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Following the efficiency data, one notes that it very gradually deteriorates (due to normal wear and tear) until between service
months 18 and 34.  What happened was that prior to service month 34, cooling water leaked into the Low Pressure
compressor body from the first stage inter-cooler during a plant start-up, due to a tube leak in the cooler.  The compressor
body substantially filled with water while operating in start-up between slow roll and minimum governor.  Of course the
equipment was not designed to operate in this manner, so the compressor vendor was not at fault.  Following this failure and
resultant damage assessment, the Low Pressure compressor case was overhauled and replacement seals were installed at
three years of service.  Note that the High Pressure compressor body was not overhauled, so improvement did not result in
that portion of the machine.  The initial performance after the refurbishing overhaul is apparent in the 36 service month data
set.  Note that the original start-up efficiency was not fully restored, because the high pressure compressor was not worked
on.  Also, it is evident that the original start-up efficiencies for the Low Pressure body (stages 1 and 2) were not fully restored
subsequent to the overhaul.  This is because there was some cumulative wear in the impellers and diffusers which were not
replaced during the overhaul.

With the rapid run-up in energy costs following the Spring of 2000, it has become increasingly critical to monitor compressor
efficiencies and to perform maintenance at increased frequency to maintain compressor performance and overall plant energy
consumption.  The predicted penalty occurring from lack of maintaining efficiency for the Process Air Compressor example is
illustrated in Table 2, Energy Penalty for Neglecting Process Air Compressor Efficiency.
Thus, there is a very strong economic incentive to routinely monitor, calculate and sustain efficiency and performance for
centrifugal compressors resulting from ordinary wear and tear and other unintentional damages.

Retrofits for Improved Compressor Performance

Two common types of retrofit projects can improve compressor performance.  First, compressor rotors and impellers,
stationary diffusers and labyrinth seals can be replaced with state-of-the-art designs.  This retrofit often reduces energy
consumption of the compressor by about 8 percent or more for efficiency improvements greater than 6 percent.  Costs vary,
but generally fall in the $1.5-2.5 Million range for Process Air, Refrigeration and Synthesis compressor applications in large
tonnage ammonia plants.  (For plants of approximately 1500-1800 STPD ammonia capacity) Generally, energy costs
greater than $3.50-4.00 per Million Btu (HHV) are necessary to provide DCF returns greater than 20 percent, when
improved capacity is not considered in the economics.  (Economics based solely upon energy savings alone)
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