Hydrocarbon Processing - March 2022 - 17

Reliability
HEINZ P. BLOCH, RELIABILITY/EQUIPMENT EDITOR
Heinz.Bloch@HydrocarbonProcessing.com
Use available database to assist
in compressor design audits and selection
Over the last 40 yr, centrifugal compressor manufacturers
have compiled a vast array of impellers and associated stationary
components, allowing for higher efficiency staging.
Within this array, different families of impellers and stationary
parts allow application engineers to make selections to match
various process conditions. They have achieved this through
newer and more sophisticated machining technologies, and by
consistently using more accurate computational fluid dynamic
(CFD) analyses. However, even with these advanced technologies,
the matching of internals such as diffusers and return
channels with new and improved impellers can be difficult. In
instances where trial and error must be used, the end results
will be accurate, but it will take time to get there. Geometric
scaling is still the common scale-up technique-it makes use
of similitude or proportionality principles.
FIG. 1 shows two trend lines of head per stage vs. molecular
weight obtained during a 40-yr period of data mining. The
database covers molecular weight applications ranging from 4
to 104, using compressors from different manufacturers. These
trend curves have been extremely helpful in the past as a rapid
checking tool for proposals from various compressor manufacturers.
Both new and revamped (re-rated, upgraded) compressors
are represented in FIG. 1.
To reconfirm-for the most part, compressor manufacturers
do get it right when matching stages to process conditions.
However, problems arise when scaling is used to extrapolate
stages outside the vendor's normal experience or comfort
zone without proper CFD validation techniques. Therefore,
user-purchasers must check a manufacturer's proposed staging
when purchasing centrifugal compressors. The purchaser
should ensure that compressor manufacturers submit experience
lists for these stages. Alternatively, or if the user has data
from past purchases, in-house experience could be utilized.
Early delivery of the compressor train, field erection with attention
to the proper placement of temporary but fully approved
measuring instruments, and full-scale field testing are
often possible and should be on the purchaser's list of options.
Trend line A-B indicates compressors manufactured pre1990,
and trend line C-D is for compressors manufactured
post-1990 to about 2019. Points E and F were obtained from
two case histories where the compressor manufacturer initially
did not perform proper CFD to match the high-coefficient
impellers. Several iterations had to be done using high-fidelity
CFD techniques to match the stationary internal components
with the proposed impeller.
Over the years, trend lines A-B and C-D have shown that if
a compressor manufacturer's compression stage design fits on
18
16
14
12
10
8
6
4
2
4
24
44
64
Molecular weight
FIG. 1. Head per stage vs. molecular weight curves obtained
during 40 yr of data.
or below these lines, the compressor will likely perform as designed.
These lines are considered suitable audit tools to either
have confidence in a proposed offer or to raise questions on the
compressor manufacturer's proposed staging. However, these
lines are not considered fool-proof and should only be used to
raise questions when designs deviate from the lines.
Experienced third parties are sometimes brought in to assess
certain risks or to assist in resolving different opinions
before they become more serious disputes. Engaging in structured
machinery quality assessment (MQA) will be of great
benefit to the user-purchaser.1
It has been established that budgeting an additional 5%
of the cost of critically important process machines to fund
MQA activities pays back handsomely. The tendency of an
MQA to catch vulnerabilities before they reach the field is
the primary reason for a facility to be later counted among the
best-in-class performers.
LITERATURE CITED
1
Elliott, H. G. and H. P. Bloch, " Compressor technology advances: Beyond 2020, "
Walter DeGruyter, Berlin, Germany, ISBN 978-3-11-067876-5, 2021.
HEINZ P. BLOCH resides in Montgomery, Texas. His professional
career commenced in 1962 and included long-term
assignments as Exxon Chemical's Regional Machinery
Specialist for the United States. He has authored or co-written
more than 780 publications, among them 24 comprehensive
books on practical machinery management, failure analysis,
failure avoidance, compressors, steam turbines, pumps, and the
just released " Optimized Lubrication, Oil Mist Technology, and Standstill
Protection, " (DeGruyter, Berlin/Germany, ISBN 978-3-11-074934-2). He holds BS
and MS degrees (cum laude) in mechanical engineering and is an ASME Life
Fellow with lifetime registration as a Professional Engineer in New Jersey.
Hydrocarbon Processing | MARCH 2022 17
84
104
Head per stage - FT-LB/LB x 1000
Hydrocarbon Processing - March 2022

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