Buffer tank
30. September 2025
Expansion joints, bending radii & linear expansion
ROUTING WITH MECHANICAL LOGIC
Route planning not only determines where alongbut also how the line may operate mechanically.
If you think about expansion paths, expansion zones and changes of direction at an early stage, you reduce constraints and create a more robust design - especially for long strands, narrow corridors and close to buildings.
Note: This article is based on the information in the district heating planning manual (thermal networks) and common planning practice in pipeline construction.
📐 1. linear expansion: magnitude & consequence
It is helpful for the design to quickly estimate the movement:
ΔL = α - L - ΔT
α = Coefficient of thermal expansion of the carrier pipe
L = Considered pipe length
ΔT = Temperature change between installation/standstill and operation
The calculation formula is just the start. The crucial question is: Where can this movement go - and where is it hindered by the route, backfilling, structures or installations?
🧭 2. natural expansion compensation: geometry first
In practice, expansion is absorbed as far as possible via the route geometry - i.e. via deliberately planned changes of direction and expansion legs.
L-bend (one change of direction)
Z-bend (two changes of direction)
U-bend (return, often particularly "flexible")
Planning notes:
Use clear 90° geometries where possible.
Sharp angles are often unfavorable (less expansion path, more difficult to install).
Position expansion legs so that movement does not "run into" structures/installations.
3. buried rigid systems: Expansion zones are also underground
In underground rigid systems, the pipe acts as a unit. At the same time, the environment (cover, friction/adhesion, compaction) determines whether the pipe can move "freely" or whether constraints build up.
Two consequences for the route:
Expansion zones need space: In areas that are to absorb expansion (e.g. bends/expansion legs), a sufficient clear trench width is also relevant for installation.
Create a profile early on: Height and length profile helps with collisions with utility lines, venting/draining, construction sequence and tendering - and shows where expansion sections are sensibly located.
4. cold installation vs. thermal prestressing
The installation method influences where and how controlled movement takes place.
Cold installation: Expansion movement is set via operation/standstill.
Thermal pre-stressing: After installation, the cable is heated in a targeted manner, filled to a defined state and then cooled - this can help to "steer" expansion behavior more strongly into planned areas.
The "favorite method" is less important than the coordination with the route, construction stages, space and detailed design: the laying method and expansion strategy must fit together.
Construction site on Buchholzstrasse, 3604 Thun
🧩 5. expansion joints: when geometry is not enough
Expansion joints are more of a special tool - e.g. for narrow corridors without expansion joints, for structural penetrations or complex transitions.
They work, but place additional demands on installation and quality assurance.
The following therefore applies: first check the route geometry (and pretensioning where appropriate) - then check the expansion joint.
🔁 6. bending radii: rigid planning, flexible clean guidance
Rigid cables
Change of direction via bends/formed parts; the decisive factor is the interaction between the expansion leg, space requirement and assembly sequence.
Flexible pipe systems (often smaller dimensions, e.g. house connections)
Lines are usually simpler - the key is to maintain the minimum bending radius according to the manufacturer's specifications.
Integrate rigid/flexible transitions cleanly (details and space reserves).
✅ Practical checklist for planners & general contractors
Define the stretching concept: Where are stretching sections, where should movement go?
Use geometry consciously: Plan L/Z/U arcs, avoid acute angles.
Think about civil engineering: Expansion zones need space (trench width/installation).
Create profile: Height/length profile for coordination and operation.
Coordinate the installation method: Cold laying vs. pre-tensioning to suit the expansion strategy.
Targeted expansion joints: only if geometry/preload is not sufficient.
Flexible systems: Ensure minimum bending radius and clean transitions.
✅ Conclusion: route = mechanical concept
A good route is not only "short" or "simple", but also mechanically plausible: it absorbs expansions in a controlled manner, minimizes constraints and remains installable. Integrating these points at an early stage saves discussions on the construction site - and often also costs during operation and maintenance.