Welded vs Seamless Instrumentation Tubing: Selection and Pressure Differences
Jan 9th 2024
Instrumentation tubing is commonly selected as welded or seamless stainless steel tubing. Both can be useful, but the manufacturing method affects cost, surface quality, wall consistency, pressure capability, and inspection concerns.
The right choice depends on pressure, corrosion resistance, criticality of the process, and the fitting system used with the tubing.
Welded Tubing
Welded stainless steel tubing is often more cost effective than equivalent seamless tubing. It is made from flat rolled material that is formed and joined with a longitudinal seam.
Because welded tubing starts from rolled material, it can have good outside and inside surface quality. Wall thickness is often more consistent than seamless tubing, and the internal surface can be checked before manufacturing.
Limits of Welded Tubing
The weld seam must be suitable for the service. Applications that require very high pressure, high corrosion resistance, or critical leak avoidance may call for seamless tubing instead.
Weld orientation, weld integrity testing, and pressure derating should be reviewed when welded tubing is used in demanding instrumentation systems.
Seamless Tubing
Seamless tubing is formed without a longitudinal weld seam. It usually costs more, but the homogeneous tube structure can support higher pressure capability in critical service.
Seamless tubing does not present the same weld-seam orientation concerns. It is often preferred where corrosion resistance, pressure reliability, and uniform structure are priorities.
Pressure and Standards
Seamless 304 and 316 stainless tubing made to ASTM A269, ASTM/ASME A213, or equivalent specifications is commonly used for instrumentation systems. The referenced working temperature range is -20 F to 100 F for the stated pressure data.
One cited basis uses tensile strength of 75,000 PSI and allowable stress of 20,000 PSI under ASME B31.3-2002. Actual rating still depends on tube size, wall thickness, temperature, and the fitting connection.
Installation Preparation
Recommended tubing practices include using annealed 304 or 316 stainless steel tubing, keeping tube hardness at RB 80 or below, and preferring a hardness range around RB 70 to 78.
Before installation, tubing should be cut square, deburred, cleaned, and checked for surface defects. Burrs, scratches, and poor cuts can damage ferrules or sealing surfaces.
Matching Tubing and Fittings
Instrumentation tubing performance depends on the tube and fitting combination. Certified fittings and properly specified tubing help reduce leaks, pressure loss, and maintenance problems.
For critical systems, choose the tubing process, material grade, wall thickness, hardness, and fitting style together rather than selecting tubing as a separate commodity item.
Which One to Choose
Welded tubing can be a practical choice where cost, surface quality, and consistent wall thickness fit the system requirements. Seamless tubing is usually favored for critical pressure, corrosion, and reliability demands.
The safest selection follows the process conditions, code requirements, pressure rating, and fitting manufacturer's installation guidance.