Page 190 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 3 .15 Compressor Best Practices

Table 3.14.1 Examples of fouling
Service Preventive action Cleaning method
Air Good inlet filtration Water wash catalyst or
nut shell
Cracked gas Process gas temperature Solvent wash
control/continuous injection
FCC wet gas
TCC wet gas

Coker gas Continuous injection Solvent wash
Polyethylene Control process conditions Stationary impeller
and possible impeller coating Heating (off-line)
Reformer recycle Prevent carry-over Steam/water wash

Best Practice 3.15Practice 3.15
Best
Restrict the use of low solidity diffusers (LSD) to positively from the mismatching of the LSD vane angle and the im-
eliminate gas disturbances which can result in reduced peller exit gas angle.
centrifugal compressor safety and reliability.
Low solidity diffusers are used to direct the gas out of the impeller in Benchmarks
a radial direction and not in the log spiral, as vaneless diffusers do. The This best practice has been used since the late 1990s, when I was
result is reduced friction in a LSD, producing increased head and ef- involved with a LSD type propylene compressor (molecular weight ¼
ficiency, but less flow range when compared to a vaneless diffuser. 44) that could not operate beyond 103% flow from the rated point
If the fixed angle of the LSD vanes do not match the impeller exit without tripping the compressor in the field.
gas angle, gas disturbances can occur (stall cells) at frequencies less During the Factory Acceptance Test (FAT), which was performed on
than approximately 40% of operating speed, which can excite the rotor nitrogen gas, it was discovered that there was a sub-synchronous vi-
to produce large external forces and shaft vibration. bration frequency component greater than 25% of the overall vibration
The forces exerted on the rotor by LSD stall increase with the at a frequency of 40% of operating speed. The recommendation was to
molecular weight and density of the gas, and can be large if the mo- reject the compressor, in accordance with project and API 617 spec-
lecular weight of the gas is greater than 40. ifications, modify the LSD vane angles and retest. The project team
decided to accept the compressor based on start-up schedule
Lessons Learned considerations.
The use of low solidity diffusers (LSD) has resulted, in some The compressor had to operate at flows less than 103% of rated
cases, in severe production reductions and compressor flow for 3 years until the problem was corrected. During this period,
many compressor trips occurred that resulted in losses of revenue
vibration trips, due to gas disturbances (stall) that result
totaling millions of dollars.

B.P. 3.15. Supporting Material forces on the impeller e which would otherwise result in im-
peller cover breakage.
Low solidity diffusers (LSD) have been used since the late
1980s to increase the head produced per stage, and potentially
reduce the number of stages in a multistage centrifugal com- Surge facts
pressor, and so reduce the quoted capital cost.
The name ‘low solidity diffuser’ comes from the fact that the Surge can be a very dangerous phenomenon if it is allowed to
vanes do not extend through the entire length of the diffuser, continue. The damage caused by surge in any type of dynamic
but are located approximately in the middle of the diffuser compressor can render it inoperable for periods of time in excess
passage for purposes of avoiding pulsating impeller/LSD vane of two (2) months. Considering that most dynamic compressors

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