Threaded Pipe Coupling Pin Nose Rigidity and Seal Design
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Solution Overview
Problem
The existing threaded joints for steel pipes, particularly in oil and gas well applications, face challenges in maintaining sealability due to plastic deformation and compression-induced diameter reduction at the seal point, leading to insufficient contact pressure and potential galling issues.
Innovation Solution
The design involves a pin with an outward convex curved seal surface and a box with a tapered sealing surface, where the cross-sectional area at the seal point is 35% or more of the original pipe area, and the seal point is positioned such that the angle with the joint axis is less than 45 degrees, reducing deformation and enhancing sealability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the seal taper angle is reduced to increase radial seal contact pressure, then sealability is improved, but galling is prone to occur during make-up
Solution Approach 1:
The invention applies different geometric characteristics to different portions of the seal surface. The forward portion has a first seal taper angle while the rearward portion has a second seal taper angle, creating localized zones with different pressure distribution and friction characteristics. This allows the forward portion to generate sufficient radial seal contact pressure for sealability while the rearward portion with its smaller taper angle reduces the tendency for galling during make-up operations.
Solution Approach 2:
The seal surface is divided into two distinct segments: a forward seal portion and a rearward seal portion. Each segment has its own optimized taper angle designed to address specific functional requirements. The forward portion handles the primary sealing function under high pressure, while the rearward portion facilitates smoother engagement and reduces galling risk, effectively segmenting the sealing function to resolve the contradiction.
2Strength
If axial compression is applied to secure the threaded joint, then connection strength is improved, but plastic deformation occurs at the seal point causing diameter reduction and insufficient contact pressure
Solution Approach 1:
The threaded joint design incorporates a make-up torque mechanism that preliminarily compresses the joint components in a controlled sequence. The box and pin are threaded together with sufficient torque to compress the seal portions before the joint is fully loaded in service. This preliminary compression action ensures that the seal surfaces are already in firm contact, creating a pre-compressed state that maintains sealability even when additional axial compression is applied during operation, thereby preventing plastic deformation that would compromise sealing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration improves the rigidity of the pin nose, preventing significant diameter reduction and ensuring high sealability even under large axial compression, while minimizing galling and maintaining effective seal performance.
Implementation Method 1
the pin and the box are threaded-joined so that the outer circumferential surface of the nose of the pin and the inner circumferential surface of the box are brought into metal-to-metal contact, and the contact portion serves as a seal portion
Implementation Method 2
an externally threaded portion (7), a nose (8) extending from the externally threaded portion (7) to a pipe end, and a shoulder portion (12) provided at a distal end of the nose (8)
Implementation Method 3
a box (1) having an internally threaded portion (5) that is threaded-joined to the externally threaded portion (7) to form a threaded portion
Data Source
Figure 1~2
Figure 3A~3B
Figure 4~5
AI summary
Radial seal type threaded joints for steel pipes have still room for improvement in ensuring sealability in terms of the rigidity of the pin nose. Specifically, the outer circumferential surface of the nose 8 of the pin 3 has an outward convex curved shape, and the sealing surface of the box 1 has a tapered shape; and the cross-sectional area of the pin 3 at a seal point sp that is a portion on the outer circumferential surface of the nose 8 of the pin 3, the portion first coming into contact with the sealing surface of the box 1 when threaded-joined, is 35% or more of the cross-sectional area of an original pipe portion corresponding to an unprocessed portion.