Structural fatigue of agitator build

Xi Engineers identify cause of building and pipe work failures

Issue

An agitator building at Grangemouth Refinery was experiencing high level of vibration leading to ruptures in the concrete foundations, cracking of the steel support structures and the risk of pipe work failure. The building houses four agitator units used for mixing polymers supported on a mezzanine floor. During operation of the agitators, high levels of vibration could be felt and observed throughout the building leading to concern of frequency matching between the machines and resonances of the structure. Xi engineers were asked to determine; the resonance of the building, which agitators were responsible for the high levels of vibration, and to recommended possible solutions.

Skill Set Used
Low frequency vibration sensors
Multi-sensor signal processing
Campbell diagram analysis
Finite element eigenfrequency analysis
Fatigue analysis
Other Benefits
Modelling approach removes the need for ad hoc alterations to the building.
Identify potential faults allowing mitigation before costly failures and down time arise.
A variety of design changes could be trialled in a virtual environment without the requirement of plant shut down for testing.

Solution

Xi engineers deployed geophones and accelerometers to determine that the high levels of vibration occurred at 7 Hz and were caused by frequency matching between two of the four agitators and a structural resonance of the building. A finite element (FE) computer model was built of the agitator building and calibrated using the measured data.

The FE models were used to calculate the cyclic stress and fatigue life for structural steel elements of the agitator building. Stress hotspots were identified in a number of steel beams close to the base of the agitators. The analysis was used to recommend where structural support should be improved.

Further Comments
The installation of plant machinery onto steel frame buildings and mezzanine levels commonly causes vibration issues. The use of FE models that a calibrated with measured data can reduce the risk of plant and building infrastructure failure.