Cell glass can be used as an alternative to pipe gravel when new district heating pipelines are installed

Is cellular glass an alternative to pipe gravel when new district heating pipelines are to be established?

When district heating plants establish new district heating pipelines, the district heating pipes are laid in the ground with pipe gravel around them to ensure that they stay in place and lie stable and secure. This has been done for many years, but what if there were an alternative to pipe gravel that was just as good and perhaps both more cost-effective and more environmentally friendly?

This is what Dansk Celleglas ApS has set out to investigate with their product, Durapor cellular glass, which is made of 100% recycled glass. To get the documentation in place, they have asked Force Technology to carry out tests as a third-party testing institute with both pipe gravel and Durapor cellular glass in order to compare the results.

The tests have been prepared under the performance contract project Surfaces and interfaces in industrial products (OFGRIP), which is to increase knowledge about surfaces within, among other things, the characterisation areas of friction and wear resistance.

Dansk Celleglas has previously carried out tests with their product Durapor cellular glass for Næstved Fjernvarme with good results. Dansk Celleglas now wanted documentation of whether the product can replace pipe gravel in the future when future district heating pipelines are to be designed.

Specially designed test shows whether Durapor wears more than pipe gravel

Force Technology has specially designed a test that is to investigate how much friction and thus wear there is on the district heating pipe jackets if pipe gravel is used compared with if Durapor cellular glass is used.

The test is based on the standards DS/EN ISO 12957-1:2018 as stated in DS/EN 13941-1:2019 section 6.5.3.2. FORCE Technology is to carry out a friction test that is to demonstrate how much wear there is on the district heating pipe jackets. The test is carried out as a pull-out test with the normally used pipe gravel and with the new alternative Durapor cellular glass.

The pipe gravel has a particle size of 0.063–8.0 mm and Durapor cellular glass has a particle size of 10-60 mm. This means that the pipe gravel will produce an even wear on the entire surface, whereas Durapor cellular glass will probably create point loads.

According to the standard, three scenarios are to be investigated, which indicate where the district heating pipes are buried:

• Verges and similar areas
• Residential road
• Heavily trafficked road

Depending on how much traffic there is over the district heating pipes, they become more or less pressure-loaded.

The test has been carried out with a fixed pulling speed of 1 mm/min for a total of approx. 3000 s (50 min.). The normal force is provided with a hydraulic press, where the resulting normal force is measured continuously. The applied normal force is respectively 8 kPa, 20 kPa, and 50 kPa cf. DS/EN 13941-1:2019 section 6.5.3.2.

Friction test of district heating pipe jacket material

As it is not possible to carry out tests with round pipes according to the standard, tests have been carried out on flat PEHD plates and the coefficient of friction has been adjusted accordingly.

Friction, wear and roughness measurements have been carried out on the flat plates of district heating pipe jacket material. Friction testing of pipe gravel against jacket material showed that the tested pipe gravel has a static coefficient of friction of approx. 0.20–0.28, as well as a dynamic coefficient of friction of approx. 0.15–0.20, whereas Durapor cellular glass against jacket material has a static coefficient of friction of approx. 0.28–0.45, as well as a dynamic coefficient of friction of approx. 0.30–0.38 at 8 kPa normal force.

Both the static and dynamic coefficients of friction decrease with increasing normal force, and are at the same level as pipe gravel at 20 kPa normal force.

The wear on the surfaces is evenly distributed and uniform for the pipe gravel, whereas Durapor cellular glass exhibits local wear due to the point loads. Measurement of the surface roughness in the wear tracks shows that the surface for the pipe gravel is more uniform than for Durapor cellular glass.

Friction test of district heating pipes

For the test of district heating pipes, the type ISOPLUS TwinPipe, series 2, ø76+76/250 mm, has been used. The pipe is approx. 450 mm long, and a new pipe has been used for each test.

Measurement of the relative friction force of respectively pipe gravel and Durapor cellular glass against district heating pipes at 20 kPa normal force showed that the relative friction force of the two backfilling materials is at the same level.

Both filling materials create a tunnel effect

Cycle testing with (15+1) back-and-forth runs showed that Durapor cellular glass tends towards a so-called 'tunnel effect', whereas pipe gravel remains constant. The pipe gravel was 'relatively wet' in the test, and it therefore cannot be ruled out that drying of the pipe gravel influences the occurrence of tunnel effect.

Therefore, an additional test of pipe gravel has been carried out to confirm or disprove whether drying of pipe gravel can influence the coefficient of friction. It is seen very clearly that there is tunnel effect in pipe gravel, but not because of the movement of the pipe, rather because of drying of the pipe gravel, which occurs when heat is sent through the district heating pipes.

The tests show that the relative coefficient of friction, when there is tunnel effect, is almost identical between pipe gravel and Durapor cellular glass.

Visual inspection of the surface of the jacket material used for friction measurement as well as district heating pipes showed that the wear level is comparable.

Reduced heat loss and improved working environment

Dansk Celleglas has, together with Næstved Fjernvarme, tested the insulation capability on a section made with a Series 3 pipe from Isoplus. Testing was carried out over one year, and the result showed that the heat loss was reduced by 38% on average, covering a reduction of more than 50% in winter and approx. 25% in summer. The most significant reason why Durapor cellular glass reduces heat loss is that the material cannot absorb water and at the same time has an insulating effect. In contrast, pipe gravel in a moist state conducts heat away from the district heating pipe and out into the ground.

As Durapor cellular glass weighs a factor of 10 less than pipe gravel per m3, it does not physically burden the workforce as heavily. It also causes less wear on the equipment used to establish the district heating pipeline.

In addition, there is room for 92 m3 in one delivery by one haulier at a time. So fewer trips are needed, and the roads become less pressure-loaded than if pipe gravel were transported.

The article was published in Fjernvarmen no. 1, January 2021.