The strength, imperviousness and service life of bonded jackets
The starting point of this project was that the joints should fulfil the requirements during their entire expected life. Therefore, the work has concentrated on investigations connected with ageing and stress. The report divides the products into welded joints and sealed joints.
The purpose of the project is to measure the stress and tensions occurring in bonded jackets and thus obtain a better basis for specifying the necessary and sufficient requirements for bonded jacket pipes and testing methods. The project comprises know-how collection, setting up of theoretical models, testing of these and conclusions according to the following plan: 1) Detailed planning. 2) Damage assessment carried out under impartial condition. 3) Mechanical stress and strength: Study of existing know-how including previous projects, setting up of theoretical models concerning the spreading of tensions in joints, checking the effect of the test parameters, concluding and setting up of test conditions and specification of requirements. 4) Tightness test: Functional description, going through the methods for the measuring of tightness, including sensitivity, testing of selected systems, conclusion and specification of requirements for products and method. 5) Lifetime: Study of accelerated methods for lifetime measurements and notch sensititily, full scale investigations of shrunk bonded jackets' lifetime under accelerated conditions. 6) Evaluation of results: Complete evaluation and possibly adjusting initiatives concerning results as a whole and setting up of recommandations for the future requirements for all types of bonded jackets and test methods. 7) Reporting. The results of the project will be put before the CEN/TC107 being the European group for standardization of district heating systems. The project is managed by the DTI. A steering group is attached to the project consisting of firms and district heating plants
Sealed joints: Examinations have been carried out on a sealant made of polyisobutylen and they show that the connection between shear strength, time and temperature can be calculated. The investigations ended before the expected life of the joint expired and therefore it is recommended to let the investigation continue during the entire period calculated as the lifetime of the sealant. In the light of tests carried out at elevated temperatures it can be expected that the hoop stress present in shrinked joint casing will be present after artificial ageing. It has been demonstrated that the force in the sandbox can be reproduced. The shear force on the joint can be calculated. The shear force per m2 of the applied joint is 5 times greater than the shear force per m2 of the casing pipe. In the light of the measurements it is recommended to only use a test speed of 10 mm min-1. With a starting point in literature it is suggested to maintainthe displacement and cycle number. Investigations of the grain distribution of sand concentrated under the joint casing have resulted in the recommendation to use sand with a grain size of 2 mm or less. As the joint must not create a tunnel in the sand it is recommended touse dry sand. It is recommended to carry out the sandbox test on artificially aged, tightened joints. Considerations have arisen on behalf of the theory about the flow of water in capillaries and therefore it is recommended to increase the tightness testing to 7 days. It is recommended to use 2 temperatures and 2 pressures. With regard to ensuring optimum corrosion protection of steel joints it is suggested how corrosion protection of polyethylene on steel joints can be tested. Proposals for the requirements to the test results are stated. With regard to ensuring the quality of welded and mechanically sealed plugs of polyolefins a test method has been stated. Finally, another type of measurement is shown for properties that today only can be revealed by means of a sandbox test. Welded joints: An evaluation has been made on the stresses and strains in PE casings and joints of pre-insulated pipes based on a FEM analysis and formulas for stress-strain relationships. The design of components of PE is normally based on the creep modulus, but the stresses and strains resulting from the actions on a welded joint on a district heating pipe depend almost solely on the relaxation modulus. Within the lifetime of a district heating pipe the relaxation modulus is reduced with a factor of up to 10, and therefore stresses and strains can only be evaluated taking this reduction into account. Based on available literature a table with relaxation modules for time span, temperatures and strains relevant for district heating is calculated. The following conclusions are made: 1) The calculations are connected with much uncertainty most substantially because of lack of information on relaxation curves, limit values for PE subject to constant strain and uncertainty connected with possible methods for combining the strains from different actions. 2) Short-term analyses of stress in PE casings and joints are not relevant. 3) The durability of welded joints is a strain related life problem. 4) Strains originating from operation are normally low compared to the strain arising from production and installation. 5) Fatigue is not a problem within the range of temperature variations relevant for pre-insulated district heating pipes. 6) The sandbox test is a short-term strength test therefore not relevant for welded joints. 7) A long expected life can be assured by reducing the strains that arise during production and installation and by using material with low sensitivity to RCP (Rapid Crack Propagation) and SCG (Slow Crack Growth). The resulting conclusion is that if high quality PE is used, then normal actions on a welded joint will not be decisive for the expected life. The crucial factor is the welding quality. Therefore, it is suggested that the short-term sandbox test should be replaced by an accelerated durability/lifetime test, e.g. a modification of the FNCT test (Full Notch Creep Test), where the notch is the weld itself
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| ABB A/S | ||
| Tarco Energi A/S | ||
| Aalborg kommune | ||
| Affald Varme Århus | ||
| Herning Kommune, Herning (DK) | ||
| VESTEGNENS KRAFTVARMESELSKAB I/S | ||
| Rambøll | ||
| Dansk Fjernvarme |
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