Premium connection generally has multiple seals, including main seal (mainly realized by metal / metal radial seal structure), auxiliary seal (generally realized by torque shoulder), elastic seal (generally realized by teflon seal ring), etc. in addition, although the thread does not play a major role in sealing, it still has a certain sealing effect. Because of these sealing structures, special threaded joints generally have good sealing performance.
For example, nk3sb premium connection is a product of Japan NKK company. Its main seal adopts the spherical / conical sealing form, and the torque shoulder is a right angle shoulder.
Nscc premium connection is produced by Nippon Steel Corporation of Japan. This kind of joint is suitable for casing. Correspondingly, NSCT joint suitable for oil pipe has been developed by the company. The basic structure of the two joints is the same. The characteristic of this joint is that it adopts double torque shoulder structure and can bear large compression load. The main seal adopts a conical / conical radial metal seal structure, with a sealing taper of 1:10 and its position between two shoulders. The sealing performance is not affected by the plastic deformation of the main torque shoulder. Although the joint has two torque shoulders, in general, only the main shoulder plays a role, while the auxiliary shoulder only plays a role when the over torque is buckled up and bears a large compression load.
TM premium connection is a product of Sumitomo Metal in Japan. The main seal of this kind of joint adopts the conical / conical radial metal interference seal, and the sealing taper is between 1:10-1:6. The torque shoulder adopts the form of right angle shoulder. After the joint is tightened, the sealing cones of the internal and external threads are interference with each other within the elastic range, generating a certain contact pressure, forming a seal. The right angle torque shoulder is mainly used for buckle positioning.
In general, the main sealing structure of premium connection mainly includes cone / cone, Cone / sphere, cylinder / sphere and so on. After the joint is tightened, the main sealing structure forms interference fit, which produces large contact pressure and plays a main sealing role. The auxiliary sealing structure is mainly realized by the torque shoulder. After the joint is tightened, the torque shoulder of the internal and external threads contacts and generates a certain contact pressure, which plays the role of sealing. The reason why the torque shoulder seal is called the auxiliary seal is that under the action of tensile load, the contact pressure at the shoulder will obviously decrease, which makes its sealing performance significantly reduced. However, through the use of elastic sealing structure of sealing ring, due to the limited service life of sealing ring and the great stress concentration at the sealing ring after the joint is tightened, it is seldom used in premium connection, and is generally not recommended.