Elastomer gaskets
Elastomers
EPDM (Ethylene Propylene) elastomers are normally used for the joint gaskets of PAM potable water and irrigation pipes.
They are rigorously selected on the basis of criteria directed at maintaining their long term physico-chemical properties.
Long term performance
Elastomer ageing
The purpose of elastomers used in jointing systems is to guarantee that the gaskets will be perfectly tight as long as the pipeline is in service. Pipeline experience acquired by PAM has allowed its Research Centre engineers to understand and measure the change in properties with time of various types of elastomers, in order to find the ones with the best performances.
The change in the mechanical properties of elastomers with time can be indicated by two phenomena:
– creep (increasing deformation under constant loading),
– relaxation (compression relaxation under constant deformation).
In the case of socket joints, the seal is obtained by the contact pressure between the metal and gasket. The elastomer deformation produced during jointing remains constant. The relaxation phenomenon is therefore the only one of interest.
Relaxation measurement
Relaxation of elastomers is determined by a procedure which measures the change with time of the force required to compress a specimen of fixed deformation.
The diagram below shows the relaxation at ambient temperature of the EPDM used in the joints of PAM potable water supply and irrigation systems.
It can be seen that:
– the EPDM used by PAM ages less rapidly than a material that exactly meets the requirements of EN 681.1 and ISO 4633,
– long term, the contact pressure remains well above the leakage danger threshold.
Examination of specimens taken from mains after several years’ service has confirmed the excellent long term performance of PAM gaskets: all the physical and chemical properties of the gaskets tested had been maintained after many years of service.
Physico chemical properties
The principal properties of elastomers used by PAM are given in the table below.
Unless otherwise specified, EPDM elastomer gaskets are supplied.
Maximum usage temperature : 50 °C.
Certain storage precautions need to be taken.
| Properties | EPDM (Ethylene propylene polymer) |
|---|---|
| Hardness range (Shore A) | 40-90 |
| Density (basic product) | 0.86 |
| Tear strength | good |
| Abrasion resistance | good to excellent |
| Compression set resistance | good |
| Oxidation resistance | excellent |
Specifications and quality management
Specifications
The properties and minimum fitness for purpose requirements for gaskets are standardized.
| Criteria | Reference Standards |
|---|---|
| French International | |
| Tensile (strength and elongation at break) | ISO 37 |
| Creep | ISO 815 |
| Relaxation | ISO 3384 |
| Tear strength | ISO 816 |
| Resistance to water and chemicals | ISO 1817 |
| Ageing | ISO 188 |
| Resistance to ozone | ISO 1431-1 |
| Minimal fitness for purpose requirements | ISO 4633 EN 681-1 |
In view of the importance of gaskets in sealing pipe systems, PAM has instituted a more demanding specific quality control procedure, including:
- supplier acceptability, following an evaluation of his ability to supply a product on a regular basis, meeting PAM’s technical requirements, proof of the elastomer suitability,
- acceptance of the manufacturing moulds (design and dimensions),
- checking the quality of prototype gaskets, then of pre-production runs,
- constant monitoring of the supplier’s quality control results and parallel tests in our own laboratories.