Pipe Systems Compared

Seven pipe system families are available for thermal networks: the pre-insulated bonded pipe (KMR) as the standard for district heating (up to 160 degC, PN 25), the flexible corrugated metal pipe (MMR) for service connections, the cost-effective plastic medium pipe (PMR) for local heating (up to 95 degC), PE pipes without insulation for anergy networks (up to 50 degC), ductile cast iron pipes, GRP pipes for corrosive media, and the steel casing pipe (SMR) for high-temperature transmission lines (up to 400 degC). Since the pipe network typically accounts for 50 to 60% of total investment costs (VDI 2067), the system selection is one of the most consequential planning decisions.

Pre-insulated Bonded Pipe (KMR)

The pre-insulated bonded pipe (Kunststoffverbundmantelrohr, KMR) is the standard system in district heating pipeline construction. It consists of a steel carrier pipe, PUR foam insulation, and an outer casing made of HDPE (high-density polyethylene). It is standardized according to SN EN 253.

KMR pipes are rated for operating temperatures up to 160 degC and operating pressures up to PN 25. Nominal diameters range from DN 20 to DN 1200. They are delivered as straight lengths of 6, 12, or 16 m. Connection is made by welding the steel pipes; the joints are then post-insulated with PUR joint foam and shrink sleeves.

A key characteristic: KMR pipes are not self-compensating. Thermal expansion must be accommodated by suitable measures such as expansion pads, L- or Z-bends, or pre-stressing (cold bending). The thermal expansion of a steel pipe is:

where $\alpha \approx 1{.}2 \times 10^{-5}$ K$^{-1}$ for steel. For a 100 m long pipe with a temperature difference of 100 K, this results in an expansion of approximately 120 mm.

The expected service life is at least 30 years. KMR systems are available with integrated leak detection (copper wire in the insulation), enabling early identification of damage.

Corrugated Metal Pipe (MMR)

The corrugated metal pipe (Metallmediumrohr, MMR) uses a flexible, corrugated copper or steel pipe as the carrier pipe, surrounded by PUR insulation and a PE casing. The operating limits match those of KMR at up to 160 degC and PN 25.

The decisive advantage lies in its flexibility: MMR pipes are delivered as coils, typically up to DN 50 in lengths of up to 1000 m. This eliminates numerous joints, and installation in curved routes or confined spaces is significantly simplified. The corrugated carrier pipe is self-compensating — thermal expansion is absorbed by the corrugations, so elaborate expansion measures are unnecessary.

MMR pipes are particularly suitable for service connections and smaller distribution networks (network topology) where frequent changes of direction occur and short construction times are required.

Plastic Medium Pipe (PMR)

In the plastic medium pipe (Kunststoffmediumrohr, PMR), the carrier pipe is made of cross-linked polyethylene (PEX) or polybutylene (PB). The PUR insulation and PE casing follow the familiar design.

In the standard variant, PMR pipes are rated for a maximum of 95 degC and PN 6. Reinforced versions achieve 115 degC at PN 10 to PN 16. Nominal diameters extend to DN 150; delivery is predominantly as coils.

Connection is made via press fittings, which significantly reduces installation effort compared to welding and requires no specialized welding qualification. PMR pipes are self-compensating and less expensive to purchase than KMR. They are preferably used in local heating networks with moderate temperatures.

PE Pipe

Polyethylene pipes (PE 100, PE-RC) are used without thermal insulation. They are rated for maximum medium temperatures of approximately 50 degC and pressure classes of PN 10, PN 16, or PN 25.

By dispensing with insulation, PE pipes are particularly suitable for low-temperature networks and anergy networks where the temperature difference to the surrounding soil is small and heat losses play a minor role. The material is robust, chemically resistant, flexible, and enables trenchless installation methods (e.g., ploughing or horizontal directional drilling). Connection is made by electrofusion or butt welding.

