Root Cause Analysis of an Aerzen Positive Displacement Blower â€“ A Case Study by CPM Enginering Root Cause Failure helps analyse reason for the break-down of an Aerzen Blower
Background: An Aerzen Blower seized in operation and a site investigation was called. The Aerzen Positive displacement blower operated in the conveying of bulk materials up to 65.000 mÂł/h. A site investigation showed that the machine had been subjected to significant heat from a generated source, the case was slightly discoloured and contaminated with product but no physical damaged was evident. The inline filters had been replaced within the past month and were found to be in good condition. They had been changed as a direct result of report that there was a reduction in the efficiency of the machine. It is a known factor that restricted filters will result in the air within the blower to heat up and thus causes the lobes to expand. Further investigation revealed that the machine had been inefficient even following the previous filter change. Investigations then turned to a possible restriction / blockage in the discharge pipe work from the Aerzen Blower. There was found to be a restriction. Investigation: In addition to the site investigation the machine was investigated within a workshop environment a view to a Root Cause Failure being determined. The Aerzen Blower was not dynamically tested as the lobes had seized. A decision was taken to dismantle the machine. The Oil level in the machine was satisfactory and the oil appeared in good condition. Careful inspection was performed during the dismantle operation, examination and recording of the findings carried out on all components parts and assemblies. The lobes showed signs of axial expansion and had and touched the end plates of the blower.
Although the Aerzen Blower had seized in operation damage to both lobes and end plates was minimal. (See Figs 1-3) All parts were thoroughly cleaned with service metal being exposed and visually Crack detected. NDT methods were employed to detect cracks within the lobes and end plates as signs of heat and distress could be seen on the component parts. (See Figs 4 & 5) The bearings showed normal wear patterns consistent with normal operating conditions, environment and running hours. The bearing seatingâ€™s and housings were measured and all found to be within tolerance, likewise the lip seal seatingâ€™s and housings were found to be within tolerance. Conclusions: Root Cause Failure concluded that the eccentric lobes had seized due to a buildup of heat caused by restrictions within the discharge pipe work. As stated in the body of the report it is highly unlikely that the filters had any significant impact in this particular failure. The likelihood is that the Aerzen Blower has run against a partially closed valve or that the line has been partially blocked with product. The expansion within the lobes was consistent with the machines design characteristics. Fortunately a repair was able to be conducted with minimal effect on the efficiency of the machine using good engineering practice and Original Equipment components. In other experiences the lobe expansion can be sufficient to close the rotary clearances cause catastrophic damage to the blower case deeming the machine non-repairable. Solution: As a result of these experiences the customer was recommended to make changes to his maintenance regime. Thermal imaging of the ducting whilst the Blower is in operation is non-invasive and should reveal partial blockages (these reveal themselves as cool spots in the ducting) thus enabling a targeted clean of the trouble spot rather than a random search of the whole ducting. