Putting in the groundwork

Published:  07 February, 2019

Building Solutions at REHAU, District heating networks, heat interface unit, heat networks, HIU, Steve Richmondv
The benefits of flexible piping in a trench

Steve Richmond addresses some misconceptions surrounding district heating pipes in the UK, and explains some of the key design principles for a scheme which is built to last.

Although the UK government identified district heating as carrying the potential to enhance efficiencies, lower carbon emissions, and reduce fuel poverty, familiarity with the concept in the UK construction industry remains relatively low. The UK is home to approximately 17,000 heat networks, but they only contribute around 2% of the UK’s total heat demand. This figure is relatively low compared to other European countries, where district heating systems have grown rapidly over the last 20 years.

District heating could provide as much as 17% of heat to our homes and 24% of heat to commercial buildings by 2050, according to latest estimates. The knowledge base in the industry and the general awareness from the end user needs to be increased to help us reach these targets. We now need ensure designers’ and contractors’ familiarity with heat networks improves so they are able to provide successful and long-lasting heat networks.

District heating networks have the capacity to provide heat for a number of buildings from at least one energy centre. A variety of sources can be used for its heat, from gas, combined heat and power (CHP) to energy from waste, industrial waste heat and renewable energy such as heat pumps. These sources could be combined or changed at any time without the need to replace the whole network.

Hot water travels through an underground pre-insulated pipe network connected typically to a heat interface unit (HIU) located in each building. This provides heating and hot water to each building and then the cold water returns back to the energy centre to be heated and circulated again.

One the key benefits of district heating is the ability to reduce carbon emissions through local generation. Other benefits can include greater fuel security, lower bills for consumers, flexibility in heat sources used, and reduced maintenance costs from using one central plant.

To ensure we’re developing efficient heat networks, we need to address some of the common misconceptions which may have hindered many contractors’ familiarity with district heating in the past.

#1: Polymer pipes can only be used for a maximum 70°C flow temperature

This misconception comes up regularly in discussions with designers but, with over 45 years of experience producing PE-Xa pipes as one of its pioneers, REHAU can debunk this myth. Many schemes we work on today operate at continuous flow temperatures of 80-85°C but the most efficient schemes are already being designed at 70°C where possible. According to BS EN 15632, PE-Xa district heating pipes have a minimum service life of 30 years at 6 bar and 80°C continuous flow temperature with an allowance of 90°C for one month per year and 95°C for 100 hours per year.

#2: Specialist welding teams are required for installation

Another common misconception is that all district heating pipework requires specialist welding teams on site for the jointing, however this is only true for steel and large diameter PP-R pipes. Small polymer pipes are on the market now and are joined using a permanent mechanical compression sleeve. This mechanical jointing is much faster on site than welding, not least because it is safer, simpler, with fewer specialist tools and less room required in the trench. This means the mechanical or civil contractor on site can install the pipes themselves rather than subcontracting the jointing to a specialist welding team and having additional trades on site.

#3: Polymer pipes are only suitable for small networks and house connections

Contrary to popular belief, steel is not the only material suitable for district heating schemes. Polymer pipes are available up to 160mm and PP-R pipes are available up to 355mm. A 355mm can provide up to 16MW of heat at 80/50°C which allows many larger district heating projects to be a fully polymer-based network which reduces the risk of corrosion and removes the need for expansion mitigation in the trench.

Building Solutions at REHAU, District heating networks, heat interface unit, heat networks, HIU, Steve Richmond
Making a joint in a trench with the latest piping technology

While addressing these misconceptions, further considerations need to be made at the planning stages of a project to ensure a simple, quick, and cost-effective installation that stands the test of time.

For the pipework-specific procedures, the following tips will act as a solid foundation when it comes to implementing a well-designed project on site.

1. Ensure that the network is designed on the actual heat loads, not peak assumptions. The more consumers a heat network has, the greater the impact on the total heat load.

2. Planning of storage and vehicle access on site is absolutely essential. Working closely with the pipe supplier from the very start of a project helps prevent issues, which ensures cost-effective completion of the project.

3. Transport, handle, and store responsibly to avoid pipe damage. It is vital that considerations are made to ensure the pipe isn’t damaged during storage to give the pipework longevity.

4. Choose the right installation technique. A common technique to laying the pipe is to use an open trench but there are alternative techniques available such as pull-through, ploughing-in or horizontal directional drilling (HDD).

5. Ensure a commissioning procedure is in place so there is opportunity to identify and resolve any potential snagging issues and ensure the system performs as expected for the project lifespan.

In reality, a heat network system is only ever as good as its design, installation and operation and in that respect, thorough planning of a project is needed to ensure pipework installation can play a vital role in the long-term performance of a system. Choice of pipework is critical, and we now find ourselves in a position where there are more options on the market than ever before.

It is important that designers and contractors familiarise themselves with exactly how certain materials, jointing techniques, and manufacturer-led support can benefit them. By working closely with industry partners such as REHAU, designers and contractors working in the industry can play a key role in ensuring district heating schemes fulfil the potential they have to contribute significantly to the UK carbon reduction targets for years to come.

Steve Richmond is head of marketing & technical - Building Solutions at REHAU



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