When planning pre-insulated piping systems, it is possible to choose between several laying methods depending on physical conditions and operating principles:
Method 1 – Expansion bends (L, Z and U bends)
Method 2 – Heat prestressing
Method 3 – Prestressing elements
Method 4 – Cold laying
As factors such as operating temperature, system type, laying depth, flow requirements and local conditions affect the choice of laying method, we recommend that guidance be sought, either from isoplus or from a consulting engineer.
The various methods, and the rules that must be followed in their use, are described in detail in the following sections.
By way of introduction, a few decisive parameters which influence the choice of laying method will be described here.
Pre-insulated pipes consist of a sandwich construction in which the three components (i.e. the carrier pipe, polyurethane foam and jacket pipe) are bonded together.
As a result, the entire construction behaves as a single integrated unit when, for example, moving in response to temperature fluctuations.
The stresses accumulating in the carrier pipe are thus determined by the ability of the pipeline to expand freely in response to temperature fluctuations, the pressure within the pipe, and the weight of the pipe and backfill material.
In the ”old days”, plastic yield (i.e. the stress at which a material becomes plastic and is permanently deformed) was the limiting factor in the design of district heating systems.
As the conditions applying to district heating systems have significantly changed, it has now been accepted for many years that the plastic yield of the material can be exceeded without influencing system performance. The limiting factor is therefore no longer (or not necessarily) plastic yield, but rather the material’s ability to withstand repeated breaches of the yield point.
Whenever the plastic yield point is exceeded, the material will become slightly deformed (i.e. strained), resulting in subsequent stress relief. No risk of fatigue fracture will, however, occur until the number of such strains exceed the limit for repeated stress failure.
In practice, this limit is high enough to prevent any problems if modern district heating pipes are laid without any form of compensation.
However, certain practical conditions must be taken into account when choosing laying method.
Due to high stresses in the piping system, particular care must also be taken when excavating around or parallel to the system in order to prevent uncontrolled buckling.
In situations where it is necessary to perform regular excavation parallel to the district heating system with no limiting restraints, laying method 4 (cold laying) should be avoided as this method results in considerable axial stresses.
In situations where it is necessary to perform regular excavation with no limiting restraints, but where, during installation, it is not possible to leave the trench open for long periods of time and/or to install long runs, laying method 2 (heat prestressing) should also be avoided. This method consists of preheating the pipes before covering them with backfill material, thus effectively preventing stresses from arising in the pipeline below the preheating temperature.
In such situations, the system should be installed using laying method 3 (prestressing elements/single-use compensators) or method 1 (expansion bends).
Although both these methods are suitable, method 3 will often be preferable to method 1, which typically requires additional components, making it relatively more expensive.
It is thus evident that no ”black and white” solution exits, and there will usually be several alternatives which are equally suitable from a technical point of view. There is therefore no universal answer to the question of which laying method is best.
A general point of departure would, however, be to start with the cheapest and simplest method, i.e. method 4 (cold laying), combining this with one or more of the other methods where necessary and exploiting natural expansion zones in connection with direction changes.
The four methods are described in detail in section 4 together with laying rules for specific systems.