. On-line wash should be done whenever compressor performance diminishes by 2-3%. It would be imprudent to let foulants build up before commencing water wash.
. Stainless steel fortanks，nozzles， and manifolds are recommended to reduce corrosion problems.
. Spray nozzles should be placed where proper misting of the waterwould occur， and minimize the downstream disturbance of the flow. Care should be taken that a nozzle would not vibrate loose and enter the flow passage.
. After numerous waterwashes， the compressor performance will deteriorate and a crank wash will be necessary.
Fuel Economics
Because gas turbine fuel properties are not the ones that determinecost， in some instances the better gas turbine fuel will sell for less than the poorer one.The selection of the most economical fuel depends on many considerations， of which fuel cost is but one. However， users should always burn the most economicalfuel， which may not be the cheapest fuel.
Fuel properties must be known and economics considered before a fuel is selected. The properties of a fuel greatly affect the cost of a fuel treatment facility. A doubling of viscosity roughly doubles the cost of desalting equip-ment， and having a specific gravity of greater than .96 greatly complicates the washing system and raises costs. Trying to remove the last trace of a metallic element affects the cost of fuel washing approximately as shown in Table 12-7. The high cost of fuel treatment systems is the fuel washingsystem， since the ignition system costs about 10% of that amount. The fuel flow rate as well as the fuel type affect the fuel treatment system investment cost as shown in Figure 12-11.
Gas turbines， like othermechanical devices， requireinspection， main-tenance， and service. Maintenancecosts include the combustionsystem，hot-gaspath， and major inspections. (See Chapter 21.) The effect of fuel type on maintenance costs is shown in Table 12-8. A cost factor is shown using natural gas as unity. The cost of maintenance is subject to great variations. Recognizing the great difficulty in establishing expected maintenance costs
Table 12-7
Effect of Washed Fuel Quality on System Cost
Sodium .eduction Washing System Cost
100 → 5 ppm Na x dollars 100 → 2 ppm Na 2x dollars 100 → 1 ppm Na 4x dollars 100 → 1坦2 ppm Na 8x dollars
Figure 12-11. Gas turbine fue. treatment p.ant in.estment costs. (Courtesy of Genera. E.ectric Company.)
Table 12-8 Average Total Maintenance and Cost Factor for a Gas Turbine
Expected Actual Expected Fuel Maintenance Cost Maintenance Cost (mils/kWh) Factor
Natural gas 0.3 1.0二 Base line No. 2 distillate oil 0.4 1.25 Typical crude oil 0.6 2.0 No. 6 residual oil 1.0 3.33
Table 12-9
Economic Factors Influencing Fuel Selection
I. Fuel Cost
II. Operation
1. Power output for given turbine
2. Efficiency degradation
3. Outstage (Downtime)
III. Capital Investment
1. Fuel washing and inhibition
2. Fuel quality monitoring
3. Turbine wash and cleaning
IV. Duty Cycle
1. Continuous duty required
2. Total annual operation
3. Starts and stops
for different applications， Table 12-8 should be used as a rough guide in estimating costs. These data are based on actual maintenance costs for heavy-duty gas turbines.
As has beenshown， the selection of the most economical fuel can depend on many factors besides cost. Table 12-9 summarizes the major economic considerations in fuel selection.
Operating Experience
Early U.S. experience in residual operation dates back to the early 1950s. Several companies adapted gas turbines to run on residual fuel for loco-motive application. Operating with a low inlet temperature 1350 0F (732 0C)，low-sulfur residual corrosion was limited; however， it was noted that any increase in firing temperature was accompanied by serious corrosion.Because of the advantage of increased firing temperatures， research on fuel
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