International standards for casing pipe threads require that when the inner and outer tapered threads are fitted together, they must meet the face allowance requirement, allowing axial displacement to create an interference fit at the connection surface. This interference enhances the processing quality in the connection area, increases axial force, and improves the contact pressure in the sealing area. As a result, a tighter ring-shaped contact area is formed, reducing potential leakage paths and ensuring a more reliable seal. The larger the contact area, the less likely it is for leaks to occur, strengthening the overall sealing performance. Casing pipe threads use a special tapered sealing method known as the taper pipe thread seal, which features a thread groove for added sealing capability.
When the tapered pipe threads meet, fulfilling the face allowance (contact distance), the axial displacement caused by screwing the threads together creates a preload on the sealing surface. This preload helps establish the required sealing force. For effective sealing, the matching inner and outer tapered threads must form a continuous or discontinuous ring-shaped contact area at the connecting surface.
In the casing pipe’s thread cone, the presence of threads increases the surface area of the conical helical surface, creating a labyrinth-like structure that makes it difficult for pressure media to pass through, which aids in sealing. However, this also introduces potential contact surface irregularities that could lead to leaks. Therefore, the elements of the tapered thread must be as precise as possible during design and manufacturing. Additionally, when tightening the threads, a certain amount of interference is required between the inner and outer threads, which causes the thread teeth to undergo complex deformations such as compression, stretching, and twisting. These deformations help compensate for any imperfections on the contact surface, effectively sealing gaps and preventing leaks.
