Blading

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.

in a new compressor, the wires should not be used because the stress problems should be solved in a fundamental design manner without hav ing to resort to "fixes." When the manufacturer designs the blades, the vibrational characteristics for both rotor and stator blades should be established. The basic bending resonances and the higher orders to which the blades may be excited should be established. Care should be taken to avoid any direct excitation sources, such as splitter vane or stator and guide vane passing frequencies. If possible, any fundamental and lower order resonances should be at least three to four times higher than any of the running speed. The manufacturer should supply a Campbell diagram to demonstrate that the compressor is free of direct excitations. When resonances do exist in the operating range, the vendor should demonstrate his understanding of the stress level and provide some assurance to the user that the compressor will not have premature blade failures. One method to convey the information is with a Goodman diagram. Some users ask for Goodman diagrams for all stages, regardless of any resonant interferences, to demonstrate that a conservative design concept was used throughout the blading design. While stress levels in the rotor blading are by far the most severe, resonances can occur in the stator blades that have been known to fail when excited by one of the compressor's operating frequencies. This is a rare occurrence, but must still be considered. Most reported failures are caused by rubs or foreign object damage. Regardless of the cause, if a stator blade should break and drop into the gas path where it can be struck by the rotor blades, the wreck is just as traumatic as a direct rotor blade failure.

While much has been said about the axial compressor that would give the impression that the machine is not durable, nothing could be farther from the truth. The compressor has logged hundreds of thousands of hours in trouble-free operation, and will do so if properly designed and operated with reasonable care. It may not be quite as abuse-resistant as the centrifugal. However, as the efficiency and performance of centrifugals is upgraded they tend to become less abuse-resistant. The object of any operating group should be a conscientious effort to properly operate and maintain the equipment so that everyone will benefit.

Bearings

The bearings used in axial compressors are the same journal and thrust type used in the centrifugal compressor. Refer to Chapter 5 for a complete description of these bearings.

For axial compressors, the journal bearings are of the plain sleeve type for the larger, slower speed compressors. They are of the tilting pad type for the smaller, higher speed machines. The sleeve bearing is normally housed in a spherically seated carrier. The bearings require pressure lubrication as do most of the other compressors.

The thrust bearing is generally the tilting-pad type bearing. Most vendors apply the recommendation that the thrust bearing be of the symmetrical design with leveling links. Axial compressors have a high inherent thrust load, so the thrust bearing is quite important in the overall reliability of the compressor. While this is true for the other compressors as welt, it deserves an extra emphasis here.

Balance Piston

The axial compressor is inherently always a reaction type of machine. In regards to axial thrust, this means the rotor is subjected to a differential pressure across each rotating blade row. The differential pressures convert to an axial force at each rotor row that totals to a rather high value when taken over the normal number of stages. A thrust bearing would be prohibitive in size to carry the generated thrust. Fortunately, the geometry of the axial provides space for a large balance piston at the discharge end ol the compressor. In fact, the construction of the axial compressor rotor is such that the placement of a labyrinth seal on the hub diameter and another labyrinth seal on the shaft forms a balance cavity. The balance piston cavity is normally vented to the suction end of the compressor or to the atmosphere on air machines. The balance piston seal leakage is charged to the compressor as a loss. As in the centrifugal, the return gas represents a head loss due to the heating effect of the return gas, and a direct loss in capacity due to the quantity of gas bypassed.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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