PE pipes are an economically attractive solution for cold networks with long pipe runs, for example for connecting groundwater wells or borehole heat exchanger fields.

Ductile Cast Iron Pipes

Ductile cast iron pipes are made of spheroidal graphite cast iron and are also used without thermal insulation. They are characterized by extremely high pressure resistance (25 to 100 bar) and robustness. Nominal diameters range from DN 80 to DN 700; connection is made via push-fit joints.

A particular advantage is the long service life of up to 140 years, as demonstrated by experience from the drinking water supply sector. Ductile cast iron pipes are suitable for trenchless installation by horizontal directional drilling and are resistant to settlement and ground movement.

Their use in thermal networks is limited to low-temperature applications. They are an option where special requirements for mechanical load-bearing capacity exist, such as crossings beneath traffic routes or in difficult ground conditions.

Glass Fibre Reinforced Plastic Pipe (GRP)

GRP pipes consist of an epoxy resin-based carrier pipe fitted with PUR insulation and a PE casing. They are rated for temperatures up to 160 degC and pressures up to PN 16.

The key advantage over steel pipes is corrosion resistance. GRP pipes are therefore particularly suitable for networks with corrosive media, such as geothermal plants with high salt content in the thermal water. Connection is made via adhesive joints or mechanical couplings.

Steel Casing Pipe (SMR)

The steel casing pipe (Stahlmantelrohr, SMR) uses a steel casing instead of a PE casing. The annular space is fitted with vacuum insulation, which enables particularly low heat losses. SMR systems are rated for temperatures up to 400 degC and pressures up to PN 64.

Due to the high costs and the elaborate installation procedure, SMR pipes are primarily used in large district heating transmission lines and industrial steam networks. The vacuum insulation requires regular monitoring of the vacuum.

Comparison Table

Selection Criteria

The choice of a suitable pipe system depends on numerous project-specific boundary conditions. The most important criteria are:

• Maximum operating temperature — High-temperature networks ($>$ 100 degC) require KMR, MMR, GRP, or SMR; for low-temperature and anergy networks, PMR, PE, or ductile cast iron pipes are also viable.
• Operating pressure — Particularly for long transmission lines and large geodetic elevation differences, the pressure rating of the system is decisive.
• Leak detection — Not all systems offer integrated monitoring capabilities; for large networks, this is an essential selection criterion.
• Customer density and number of connections — In densely built-up areas with many service connections, flexible systems (MMR, PMR) offer advantages during installation.
• Pipe dimension — Large nominal diameters ($>$ DN 200) limit the choice to KMR and SMR; flexible systems are only available up to limited diameters.
• Planned network expansion — For phased expansion, a system that facilitates simple branches and extensions may be advantageous.
• Space constraints in civil works — In confined routes (city centres, crossing areas), flexible systems or trenchless installation methods are often the only economical solution.

For a more detailed comparison of the systems, refer to Table 4.1 in the Planungshandbuch Thermische Netze 2.0 (Planning Handbook for Thermal Networks 2.0).

Conclusion

No pipe system is universally optimal — the right choice results from the combination of operating parameters, installation conditions, and economics. It is common for multiple systems to be used in parallel within a single network: KMR in the main network, MMR or PMR for service connections, and PE pipes in cold network sections. Well-founded planning and hydraulic analysis with suitable software such as VICUS Districts helps to identify the right combination for each project.

Further reading: Pipe Dimensioning explains how to determine the appropriate nominal diameter from the selected pipe system family, Heat Loss Calculation shows the influence of insulation quality of different systems on network losses, and Dimensioning of 5GDHC Networks covers the special requirements for pipe systems in low-temperature and anergy networks.

References and Standards

• AGFW FW 401 — Installation and Statics of Pre-insulated Bonded Pipes in District Heating Networks
• DIN EN 253 — District Heating Pipes — Factory-insulated Bonded Pipe Systems — Pipes with PUR Insulation
• DIN EN 15632 — District Heating Pipes — Flexible Pipe Systems