Introduction

Connection methods create interfaces where leaks and contamination commonly occur. This article compares threaded, welded, compression/ferrule, and VCR fittings and recommends best practice patterns for UHP systems.

Threaded fittings

 • Advantages: Low cost, easy to install for temporary or low-purity lines.

 • Disadvantages: Require sealing compounds or PTFE tape; potential particulate generation and permeation; not recommended for UHP or hazardous gases.

 • Use: Low-risk, non-critical lines.

Welded connections (butt-welds, orbital welding)

 • Advantages: Smooth internal flow, minimal dead volume, highest integrity and lowest leak potential. Orbital welding yields consistent welds for thin-wall tubing.

 • Disadvantages: Requires skilled welding and often cleanroom or controlled-environment procedures for UHP systems. Difficult to modify in the field.

 • Use: Main trunks and permanent high-purity lines.

Compression / ferrule fittings (e.g., Swagelok style)

 • Advantages: Removable, reasonable sealing for many HP applications; good for instrumentation and service lines.

 • Disadvantages: Repeated disassembly can wear ferrules and degrade sealing; potential small dead volume compared with a welded line.

 • Use: Instrumentation, sampling lines, non-critical branches.

VCR fittings (metal gasket, face-seal)

 • Advantages: Metal-to-metal seal with soft metal gasket (e.g., gold, silver-plated nickel), extremely low dead volume, repeatable high integrity sealing. No elastomer exposure.

 • Disadvantages: Higher cost, requires correct torque and installation practice.

 • Use: POU, analytical instrument interfaces, any circuit demanding ppb/ppt purity.

Design recommendation (best practice)

 • Main trunk: Welded (butt-weld) 316L EP where feasible.

 • Distribution and POU: Use VCR fittings and VCR-compatible valves for last-meter flexibility.

 • Sampling/Instrumentation: High-quality compression fittings (metal ferrules) or VCR depending on sensitivity.

Field experience note

A corrosive gas line that used PTFE-sealed compression fittings experienced micro-leaks after several months. Replacing the interface with VCR fittings and metal gaskets eliminated the leaks and kept the system within detection limits over multiple years.

Conclusion

Choose connection methods that match your purity and service needs. For semiconductors and analytical labs, a mixed approach (welded trunks + VCR at endpoints) is the accepted industry standard.