Page 176 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 3 .12
Be st Practice 3 .13 Compressor Best Practices

Best Practice 3.12

Select individual impellers or blade rows to operate as Lessons Learned
close to best efficiency point (BEP) as possible. Root Cause Failure Analysis (RCFAs) of impeller failures
This will ensure: confirm the cause to be operation in the overload region
Maximum overall compressor curve flow range (more than 20% greater than rated flow) of the individual
Trouble free heavy gas impeller operation for gas molecular weights impeller curves.
above 40
Ensure that each proposed impeller, when operating at the rated Benchmarks
condition, is a maximum of þ/ 10% from its best efficiency point, and Since the mid-1980s I have implemented the best practice of review in
þ/ 5% from its best efficiency point for gases above 40 molecular the pre-bid phase of individual proposed performance curves for each
weight. impeller, and have required vendor re-selection of impellers if the rated
Agree to review and not retain the individual vendor individual operating point for each impeller was greater than þ/ 10% from the
performance curves since they are proprietary. impeller best efficiency point, or þ/ 5% away if the gas molecular
Many compressor vendors can presently (2010) manufacture each weight was greater than 40. This best practice has resulted in trouble
impeller to operate at its best efficiency point when operating at the free Factory Acceptance Tests (FATs) and compressor field reliabilities
specified compressor rated operating point. in excess of 99.7%.

B.P. 3.12. Supporting Material

See the supporting material for B.P: 3.11.

Best
Best Practice 3.13Practice 3.13
Gas density changes of more than D/L 20% affect cen- inspected. After this, it was discovered that the operating
trifugal compressor performance curve (head vs. actual gas density conditions had changed by more than 20%,
flow) shape. Require the vendors to re-issue the perfor- and a performance recheck using the vendor revised
mance curve in these instances. performance curve for the actual gas density showed the
Impeller head is produced by the velocity of the impeller tip and the compressor to be in good condition.
tangential velocity of the gas through the impeller (Euler’s turbo There are many cases where field performance checks, carried out
equation). where a compressor was thought to be in need of maintenance, have
Unlike pump impeller head, where the liquid is incompressible, showed that it was the performance curve that was in error, because
compressor head is affected by the gas density. This is the reason that the gas density had changed by more than 20%.
a centrifugal pump can be tested on water and, provided the field liquid Only the vendor can re-issue a new overall performance curve
is not viscous, will yield exactly the same performance head vs. flow because only the vendor is in possession of the individual impeller
curve in the field. By Euler’s turbo equation, the tangential velocity of performance curves.
the liquid through a pump impeller is the same regardless of the liquid
density, since the liquid is incompressible and the impeller geometry Benchmarks
(flow area) is unchanged.
This best practice has been used successfully since the mid-1990s, for
However, if the gas density in a centrifugal impeller or axial blade
field testing prior to turnarounds. In some cases, field performance
changes by more than 20%, the tangential velocity of the gas will
where gas density has changed by 20% or more was in fact accept-
significantly change and the individual impeller performance curve able, when compared to the re-issued curves for the correct gas
shape (head vs. flow) will change. density.
In these cases, time-consuming compressor dis-assembly was
Lessons Learned averted, thus saving turnaround time and allowing additional pro-
Field performance checks, which did not consider gas duction revenue (3 e 7 days depending on the type of compressor
density change, originally confirmed that a compressor case and nozzle orientation e top nozzle orientation on horizontal split
required maintenance, only to find that the internal condi- compressor cases being the longest time).
tion of the compressor was in good condition when

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