Page 170 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Best Prac tice 3.9
Be st Practice 3 .10 Compressor Best Practices

Best Practice 3.9

During the pre-bid project phase (before priced vendor Lessons Learned
bids are received), screen for proven impeller or blade row The failure to review for impeller/blade experience prior to
flow, head coefficient and tip speed experience on similar vendor acceptance can result in extended FAT time, delayed
gas compositions. field start-up and continual safety and reliability field issues.
Centrifugal and axial compressors are custom designed, but should Accepting vendor-proposed impellers and blades without a review
incorporate proven impellers or blades and gas path parts. of their experience and of the relevant design details has led to many
The success of the Factory Acceptance Test (FAT), field start-up unexpected surprises during factory acceptance performance testing
and process life safe and reliable compressor reliability is directly re- and field start-ups.
lated to impeller, blade and gas path component integrity. Once impellers or blade rows are designed and operating, field
As a result, vendor requirements should be noted in the invitation to changes are difficult, time consuming, will produce revenue loss and
bid (ITB), to provide the following parameters for each stage (impeller or are difficult to confirm, since field instrumentation does not have the
blade row). This will ensure optimum FAT results and field safety/ same accuracy as FAT instrumentation.
reliability:
Flow coefficient experience Benchmarks
Head coefficient experience I have used this best practice since the mid-1970s to achieve problem
Tip speed experience free FATs, smooth start-ups and optimum centrifugal and axial com-
Impeller inlet (eye) Mach number experience pressor safety and reliability (of over 99.7%).
Above parameters for similar gas compositions

B.P. 3.9. Supporting Material

The review of each impeller stage or blade row for vendor in- Flow coefficient map of all experienced flow coefficients
stalled 2 year operating experience during the pre-bid stage of Head coefficient map of all experienced head coefficients
the project requires each quoting vendor to provide the fol- Impeller tip speed, map of all similar flow and head
lowing parameters for each quoted impeller or blade row: coefficient impellers

Best
Best Practice 3.10Practice 3.10
Limit closed centrifugal compressor head per impeller occurred when the above head limits were exceeded in the
values for optimum impeller performance and reliability in compressor design.
the project specification. Centrifugal compressor head vs. flow performance curves fell off
Limiting centrifugal compressor impeller head per stage will ensure sharply in head and flow beyond the rated point, but had to be ac-
the optimum flow range for each impeller and the overall compressor cepted since the centrifugal curve shape is not guaranteed. This re-
performance flow range. In addition, it will ensure that all impeller duction in flow means a lower production rate for the life of the
stresses are within safe, proven limits that will result in failure-free compressor, which amounts to a significant loss of revenue.
impeller life. Closed impeller failures in the impeller cover, hub side plate and
Limit heavy gas applications (molecular weights above 40) to scallop failures (between the impeller vanes at the outside diameter on
24 kJ/kg or 8000 FT-lbf per impeller. the cover or hub) were experienced when the head per impeller limits
Limit other services below 40 molecular weight to 3000 meters or were exceeded.
30 kJ/kg or 10,000 lbm per impeller.
In applications where the use of these head limits forces the Benchmarks
compressor into 2 cases FT-lbf/lbm, be prepared to justify this re- I have used this best practice since the mid-1970s and it has resulted
quirement based on the loss of daily revenue. Reducing the head per in:
impeller in the field will either require reduced operating speeds and Meeting or exceeding predicted compressor predicted curve shape
proportional lower revenue or a new compressor train design. beyond the rated point
Do not accept impeller heads above these values even if vendor Fault-free closed impeller performance (zero compressor impeller
experience is proven. failures)
Lessons Learned
Failure to meet predicted performance curve flow limits
beyond the design point and field impeller failures have

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