Short answer: Bends cut the number of joints, and every joint you remove is one less potential leak and one less weld or seal to inspect. Fittings and elbows are compact and repeatable where space is tight or the route must be modular. Use bends where the tube manufacturer's minimum bend radius fits and your spec allows bending of EP tubing. Use weld elbows or fittings where it doesn't fit, or where the bend could damage the internal surface.
Steps
1. Check the spec. Some UHP specs restrict field bending of electropolished tube, because bending can disturb the internal surface. Others allow it with qualified tooling.
2. Get the minimum bend radius for the tube size and wall from the tube or bender supplier. Don't use generic values.
3. Lay out the route with bends where the radius fits, and count the joints you save.
4. Use elbows or fittings where the space is tighter than the bend radius allows, or where modules have to break apart at a defined joint.
5. Inspect bends for ovality, wrinkling and surface marks, to your spec.
6. Show bends on the drawing with centerline radius, angle and bend sequence where they are made in the shop.
Worked example (generic)
A route between two gas sticks needs three 90° turns. The available envelope fits a bend at the supplier's radius for two of the turns but not the third. The designer bends two turns and uses a weld elbow for the tight one. That removes several joints compared with all-elbow construction, and the third turn still fits.
Common mistakes
- Bending below the minimum radius, which flattens the tube.
- Using hand benders on EP tube when the spec requires qualified tooling.
- Forgetting the straight length needed at each end of a bend for clamping and for the next joint.
More steps: deciding bend or fitting per segment
1. Identify each direction change in the route.
2. Check the space for the required bend radius and the straight lengths before and after the bend that the bender and the weld head need.
3. Check the spec for whether bending is allowed for this tube grade and finish. Some specs restrict bending of electropolished tube, or require post-bend inspection.
4. Check maintenance needs. Where a segment has to be removable, a face seal fitting may be needed anyway.
5. Count the joints each option creates. Fewer welds generally mean fewer inspection points and fewer potential leak paths.
6. Pick and document the choice per segment, so the BOM and fabrication drawings agree.
Edge cases
- Bends near welds. A bend too close to a weld stresses the heat-affected zone and leaves no straight length for the weld head. Respect the minimum straights from the bender and weld equipment.
- Multi-plane bends. These are hard to measure and inspect. Consider splitting them with a fitting when the tolerance is tight.
- Surface finish after bending. Bending can roughen the inner surface on the outside of the bend. Specs for electropolished tube may limit it or require inspection.
- Springback. Tube springs back after bending, so the bender is set past the target angle. Fabrication handles this, but the model should hold the final angle.
- Small spaces. Prefabricated elbows and weld fittings allow tighter turns than bends, at the cost of extra welds.
A second worked example (generic)
A route from a gas stick outlet to a bulkhead needs two direction changes in a tight corner.
- The first change has room for a standard bend radius plus straights, so it becomes a bend.
- The second change sits right before a face seal fitting, with no room for a straight after a bend. It becomes a prefabricated weld elbow.
That leaves one bend and one elbow with two welds, instead of two elbows with four welds. The model shows the bend as a tube segment with a bend radius and the elbow as a separate part, so the BOM lists the elbow and the cut length of the bent tube.
FAQ
Are bends cleaner than fittings?
They remove joints and internal steps. Whether bending is acceptable depends on your spec and process.
Do bends save cost?
Often, by reducing the fittings and the welds to inspect. Measure it for your shop, though.
How is this modeled in SOLIDWORKS?
With routing (tubing routes) or swept tube bodies. Use a controlled bend radius so the drawing and the shop agree.
Does the model need the bend radius?
Yes, for cut length, clearance and fabrication drawings.
Can I bend electropolished tube?
Depends on the spec. Many allow it with limits. Check yours.
Are bends always cleaner than fittings?
Not always. Bend quality and inner-surface condition matter.
Who decides between bend and fitting?
The designer proposes. The owner's spec and the fabricator's capability decide.
How do I show bends on a fabrication drawing?
Usually with a bend table or a dimensioned isometric that gives the bend positions, angles and radius.
Do bends change the BOM?
The bent tube becomes one item with a cut length. A fitting is a separate line.