ASTM A213 TP316L U-Tubes for Heat Exchangers: Guide
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What Are ASTM A213 TP316L U-Tubes?
ASTM A213 is the standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater and heat-exchanger tubes, and TP316L is its low-carbon austenitic stainless grade with the UNS designation S31603. U-tubes are straight seamless tubes bent into a U shape so that both ends terminate in the same tube sheet. This configuration allows the tube bundle to expand and contract freely with temperature changes, eliminates the need for a second tube sheet and return bonnet, and gives a large heat transfer surface within a compact shell, which is why U-tube bundles are widely used in shell-and-tube heat exchangers for heating, cooling or reheating fluids, gases and air.
TP316L contains molybdenum in the range of 2.00 to 3.00 percent, which provides strong resistance to pitting and crevice corrosion in chloride-bearing cooling waters and process streams, while the low carbon limit of 0.035 percent maximum prevents sensitisation during welding and bending operations.
Chemical Composition and Mechanical Properties
The chemical composition limits for ASTM A213 TP316L U-tubes are shown below.
| Element | Composition, % |
|---|---|
| Carbon, max | 0.035 |
| Manganese, max | 2.00 |
| Phosphorus, max | 0.045 |
| Sulfur, max | 0.030 |
| Silicon, max | 1.00 |
| Chromium | 16.0 - 18.0 |
| Nickel | 10.0 - 14.0 |
| Molybdenum | 2.00 - 3.00 |
Mechanical requirements in the solution-annealed condition are a minimum tensile strength of 485 MPa, a minimum yield strength of 170 MPa, a minimum elongation of 35 percent in 50 mm and a maximum hardness of 217 HB. The specification also permits the tube to be ordered on a minimum wall or average wall basis, which affects the permissible wall thickness tolerance.
Available Size Range
Outside diameter: commonly 9.53 mm (3/8 inch) to 25.4 mm (1 inch); other sizes available on request.
Wall thickness: commonly 0.8 mm to 2.5 mm, matched to the outside diameter.
U-bend radius: customised according to the customer's design and tube bundle layout.
Straight leg length: precisely cut to the customer's drawing.
Manufacturing and Bending
TP316L U-tubes are produced from stainless steel billets by hot drawing or cold drawing and cold rolling to achieve the required dimensions and mechanical properties, followed by solution annealing and, when required, bright annealing. The straight tube is then bent on a mandrel or by rotary draw bending to the specified bend radius, normally with a bend allowance that compensates for wall thinning on the outside of the bend. For seamless tubes the bend can be made without any weld in the curved section, which removes the risk of weld defects in the highest-stress region of the bundle.
Typical Applications
Chemical processing heat exchangers and condensers.
Oil refining coolers and reboilers.
Power generation feedwater heaters and condensers.
Boiler and superheater duty where seamless integrity is required.
Process heating and cooling coils for gases and liquids.
Testing and Quality Control
Tubes supplied to ASTM A213 must pass chemical analysis, tensile and hardness testing, flattening and flaring tests, and either hydrostatic or non-destructive electric testing. For U-bends, the bend region should be examined for surface defects and the ovality of the bend cross-section checked against the design requirement. The mill certificate should record the heat number, heat treatment details and all test results so that every tube in the bundle is traceable.
Frequently Asked Questions
Why are U-tubes used in heat exchangers?
U-tube bundles have both tube ends in the same tube sheet, so the tubes are free to expand and contract with temperature changes, thermal expansion stresses are low, and the shell can be removed for cleaning without disturbing the bundle.
Why is TP316L preferred for many heat exchanger services?
The molybdenum addition gives reliable resistance to chloride pitting and crevice corrosion in cooling water and process streams, and the low carbon content keeps the material corrosion-resistant after welding and bending without post-weld heat treatment.
Can the U-bend be made from welded tube?
Yes, welded tube can be bent, but seamless tube is normally preferred for U-bends because the curved section then contains no longitudinal weld and the bend is subjected to less risk of weld-related defects under cyclic service.
What bend radius can be supplied?
The bend radius is customised to the heat exchanger design and tube bundle layout. A common rule is a centreline bend radius of at least twice the tube outside diameter, but tighter radii can be produced with appropriate tooling and quality control.
What tests are required before U-tubes are shipped?
Chemical analysis, tensile and hardness testing, flattening and flaring tests, and hydrostatic or non-destructive electric testing are required by the standard, with all results documented on the mill certificate.
