Turbomachinery Clearance Control [chapter]

Raymond E. Chupp, Robert C. Hendricks, Scott B. Lattime, Bruce M. Steinetz, Mahmut F. Aksit
2014 Turbine Aerodynamics, Heat Transfer, Materials, and Mechanics  
Acronym List ACC active clearance control APS air plasma spray BOM bill of material EB-PVD electron beam plasma vapor deposition FEA finite element analysis FOD foreign object damage HCF high-cycle fatigue FSN first-stage nozzle HFBS hybrid floating brush seal HPC high-pressure compressor HPP high-pressure packing HPT high-pressure turbine Turbomachines range in size from centimeters (size of a penny) to ones you can almost walk through. The problem is how to control the large changes in
more » ... y between adjacent rotor/stator components from cold-build to operation. The challenge is to provide geometric control while maintaining efficiency, integrity and long service life (e.g., estimated time to failure or maintenance, and low cost 1 ). Figure 1 shows the relative clearance between the rotor tip and case for a HPT during takeoff, climb, and cruise conditions. 2 The figure shows the dramatic effect of clearance control via applied cooling to the casing. A critical clearance requirement occurs at "cut-back" (about 1000 s into climb-out) when takeoff thrust is reduced. Using thermal active clearance control (ACC), the running clearance is drastically reduced, producing significant cost savings in fuel reduction and increased service life. However, designers must note that changing parameters in critical seals can change the dynamics of the entire engine. 3 These effects are not always positive. B. Sealing Benefits Performance issues are closely tied to engine clearances. Ludwig 4 determined that improvements in fluid film sealing resulting from a proposed research program could lead to an annual energy saving, on a national basis, equivalent to about 37 million barrels (1.554 billion = 1554 million U.S. gallons) of oil or 0.3 percent of the total U.S. energy consumption (1977 statistics). In terms engine bleed, Moore 5 cited that a 1-percent reduction in engine bleed gives a 0.4-percent reduction in specific fuel consumption (SFC), which translates into nearly 0.033 (1977 statistics) to 0.055 (2004 statistics) billion gallons of U.S. airlines fuel savings and nearly 0.28 billion gallons world wide (2004 statistics), annually. In terms of clearance changes, Lattime and Steinetz 6 cite a 0.0254 mm (0.001 in.) change in HPT tip clearance, decreases SFC by 0.1 percent and EGT (exhaust gas temperature) by 1 °C, producing an annual savings of 0.02 7 billion gallons for U.S. airlines. In terms of advanced sealing, Munson et al. 7 estimate savings of over 0.5 billion gallons of fuel. Chupp et al. 8 estimated that refurbishing compressor seals would yield impressive improvements across the fleet ranging from 0.2 to 0.6 percent reduction in heatrate and 0.3 to 1 percent increase in power output. For these large, land-based gas turbines, the percentages represent huge fuel savings and monetary returns with the greatest returns cited for aging power systems. C. The Sealing Environment 1. Seal Types and Locations Key aero-engine sealing and thermal restraint locations cited by Bill 9 are shown in figure 2. These include the fan and compressor shroud seals (rub strips), compressor interstage and discharge seals (labyrinth), combustor static seals, balance piston sealing, turbine shroud and rim-cavity sealing. Industrial engines have similar sealing requirements. Key sealing locations for the compressor and turbine in an industrial engine are cited by Aksit 10 and Camatti et al. 11,12 and are shown in figures 3 and 4, with an overview of sealing in large industrial gas turbines by Hurter, 13 figure 3(c). Figure 3 shows high-pressure compressor (HPC) and HPT tip seal (abradable) and interstage seal (brush seal) locations, while figure 4 shows impeller shroud (labyrinth) and interstage seal (honeycomb) locations for the compressor. Compressor interstage platform seals are of the shrouded type (figs. 5 and 6). These seals are used to minimize backflow, stage pressure losses and re-ingested passage flow. Turbine stators, also of the shrouded type, prevent hot gas ingestion into the cavities that house the rotating disks and control blade and disk coolant flows. 8 Designers need to carefully consider the differences in thermal and structural characteristics, pressure gradient differences, and blade rub interfaces. Characteristically the industrial gas turbine can be thought of as a heavy-duty derivative of an aero engine. Still industrial and aero-turbomachines have many differences. The most notable are the fan, spools and combustor. Aero engines derive a large portion of their thrust through the bypass fan and usually have inline combustors, high and low pressure spools, drum rotors and high exhaust velocities, all subject to flight constraints. Large industrial engines (fig. 7) have plenum inlets, can-combustors, single spools, through-bolted-stacked disc rotors and exhaust systems constrained by 640 °C (1180 °F) combined cycle (steam-reheat-turbine) requirements. In both types of engines, core requirements are similar, yet materials restraints differ. Materials and Environmental Conditions Over the years, advances in new base materials, notably Ni-based single crystal alloys, and coatings have allowed increased operating temperatures of turbine engine components. Complementary to the thermal and pressure profiles, materials used range from steel to superalloys coated with metallics and ceramics. Variations in engine pressure and temperature of the Rolls-Royce Trent gas turbine * are illustrated in figure 8 which also has an intermediate pressure turbine (IPT). The lower temperature blades in the fan and low-pressure compressor (LPC) sections are made of titanium, or composite materials, with corrosion resistant coatings * (Data available from Sourmail, T., "Coatings for Turbine Blades," University of Cambridge, at
doi:10.2514/5.9781624102660.0061.0188 fatcat:kzbiax6txfabjczdeubmrulxni