Seal coupling system for valve, said seal coupling system comprising: - a coupling portion protruding from a body of said valve along a longitudinal axis and suitable for engaging with a fitting seat obtained on a connection of a plant along which a fluid runs, - said coupling portion being provided with a first thread suitable for engaging with a second thread of said fitting seat, - an annular sealing gasket element positionable on said coupling portion, - said coupling portion comprising a zone on which there is obtained said first thread and a further zone on which there is obtained a housing seat shaped for housing said annular sealing gasket element, said housing seat being cylindrical and extending between said first thread and an abutting surface transverse to said longitudinal axis, characterised in that, - said annular sealing gasket element has a cross section, according to a plane passing through said longitudinal axis, that has a zone of greater thickness, measured in a direction that is orthogonal to said longitudinal axis, and a zone of lesser thickness configured for penetrating, and being tightened, between said second thread and an external cylindrical surface of said coupling portion when said coupling portion is screwed on said fitting seat, - wherein, in a non-deformed rest configuration, said cross section has in said zone of lesser thickness a tapered profile converging towards said longitudinal axis according to a direction that points towards said first thread, - constraining means being further provided which are configured for preventing a relative rotation of said annular sealing gasket element with respect to said coupling portion, said constraining means comprising protruding means and/or recess means obtained on said cylindrical housing seat and distributed around said longitudinal axis for at least one angular portion, said protruding means and/or recess means being arranged for coupling with part of said annular sealing gasket element in a work configuration in which said annular sealing gasket element is deformed upon a tightening screwing of said coupling portion. - a coupling portion protruding from a body of said valve along a longitudinal axis and suitable for engaging with a fitting seat obtained on a connection of a plant along which a fluid runs, - said coupling portion being provided with a first thread suitable for engaging with a second thread of said fitting seat, - an annular sealing gasket element positionable on said coupling portion, - said coupling portion comprising a zone on which there is obtained said first thread and a further zone on which there is obtained a housing seat shaped for housing said annular sealing gasket element, said housing seat being cylindrical and extending between said first thread and an abutting surface transverse to said longitudinal axis, - said annular sealing gasket element has a cross section, according to a plane passing through said longitudinal axis, that has a zone of greater thickness, measured in a direction that is orthogonal to said longitudinal axis, and a zone of lesser thickness configured for penetrating, and being tightened, between said second thread and an external cylindrical surface of said coupling portion when said coupling portion is screwed on said fitting seat, - wherein, in a non-deformed rest configuration, said cross section has in said zone of lesser thickness a tapered profile converging towards said longitudinal axis according to a direction that points towards said first thread, - constraining means being further provided which are configured for preventing a relative rotation of said annular sealing gasket element with respect to said coupling portion, said constraining means comprising protruding means and/or recess means obtained on said cylindrical housing seat and distributed around said longitudinal axis for at least one angular portion, said protruding means and/or recess means being arranged for coupling with part of said annular sealing gasket element in a work configuration in which said annular sealing gasket element is deformed upon a tightening screwing of said coupling portion.