Large Diameter Aluminium Tube and Pipe

What is large diameter aluminium pipe?

Most extruded aluminium tube and pipe is below about 300 mm (NPS 12) in diameter, the practical limit of ordinary horizontal presses. Larger sizes are made by heavier presses or by entirely different processes, and each process brings its own limits on wall thickness, length, alloy and quality.

Aluminium is chosen for large pipe because it is light (about one third of the weight of steel), does not rust, stays tough at cryogenic temperatures and is non-magnetic and non-sparking. That makes it a standard material for LNG and industrial gas lines, cryogenic transfer, ship and offshore piping, chemical process lines handling media that attack steel, large ducts, fire mains on vessels, and structural uses such as columns, towers and rollers.

In this guide, "pipe" refers to products sized by nominal pipe size (NPS) and schedule, and "tube" to products ordered by outside diameter and wall thickness. In large sizes the two often overlap, and most buyers specify outside diameter and wall directly.

Manufacturing routes compared

As a rule, choose seamless pipe when the line carries pressure, cyclic loads or hazardous media, or when a code requires it. Welded pipe is economical for large, thin-walled lines at low pressure, provided the joint efficiency of the weld is taken into account in the design. Forged and bored tube is reserved for short, heavy, critical parts.

Alloys for large diameter pipe and tube

For cryogenic and marine service, 5083 is usually the first choice because it keeps its strength after welding and has no ductile-to-brittle transition. For general process piping, 6061-T6 gives more strength in the as-supplied condition, but welded joints fall to a lower strength, which codes such as ASME B31.3 account for in allowable stresses.

Large NPS pipe sizes and weights

Large aluminium pipe is often ordered to the dimensional system used for stainless steel pipe (ASME B36.19 schedules 5S and 10S, and ASME B36.10 schedule 40). The table gives calculated weights in aluminium at 2.70 g/cm³. Diameters beyond NPS 24, and non-standard walls, are ordered by outside diameter and wall thickness.

Walls converted from ASME B36.19 / B36.10 inch values; confirm the edition required by your project specification. Weights exclude coatings and are for plain-end pipe.

For sizes beyond the NPS range, weight per metre is π × (OD − t) × t × 2.70 × 10⁻³ kg (OD and t in mm). A 1,000 mm OD tube with a 50 mm wall weighs about 403 kg/m; a 1,500 mm spiral-welded pipe with a 10 mm wall about 126 kg/m.

Standards and codes

When a line is designed to a pressure code, the pipe grade and temper, weld joint efficiency, corrosion allowance and testing (hydrostatic test, radiography or ultrasonic testing of welds) all come from the code. Specify these on the purchase order so that certification matches the design basis. For pressure vessels approved by Lloyd's Register, see our pressure vessel pipe guide.

Wall thickness and pressure: how it is worked out

For a thin-walled pipe, the hoop stress from internal pressure is approximately p × D / (2 × t), where p is pressure, D the outside diameter and t the wall. Codes such as ASME B31.3 turn this into a required wall thickness using an allowable stress for the alloy and temper, a weld joint quality factor, and allowances for corrosion, threading and manufacturing tolerance.

Three aluminium-specific points change the answer compared with steel. First, allowable stresses are lower, so walls are thicker for the same pressure. Second, for welded construction in heat-treatable alloys such as 6061-T6, codes base allowable stress on the much lower as-welded strength, which often makes 5083 the more efficient choice for welded systems. Third, allowable stresses fall faster with temperature above about 100 – 150 °C, so aluminium piping is generally used from cryogenic temperatures up to moderate service temperatures.

Fittings for large diameter aluminium pipe

Inspection and testing

• Dimensional checks: outside diameter, ovality, wall thickness at several positions, straightness and end squareness
• Mechanical tests: tensile tests per lot, flattening or expansion tests where the standard requires
• Non-destructive testing: ultrasonic or eddy-current testing of seamless pipe; radiography or ultrasonic testing of welds in welded pipe
• Pressure test: hydrostatic or pneumatic test to the standard or project specification
• Surface: visual inspection for dents, scratches and die lines; internal cleanliness for gas and cryogenic service
• Certification: EN 10204 3.1 mill test certificate; 3.2 with third-party or class surveyor witness on request

Selection tips and common mistakes

• Specifying seamless where welded would do: for large, low-pressure ducts and cooling water lines, spiral-welded pipe is often far cheaper and available in longer lengths.
• Specifying welded where code needs seamless: check the design code and the classification society before choosing welded pipe for pressure service.
• Ignoring weld strength in 6061: the heat-affected zone of welded 6061-T6 drops to well below parent strength; 5083 keeps more of its strength when welded.
• Mixing metals without isolation: use insulating gaskets, sleeves or bonded transition joints where aluminium pipe meets steel or copper.
• Underestimating handling: thin-walled large pipe dents easily; specify internal spiders or end protectors and padded supports for transport.
• Forgetting thermal expansion: aluminium expands about twice as much as steel (about 23 × 10⁻⁶ per K), so long lines need expansion loops or bellows.

How to order large diameter pipe from Metline

1. SpecifyOD and wall (or NPS and schedule), length, alloy and temper, governing standard or code, and service conditions.
2. Choose routeWe confirm whether seamless, forged or welded pipe meets your requirements and is economical for the size.
3. Add fittings and processingCutting to length, end bevelling for welding, flanges, bends and reducers as needed.
4. Inspect and certifyDimensional, mechanical and NDT checks with EN 10204 3.1 certificates; third-party inspection on request.
5. Pack and shipEnd caps, internal supports and timber cradles; shipped in containers or on flat racks on EXW, FOB, CIF or DAP terms worldwide.

Related pipe and large product guides

Large diameter aluminium (aluminum) pipe: FAQ