Abstract: Selective valves to pass fluids consisting of a body and a housing, provided with a midway communicating passage in which is placed a dynamically governed mechanical system able to separate gases from fluids compressed by a subsurface oilfield pumps. The mechanical system utilizes a pair of half-shells, a ball or equivalent, provided with or acting on a roughness controlled surface which allows only a gas to pass.

Abstract: A selective valve to pass fluids consists of a body, and a housing, and is provided above and in the middle with a communicating passage for low surface tension and viscosity fluid, there being shells inside the body, seated upon the aforesaid housing, said shells consisting of a surface of a shape and a size that is controlled, a soft surface, and orifices, a sealing surface created in the control area of the shells where it meets the housing. There are below and in the middle, a communicating passage in touch with the fluid compressed by the pump. Preferred versions of the selective valve are provided with controlling blades and a ball and roughness controlling surfaces instead of the shells.