A brief introduction to district heating pipe technologies

Although district heating is a standard utility throughout much of the EU, it remains a niche technology in Ireland, typically limited to communal schemes rather than large-scale networks. Greentherm Managing Director ENDA RUXTON breaks down the technical composition of these piping systems, emphasising that their installation occupies a unique ‘skillset crossroads’ that demands expertise in both mechanical contracting and civil engineering.

District heating pipe technologies

District heating pipe technologies, by which I mean pre-insulated rigid or flexible pipes, have had a minimal usage in the Irish market, comparison to other EU countries. There are a handful of district projects implemented in the entire country.

There are many more communal projects utilising district piping, for the purposes of clarity, communal, by definition is a centralised system serving apartment/housing within the same development.

Pre-insulated pipe installations by there nature sit on a skillset crossroads between the works typically undertaken by a mechanical contractor and civil engineering works.

Pre-insulated pipes can be divided into two key categories, rigid pipes and flexible pipes. There is some common terminology to both types of products.

Carrier pipes

This is the encased pipe used to carry the liquid for heating or cooling. Flexible pipes are typically cross-linked polyethylene (PEX) or multilayer piping. Rigid pipes can be P235GH steel, stainless steel, copper, high-density polyethylene (HDPE), PPR, etc, with pipes ranging in size from DN20 to approximately DN1100.

As a general rule, carrier pipes are manufactured by third-party manufacturers, procured by the pre-insulated pipe manufacturer and encapsulated in the finished product. In essence, in almost all cases, the pre-insulated pipe manufacturer’s expertise is in the application of insulation and the external casing, along with features such as leak detection and trace heating cables.

Insulation

The most common insulation utilised in rigid and flexible pipes is polyurethane foam. Typically, this has a maximum operating temperature between 120- 140C. Polyisocyanurate (PIR) insulation is sometimes employed as an alternative to achieve temperatures up to 150C. Some manufacturers offer hybrid insulation solutions using, for example, rockwool, adjacent to the pipe surrounded by a rigid insulation product to achieve higher operating temperatures for applications such as steam.

The insulation density for rigid pipes is defined in EN253 as a minimum value of 55 kg/m3. Depending on the manufacturing methods employed by the producer, insulation density can range from 55 kg/m3 to approximatley75 kg/ m3. There are specific technical reasons for this based on the particular process utilised.

Pipe casing

The normal external pipe casing used in district pipes is typically manufactured with HDPE. The purpose is to encase the other product elements with the benefit of durability and UV resistance. Various manufacturers use different HDPE densities, typically ranging from PE80 to PE100. The higher the numerical value the greater the material density, hence typically more durable.

Other pipe casing options are available depending on the application such as spiral wound galvanised finish.

Leak detection systems

There are two fundamental types of leak detection systems; basic systems that alert to a general system problem; and, accurate systems that provide an exact location.

Basic leak detection typically consists of two copper wires embedded in the insulation layer, used in conjunction with an electronic alarm. This provides a simple leak detection alarm, which highlights an issue without disclosing the position of the problem in the network.

A more accurate solution exists that can provide the exact position of a network leak such that provided by manufacturers such as Ratmon. Two x 2 wire MSC-1 cables are embedded in the pipe insulation during manufacture. When the pipes are installed the cable connections between pipes are crimped and heat shrunk. Once the finished circuit is coupled with the necessary electronic supervision hardware, it provides accurate location detection. The MSC-1 cable is fully insulated ensuring that condensation cannot occur, causing a false alarm.

Some manufacturers of flexible pipes claim to provide embedded leak detection cables. At the time of writing this is not yet known to have been reliably proven to work reliably, with some reports of failures in the field.

Flexible pipe systems

Flexible pipes are constructed generally in two different configurations. The first is layered expanded polyethylene and other is PUR foam.

The following chart shows a performance comparison based on the same pipe size using the two types of insulation. It can be seen that there is a significant different in thermal losses between the two product types which increases with system temperature.

It is worth noting that flexible pipes typically use cross-linked Polyethylene carrier (PEX) pipes ranging in sizes from DN20 up to DN160. These pipes have temperature and pressure operating limits, with the lifetime performance degrading with increased temperature.

Some manufacturers offer high temperature/pressure flexible pipes capable of higher performance with pressures as high as 25 bar and temperatures as high as 100 degrees. These pipes are appropriate for applications such as high temperature heat pumps, biomass boilers or waste heat applications, where elevated temperatures can adversely affect the pipe service life.

Flexible pipe systems typically utilise two different methods of jointing connections; bolted compression and axial press.

Axial press requires the use of a proprietary press gun, where pressing is a one-time attempt. Bolted compression can be fitting with the use of ordinary hand tools and can be retightened if required. Pressing fittings in restricted spaces can also pose a significant challenge.

Rigid pipes

Rigid pipes with steel carrier pipes are welded and are finished using various types of jointing kits, such as shrink jointed and electrofusion sleeves. This provides a continuation of the pipe casing at the welded joints. Once applied a two-part insulation compound is poured inside the sleeve to provide insulation continuity.

Many jointing systems allow the recording of electrofusion weld quality, by scanning a local barcode and recording the weld quality to a cloud service. This provides an auditable quality trail for a given installation.

There are many additional system components such as a wide variety of fittings and valves used in a typical installation, along with technical considerations, like expansion, which are simply too extensive to detail within this article.

About the Author

Enda Ruxton, Founder and Managing Director, Greentherm, is a sustainable heating expert. specialising in renewable technologies like heat pumps and solar. He leads the design and commissioning of mechanical systems for diverse projects, utilising intelligent controls and energy monitoring. He combines mechanical engineering and controls expertise to deliver efficient, eco-friendly energy solutions across Ireland.

Greentherm offers CPD training on DHS design. For details, phone 01 531 4781, or email info@greentherm.ie