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Rows of blades in a gas turbine generator cost millions of pounds, yet need to be replaced frequently. So major power operators and generator manufacturers in the UK have started
using the services of companies like Qinetiq to redesign blades in order to prolong their service life as well as increase their operational efficiency, thereby saving a considerable amount of money.
Key to improving the designs is reverse engineering of the original blades, which is carried out at Qinetiq's hydrodynamic test centre in Gosport. Project design engineer, Kevin Brown, uses an LK G-80C co-ordinate
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measuring machine (CMM) to collect data from blade samples sent by the group's Power Division in Farnborough. The data is used to create CAD models of the blades which form the basis for redesigning them to improve their operation, making them last longer and convert energy more efficiently and cleanly. The same technology has significant potential in aerospace and marine applications.
Indeed, the test centre on the Haslar Marine Technology Park in Gosport, formerly part of the Government's defence evaluation and research agency (DERA), has been a development facility for the design of ships' hulls and propellers and latterly submarines and submersibles since 1887. The shed which houses No 1 ship tank for towing and testing ship models was designed by Isambard Kingdom Brunel, who was born in nearby Portsmouth.
The original use of the LK CMM, installed in 1992, was for quality assurance of propellers and other items manufactured on site, such as rudders and hydroplanes. The CMM took over from a hand-operated machine, measuring in two hours a propeller that used to take two weeks to inspect. This was because it was necessary constantly to reposition the component and wind in a height gauge clock to accumulate typically 140 point readings on both sides of the component.
The original use of the LK CMM, installed in 1992, was for quality assurance of propellers and other items manufactured on site, such as rudders and hydroplanes. The CMM took over from a hand-operated machine, measuring in two hours a propeller that used to take two weeks to inspect. This was because it was necessary constantly to reposition the component and wind in a height gauge clock to accumulate typically 140 point readings on both sides of the component.
The gradual demise of shipbuilding in the UK coupled with the ability of modern simulation software to replace physical tests meant that the amount of marine work passing through Kevin Brown's department fell off dramatically a couple of years ago. This was when Qinetiq Power, headquartered in Farnborough, took the opportunity to harness the underused CMM at Gosport for creating CAD models of gas turbine generator blades by reverse engineering.
As part of the project, Qinetiq sanctioned a £28,000 upgrade of the CMM, which was carried out on-site in just one week by LK during July 2003. The old VAX computer with CMES software requiring laborious, off-line programming was replaced by a PC running LK's CAMIO Studio DMIS-based, on-line programming and measurement software, allowing faster inspection cycles and more efficient data management. At the same time, a new LK 2000 controller was fitted and the machine was rewired.
The most notable operational improvement following the upgrade has been the ability to digitally scan the surface of blades at much higher speeds than was previously possible. (Note that this is measurement by discrete-point pecking rather than continuous-path analogue scanning). CAMIO's freeform digitising can be performed on any workpiece that is unspecified mathematically, collecting data on successive curves across the surface. Mr Brown has only to define the required point density and the software automatically generates the digitising part-program.
A set of five identical turbine blades is normally sent to Gosport for analysis. On three, the root section only is checked by taking 300 to 400 points in a two-hour cycle, and the features are generally found to be within the accepted 5 microns tolerance. All surfaces on the other two samples are scanned in an 8-hour cycle, capturing 3,000 to 4,000 points including blade profile and all fine detailing down to the smallest groove. The average of the two blade profiles is taken and combined with the root and additional surface data to create a master CAD model which Qinetiq Power uses to modify the blade design, followed by testing and finally subcontract remanufacture.
Commented Mr Brown, "Using the LK machine as originally supplied, I would have had to make point-to-point measurements as digital scanning would not have been feasible in practice. A blade aerofoil alone would have taken a day to inspect whereas the upgraded machine scans it in an hour – an eight-fold saving. It would then have been necessary to write all points manually into the 3D model."
"Now, feature detail is much easier to visualise and data is sent directly and automatically into the CAD model. A report is generated as the cycle progresses, rather than at the end, allowing erroneous readings to be spotted straight away. This is a big help, as initial propeller set-ups were sometimes incorrect and the final readings were all over the place, so two hours' work would be wasted."
In addition to reverse engineering on the LK CMM, two additional checks are made on turbine blades before the results are fed back to Qinetiq Power in Farnborough. They are X-ray analysis to map the internal cooling channels, and the use of a 6-axis, articulating measuring arm to capture the general shape of the component.
Download: Qinetiq uses LK CMM for reverse engineering of turbine blades (PDF file)
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