Why pre-insulated pipeline systems need thermal pre-stressing

When an investor decides to build a heat distribution network and picks a pre-insulated pipe system, the assumption is that they are buying a quality product with a long service life. In many cases that turns out not to be true — because of poor design and unskilled installation that follows.

Pre-insulated systems are reliable only when they are properly designed, checked for strength, correctly built and then operated the way they were meant to be.

A common argument for turning away from pre-insulated pipelines is that they fail often and do not last. Our experience says the components from leading Czech suppliers match foreign manufacturers in quality. So why do we see up to a hundred times more failures than our southern and western neighbours?

The problem has a simple common denominator: lack of knowledge, and lack of discipline — and I will leave the second one aside today. A pre-insulated pipeline cannot be built like a conventional heat main, a water main, a gas pipeline or a sewer. It has specifics no other type of pipeline has.

Thermal pre-stressing of a pre-insulated pipeline is a basic precondition for using it correctly in roughly 80 % of cases. The remaining 20 % can be laid cold, thanks to the specific local parameters of the route, or have to be handled with a different type of compensation.

So why is pre-insulated pipework not routinely pre-stressed?

Because the importance of this construction step is not understood well enough. Neither the cost nor the effort is significant within the scope of the whole contract.

With electrical pre-stressing the effort involved is minimal. If the site is prepared according to our instructions, we can start heating the pipeline within two hours of arriving. Diameters from DN 50 to DN 150 usually reach the required temperature in about four hours. Once the check temperatures and expansions are verified, the contractor can start backfilling with sand straight away. For DN 200 and above, the pre-stressing time depends on the specification of the section. For a DN 500 pipeline over a 650 m route, insulation class 1 and a target temperature of 92 °C, heating takes up to 24 hours.

The whole process is monitored and checked throughout. Electrical pre-stressing makes many pre-insulated pipeline construction scenarios workable that could not be handled efficiently before: long thrust bores, cold laying at complicated traffic junctions, tie-ins to existing routes without losing pre-stress, and many others.

An emergency pipeline repair often costs millions to tens of millions of crowns.

The cost of electrical pre-stressing is directly proportional to how well the contractor has prepared for it. Short sections and smaller diameters can be pre-stressed and backfilled in one or two days, which puts the price under a hundred thousand crowns. Larger diameters or longer, more demanding routes can be managed within a few days.

An emergency repair, on the other hand, where the pipeline was not designed and built correctly, often runs to millions or tens of millions.

To paraphrase Mr Maurer: „Do not design, do not build and do not live badly!“

Authors: Ing. Michal Homola and Ing. Ondřej Homola