Stator blades

The so-called propeUer-type fans with blades that are aerodynami-caUy designed (as seen in Fig. 10-68) can consist of two or more stages. The air in these fans enters in an axial direction and leaves in an axial direction. The fans usually have inlet guide vanes followed by a rotating blade, followed by a stationary (stator) blade. 

Figure 4-10. Compressor train for dual-pressure installation consisting of an axial flow air compressor with adjustable stator blades and a radial flow nitrous gas compressor and expander. Mass flow air = 139,000 Nm /h, nitrous gases = 122,500 Nm /h Pressure air = 0.82/4.75 bar, nitrous gases = 4.38/10.8 bar Power input total = 16,840 kW Power recovery by expander = 10,950 kW.

Figure 4-15. Axial compressor with fixed stator blades, double-casing design, and forged monoblock rotor. Particularly suitable for compressing nitrous gases. There are no dead spaces provoking buildup of ammonium nitrate salt.

When used to eompress air, the axial eompressor may be equipped with adjustable stator blades, either partially (Figure 4-16) or on all stages. In the ease of a motor serving as the assoeiated driver or with a generator eoupled to the train, the adjustable stator blade feature allows more effieient eontrol than suetion throttling. With a steam turbine as the assoeiated driver, stator blade adjustment may be eombined with the obvious shaft speed eontrol. This eombination allows for speeial operating eonditions, sueh as startup, or extreme part-load operation. 

A well-designed stator blade adjusting meehanism is absolutely maintenanee-free, requires no lubrieation, and is eompletely proteeted against the ingress of dirt or moisture. 

Figure 4-16. Axial compressor with adjustable stator blades, either partially or on all stages. Double-casing design and forged monoblock rotor. This design offers a wide operating range especially in combination with speed control.

Wide useful flow range by varying stator blade angles... 

Adjusting the stator blades leads to full are admission at all operating eonditions, and blade exeitations due to partial admission are thereby avoided. On well-designed hot gas expanders the adjusting meehanism is generally loeated between the blade earrier and outer... 

Stator blades X20Crl3 X22CrNil7/V80 X20CrMoV121 NiCr20TiAL... 

The stator blades shall be made from a material suitable for the expeeted serviee eonditions. A ehromium-earbide eoating shall be applied to the stator blades if the Seller has typieally experieneed erosion in this area due to the eatalyst partieulate. 

Two 1-in., Class 300 borescope connections shall be provided on the inlet casing to inspect the condition of the stator blades and the upstream side of the rotor blades during periods the expander is shut down without disassembly to the expander. 

The injection of coolant air in the turbine rotor or stator causes a slight decrease in turbine efficiency however, the higher turbine inlet temperature usually makes up for the loss of the turbine component efficiency, giving an overall increase in cycle efficiency. Tests by NASA on three different types of cooled stator blades were conducted on a specially built 30-inch turbine cold-air test facility. The outer shell profile of all three blade types was the same, as seen in Figure 9-24. 

Behning, F.P., Schum, El.J., and Szanca, E.M., Cold-Air Investigation of a Turbine with Transpiration-Cooled Stator Blades, IV—Stage Performance with Wire-Mesh Shell Blading, NASA, TM X-2176, 1971. 

If the air inlet is subjeeted to saltwater eontamination, the rotor and stator blades should be eheeked for pitting. Severe pitting near the blade roots may lead to struetural failures. The manufaeturer should be informed of severe pitting. 

Stator blades are as important as rotor blades. All the same eleaning, inspeetion, and nondestruetive test proeedures should apply. It should be noted that the wear pattern is somewhat different on the stator blades. Again, the manufaeturer should be informed of the wear eonditions and should in turn make reeommendations eoneerning eontinuous operation or replaeement. 

The energy from the rotor is transferred to the gas by rotating blades — typically, rows of unshrouded blades. Before and after each rotor row is a stationary (stator) row. The first stator blade row is called the guide vane. 

Figure 6-5(b) is a case for a reaction of unity, that is, all the pressure rise is in the rotor, with the stator blades acting only as guide vanes to deflect the gas. A reaction of unity is aerodynamically the equivalent of R = 0 or impulse, as shown at Figure 6-5(f) since it corresponds to an interchange of the moving and fixed blade row. 

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. 

Because most axial compressors are in air service, most are equipped with labyrinth type end seals. There are no interstage seals in the machines with unshrouded stator blades. The balance piston seal, a labyrinth type, is the only internal seal. There is no reason that axials cannot use some of the other seals as described in Chapter 5, such as the controlled leakage or the mechanical contact type, if the gas being handled by the compressor needs a more positive seal. If there is any prob-... 

Figure 6-11. Adjustable stator blade linkage. Courtesy of Elliott Company)...

If movable stator blades were used, the linkage should be checked for binding and wear. If a clean gas purge was used to keep the linkage clean and dry, the source and supply lines should be inspected to make sure they all work as intended when the compressor is restarted. 

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