provided on the face of seat 200 that is opposite ball 300 said sealing elements coming into contact with said ball 30 when the sealing system is assembled in a valve said sealing element made of elastomeric material 500 consists of a ring of triangular cross section similar to the greek capital letter delta and therefore called a u201c delta ring u201d this sealing system is characterized in that it comprises a second element made of thermoplastic material 600 also positioned between said seat 200 and said ball 300 in particular said thermoplastic element has the shape of a ring preferably of trapezoidal section and is housed in a second seat 600 a again suitably provided on the face of said seat that is opposite ball 300 said ring made of thermoplastic material indicated in the drawing with 600 is advantageously positioned close to the elastomeric element 500 for the purpose of retaining it in the seating and of becoming a second sealing element as the pressure rises the sealing system according to the present invention is thus more compact than the system provided with a retaining element 50 known from the prior art and the compactness of the seat translates into a reduction of the final costs of the seat and of the whole valve in which the seat is assembled in this way there is the further result that it allows the technician to install the seat according to.