Materials compressor blades

Sinee the design of turbomaehinery is eomplex, and effieieney is direetly related to material performanee, material seleetion is of prime importanee. Gas and steam turbines exhibit similar problem areas, but these problem areas are of different magnitudes. Turbine eomponents must operate under a variety of stress, temperature, and eorrosion eonditions. Compressor blades operate at relatively low temperature but are highly stressed. The eombustor operates at a relatively high temperature and low-stress eonditions. The turbine blades operate under extreme eonditions of stress, temperature, and eorrosion. These eonditions are more extreme in gas turbine than in steam turbine applieations. As a result, the materials seleetion for individual eomponents is based on varying eriteria in both gas and steam turbines. 

Axial compressor blades are usually forged and milled. Precision casting has been used on occasion. The most common material used is a 12 chrome steel, in the AISI 400 series, and is also known as 400 series stainless steel. While the stator blades are occasionally shrouded, the rotor blades are free-standing. Lashing wires have been used on rotor blades, but are generally used to solve a blade vibrational stress problem. 

The impellers for centrifugal compressors are assemblies consisting of three parts the hub disc, the blades, and the cover disc. The hub and cover disc are machined from single-piece forgings of an alloy steel suitable for the application of the compressor. Blades are machined from forged steel plates of identical material. Each forging is checked with either sonic or X-ray machines to detect flaws or inclusions. 

Sawyer, . C.,Pekarek, E. G. (1955), Study of Therme-nol as a Compressor Blade Material, Final Report by Thompson Products, Inc., oh BuAer Contract No. NOa (S) 51-1257-C. 

Fouling deposits. Most fouling deposits are mixtures of water wettable, water soluble, and water insoluble materials. Very often pH 4 and lower can be measured in compressor blade deposits. This represents a risk of pitting corrosion. Furthermore, these deposits become more difficult to remove if left untreated as the aging process bonds them more firmly to the airfoil surface, thus reducing deaning effldency. 

Turbine-Blade Cooling The turbine inlet temperatures of gas turbines have increased considerably over the past years and will continue to do so. This trend has been made possible by advancement in materials and technology, and the use of advanced turbine bladecooling techniques. The olade metal temperature must be kept below 1400° F (760° C) to avoid hot corrosion problems. To achieve this cooling air is bled from the compressor and is directed to the stator, the rotor, and other parts of the turbine rotor and casing to provide adequate cooling. The effect of the coolant on the aerodynamic, and thermodynamics depends on the type of cooling involved, the temperature of the coolant compared to the mainstream temperature, the location and direction of coolant injection, and the amount of coolant. 

Titanium is an interesting material that, due to its high price, has not been particularly widespread in use, but the number and extent of applications are increasing. For demanding eonditions, selection of titanium may be an economic solution because it has a series of good properties. Its primary field of application has been in the aerospace industry compressor wheel discs, blades in gas turbines, etc. It has traditionally been used also in the chemical industry. Its use in the oil production industry has increased due to conditions dealt with in the following paragraphs. 

As the observed power deficit had been explained to a very large extent, several measures were identified to enable its reduction. One measure would be to optimize the flow conditions for the inflow and outflow areas of the compressor and turbine sections in order to minimize the pressure drop losses and to achieve an optimum inflow into the blading. Moreover, a reduction of the blade gap losses would be required. That could be achieved by a reduction in rotor vibration and better selection of materials. The materials for both the rotor and the stationary blade carrier should be selected to optimize thermal expansions to achieve minimum gaps at operating conditions. One approach would be the replacement of the non-cooled austenitic stationary blade carrier in favor of a ferritic one, with cooling provided at necessary locations. Preferably no cooling at 750 C should be provided at all, taking into account possible improvements in avaible blade and rotor materials. A third approach that seems to be possible would be a further optimization of the stationary blade profiles and rotor blade profiles. 

The compressor is a highly stressed material, though operates at low temperature. Thermal creep is not considered a failure mode at this low temperature (<550 K). Compressor stability through transients is known to cause imbalanced blade loading, causing the compressor to potentially rub the shroud. Erosion wear from potential high velocity particulate flow on the diffuser vanes could also reduce compressor performance. 

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