By Tom Axon-Smith, senior estimator and project planner at PPSL District Heating
When designing buried district heating systems, one of the most important challenges we face is managing thermal expansion and contraction. Pipes carrying high-temperature fluids (often at 100°C or more) naturally expand as they heat up and contract as they cool. If these movements aren’t properly accounted for they can put significant stress on the network, leading to casing damage, leaks or system failure.
In this guide, I’ll take you through the principles of expansion and contraction management in buried systems, focusing on SIS-KMR pre-insulated pipe systems and why materials such as foam pads are becoming the preferred choice over traditional sand bedding.
Steel expands by about 1.2 mm per metre for every 100°C rise in temperature. That might not sound like much, but on a 100-metre pipe run, a 120°C temperature rise could create 144 mm of expansion. Left unmanaged, this movement can strain the pipe, insulation and outer casing significantly shortening system life.
When designing a district heating system, several methods can be used to control thermal movement:
1. Natural Bends or Loops
Expansion loops, offsets, or z-bends built into the layout help absorb movement naturally
2. Sliding Supports and Anchors
Anchors fix the pipe at strategic points, while sliding supports allow controlled movement elsewhere
3. Pipe Length Limitation
Restricting the length of straight runs reduces the risk of excessive force build-up
4. Material Selection
Using high-quality, pre-insulated bonded pipe systems helps minimise heat loss and maintain structural integrity
SIS-KMR (and similar pre-insulated systems) are widely used in district heating networks because of their robust construction:
Bonding the layers together prevents differential movement, which reduces stress points and improves reliability. To get the best from these systems you should always carry out detailed expansion calculations during design and use pre-fabricated bends or compensators at high-movement zones. You must also follow the manufacturer’s recommendations on maximum pipe lengths between anchors.
Traditionally buried district heating pipes have been laid in sand bedding. While sand provides protection and drainage, it isn’t particularly effective at accommodating pipe movement. In contrast, foam pads are specifically designed to handle thermal expansion and provide additional benefits:
| Criteria | Foam Pads | Sand Pads |
|---|---|---|
| Compression Flexibility | High – compressible under pressure | Low – rigid |
| Thermal Insulation | Excellent – minimises heat loss | Poor – promotes thermal bridging |
| Expansion Accommodation | Excellent – allows free pipe movement | Limited – resists pipe movement |
| Stress Reduction | Reduces stress on casing and joints | Increases mechanical stress |
| Installation Speed | Quick, pre-formed pads | Labour-intensive |
When incorporating foam pads into your design:
Thermal expansion and contraction are inevitable in buried district heating systems, but with the right design approach, they don’t have to be a problem. By combining SIS-KMR pre-insulated systems with modern movement management techniques, and by replacing traditional sand bedding with foam pads, you can significantly extend the life of the network, reduce stress on components and improve overall performance.
At PPSL, we bring experience and technical expertise to every project, ensuring that design considerations like expansion and contraction are factored in from the very start. If you’d like more advice on any of the topics covered, or we can help with a district heating project, then please get in touch.
