The construction of middle oil-gathering facility, in which technological processes are managed in a closed medium is necessary for environmental protection to control highly corrosive medium in oil and gas production. Associated gas separated from the fluid in initial separation unit within middle oil-gathering facility enters gas-gathering point with low pressure, and the liquid - into the pig of oil, water and sand, which should be constructed from iron concrete for cleaning from mechanical impurities sediments and salt as well. The liquid charge from the separation unit and pig of oil, water and sand is based upon the law of communicating vessels. To supply long-life for reservoirs, the inner and outer walls should be covered with a special coating and additionally, electrochemical protection should be provided as well.