Instrument Tubing Selection Guide

How to Select Instrument Tubing?

Instrument tube with fittings have been meticulously engineered to meet the diverse demands of various applications, ensuring optimal performance. However, the selection of tubing plays a pivotal role in maintaining system integrity and reliability. This guide aims to aid designers in choosing and ordering high-quality tubing, emphasizing proper selection and installation as essential for constructing leak-free and dependable tubing systems.

Instrument Tubing General Selection Criteria

The primary consideration in tubing selection is the compatibility of the tubing material with the intended media. Table 1 provides a comprehensive overview of common materials and their recommended applications, including the minimum and maximum operating temperatures. Its crucial to match tubing materials with like materials; for instance, stainless steel fittings should only be used with stainless steel tubing, except for brass fittings with copper tubing. Mixing materials is discouraged due to potential galvanic corrosion and variations in hardness, which can compromise fitting seals.

Instrument Tubing for Gas Service

Selecting tubing for gas service requires special attention to achieve a gas-tight seal. Ferrules in instrument fittings seal imperfections on the tubing surface by penetrating it. This penetration necessitates tubing with radial resistance softer than the ferrules, ideally achieved through thick-walled tubing. Tables 2-7 specify the minimum acceptable wall thickness for various materials in gas service. Additionally, Table 9 outlines acceptable tubing hardness, with softer materials yielding better results, particularly for gas service.

Instrument Tubing for System Pressure

System operating pressure dictates tubing material selection and sizing. High-pressure installations typically demand robust materials like steel or stainless steel, although copper may suffice if chemically compatible with the media. Steel or stainless steels superior strength allows for thinner tubes without compromising system integrity. Its imperative to adhere to recommended working pressures to ensure safety. Tables 2-7 provide maximum suggested working pressures for different tubing sizes and materials.

Instrument Tubing Consider System Temperature

Operating temperature influences tubing material suitability, with copper and aluminum suitable for low temperatures and stainless steel or carbon steel recommended for higher temperatures. Special alloys like Alloy 600 are ideal for extreme temperatures (refer to Table 1). Table 8 presents derating factors for elevated temperature conditions, enabling the adjustment of working pressures listed in Tables 2-7 accordingly.

Instrument Tubing Ordering Guidelines

Ordering tubing for instrument fittings requires careful consideration to ensure performance and quality. Purchase orders should specify material, nominal outside diameter, wall thickness, and compliance with ASTM specifications to guarantee dimensional, physical, and chemical consistency. Additionally, users may impose more stringent requirements, such as scratch-free surfaces and suitability for bending. Table 9 provides specific ordering information for each material.

A well-structured purchase order should include the following details:

1/2" x .049" 316 stainless steel, seamless, or welded and redrawn per ASTM A-269. Fully annealed, 80 Rb or less. Must be suitable for bending; surface scratches and imperfections (incomplete weld seams) are not permissible.