Taking the Stress out of District Heating

Mark Whettall, managing director, CPV Ltd discusses the issues behind the use of stress analysis techniques in the UK.

Spurred-on by ever-increasing carbon targets for the built environment, more and more developers are turning to district heating as a means of delivering low-carbon heating and hot water to consumers. So with the technology finally looking like it’s going to enter the mainstream, it’s important that the expanding sector does not repeat the same mistakes that were made when local authorities first adopted the technology during the 1970s and 80s.

As with any industry that is experiencing growth, it’s attracting a lot of attention from companies wishing to get on board – if you want an example, just think back to what the solar PV sector was like in 2010. Whilst this is good for healthy competition, one area that is already suffering is the quality of design and installation of pre-insulated district heating pipe networks. As a manufacturer, I’m of course delighted to sell more products, but not if it’s designed and installed incorrectly and I’m afraid to say that this seems to be happening throughout our industry. 

One such example of this is an apparent lack of awareness of the whole issue of thermal expansion of the pipe systems – something which, if ignored, in many cases will result in systems failing and consumer supplies being disrupted. It does not take much effort to imagine the fallout from the negative PR created by a whole community without heating and hot water in the depths of winter.

In more mature markets for district heating such as the Nordic countries, stress analysis is a pre-requisite and it allows for not only a system to be designed correctly – keeping stress within safe working limits and thereby maximising the system’s life expectancy – but it also needs to be applied during the construction phase. When excavating trenches to install buried networks, there will often be unforeseen obstructions which necessitate the pipe route to be changed. Usually done on the hoof, this is an area that can have serious consequences if the stress calculations are not revisited.

Although it may not be apparent straight away, as soon as the system is operational and the temperature cycles commence, the stress levels increase dramatically and in the worst cases, stress fatigue can occur.

To illustrate this point, something as simple as adding in an unplanned branch connection can also seriously impair a system’s ability to cope with the huge forces exerted on the pipe from thermal expansion. To put it into context, a 200-metre length of buried DN250mm pre-insulated steel pipe, heated to an operating temperature of 120°C will expand by around 150mm – exerting a stress of some 171MPa – the equivalent of 52 tonnes.

Experienced network designers, manufacturers and their distributors, should be providing detailed stress analysis to its clients as standard throughout a project’s entire development - providing support in calculating the effects that these system changes can have – so that necessary steps can be taken to ensure that the thermal expansion stays within the requirements of both the manufacturers’ recommendations and the requirements of the EN 13941 standards for the design and installation of pre-insulated bonded pipe systems for district heating.

It’s no black art, just straight forward engineering calculations. Advanced stress analysis tools such as the industry-leading sisKMR software package is perfect for analysing and correct a client’s proposed design so that its performance and life expectancy can be maximised.

In an ideal world, installers should pre-stress a pre-insulated pipe system by heating the network before it’s buried – then backfilling and compacting the trenches whilst the system remains heated in order to reduce the initial stress to acceptable levels. Unfortunately though, site conditions – particularly when working in busy city-centre highways or highly-populated residential areas will often prevent this approach, so other methods of dealing with expansion have to be employed as dictated by stress analysis calculations.

This can be simple expansion loops, foam expansion pads which allow a degree of expansion movement or single-use expansion compensator bellows that allow the pipe to expand when heat is first circulated, then, when it reaches a pre-determined temperature, it’s fixed in place by circumferential welds– with the stress levels then kept to safe limits and the threat of expansion bellows failing being completely removed.

As the UK’s district heating sector continues to gather apace, we must all remain vigilant and ensure that these fundamental calculations are not only carried out, but implemented. Project teams will always be under pressure to get a trench backfilled and the surface re-instated – particularly if it’s in an urban area – so developers need to select a supplier that will provide them with quick and accurate calculations and advice.

With this upsurge in district heating projects, we are inevitably seeing traditional utility contractors with experience in MDPE water and gas pipes move into the district heating sector and it’s imperative that the project teams – particularly those in supervisory roles – receive training in installing district heating networks. Many are largely unaware of the importance of dealing with thermal expansion and, as such, are prime candidates to attend manufacturers’ training courses.

Earlier this year, I called for the creation of UK-wide quality standards to cover every aspect of a district heating systems design, construction, operation and maintenance and the issue of stress analysis is yet one more reason why this must happen.

The capital cost of installing a buried pre-insulated pipe network forms only part of the whole-life cost of owning a system and relatively small savings made at the time of installation – by developers taking the least-cost tender from a contractor will have serious implications in later years – particularly if systems fail and repairs are required. Having to close busy roads, locate and excavate the point of failure and then carry out repairs can so easily be avoided by getting it right first time when designing and installing a network. It’s really quite simple!