Reciprocating Gas Compressors The single stage, reciprocating compressors create a low-pressure area in the pumping chamber when the piston moves from the highest point of the stroke to the lowest point. Gaseous product (at a higher pressure in the tank) moves into the pumping chamber. This volume of gas is then displaced when the piston moves to the top of the stroke. Equipped with high efficiency valves, ductile iron pistons, self-adjusting piston rod seals and other robust features, these compressors are designed to provide maximum performance and reliability under the most severe service conditions. They are also a critical component in the design of oil and gas vapor recovery units (VRUs).