# Stainless Steel U-Tubes: What the Enquiry Drawing Must Define

Canonical: https://jundapipe.com/en-AU/blog/stainless-steel-u-tube-drawing

Define U-tube bend radius, leg spacing, lengths and final inspection, with an illustrative centreline geometry calculation for a semicircular bend.

A stainless steel U-tube enquiry should define tube OD and wall, bend centreline radius, leg spacing, straight lengths, end alignment and required inspection after forming. A photograph or the words “U-shaped tube” cannot establish those dimensions.

Junda lists a [stainless steel U-pipe](https://jundapipe.com/products/heat-exchanger-u-bend-tubes) entry. [Catalogue source](https://jundapipe.com/products) Use a controlled drawing to confirm whether the required tube geometry, grade and inspection scope can be supplied.

## How are bend radius and leg spacing related?

For an ideal 180-degree semicircular centreline bend, leg centreline spacing equals 2R, where R is the centreline radius. The curved centreline length equals πR.

For an illustrative R = 100 mm and two straight legs of 1,000 mm measured from the bend tangent points:

| Geometric quantity       | Calculation            |                    Result |
| ------------------------ | ---------------------- | ------------------------: |
| Leg centreline spacing   | 2 × 100                |                    200 mm |
| Curved centreline length | π × 100                |   Approximately 314.16 mm |
| Total centreline length  | 1,000 + 1,000 + 314.16 | Approximately 2,314.16 mm |

This is ideal geometry, not a cutting instruction or a confirmed bend capability. Forming allowances, end trimming and the actual bend profile require the manufacturer’s process review. Label whether a drawing dimension refers to a centreline, inside surface or outside envelope.

## Which drawing details prevent ambiguity?

Specify whether legs are equal, how straight lengths are measured and where end faces must align. Include tolerances, required end preparation and any assembly datum. If a bundle contains different bend radii, give each configuration a separate identifier and quantity.

State the required material and product standard. [ASTM A213/A213M-25](https://store.astm.org/a0213_a0213m-25.html) covers seamless boiler, superheater and heat-exchanger tubes, but the full equipment requirements must establish what applies before and after bending. A straight-tube certificate alone does not describe every finished U-tube feature.

## What should be checked after forming?

Separate starting-material acceptance from final geometry and inspection. The project should define any required wall measurement in the bend, ovality, surface examination, thermal treatment, pressure or nondestructive examination and documentation.

| Stage             | Question to close                                                            |
| ----------------- | ---------------------------------------------------------------------------- |
| Before forming    | Does the starting tube meet material, dimensions and condition requirements? |
| After forming     | Does the bend and leg geometry meet the drawing?                             |
| Final examination | Which required tests cover the finished configuration?                       |
| Release           | Can each configuration and batch be traced to its records?                   |

This table requests a plan; it does not assert that every listed examination is mandatory. Agree the plan under the selected product and equipment specifications. [ASTM A1016/A1016M-24a](https://store.astm.org/a1016_a1016m-24a.html) provides the requirement hierarchy for the tube standards it covers.

## What should be sent with the enquiry?

Provide the drawing revision, material requirement, configuration list, quantities, inspection plan and packing constraints through the [Junda contact page](https://jundapipe.com/contact). Ask for proposed deviations in writing. For long straight starting lengths, coordinate the [length and delivery review](https://jundapipe.com/blog/extra-long-stainless-tube-length-planning) with the final U-tube geometry.