Figure 21-7. Effect on planned maintenance .ith usage of borescope.
turbine governor valves and trip and throttle valves are suspected of causing overspeed failures. Fouling of balance piston labyrinths and balance lines has caused thrust-bearing failures in centrifugal machines. Any rotor deposit can cause vibration from unbalance if it is not laid down uniformly or if itsluffs off nonuniformly. There can beother， similar effects which will cause failure of a unit.
Fouling Indicators
A prerequisite of a cleaning program is some kind of fouling detectionsystem. Naturally， this system must cover the prime reason for cleaning. Ifthe machine is a gas turbine， the prime reason may be horsepower capability，or it may be efficiency. On a centrifugal compressor， the prime reason for cleaning may be to restorecapacity， to improveefficiency， or to reduce thrust loading.
The selection of a fouling detection system will be strongly influencedby the safety and complexity of the cleaning procedure. Forexample， the procedure may be to throw 10 pounds of spent catalyst into the suction of agas turbine. Or， it may involve injecting a quart of water into a single-stage mechanical-drive turbine with a 30 0F superheated inlet. In either case the risk of damage and the personnel required are low. The cleaning should be frequent and routine.
Fouling indicators include:
1. Gas turbine exhaust temperature
2. The exponent n ι 1.n on a compressor or gas turbine where . is either known or is relatively constant
3. The exponent n ι 1.n in one section of a machine relative to another section handling the same gas
4. The pressure ratio in one section of a machine relative to another section
5. Thrust loading or thrust-bearing metal temperature
6. Balance line to suction differential pressure
.. Compressor discharge pressure and temperature
8. High vibration readings
.leaning Techniques
There are two basic approaches to cleaning: abrasive cleaning， and solvent cleansing. Details of cleaning are given in Chapter 12. Abrasion is the simplestof the two methods， but it is usually the least effective. Figure 21-8 shows that abrasive cleaning does not bring the unit back to full performance and that there is a deterioration in the maximum performance after repeated cleanings. The more common abrasives are 1.64-inch nut shells or spent catalyst. The abrasive must have sufficient mass to achieve the momentumrequired to dislodge the dirt. However， high-mass particles do not follow thegas stream.Also， they are hit by the leading edge of the moving wheels andblades. Consequently， the trailing edges are not abraded. The closer the dirtis to the injectionpoint， the less significant the asymmetrical distribution.
The abrasive must also be sufficiently tough to resist breakage on impact. Rice is apoor substitute， since it tends to shatter on impact and smallparticles lodge themselves in bearings and seals. Again， the closer the injec-tion to the deposit， the less significant the toughness.
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