A fluid control system in pneumatic conveying ducts of powdered or granular material, comprising: a supply source of a pressurized gas for transporting said powdered or granular material; an injection line of said gas to put said source in fluid communication with a conveying duct inside which the material is conveyed; a self-regulating pressure valve arranged on the injection line between the source and said duct; a control member of the self-regulating valve to provide a control signal to said valve representing a pressure value, said valve maintaining the gas flow supplied in said duct constant; and a flow and flow rate meter, arranged on the injection line upstream of the valve to transmit a flow and flow rate value to said control member.