Page 88 - Improving Machinery Reliability

60 Improving Machinery Reliability

where
Do = steam, kghr
NTK = coupling power, kw
Aiad = enthalpy drop at constant entropy, kcal/kg

Knowing the “as-clean’’ efficiency of the turbine, we now also know the energy loss
due to inefficiency in the one-minute interval. Since one HP minute equals 42.44 Btu,
and one kw minute equals 14.34 kcal, the summation of these losses can be refer-
enced to utility-cost debits and, finally, compared to the cost of on-stream cleaning.
Figure 2-4 can be used for screening studies showing the approximate cost of
operating prime movers with an assumed efficiency decay for relatively short time
periods. The potential cost of losing efficiency or output capability can be stagger-
ing. Howell and McConomy7 documented a 7.2% capability loss that occurred on a
138 MW steam turbine within approximately 360 days. The same machine experi-
enced a 3.6% loss in a 90-day period.
Axial-compressor fouling continues to be a common and persistent cause of
reduced gas turbine efficiency. A I % reduction in axial-compressor efficiency
accounts for approximately 1!4% increase in heat rate for a given power output. Even
compressor stations that are not subjected to industrial pollutants or salty atmos-
pheres are frequently prone to fouling. Typical of these conditions would be pipeline
operations in the northern hemisphere where insects and fine dust are found to
adhere to inlet guide vanes, compressor blades, and stator blading.8
Performance deterioration of gas turbines can be detected by combining turbine
fuel flow rate with horsepower output. Display modules can thus incorporate read-
outs of horsepower produced per cfm of fuel consumed. Since the turbine manufac-

Figure 2-4. Cost of operating inefficient prime movers.