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

Figure 5.9. 
Notice three things about the compressor in Figure 5.9: 
• the impellers get smaller as the gas pressure increases and its volume is 
compressed, just like the cylinders of reciprocating compressors 
• this compressor has four impellers so a mechanical engineer would describe 
it as a four-stage compressor 
• the gas only enters the casing at one point and leaves the casing at one point so 
a process engineer would call it a single-stage compressor 
Figure 5.9: Back-to-back Impellers 
Single Process Stage 
Suction nozzle 
Discharge nozzle 
Stage 1 
Stage 2 
Stage 4 
Stage 3 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 52 of 90
If impellers are all facing the same way, balancing drums or balancing discs are fitted 
to reduce this out-of-balance force. They are located at the discharge end of the 
impeller shaft as was shown in Figure 5.8. Gas at final discharge pressure pushes 
back against the impeller thrust to reduce the load on thrust bearings. 
If some gas is needed at a pressure lower than the final discharge pressure it can be 
taken out of the compressor between stages. Also, if some of the feed gas is at a 
pressure higher than the main feed it can enter the compressor between stages, as 
shown in Figure 5.10. 
This is called a four-stage compressor by mechanical engineers and a three stage 
compressor by process engineers. 
Figure 5.10: Inter-stage Side Streams
Initial suction 
nozzle 
Final discharge 
nozzle 
Stage 1 
Stage 2 
Stage 3 
Stage 4 
Side-stream in at 
stage-1 discharge 
pressure 
Side-stream out at 
stage-3 discharge 
pressure 
First Process 
Stage 
Second