How to Weld Sanitary Stainless Steel Tubing for Hygienic Processing
Understanding Sanitary Stainless Steel Tubing
Sanitary stainless steel tubing refers to tubing systems designed for hygienic processes in industries like food, beverage, dairy, and pharmaceutical. This tubing is made from austenitic stainless steels like 304 and 316L due to their corrosion resistance and ease of cleaning. The inside surfaces are highly polished to allow thorough cleaning and sterilization.
Characteristics of Sanitary Stainless Tubing
There are some key properties that define sanitary stainless steel tubing:
- Highly polished interior surfaces with a surface finish of < #20 Ra and low particle counts for cleanability.
- Low austenitic stainless steel grades like 304 and 316L that limit carbon content to avoid carbide precipitation.
- Sanitary tube fittings like clamps and gaskets that enable rapid disassembly for inspection and cleaning.
- Orbital welding methods used to create smooth, crevice-free joints.
Common Sizes and Standards
Sanitary tubing comes in sizes matching the nominal internal diameters. Common imperial sizes are 1⁄2”, 3⁄4”, 1”, 1 1⁄2”. 2”, and 3”. Metric sizes in mm are also used like 10mm, 20mm, 25mm etc.
Key sanitary stainless tubing system standards are:
- ASME BPE - Bioprocessing Equipment standard
- ISO 1127 - Stainless steel tubing dimensions
- ASTM A270 - Seamless and welded austenitic tubing
- ASTM A789 - Welded duplex and austenitic tubing
Welding Methods for Stainless Steel Sanitary Tubing
To meet strict cleanliness and sterility demands, stainless steel sanitary tubing systems use orbital TIG welding to create smooth interior welds. The common methods are:
Manual GTAW Welding
Manual GTAW (TIG) welding is the most basic and economical option. The welder fuses tube ends together for short vent or drain lines. It allows good penetration, fast welding for small batch work. But internal alignment and controlling heat input for wall thickness can be difficult.
Orbital GTAW Welding
Orbital TIG systems use an enclosed welding head that rotates around a joint with an automatic arc. This creates even fusion with less welder skill dependence. Automated systems help control parameters like speed, arc gap, heat input etc. It allows better process repeatability for large production batches.
Automated TIG Welding
Fully automated orbital TIG welding uses tube rotators and weld head carriages for welding multiple joints. It incorporates automatic seam tracking and parameter control for complete consistency and reliability across hundreds of tube welds.
Other Advanced Methods
Some other advanced welding options like laser beam welding and micro-plasma welding suit thinner wall sanitary tubing. They offer extremely concentrated heat input for lower thermal impact, but require very expensive equipment.
Welding Procedure for Stainless Steel Sanitary Piping
The general steps for manual and automatic orbital TIG welding procedures for stainless sanitary tubing are outlined below:
Cutting and Prepping Tube Ends
The tube ends to be welded are cutoff at right angles and deburred. Facing tools can trim and bevel the mating ends for better fusion. Cleaning using citric or nitric acid removes surface impurities.
Fixturing and Fitting Tubes
The tubes are tightly fixtured in adjoining position, oriented and aligned using tube manipulators. Automated carriages may also shift tubes under welding torches.
Setting up Welding Parameters
The optimal settings for current, arc voltage, torch angle, weld speed, filler metal addition etc. are set up. Gas flow rates, arc gaps and joint fitup are also verified.
Performing Orbital Welding
During welding, a focused arc melts the joint as the weld head rotates evenly around the weld, fusing the metals. Filler metal is added manually or automatically. A backing strip prevents contamination at the tube ID.
Welding Completion and Testing
After completing the circumferential weld, the joints are tested for weld faults using methods like helium leak testing, X-ray tests etc. The system can then be hydrostatically tested before final use.
Conclusion
Sanitary stainless steel tubing requires stringent welding methods like GTAW to achieve standards for cleanability, sterility and contamination control. Orbital TIG welding with automated sequencing provides reliable, repeatable welding for cost-effective production of high purity process systems.
FAQs
What are some key standards for sanitary stainless steel tubing?
Key standards are ASME BPE, ISO 1127, ASTM A270, and ASTM A789. These specify requirements for tube dimensions, materials, surface finishes and welding.
Why is orbital GTAW the preferred welding method?
Orbital GTAW creates smooth, crevice-free interior welds meeting surface finish and cleanliness requirements. It offers better control and consistency than manual welding.
What metals are used to make sanitary tubing and fittings?
Austenitic stainless steels like 304 and 316L are predominantly used. Their low carbon content and corrosion resistance facilitate cleaning and sterility.
How are tube welds tested for flaws?
Helium mass spectrometry detects leaks. X-ray and gamma-ray testing reveal internal flaws. Destructive hydrostatic testing checks weld integrity at 1.5 times rated pressure.
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