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Generally speaking, within the petroleum marketing industry, coaxial is used to describe a piece of equipment that consists of a tube within a tube. A coaxial hose, for instance, is an outer hose with a smaller hose inside. A coaxial fitting is one that can be attached to a single opening, but which will permit liquid to flow through one channel in the fitting while vapors are simultaneously flowing through another channel in the same fitting.

Coaxial design has become important within the industry in recent years because of gasoline vapor recovery requirements. When gasoline is being unloaded into an underground storage tank, for instance, as liquid flows into the tank displaced vapors flow out. These vapors must be captured and returned to a terminal for processing.

In some locations, the transport driver hooks a delivery hose into one opening in the tank, and connects a second hose–a vapor-return hose, to another opening. This is referred to as a two-point connection. At locations designed to accommodate coaxial delivery, however, the driver connects a single fitting to the tank opening. To one port on this fitting, the driver would connect a gasoline delivery hose; to another port, the driver would connect a vapor-return line. Thus, the coaxial fitting permits simultaneous product delivery and vapor retrieval through a single opening in the tank.

Dispenser hoses, used in gasoline stations where Stage II vapor recovery is required, are generally coaxial. The hose connecting the nozzle to the dispenser appears to be a single hose. Actually, it is a hose within a hose. Gasoline flows through the inner hose, into the motorist’s tank. Displaced vapors, from the tank, enter the nozzle and then flow back toward the dispenser by passing through the space between the inner and outer hose. In some coaxial hoses, liquid flows through the outer hose and vapor through the inner hose.