Abu Dhabi Gas Liquefaction Company Ltd Job Training Mechanical Technician Course Module 9 Compressors

Figure 5.2: Booster Compressor Platform, Trains I and II 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
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The principle of operation and construction of centrifugal compressors is similar to 
that of centrifugal pumps. The drawing of the basic construction shown in Figure 5.3 
is exactly the same as for a pump. 
Gas enters the low-pressure area at the eye of the impeller. As the impeller rotates, 
the gas moves outwards by centrifugal action and its pressure starts to increase. It 
then passes between the vanes of the diffuser, where pressure increases more. Finally 
it travels around the volute, the pressure continuing to increase as it makes its way to 
the discharge. Figure 5.4 shows a medium-sized, single-stage centrifugal 
compressor. 
Figure 5.3: Basic Centrifugal Compressor
Figure 5.4: Single-stage Centripetal Compressor 
Impeller 
Gas 
flow 
Diffuser vanes 
Impeller 
eye 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
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5.1.1 Impellers 
Centrifugal compressor impellers work in exactly the same way as a pump impeller 
but they are a little different in design. Impellers may be: 
• open—with no shrouds 
• semi-closed—with a shroud on one side 
• closed—with shrouds on both sides 
These three types of impeller are shown in Figure 5.5. 
Most centrifugal compressors use semi-closed or closed impellers. Multi-stage 
compressors may have an open-impeller first stage, with closed impellers for the other 
stages. The vanes at the impeller eye are often more steeply angled to give a 
smoother entry path for the gas, as shown in Figure 5.5(b). 
Figure 5.5: Centrifugal Compressor Impellers 
(a) Open 
(b) Semi-closed 
(c) Closed 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 47 of 90