Doosan Babcock contracted Advanced Mechanical Systems (AMS) to develop a deployment system for the inspection of Heat Recovery Steam Generator (HRSG) finned tube bundles. A robotic snake system was developed which allows non-destructive inspections to be carried out without cutting open the header vessel to gain access. Instead an inspection stub or riser is utilised which can be as small as 40mm in diameter.
An eddy current Near Field Array (NFA) system and a Remote Visual Inspection system (RVI) was developed. The equipment is deployed into the header through the access diameter to locate the selected tube bore and feed through a position encoded NFA probe or steerable endoscope camera. The NFA probe is specialised for the finned tube application and provides high resolution scan of the entire inner tube surface.
Several finned tube test pieces were produced to evaluate the capability of the NFA system. The images above show sample tube geometry (fins not represented) and NFA display window.
James Ireland (Design Engineer at AMS) said: "We worked alongside Doosan Babcock NDT Engineers from development of the initial concept through to manufacture and testing. The development included a prototyping stage where revisions were made to prove deployment would be practical. The equipment evolved until it was demonstrated that the technicians could easily deploy and retrieve it from the vessel. Finally, the eddy current and remote visual inspection equipment was integrated into the latest versions".
The extremely restrictive access to HRSG tubes made this a very challenging project. However, it was considered worthwhile as there are no other effective solutions for in-service inspection of finned tubes. Degradation leading to failure is unpredictable and, when it occurs, is a heavy financial blow to Combined Cycle Gas Turbine (CCGT) owners. The hard work has resulted in a unique capability for Doosan Babcock to reduce inspection time and maximise HRSG overall plant availability.
A mock-up based on typical header specifications was used to trial the deployment system.