Page 89 - Improving Machinery Reliability

Vendor Selection and Bid Conditioning 61

turers normally furnish test-stand verified performance curves (Figure 2-S), the
degree of the performance deterioration is readily evident from a comparison of
actual (fouled condition) and ideal (clean condition) fuel-consumption rates. As in
the case of steam turbines, the average turbine efficiency, or average fuel wastage,
could be calculated in convenient time intervals. The summation of energy losses
can be referenced to fuel-cost debits and on-stream cleaning initiated when cumula-
tive fuel costs outweigh cleaning costs.
In-depth performance analyses on pipeline drivers also established that exhaust-
temperature measurement errors are virtually unavoidable on power turbines. These
errors can be traced to slight changes in gas flow pattern and have been shown to
cause considerable change in average temperature readings, even from a group of
thermocouples. In turn, the errors can influence the power capability of a gas turbine
if exhaust-temperature limits are observed or made one of the controlling parame-
ters. With limit increases reported as high as 30°F (17"C), 7% increases in high
ambient temperature power capabilities have been produced. The corresponding
decrease in heat rates is 2%.8
The merits of performance monitoring via on-stream torque-sensing devices in
combination with inlet-temperature and fuel-rate metering are thus evident.

Performance Optimization and Determination of Fault, Driver Versus Dri-
ven. Continuous torque-sensing devices can provide valuable information about
other areas as well. While in the case of torque limitations on one or both of the cou-
pled shafts the devices might serve as constraint controls, torque sensing may allow
process optimization in computer-controlled compressor situations where several
levels of refrigeration may be available.
In another application, turbocompressors may be configured to achieve desired
flow and head by such methods as varying speed, varying stator-blade angle, and
varying guide-vane angle. On large axial compressors, a combination of stator-blade

Fuel rate wasted due
to fouling
Speciflc fuel consumption,
. -.-.
--, ~. -. -.
-
- Specific fuel consumption,clean
I I R I I 1 I I I I I I I I I I

Figure 2-5. Gas-turbine fuel consumption versus output horsepower.