Wind Tower Flange Flatness: Why Tower Sections Are Accepted or Rejected at the Flange

A wind tower is a stack of steel cans bolted together at ring flanges. Every joint in that stack is a pre-tensioned bolted connection between two machined rings, and the bolts in that joint are among the most fatigue-critical components in the turbine. If the flanges are not flat and not parallel, the bolts see bending they were never designed for, and the tower OEM rejects the section.

This guide is for buyers sourcing tower sections, transition pieces or any flanged tubular steel for wind. It covers what the flange tolerances mean, why the rolling and welding sequence decides them and what to check before acceptance. The wind energy structures page lists the related steel.

What the flange tolerances are

Tower OEM specifications and the standards behind them, principally DNV-ST-0126 for support structures and EN 1090-2 for execution, define the flange geometry with a handful of terms:

• Flatness of the flange face, measured as the deviation from a best-fit plane around the circumference. Specifications often allow a total flatness in the order of 1 to 2 mm across a flange several metres across, with a tighter limit over any short arc.
• Local gap, the largest gap between two mated flanges before bolt tensioning, often limited to a fraction of a millimetre over a stated arc length.
• Inclination, how far the flange plane leans away from perpendicular to the can axis. This is what makes a tower lean.
• Ovality of the can at the flange, which decides whether the bolt holes line up.
• Bolt hole position, on a pitch circle of several metres.

The exact numbers are set by the OEM and vary by turbine, hub height and flange design. The point for a buyer is that they are tight, they are checked with a laser tracker or a precision level around the full circumference, and there is no site remedy for a bad flange other than machining or replacement.

Why the flange moves

A tower flange starts life as a forged or rolled ring, machined flat. It is then welded to a rolled can with a large circumferential weld. That weld shrinks, and the shrinkage pulls the flange face out of flat. The three things that decide how far it moves:

1. Weld sequence. Balanced welding around the circumference, in segments, limits the accumulated distortion. A single continuous pass builds it up.
2. Restraint. Flanges are usually welded with the ring clamped to a machined fixture or with the can held in rotators that keep it round.
3. Heat input. Submerged arc welding is the norm for the circumferential seam; the parameters are chosen to fill the joint in the fewest passes consistent with the quality level.

Shops that build tower sections regularly know how much their process pulls a flange and set the flange up with a pre-set, so that it comes out flat after welding. That knowledge is process-specific, and it is one reason why a shop’s first tower section is rarely its best one.

Can rolling and the longitudinal seam

The can itself comes from plate rolled into a cylinder or a cone on a plate roll, with a longitudinal seam. Rolling accuracy decides ovality, and ovality at the flange decides whether the bolt holes line up. The seam is a full penetration butt weld, usually submerged arc, usually with 100% UT.

Tower plate is thick at the bottom sections, sometimes beyond 50 mm, and the roll has to handle both the thickness and the width. A shop that can roll 30 mm plate at 3 m wide may not be able to roll 60 mm at the same width. Ask for the roll capacity in plate thickness at the widest plate the design uses.

Machining after welding

Some flange designs are machined after welding to the can, which removes the welding distortion entirely. It needs a vertical lathe or a large boring machine that can take a full can section, and it adds cost. Whether the specification calls for post-weld machining or for controlled welding to a tolerance decides which shops can quote.

Where machining after welding is not specified, the weld sequence and the inspection are the only controls. The inspection is a laser tracker survey of the flange face relative to the can axis, and a buyer should ask to see the survey report for every section before it ships. The dimensional inspection guide describes what a good report contains.

Fatigue and the flange weld

The flange-to-can weld is a fatigue detail. It carries the tower bending moment through the flange and into the bolts, and the tower’s fatigue life at that joint depends on the weld profile. Specifications typically call for:

• Full penetration, with the root ground or back-gouged.
• Toe profile within a stated limit, sometimes with toe grinding.
• 100% UT for internal defects and MT on the toes.

Our weld standards guide covers how quality levels are specified. The NDT guide covers what the inspection finds.

Coating

Tower sections are coated inside and out, generally to an ISO 12944 C5 system for onshore sites and C5-M or CX for offshore, on a Sa 2½ blast. Flange faces are masked and protected. Bolt holes are inspected for coating overspray, because paint in a hole affects the bolt pre-tension. The surface finish guide explains the systems.

Transport

Tower sections travel by road on specialised trailers and by sea as breakbulk. The largest bottom sections exceed road limits in most countries and are the reason tower factories sit near ports. A shop quoting tower sections should be asked how it gets a section to the quay, and the answer should be part of the price. The breakbulk shipping guide covers the questions to ask.

What to check before acceptance

For every section:

• Laser tracker report on both flanges: flatness, inclination, ovality, bolt hole position.
• UT report on the longitudinal and circumferential seams.
• MT report on the flange weld toes.
• Material certificates for plate and flanges, traced to heat.
• Coating report with thickness readings inside and out.
• Photographs of the flange faces after masking is removed.

Getting sections quoted

Tower sections need plate rolls to the thickness, rotators to the weight, a submerged arc line, a laser tracker and a route to a quay. We check all of those before a tower drawing goes to a shop. Send yours through the RFQ form and within 24 business hours we tell you which shops in the network fit; up to three of them quote you directly. The wind energy structures page lists the rest of the turbine steel this covers.