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

4.1 Reciprocating 
Compressors 
Most reciprocating compressors are piston-type. Piston compressors are very similar 
to piston pumps in their basic design.
ADGAS uses piston reciprocating compressors to: 
• supply instrument and service air for the Sulphur STOREX area 
• supply emergency instrument air for all other plant
• compress vapour that evaporates off the liquefied gas held in storage tanks in 
the STOREX area 
Vapour from the LNG tanks is called boil-off gas (BOG). This is compressed in one 
of the two BOG compressors and used as fuel gas on the plant.
Vapour from the C
3
(propane) LPG tanks is compressed in the vapour recovery unit 
(VRU) compressor before cooling and converting back to liquid. 
Figure 4.2 shows one of the two air compressors in the Sulphur STOREX area. 
Figure 4.2: STOREX Air Compressor (Tag# 28-K-70) 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 16 of 90
4.1.1 Single- and Double-acting Compressors 
Reciprocating compressors can have single- or double-acting cylinders. The principle 
of operation of these two arrangements is exactly the same as for pumps. Figure 4.3 
shows a single-acting compressor with the main components labelled. 
The single-acting compressor discharges gas on every second stroke, as the piston 
moves away from the crankshaft. This gives a pulsating flow. 
The section drawing in Figure 4.3 shows the piston at top-dead-centre—the piston is 
as far to the right as it can go. The compressor shown is a horizontal unit with the 
cylinder axis horizontal. It is also cooled by water and the cylinder has a water jacket 
around it. 
The main components of a piston compressor are very similar to those of a piston 
pump.