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Forensic Examination of Rubber Expansion Joint

Comprehensive Expert Failure Analysis of Generation Plant Components
Project objective
An expansion joint made of silicone rubber was used in an incinerator system. The expansion joint was located between two stainless steel pipes and was secured using band clamps. The expansion joint was used on the incinerator stack, which was the exit pathway of the incinerator flue gas with known composition including hydrogen chloride. The temperature and pressure of the flue gas was approximately 173 °C and 14.6 psia, respectively. During the inspection, some cracks were observed on the expansion joint which resulted in its premature failure. The project objective was to determine the failure mechanism and the probable cause of failure.


Scope of work

The scope of work was to perform a detailed analysis to discover the root cause of the failure. Several tests were planned including: incoming inspection of the rubber sample and dimensional measurements, microscopic examination of failed areas, Fourier Transform Infrared Spectroscopy (FTIR) to identify the material, confirm the composition, and determine the presence of contaminants (molecular signature), Shore A hardness to assess any change in mechanical performance, and Scanning Electron Microscopy Energy Dispersive X-Ray Spectroscopy (SEM-EDS) to determine elemental composition (qualitative) and check for the presence of contaminants.
Kinectrics Solution
Visual and microscopic examination of the failed expansion joint showed a large crack, which elongated beneath the band clamp connecting the joint to the pipe ends. The surface of the failure site presented features characteristic of chemical etching of the material. The discoloration was also attributed to the thermal and chemical exposure. The FTIR tests confirmed that the rubber material was made of silicone rubber.
The hardness results showed that the outer surface of the failed expansion joint had almost the same hardness as the new expansion joint. However, the middle discolored part of the inner surface demonstrated noticeably higher hardness, which is a result of the exposure to high temperature flue gases.
When hydrogen chloride comes into contact with humidity, it forms hydrochloric acid, which has severe effects on silicone rubber.
Therefore, it seems that silicone rubber was adversely affected by chemical exposure to hydrochloric acid. It is assumed that the initiation of the large crack happened as a result of stress concentration under the band clamp. This stress concentration was primarily caused by the premature loss of ductility of the material through thermal and chemical exposure.
Installation of the silicone rubber joint was probably not optimal and is considered to be a compounding factor in the initiation of the large crack with the clamp being too tight. This scenario is substantiated by the failure site being located beneath one of the band clamps. Since silicone rubber starts to thermally age at temperatures above 150°C, the high temperature exposure made the material hard and brittle, and facilitated crazing and crack propagation.
This premise can be supported by the fact that the overflow of high temperature resistant silicone adhesive did not show any sign of degradation as a result of the flue gas exposure. Chemical attack, on the other hand, accelerated the crazing and crack growth processes, and led to etching and ply separation.
Silicone rubber is not resistant to the combined effects of high temperature (higher than 150°C) and hydrochloric acid exposure. A careful material selection (could be a different silicone rubber compound with higher heat resistance), and properly engineered installation, are important factors for in achieving useful service component life.