The authors would like to share their exciting experience in development and establishment of special NDE techniques adopted by M/s. BHEL for residual life assessment studies. REFERENCES

1. F. V. Ellis, et al., "Remaining Life Assessment of Boiler Pressure Parts, " Final Report RP 2253-1, Vol. 1-5, Electric Power Research Institute, Palo Alto, CA, USA (1988).
2. R. Viswanathan, J. R. Foulds and D. A. Roberts, Methods for Estimating the Temperature of Reheater and Superheater Tubes in Fossil Boilers, in Proceedings of the International Conference on Life Extension and Assessment, The Hague, June 1988
3. B. W. Robers, F. V. Ellis and R. Viswanathan, "Utility Survey and Inspection for Life Assessment of Elevated Temperature Headers", American Power Conference TIS 7795, Chicago, April 1985.