The technical service life of gaskets
In the context of the requirements of the drinking water directive and the associated national and European hygiene requirements for organic materials in contact with drinking water, the long-term behaviour of elastomer sealing materials is discussed below.
According to standards EN 805 and EN 806-2, pipes and pipe joints for drinking water supply and installation are to be planned for a working life of at least 50 years. Based on the working life of the pipe system and/or the pipe materials, the joint system must function for at least as long as the pipes themselves. The technical working life of the pipe joints, i.e. also including that of the seals, is not taken into account in this consideration as a rule.
The sealing of the push-in joints as the heart of the piping system should achieve a working life of more than 100 years, corresponding to the service life of today’s ductile iron pipelines. This is ensured among other things by appropriate physical material properties. In addition the seal must also meet all requirements and approvals as regards drinking water hygiene.
Requirements for elastomer for use in drinking water applications
Independently of the hygiene and certification aspects of an individual country, the basic requirements for a seal are covered in EN 681-1 (or ISO 4633) which addresses the physical properties of the materials.
Requirements for finished seals are defined in the relevant product and trade association standards as well as in client specifications. The performance of a pipeline seal or gasket is dependent on the material properties of the seal, its geometric form and the construction of the pipe joint.
The material of a seal, its design formulation as well as the seal itself are subject to the requirements of standards and specifications. As a rule they are certified and have all the necessary documentation and type examination certificates. However, the assessment of their behaviour over the long term is only marginal or even non-existent.
Long-term tests and measurement of the compression set
An essential criterion for a reliable statement on the durable tightness of a pipe joint is the compression set. It provides information about the viscoelastic properties of a sealing material and is measured according to the ISO 815-1 or ISO 815-2 test method.
In the context of predicting the long-term behaviour of seals, the Woco company has developed a validated calculation method and a test method in the context of a research project with the DVGW. This process has in fact been incorporated in DVGW test specification G 5406 since 2016 and has been included in the draft standard prEN 549. More far-reaching investigations show that the method is also basically suitable for elastomer seals in water applications and hence can also be used for TYTON® seals in order to carry out comparative measurements on competitive parts or to make calculated deductions about long-term behaviour.
The concrete results and the working life of a TYTON® seal/Woco seals-EPDM from Pipe System Components: The time-temperature collective is an essential basis for considering working life. An underground drinking water supply pipeline is allocated for the calculation of a temperature profile for one year. Drinking water supply pipelines typically lie at a depth of around 1.20 to 2 m, i.e. in the so-called near-surface area. The near-surface earth temperature is determined by the specific climate on the earth’s surface, the composition of the soil (thermal conductivity) and the depth.
The compression set of the TYTON® seal has been measured at 60 °C, 80 °C and 100 °C, so that for each individual temperature a number of measurement values between a 20 % and 70 % compression set have been determined over a period of one year. Then the working life is able to be calculated using the Arrhenius process based on a limit compression set of only 55 %. As a rule, one reckons with about 70 % or indeed even higher limit values, which would then increase the working life positively again. An unfavourable value was also selected for the calculation. The limit compression set gives the value of elastic deformation or recovery which still remains after the calculated working life! Thus, after “x” years, a seal still has a small compression set, this reflects a high quality level for the seal and hence of the material as regards recovery.
In the operating temperature range selected, the TYTON® seal in the Woco-Seals/EPDM material has an expected working life of 164 years with a limit compression set of 55 %.
Author: Rüdiger Werner, Woco IPS GmbH