Gas turbine blades

Abstract An Eddy current method applying a High Temperature Superconductor ( HTS ) DC SQUID sensor operating at Uquid nitrogen temperature (77K) is presented. The method is developed for the detection of surface or surface near defects. We compare the performance of the SQUID system with the performance gained from a commercial Eddy current system, while using identical probes. The experimental data are obtained on defects in gas turbine blades. The advantage of planar conformable probes for the use with the SQUID is discussed. 

Diffusion alurninide and sihcide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 10 /yr in North America and about 50 x 10 worldwide. 

Overlay coatings onto gas-turbine blades and vanes of M—Cr—Al—Y type alloys by electron beam evaporation is estimated at 10 x 10 to coat 200,000 parts at an average cost of 50 per part. 

Hard facing of various components in the aircraft gas-turbine engine and in industrial apphcations for textile machinery parts, oil and gas machinery parts, paper-shtting knives, etc, is estimated at 1 x 10 in 1995 with an estimated growth rate of 5% annually. The mix is approximately 45% aerospace apphcations, 55% industrial apphcations. Additionally, repair coatings for gas-turbine blades and vanes is estimated at 500 x 10 . These coatings are primarily deposited by plasma spray, arc-wire, HVOF, and detonation gun techniques. 

In this chapter we look first at an important class of alloys designed to resist corrosion the stainless steels. We then examine a more complicated problem that of protecting the most advanced gas turbine blades from gas attack. The basic principle applicable to both cases is to coat the steel or the blade with a stable ceramic usually Cr203 or AI2O3. But the ways this is done differ widely. The most successful are those which produce a ceramic film which heals itself if damaged - as we shall now describe. 

Steam is passed through a number of tubes embedded in the nozzle or blades of the turbine. In many cases, the steam is bled from after the HP Steam Turbine of a combined cycle power plant and returned after cooling the gas turbine blades, where the steam gets heated in the process to the IP steam turbine. This is a very effective cooling scheme and keeps the blade metal temperature below 1250 °F (649 °C). 

A design is only as effieient as the performanee of the seleeted eomponent materials. The eombustor liner and turbine blades are the most eritieal eomponents in existing high-performanee, long-life gas turbines. The extreme eonditions of stress, temperature, and eorrosion make the gas turbine blade a materials ehallenge. Other turbine eomponents present operational problem areas, but to a lesser degree. For this reason, gas turbine blade metallurgy will be diseussed for solutions to problem areas. Definition of potential solutions will also relate to other turbine eomponents. 

Meher-Homji C.B., and Gabriles G.A., Gas Turbine Blade Failures—Causes, Avoidance, and Troubleshooting, Proceedings of the 27th Turbomachinery Symposium, Texas A M University, pp. 129, 1998. 

While for many years, metal single crystals were used only as tools for fundamental research, at the beginning of the 1970s single-crystal gas-turbine blades began to be made in the hope of improving creep performance, and today all such blades are routinely manufactured in this form (Duhl 1989). 

We concentrate here on open loop cooling in which compressor air mixes with the mainstream after cooling the blade row, the system most widely used in gas turbine plants (but note that a brief reference to closed loop. steam cooling in combined cycles is made later, in Chapter 7). For a gas turbine blade row, such as the stationary entry nozzle guide vane row where most of the cooling is required, the approach first described here (called the simple approach) involves the following ... 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

The life of gas turbine blades is improved by platinum and/or rhodium, applied below or above, or co-deposited with, aluminised, thermal-barrier or AfCrAlY-type layers. The performance of modified aluminides was demonstrated in long-term engine trials . ... 

Creep is the gradual extension of a material under a steady tensile stress, over a prolonged period of time. It is usually only important at high temperatures for instance, with steam and gas turbine blades. For a few materials, notably lead, the rate of creep is significant at moderate temperatures. Lead will creep under its own weight at room temperature and lead linings must be supported at frequent intervals. 

These are the nonbumable components, typically metals and metalloids, found in fuel. Depending upon size, these particles can contribute to fuel system wear and filter and nozzle plugging. Sodium, potassium, lead, and vanadium can cause corrosion of certain high-temperature alloys such as those found on diesel engine valves and gas turbine blades. 

Poster 29. M. Morinaga, Y. Murata, R. Hashizume, A. Yoshinari and T. Kiyono (Nagoya University, The Kansai Electric Power Company, Inc., Hitachi Co. Ltd.) Design and Development of Ni-Based Single Crystal Superalloys for Gas Turbine Blades Using the Alloying Parameters Obtained by DV-Xa Method... 

Various isotope applications are used to monitor the quality of materials and structures. Isotopic tracer techniques measure wear, corrosion, moisture, leakage, and many other factors. Neutron radiography creates images of materials that are not as dense as those captured in X-ray photos. This method is used chiefly to check uranium fuel in nuclear reactors for flaws, to find cracks in the inner plastic or aluminium parts of airplanes, or to detect tiny fissures in gas turbine blades. Californium-252 is used for neutron radiography and neutron activation analyses. 

The presence of sodium and vanadium complexes in the fuel oil ash can, under certain plant operating conditions, result in considerable harm to the equipment. Spalling and fluxing of refractory linings is associated with the presence of sodium in the fuel. Above a certain threshold temperature, which will vary from fuel to fuel, the oil ash will adhere to boiler superheater tubes and gas turbine blades, thus reducing the thermal efficiency of the plant. At higher temperatures, molten complexes of vanadium, sodium, and sulfur are produced that will corrode all currently available metals used in the construction of these parts of the plant. TTie presence of trace amounts (ASTM D-1318) of vanadium (ASTM D-1548, IP 285, IP 286) in fuel oil used in glass manufacture can affect the indicator of the finished product. 

Jian, C. Y., Hashida, T., Takahashi, H., and Saito, M., "Thermal Shock and Fatigue Resistance Evaluation of Functionally Graded Coatings for Gas Turbine Blades by Laser Heating Method," Composites Engineering, Vol. 5 (7), pp. 879-889,1995. 

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