Ignoring stress effects from thermal expansion could lead to failure in district heating networks warns CPV

The lack of awareness in biomass-fuelled district heating networks regarding the stress effect on pre-insulated pipes from thermal expansion could cause systems to fail says the Hampshire-based company
Ignoring stress effects from thermal expansion could lead to failure in district heating networks warns CPV

A leading manufacturer of district heating pipe systems has highlighted serious concerns regarding the sector’s apparent lack of awareness of the effects that thermal expansion has on pre-insulated pipe networks. CPV Ltd, based in Romsey, Hampshire, warns that ignorance of such effects could lead to failure of district heating systems which in turn will risk damaging the faith placed in the technology by the Department of Energy and Climate Change (DECC).

CPV believes that stress analysis is being overlooked, particularly at site level, even though such analysis is a pre-requisite in some of the better-developed markets for district heating in Europe.

“It's all too common for civil works to commence on site and hit an unforeseen obstruction in the ground that requires a deviation from the planned route” explained Mark Whettall, CPV’s Managing Director explained. “These changes - which are often made on the hoof - can seriously affect the integrity of a system's performance. Although they 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. This of course will invariably disrupt the supply to consumers and necessitate a costly repair to be made - something that could have all too easily been avoided.”

Furthermore, 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 Mr Whettall added.

“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 force of some 171MPa - the equivalent of 52 tonnes. It's not rocket science, it's just sound engineering design and by using advanced stress analysis tools such as the industry-leading sisKMR software package, as a manufacturer we're able to analyse and correct a client's proposed design so that its performance and life expectancy are maximised.”

Experienced district heating network designers and manufacturers should always be able to assist the on-site teams with calculating the effects that these system changes can have, CPV maintains. Necessary steps can then 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.

“In an ideal situation” Mr Whittall continued,  “contractors 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 urban environments - often prevent this approach, so we have to use other methods of dealing with expansion as dictated by our stress analysis calculations.”

CPV believes that it is essential, as the UK’s district heating sector continues to expand, to remain vigilant and ensure that these fundamental calculations are not only carried out, but implemented. Given that contractors will be under pressure to get a trench backfilled and the surface reinstated, particularly if in a public highway or residential area, developers need to select a supplier that will provide them with quick and accurate calculations and advice. The problem is that with an upsurge in district heating projects the sector is invariably seeing traditional utility contractors with experience in MDPE water and gas pipes move in and therefore 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” Whettall concludes, “and as such are prime candidates to attend manufacturers' training courses.”

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