Page 195 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Best Practice 3 .17
Compressor Best Practices Best Practice 3 .18
The advantages of an integral gear compressor are the reduced advised to exercise caution and confirm satisfactory field oper-
number of stages required and higher efficiency. This is because ating experience on similar gases before purchase. The re-
all stages are intercooled, and the integral speed increasing gear quirements for gas or oil seals, varying gas composition and flow
allows all stages to operate in a higher specific speed. rates and the possibility of fouling can significantly reduce the
Multistage, integral gear compressor manufacturers have reliability of this type of compressor opposed to air or inert gas
begun to offer these units for hydrocarbon service. Readers are applications.

Best
Best Practice 3.17Practice 3.17Practice 3.17
Best
Trend individual impeller performance and intercooler Intercooler gas side differential temperature is trended
temperature rise for all integral geared compressors to The cooling media operating conditions are within the ranges
ensure maximum operating range and reliability. defined on the intercooler data sheet
It is necessary to have pressure and temperature transmitters at the
inlet and outlet of each impeller stage to trend the performance (op- Lessons Learned
erating point) of each impeller head and efficiency to determine the
need for maintenance before internal conditions (excessive clearances Many integral geared compressor failures are the result of
and/or fouling) cause impeller damage. individual impeller and/or intercooler performance de-
Most integral geared compressors have a limited flow range for terioration. Failure to monitor these items will result in low
each impeller. Many types use diffuser vanes for increased stage head, compressor safety and reliability.
which further reduce the flow range.
Operation within the allowable flow range for each impeller (above Benchmarks
the surge margin and below the choke flow margin) will depend on the This best practice has been used since the late 1990s when many
performance of the intercoolers as well as the impeller stages. Ensure apparent integral geared compressor failures were encountered, only
that the following items are present in the design and predictive to find that the root cause was usually fouled intercoolers. This best
maintenance program: practice has resulted in above average reliabilities for un-spared in-
Cooler duty is oversized by a minimum of 10% tegral geared compressors (above 97%).
The maximum cooling media conditions (water or air) are defined on
the intercooler data sheet

B.P. 3.17. Supporting Material

Please refer to the supporting material for B.P. 3.16.

Best Practice 3.18Practice 3.18
Best
Require a shaft stiffness ratio of 10 or less to ensure Lessons Learned
a stable rotor free of sub-synchronous vibration frequen- I have been involved with many RCFAs of sub-synchronous
cies and second critical speed issues. vibration and critical speed issues where the shaft stiffness
During the pre-bid phase of the project (before vendor priced pro- ratio exceeded 10. Once the compressor is designed, it is
posal), obtain the bearing span dimensions and divide by the shaft very difficult to correct the issue, since changing the shaft
diameter under the impellers and take the following action: diameter requires that impellers be re-designed and the
If the ratio is less than 10, confirm that the first and second critical bearing span cannot be changed.
speeds meet the latest edition of API-617 prior to acceptance.
If the ratio is greater than 10, require vendor experience information Benchmarks
for the same pressure ratios and performance conditions, as well as
This best practice has been used since 1972, when the concept was
a minimum of 2 installed references that have operated in the field
introduced to me by an engineer that the writer was working with in
for 2 years or more.
a joint venture project. Implementing this best practice since that time
has resulted in compressor reliabilities that have exceeded 99.7%.

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