Figure 21-1.. Compressor .ith high erosion at blade tips due to improper filtration system.
impractical. Abrasive cleaning of the compressor or a water wash usually will restore the blade surface and thus create the original design angle. In somecases， blade flutter problems are initiated due to bleed-off valves; excessive bleed-off can result in a compressor surge of the latter stages.The amount of bleed-off， which can be tolerated in most units， is between 12 and 1.%.
Centrifugal compressors can have problems at the inducer and blade tips. These parts of the blade can be excited by aerodynamic forces. Blade discs can have stresses at the rotortips， which lead to cracks. To solve thisproblem， the offending part is removed and a scalloped disc results as seen in Figure 21-20. This type of disc experiences some efficiency loss (about 2-4%).
Problems with blade fatigue are common. When interstage cooling isused， water from the cooler is carried over to the blades. These water particles
Figure 21-20. .calloped rotor.
impinge against the blades and give rise to a high-stressregion， usually nearthe blade exit. The cracks spread out from thisarea， and the blade initiallysuffers a cyclic fatigue， which leads to a rupture. Since this usually occurs inthe first-stages of the casings， a serious loss is incurred as the blade debrisgoes downstream， wiping out everything in its path. Figure 21-21 shows a cross section of such a blade. Figure 21-22 shows the effect of the blade passing through the rest of the rotor.
Other problems experienced in compressors and turbines occur in regen-erators， shafts， gearing， bearings，seals， and couplings. Problems with regen-erators often occur due to a leak in the system. Abrasive cleaning ofcompressors， which lead to regenerators， should be confined to using spent catalyst or other nonflammable-type cleaners. Abrasive cleaners such aspeanut shells and rice should be avoided， since they have a tendency to accumulate in corners where they can create hot spots or a fire that willburn through the wall. Shaft problems are usually not very common， but now and then a shaft will shear due to excessive load. This excessive load has many causes. Most often the problem results when the type of driver is changed from a turbine to a synchronous electric-motor drive. With thelatterdrive， very high torsional stresses are produced that can lead to shaft failures when the unit is brought from rest to design speed in seconds. Rotor bearings usually experience a type of instability called oil whirl. This phe-nomena has been described in detail in Chapter 5. In some cases， this problemis alleviated by a change of oil temperature， otherwise the problem requires a change in bearing design such as going to a pressure-dam bearing or in
Figure 21-21. Cross section of fatigued blade. .ote chevron markings near the trailing edge indicating cyclic fatigue.
excessive cases to a tilting-pad bearing. Thrust-bearing problems are due tomisalignment， or because the unit is running very close to the active thrustsurface. To compensate for the thrust， a balance piston is introduced. This balance piston compensates for the aerodynamic thrust. A case in point was a four-stage rotor as shown in Figure 21-23. The air to the right of the balance piston was low-pressure air from the compressor inlet. This air was originallybled-off from the inlet to the compressor. After routine maintenance， it was decided that the air be taken from in front of the inlet air cooler rather than from behind it. This small change in pressure was enough to destabilize thesystem， and the rotor screwed itself into the diaphragms， creating a great loss.
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