Inlet compression boosts a gas source to the optimal pressure for treatment at a facility. Over the course of the gas treatment, it will realize a pressure drop.
Sales compression returns the treated gas to a higher pressure for entry into a pipeline.
Inlet and sales units are sometimes combined. This would be called a multi-service unit.
Common Themes with Inlet & Sales Compressors
Inlet compression units are the highest flow and largest horsepower compressor packages at site. The entire gas capacity will run through the inlet compressor. Although dependent on supply and discharge pressures, it is typically a two or three stage reciprocating compressor.
Sales compression units typically see a smaller gas capacity than the inlet units, because the treated gas will have dropped out the heavier hydrocarbons.
Sales compression units are often low ratio, single stage. They also tend to see high quality gas since the supply has been recently treated.
Inlet and sales units are often installed in modules. This is partly due to volume restrictions, and partly due to redundancy and scalability. With four equivalent units, losing one for service or emergency shutdown only compomises 25% of the plant capacity.