Objective: To identify the mechanism of cracking and suggest remedial actions.
Situation
- The membrane is part of a specialist water filtration system and is manufactured from polyether sulphone.
- The unit consists of five membranes separated and supported by a PVC skeleton.
- In-service suction is applied to the hollow end supports and air is bubbled from the bottom of the membrane. This causes significant vibration of the membrane during operation.
- From the photographs above it can be seen that during operation cracks have appeared in the membrane structure and there had been areas of detachment.
What we did
- A thorough visual examination of the membrane was undertaken.
- Cracks were seen emanating from the junction of the membrane and the end support and junction of the PVC supports and the membrane. The geometry of both of these areas would act as stress raisers.
- Further cracks were observed emanating from rub marks where the membrane had vibrated against the PVC strips.
- It was clear that the cracking of the membrane had occurred by a fatigue mechanism where it was poorly supported by the PVC skeleton.
Impact
- To prevent fatigue cracking of the membrane in future it was suggested that:
- There should be a smaller clearance between the membrane and the vertical PVC strips.
- The whole length of the membrane should be supported by the PVC strip.
- The span between the vertical PVC strips should be reduced.
- The edge of the PVC strips clamping the membrane should have a rounded profile.
- The above recommendations were implemented and no further fatigue cracking was observed.