For API thread, there is still a spiral leakage channel after engagement, which is the inherent defect of API thread design and limits the sealing performance of API thread. Premium connection is different. The sealing function of this kind of thread is no longer realized by thread matching, but a special structure is designed for this purpose, generally a metal sealing structure. In the design of metal seal structure, the non-contact seal of API thread is improved to contact seal, so that the sealing ability of the joint is significantly enhanced. In addition, the premium connection is generally designed with a series of auxiliary sealing structures to further improve its sealing performance.
Generally speaking, for metal seal structure, the condition to prevent internal fluid leakage is that the contact pressure on the sealing surface is greater than the internal fluid pressure. Many researchers of API thread and premium connection use this condition as the basis of seal design. However, the basis for judging the sealing is based on the fact that the sealing surface is completely smooth. In fact, due to the reason of metal processing, the sealing surface can not be completely smooth, but has a certain roughness, which makes the sealing surface still has a small gap after interference fit.
Due to the addition of metal sealing structure, the premium connection relies on metal interference fit to achieve sealing. Although the metal contact surface can not be completely smooth, there is also a small gap, but the gap is far less than the engagement gap between API threads, which greatly improves the sealing reliability of special threads. It should be pointed out that due to the certain roughness of the metal sealing surface, the sealing performance of this joint still has reliability problems.
In the design of premium connection, two conditions shall be met as far as possible from the perspective of improving the sealing performance:
1. The contact pressure shall be as large as possible to make the area of leakage path smaller.
2. The contact area shall be as large as possible to make the leakage path longer.
At the same time, the surface finish of the sealing surface should be high.