Over the years of operation, steam turbine casings are repeatedly subjected to high thermally induced stress which often exceed the yield point of the material. These stresses which locked within the material, cause the casings to distort and gap open at splitline when loosen which require over torquing the bolts to obtain metal to metal seal fit. Traditional method to obtain minimum seal clearance required several episodes of putting the cover on, torqued until the gap is closed, remove the cover, measure the seal imprint, scaping the seal surface, then repeat the process until a 0.0015in or 0.038mm feeler gauge is not able to insert anywhere along the horizontal joint.
Machining scope of High and Low Pressure Casing
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Most modern turbines, with steam pressures over 100 bar and ratings greater than 100 MW, has double-shell design whereby the space between is filled with steam at exhaust conditions, allowing each casing to be designed for smaller temperature and pressure differentials.
To achieve metal to metal seal fit for both inner and outer casing, machining can be carried out using in-situ laser control milling to achieve a surface runout within 0.02mm. To work on the top inner and outer casings, customized jigs will be designed and fabricated to hold the linear milling machine as well as the inner casings.
As machining the horizontal split line lowers the turbine centerline which diaphragm fit need to be bored to allow radial clearance for diaphragm positioning at operating temperature. This can be carry out using the in-situ boring machine.
Using of large conventional instruments like vernier calipers and micrometers to carry out inspection work need to be supported by several people and prone to human error. This process can be improved by using CMM arm and laser tracker to measure the inner rings and stator blades of turbines, jigs, shafts and large parts such as the outer ring of the diaphragm as well as assist in aligning components during turbine installation.
CMM arm and laser tracker help reduced the time taken for measurements, obtained consistency results and generate digital reports automatically. It can also improve the precision of the turbine installation by measure the turbine’s original position and use the data to monitor and restore it back to its original state.
