A glass fibre tank 4m in height and 3m diameter to be used for the treatment of waste water was supplied to a waste water treatment works in the UK. After installation, the tank developed a leak. It was repaired but continued to leak. The 25,000 litres FRP vertical cylindrical tank for the storage of Ferric Chloride at atmospheric pressure was produced by mechanised spray.
Inspection found two cracks in the base, one of which was cut out for examination. The crack was approximately 600mm long on the inside of the tank and 200mm long on the outside. The sample was parted in two by hand, to reveal the failure surfaces with two distinct zones one of which was fibrous and the other which was “rock” like, without any fibre protruding from the surface. The fibrous type surface is conventional failure of a random laminate. But the rock like surface was an example of Environmental stress corrosion (ESC).
Environmental stress corrosion cracking occurs when a crack in the surface of a load carrying laminate allows an aggressive environment to gain access to the reinforcement. The filaments of fibre are degraded, fail and allow the crack to continue. The crack will continue to propagate through the laminate as long as the stress is applied and the aggressive environment has access to the reinforcement fibres. It results in catastrophic failure of the vessel unless it is taken out of service.
The risk of such an event is mitigated by the correct specification, design, installation and maintenance procedure. The proper management of such assets is supported and enhanced by reference to the FRP Asset Manual which is available from JMC Composites Ltd.
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