Boiler assessments carried out by TWI showed that defects found during the boiler’s periodic inspection did not require repair and avoided considerable expense and inconvenience.
It is common practice to inspect hot water and steam boilers annually. Previously this was limited to visual or magnetic particle inspection. However, following failures of boilers by internal fatigue crack growth from the end-plate toes, the welds are now inspected periodically using ultrasonics. This is very valuable for identifying potentially injurious defects and can also identify weld discontinuities from the original manufacture which may or may not constitute a risk to future service performance. Repairs may be difficult and costly and the repaired boiler may need full post-weld heat treatment.
Ajax Insurance Association Ltd inspected a client’s horizontal multitubular boiler and found a number of lack-of-fusion discontinuities in the shell-to-end-plate welds. Some of these exceeded the recommended limits for weld discontinuities specified by the Health and Safety Executive. At its client’s request Ajax asked TWI to perform an engineering critical assessment of the discontinuities – to determine whether they would impair the service performance of the boiler.
TWI collected the information needed for fitness-for-purpose assessment to be made using fracture mechanics techniques. This included geometric details of the boiler, the location and size of the weld discontinuities, the material specification used in the boiler manufacture, the service and standby pressure ratings, the operating temperature and the details of any cyclic load history.
The analysis was carried out using techniques similar to those described in BS PD6493:1980 Guidance on some methods for the derivation of acceptance levels for defects in fusion welded joints (now superseded by BS7910 Guide on methods for assessing the acceptability of flaws in metallic structures). These techniques required knowledge of various material properties, including yield strength, fracture toughness and fatigue crack propagation rates for the operating temperature of the boiler. The material was BS 1501/151/28A and suitable values were obtained from TWI’s extensive material databases.
The analysis considered three potential failure modes: fracture, fatigue and plastic collapse. It indicated negligible risk of failure by fracture or plastic collapse and extremely small growth of the discontinuities by fatigue. It was thus possible for TWI to assure Ajax that no repairs were needed and that the boiler could continue to operate safely. TWI also made recommendations relating to future inspection period.
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