Bends for Plastic Jackets Pipe Systems able to withstand high transverse loads
Earlier Design of Bends and Angular Deviations Today, most of the district heating pipelines are manufactured as plastic jacketed pipes. These consist of a steel medium pipe, a thermal insulation of PUR-foam and a HDPE jacket pipe. Characteristic feature of this design is the fact that district heating networks are conceived for a service life of 30 to 50 years and that the statically stressed plastics - due to the influence of age and time - are subject to strong ageing. Therefore, for design of the plastic components, material properties which are reduced due to ageing have to be applied.
Regarding the normal maximum medium temperatures between 90 °C and 140 °C, the transverse loads may act on the PUR-foam at a very small extent. In case of high outgoing temperatures such as 130 °C or 140 °C, the necessary permissible compressive strain of the PUR-foam is as high as = 0.15 MPa. Accordingly, the transverse displacement occurring at bends has to be absorbed in cavities (expansion unit) or in expansion pads (foamed cushions) in order to protect the pipe against high soil resistance forces. For conventional plastic jacket piped, angular deviations have to be limited to maximum values of 3 °C and 6 °C, depending on the line situation.
Within the next years, the application of CFC-11 will be forbidden in all of the investigated countries. In the district heating markets there are three alternative foams for plastic jacket pipes offered today by commercial production:
- CO2-foam from different manufacturers
- M90-foam from ABB I.C. Möller, blown with 1.1.1-trichlorethane and HCFC-22
- 'green foam' from TARCO, blown with HCTC-142b and CO2.
Internationally there are no restrictions regarding the application of CO2-foams.
Foams containing 1.1.1-trichloretane (such as M90) may be used for some more years; still, there has been determined a deadline in all of the investigated countries (depending on the country there will be a ban between 1994 and 2005).
Foams based on HCFC-142b and CO2 are still allowed in most countries except Switzerland; here, all HCFCs are banned from the year 2000 onwards. According to international statements and national legal projects there is a trend that the production or application of all ozone-relevant foams shall be reduced.
For the safe operation of plastic jacket pipes with CFC-11-free foams it is indispensable that the technical requirements resulting out of the thermo-mechanic stresses are met.
These requirements regard
- shearing resistance
- compression strength
- temperature resistance, and
- aging resistance (service life).
During the years 1987 to 1991 several countries carried through intensive investigations on CFC-free foams, but these investigations were substantially impaired by the following facts:
- During the above mentioned period the foams were still in a nascent stage.
- In many cases the test methods were not suitable for CFC-free foams.
- Neither the investigated foams nor the test conditions were coordinated between the testing agencies.
Not until in 1992, coordinated investigations on reference foams started. The agreement on reference foams and the coordination of the test conditions constitute important prerequisites for a global assessment. Thanks to the cooperation, investigations carried through at different testing agencies and with pipes and test cubes can be compared.
There are plans to adopt the new test concepts, which are independent from blowing agents, for the European standardization. Then, the required design approval tests for the marketed foam from the various manufacturers may be carried through. The only gap in the new test programmes which are accompanied by the European Project Group is the fact that investigations on the axial shearing resistance are missing. The axial shearing resistance is the most important characteristic of plastic jacket pipes, i.e. it allows endless-laying without expansions joints.
It would be a pity if conclusions on the axial shearing resistance could be drawn only from tangential creep tests. If there should be any financial means available for the investigation of CFC-free foams, these should be allocated for the determination of the axial shearing resistance.
There remains the hope that the development of the CO2-foams continues the trend shown during the last years, reaching gradually a higher temperature resistance so that this type of foam which is most favourable for the environment meets all the thermo-mechanic requirements.