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

4.1.7 Unloading 
Mechanisms 
Reciprocating compressors can have very high compression ratios. They can continue 
to discharge gas until the pressure is high enough to stop the driver or until something 
breaks. Pressure safety valves (PSVs) are fitted to protect equipment on the discharge 
side from overpressure and possible damage. 
PSVs are not meant to control discharge pressure during normal operating conditions.
Changing the speed of the compressor driver changes the compressor output. But all 
compressor drivers on the ADGAS plant are designed to operate at constant speed. 
Unloaders control the discharge pressure to meet the operating requirements of the 
plant. They do this by holding one or more of the suction valves open. The gas at 
suction pressure then passes freely in and out of the cylinder through the open suction 
valve. No gas is compressed in that cylinder. This has two effects: 
• reduces the capacity of the compressor so that it discharges less gas into the 
system, maintaining normal operating pressure 
• reduces the load on the compressor driver 
The second effect is useful during compressor start-up. One or more valve unloaders 
hold suction valves open to reduce the load on the driver until it reaches full speed.
Figure 4.29 shows unloaders on one of the New BOG compressors. 
Figure 4.29: Unloaders on New BOG Compressor 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 38 of 90
Some unloaders are manually operated.
Automatically controlled unloaders are pneumatically operated. Sensors monitor 
discharge pressure and activate one or more unloaders as soon as it exceeds the set 
point. Control air acts on a diaphragm that operates a rod. A set of fingers on the end 
of the rod push the valve against its spring pressure, holding it open. Figure 4.30 
shows a pneumatically controlled unloader.