Spiral Wound Pipe: Large Diameter From a Small Machine

What spiral wound pipe is

A spiral wound pipe is made by extruding a hollow profile and winding it helically onto a rotating mandrel, with adjacent turns welded together by the heat of the extrusion or by a separate hot-air or hot-plate process. The result is a continuous pipe of whatever diameter the mandrel can carry.

The advantage is size. An extruded pipe is limited by die size, cooling length and haul-off; a wound pipe is limited only by the mandrel, which is a steel drum. Diameters of 3 metres and above are routine, and the same extrusion line that makes a 300 mm wound pipe can make a 3,000 mm one with a different mandrel and wind angle.

The disadvantage is that the wall is a structure rather than a solid section. It has ribs, voids and a helical seam, and every one of those is a place where the pipe can fail if the process is not controlled.

Table 1 — Spiral wound pipe constructions. The liner carries the flow; the wound structure carries the load.

The winding process

The profile leaves the extruder at 190 to 220 °C and is still soft when it reaches the mandrel. It is laid at an angle to the axis — the wind angle — and the mandrel rotation and the traverse speed together set that angle.

Wind angle is the single most important setting. A shallow angle gives more material per unit length and higher stiffness but slower production; a steep angle gives faster production and less stiffness. Most lines run at 60 to 75 degrees to the axis.

The weld between adjacent turns is the structural joint. It is made by the residual heat in the profile plus a hot-air jet or a heated shoe. If the profile has cooled too much before it reaches the mandrel, the weld is incomplete and the pipe delaminates under load.

Cooling is by water spray on the mandrel. A large wound pipe holds a lot of heat, and a pipe that is removed from the mandrel too warm will deform under its own weight. Large diameters are often cooled on the mandrel for 20 minutes or more.

Ring stiffness from structure

Ring stiffness in a wound pipe comes from the profile height and the number of layers, not from the wall thickness in the conventional sense. A single layer of a 25 mm high profile gives SN2 to SN4; two layers give SN4 to SN8; three or four layers reach SN16.

The profile geometry matters as much as the count. A hollow rectangular profile with a 25 mm height and a 3 mm wall has a moment of inertia per unit width that a solid 3 mm wall cannot approach. This is the same principle as the corrugated pipe, applied to a wound structure.

The penalty is that the voids in the profile reduce the effective area for longitudinal bending. A wound pipe is stiff in ring compression and comparatively weak in longitudinal bending, which matters during installation and in settlement.

Table 2 — Ring stiffness by profile height and layer count. Material is relative to the single 20 mm layer.

Liners and joints

A wound structural pipe is not watertight. The helical weld is a structural joint, not a seal, and the profile itself may have gaps. A liner is either co-wound as an inner layer or inserted afterwards as a smooth tube.

The common liners are HDPE, PP and PVC-U. HDPE is the default for sewer because it is chemically resistant and can be fusion welded to the pipe ends. PP is used where the temperature is higher. PVC-U gives a smoother bore and better abrasion resistance.

The joint between pipe sections is usually a bell and spigot with an elastomeric seal, or a butt-fusion weld of the liner with a mechanical connection of the structure. For sewer, the liner weld is the watertight element and the structural connection carries the load.

Joint stiffness is the weak point. A wound pipe is stiff along its length and less stiff at a joint where the structure is interrupted. For deep burial, a coupler that continues the profile is used rather than a plain sleeve.

Where it wins and where it does not

Wound pipe wins on diameter, on short runs of large pipe, and on sites where transport of a 3 metre extruded pipe is impossible. It also wins on lead time: a mandrel change is hours, a die change is weeks.

It loses on pressure — it has none — on longitudinal strength, and on wall thickness consistency. A wound pipe's wall varies with the wind angle and the profile tolerance in a way an extruded pipe's does not.

Quality checks

Table 3 — Spiral wound pipe quality checks. The peel test is the one that catches a wind parameter that has drifted.