Description:The present invention relates to pressure vessel technology and, more particularly, to threaded connections in Type IV composite cylinders. It concerns the use of parallel thread profiles for the metallic boss that interfaces with the gas-filling valve, ensuring a safer and more durable sealing mechanism.

Background and Prior Art:
In conventional high-pressure gas cylinders, especially metallic and Type I–III composite vessels, tapered threads (e.g., ¾” NPT) are commonly used for valve-to-boss engagement. These tapered threads function by achieving a mechanical seal through interference fit. However, this technique poses serious drawbacks:
• It relies heavily on high torquing force to create the seal, leading to thread galling, plastic deformation, and even rupture under stress.
• Reusability is limited as repeated assembly/disassembly wears out the tapered profile.
• Precise torque calibration is required to avoid under-tightening (leaks) or over-tightening (material damage).
With the advancement of Type IV COPVs, which employ a polymeric liner with a metallic boss, the necessity for robust, reusable, and low-stress valve sealing systems has become paramount.
Some prior efforts have attempted to combine metallic inserts with standard tapered threads, but without solving the core issues of sealing reliability, thread damage, and field serviceability.
There exists a critical gap in the current art for a parallel-threaded valve engagement system that can offer:
• Easier assembly
• Higher sealing reliability
• Repeated serviceability without compromising safety
This invention addresses that unmet need.

Summary of the Invention:
The present invention introduces a parallel thread profile (e.g., 1.125-12 UNF 2B/2A) on both the metallic boss and the valve stem, providing:
• Straight-thread engagement that eliminates wedging forces typical of tapered threads
• Compatibility with O-rings or PTFE-based sealing systems to ensure pressure integrity
• Reduced risk of thread deformation or galling even after multiple uses
• Controlled torque levels, enhancing ease of manufacturing and field maintenance
The metallic end boss, typically integrated into the polymeric neck region of the Type IV liner, includes internal parallel threads precisely machined to industry tolerances. The valve assembly, correspondingly, features matching external parallel threads with a dedicated sealing face or groove to accommodate a secondary sealing element.

Detailed Description of the Invention:
In a typical embodiment of the invention, the pressure vessel comprises:
• A polymeric liner forming the core of a Type IV COPV
• A metallic end boss co-molded or interference-fitted into the neck region of the liner
• A valve unit made of stainless steel or aluminum alloy designed for gas filling and release
Thread Configuration:
The end boss is machined with internal parallel threads, complying with standards such as UNF (Unified Fine Threads). These threads are:
• Cylindrical in form with constant pitch and diameter
• Free of taper, ensuring even thread engagement across the entire contact length
• Designed with sufficient lead-in chamfer for easy alignment and thread starting
The valve stem carries matching external parallel threads, which:
• Mate precisely with the boss threads
• Include a machined shoulder or sealing face to accommodate O-rings or metallic seals
• Can be assembled using low-to-moderate torque, avoiding overstressing the boss or liner
Sealing Mechanism:
Since parallel threads do not self-seal, the sealing function is transferred to:
• A radial O-ring groove at the shoulder of the valve, which compresses against the flat face of the boss
• Alternatively, face seals using PTFE washers or metallic gaskets to provide high-pressure sealing up to 700 bar
• The position of the sealing surface is predetermined, ensuring precise axial location and repeatability
Advantages Over Prior Art:
• Significantly higher durability of the threads due to lower stress and friction
• Ease of serviceability — the valve can be repeatedly removed and reinstalled without damaging the threads
• Enhanced sealing security using redundant O-rings or gaskets
• Reduced installation error — no need for torque-critical interference sealing as with tapered threads
• Ideal for automated valve installation lines, improving manufacturing throughput
Use Cases and Applicability:
• CNG and Hydrogen storage cylinders for automotive and industrial use
• Portable high-pressure gas vessels for field applications, including medical oxygen systems
• Modular gas banks with frequent valve replacements or testing requirements

Preferred Embodiment:
In one preferred embodiment, a Type IV composite cylinder rated for 700 bar features:
• An aluminum boss with 1.125-12 UNF 2B internal parallel threads
• A valve unit with matching external threads and an integrated elastomeric O-ring
• The valve is tightened to a specified torque of 50–60 Nm, achieving positive stop at the sealing face
• The system is validated for zero leakage under cyclic pressure, vibration, and thermal tests
, Claims:1. A threaded connection assembly comprising a metallic end boss and a gas valve, wherein both elements are engaged via matching parallel threads.
2. The threaded connection of claim 1, wherein the sealing is achieved by an elastomeric O-ring compressed between the valve shoulder and boss face.
3. The assembly of claim 1, wherein said parallel threads are of UNF standard and ensure repeated reuse without thread degradation.
4. The assembly of claim 1, wherein said end boss is integrated into the neck region of a Type IV polymeric liner.
5. A pressure vessel equipped with the threaded connection as claimed in claim 1, designed to withstand operating pressures above 500 bar without leakage.