CFC-Free plastic Jacket Pipes for District Heating
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Introduction
For many years plastic jacket pipes proved worldwide to be and economic and reliable laying system for district heating pipelines. Especially during the 80's the state of knowledge was documented in numerous investigation reports. For the production of the pipes up to now the blowing agent CFC-11 was used, which as cellular gas was also responsible for the good heat insulation of the PUR-foam.
Considering their ozone-depleting effect, CFC-11, as well as other fully halogenated chloric chemical compounds were adopted into the Montreal Protocol, which means that their further application is limited. As a consequence, individual countries issued laws and regulations regarding the banning of these substances during the next few years.
Searching for alternatives for the CFC-containing foam the industry followed different methods, the most importants of which are described.
In view of new investigation results (climate measurements) and the political pressure, the Montreal Protocol as well as the national regulations are subject to constant changes. The present (February 1992) state of the plastic jacket pipes is described on the basis of a Swedish IEA-survey and a new German study.
The assessment of these alternatives is in most cases based on values obtained by the application of the proved CFC-containing foam. In the report a description is given of the technical prerequisites for new foams, paying special attention to:
- shearing resistance
- compression strength
- temperature resistance
- aging resistance (service life).
During the last years numerous investigations on the new foams have been carried out. The most important investigations and the corresponding results are presented.
Since the end of 1991 it is tried to internationally coordinate the investigation programmes. In view of the cross-border co-ordination in connection with investigated foam systems, the applied stresses and the testing temperatures, there is the prospect, that single investigation results may form part of a useful global assessment.
Summary
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 thermomechanic 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
