Failure of Rubber Gaskets in 300 mm bore Water Lines
July 19th, 1983
Report to Caxios Ltd.
S F Bush
Summary
Following the failure under pressure testing of rubber gaskets installed in approximately 300 mm bore water lines, tests have been carried out on samples of the gaskets against the relevant British standards. Neither type of gasket entirely meets the dimensional requirements of BS 4865 and one gasket, the Rayflex EPDM type, was well short of the tensile strength standard set by BS 2494.
Analysis, while approximate and based on assumptions, indicates that both the dimensional and strength features contributed to failure of both types of gasket, with the Rayflex estimated to fail at around 11 bar and the Corrugasket around 18 bar, broadly as observed. The analysis also indicates that merely to specify rubber gaskets to meet BS 2494 for this duty at the test pressure involved (18 bar) is inadequate. Natural rubber of the same hardness as the EPDM used, meeting BS 2494 would have performed better, but whether well enough is problematical.
It is also doubtful if any EPDM type would be really suitable for the duty. The analysis indicates that the performance of all rubber gaskets is affected by the proximity of the bolt ring to the outer gasket and to the bolting up stresses applied. In the absence of precise information on these points, estimates have been used. Neither BS 4504, nor BS 2494 refers to these matters and this seems an important omission.