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A threaded joint includes a flange, a nut and a flexible seal. The flange, the nut and the flexible seal have threads respective inner surfaces, the threads having shapes complementary to threads on an outer surface of a stem. The flange is configured to allow the stem to move by rotation through th

A threaded joint includes a flange, a nut and a flexible seal. The flange, the nut and the flexible seal have threads respective inner surfaces, the threads having shapes complementary to threads on an outer surface of a stem. The flange is configured to allow the stem to move by rotation through the flange. The nut and the flexible seal are configured to move by rotation relative to the stem. The flange has a counter bore and the nut has a groove. The flexible seal has a first portion configured to fill the counter bore in the flange, and a second portion configured to fill the groove of the nut.

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1. A threaded joint, comprising: a flange having threads on an inner surface and a counter bore, the threads having shapes complementary to threads on an outer surface of a stem, the flange being configured to allow the stem to move by rotation through the flange;a nut having threads on an inner sur

1. A threaded joint, comprising: a flange having threads on an inner surface and a counter bore, the threads having shapes complementary to threads on an outer surface of a stem, the flange being configured to allow the stem to move by rotation through the flange;a nut having threads on an inner surface and a groove, the threads of the nut having shapes complementary to the threads on the outer surface of the stem, the nut being configured to move by rotation relative to the stem; anda flexible seal having threads on an inner surface, the threads of the flexible seal having shapes complementary to the threads on the outer surface of the stem, the flexible seal being configured to move by rotation relative to the stem, and having a first portion configured to fill the counter bore in the flange, and a second portion configured to fill the groove of the nut. 2. The threaded joint of claim 1, wherein the flexible seal is made of a material resistant to corrosion due to hydrogen sulfide (H2S). 3. The threaded joint of claim 1, wherein the flexible seal is made of a polymer with a durometer value of about 75. 4. The threaded joint of claim 1, wherein the first portion of the flexible seal has an outer diameter smaller than an outer diameter of the second portion. 5. The threaded joint of claim 1, wherein the nut has an outer surface made of rectangular sides which form a hexagon in a cross-section perpendicular to a rotation axis. 6. The threaded joint of claim 1, wherein the flexible seal is completely provided within the groove and the counter bore. 7. The threaded joint of claim 1, wherein the flexible seal is covered by the flange and the nut having no outer surface exposed to ambient. 8. The threaded joint of claim 1, wherein the threaded joint is formed around the stem having a first end inside a container which confines fluid having a high pressure, and a second end outside the container, the flange having one side towards an inside of the container, and one side towards outside of the container, the flexible seal and the nut being mounted on the stem outside of the container. 9. A method of sealing a threaded joint, comprising: moving a stem having first threads on an outer surface, by rotation, through a flange having second threads on an inner surface, the second threads having shapes complementary to the first threads;moving a flexible seal having third threads on an inner surface, by rotation, along the stem, towards the flange, until a first portion of the flexible seal fills a counter bore in the flange, the third threads having shapes complementary to the first threads;moving a nut having fourth threads on an inner surface, along the stem, towards the flange, until a second portion of the flexible seal fills a groove inside the nut, the fourth threads having shapes complementary to the first threads; andapplying a predetermined torque to the nut after the second portion of the flexible seal has filled the groove inside the nut. 10. A two piece sealing nut for sealing a threaded joint including a stem having threads on an outer surface, the two piece sealing nut comprising: a nut configured to surround and move along the stem and having a groove; anda flexible seal configured to surround and move along the stem, inside a flange that holds the stem, and to have a first portion configured to fill a counter bore in the flange, and a second portion configured to fill the groove in the nut. 11. The two piece sealing nut of claim 10, wherein the flexible seal and the nut have threads on respective inner surfaces, the threads of the flexible seal and the nut having shapes complementary to shapes of the threads on the outer surface of the threaded stem. 12. The two piece sealing nut of claim 10, wherein the flexible seal is made of a material resistant to corrosion due to hydrogen sulfide (H2S). 13. The two piece sealing nut of claim 10, wherein the flexible seal is made of a polymer with a durometer of about 75. 14. The two piece sealing nut of claim 10, wherein the first portion of the flexible seal has an outer diameter smaller than an outer diameter of the second portion. 15. The two piece sealing nut of claim 10, wherein the nut has an outer hexagonal surface. 16. The two piece sealing nut of claim 10, wherein the nut is made of metal or composite material. 17. The two piece sealing nut of claim 10, wherein the flexible seal is covered by the flange and the nut, having no outer surface. 18. The flexible seal of claim 17, wherein the flexible seal is made of a material resistant to corrosion due to hydrogen sulfide (H2S). 19. The two piece sealing nut of claim 17, wherein the flexible seal is made of a polymer with a durometer of about 75. 20. A flexible seal, comprising: a first portion configured to fill a counter bore in a flange that holds a threaded stem; anda second portion configured to fill a groove of a nut, and having a diameter different from a diameter of the first portion, whereinthe first portion and the second portion have threads on a surface of an inner hole configured to allow the flexible seal to move along the threaded stem, the threads of the flexible seal having shapes complementary to threads on an outer surface of the threaded stem.