Steel or Polymer Pipes for district heating?

In this article, Mark Whettall, managing director of CPV Ltd, a UK-based district heating pipe manufacturer explores the choices behind selecting pre-insulated pipe systems.

When planning a district heating (DH) network of any size, the choice of pre-insulated pipe system is important. In the late 1960s, when the first of the modern era of bonded-pre-insulated pipe systems started to appear, steel – encased in rigid, bonded polyurethane foam and an outer casing of HDPE was the order of the day. The latter half of the twentieth century saw a rapid development in thermoplastic technologies and manufacturers soon latched onto the idea of using a service pipe that would not corrode, that could be supplied on rolls and would offer flexibility unheard of in the traditional steel systems. Now, after around a quarter of a century, both systems are still going strong, but the young upstart has been gaining a lot of ground over its rival.

So which system to choose? Of course polymer pipes don’t corrode - but unfortunately it’s not as clear cut as that. In fact you may need to use both types – depending on the size and complexity of your DH network. Temperatures and pressures are the first deciding factor.

Systems with service pipes made from polymers such PP-R and PEX will quite comfortably accommodate continuous operating temperatures up to 95°C, but at this temperature, the service life will be significantly reduced. Drop down to a more sensible temperature (around 80°C) and the life expectancy will be dramatically improved. Compare this with steel pre-insulated pipe systems and they will quite happily cope with constant operating temperatures of around 120°C and above.

When it comes to range of diameters, then steel wins hands-down – with systems ranging from DN20mm up to 1,200mm being widely available. Polymer systems are a slightly more limited – up to around DN300mm in the case of PP-R-based polymer pipes.

Larger DH schemes are now often using a hybrid of steel and polymer pre-insulated pipe systems with steel systems forming the arterial heart of the distribution network and smaller connections being made with flexible plastic systems.

With smaller diameters of polymer pipes being delivered to site on a roll, needing narrower trenches and having the ability to snake their way around obstacles, these flexible systems really prove their worth in terms of speed of installation and help minimise on-site disruption. There are no hot works required for welding, no NDT, nor the need to keep large amounts of trench open whilst hydraulic pressure tests are carried out – so a much simpler process.

It’s worth noting that the larger dimensions of polymer pipe systems are not as flexible as some manufacturers would have you believe – in fact they are positively awkward to handle and unroll – typically making much more sense to opt for straight 12-metre lengths for polymer systems over DN100mm diameter.

Polymers also have the edge on frictional losses and their low conductivity makes for reduced heat loss compared to steel alternatives something which will lower costs in terms of energy and emissions.

So in summary, many factors affect the choice of one type of system over the other – many more than discussed above - but suffice to say that whilst polymers are gaining market share, steel pre-insulated systems look set to be around for some time yet as the technology continues to increase its foothold in the low-carbon heat revolution.