What Is an Inlet Gas Separator: Purpose, Sizing and Design Guidelines Answering the core question: What is an inlet gas separator? An inlet gas separator is the first vessel a gas stream enters at a processing plant, a compressor station, a dehydration unit or a metering skid, and its job is protective: it removes the free liquids and solids that would otherwise damage, foul or shut down everything downstream. Because the liquid loading at a plant inlet is unknown by nature