Ultrasonic phased array testing in the power generation industry : novel wedge development for the inspection of steam turbine blades roots

• Main Author: Charlesworth, Chris
• Published: University of Warwick 2011
• Subjects: 620; TJ Mechanical engineering and machinery
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.560215

The thesis presented herein comprises of the work undertaken to research novel methods of Phased array ultrasonic inspection of complex steam turbine blade roots as found in the power generation industry. The research was conducted as part of the Engineering Doctorate scheme, administered by the Research Centre for Non-Destructive Evaluation (RCNDE), in conjunction with RWE npower and the University of Warwick. Steam turbine blades, and in particularly last stage blades of low pressure steam turbines, are amongst the most highly stressed components on a power generating plant. Two of the most common blade root fixing types include ‘curved axial entry fir tree roots’ (CAEFTR), and axial pinned roots, both of which are prone to cracking due to the high stresses to which they are subjected under operating conditions. Failure of the blade root fixings of such components, leading to the release of the blades, has historically led to the catastrophic failure and destruction of the whole turbine; the cost of collateral damage to plant components and the loss in generating income are seconded only by the risk these failures pose to life. Due to the high price of failure, NDT plays a critical part in the support and management of engineering maintenance, offering insight into the condition and integrity of turbine components through regular planned inspection regimes. It will be shown in this thesis how the invention of a novel continuous wedge, used to refract ultrasound into the critical regions of the blade roots, has significantly improved the ability to detect defects. Combined with the development of bespoke scanning frames these wedges facilitate the efficient and accurate acquisition of scanned data to assess the integrity of the component. By combining the latest reverse engineering, modelling and simulation tools with novel application of rapid prototyping, the author has been able to demonstrate significant reduction in design cycles whilst improving accuracy, sensitivity and repeatability of the applied inspections. Furthermore, application of this design philosophy has led to the development of inspection techniques which have facilitated the inspection of remote regions of the blade roots where manual access is limited or impossible. The developments and techniques invented during this research have been successfully deployed across numerous RWE npower and customer projects, leading to estimated savings in excess of £1m.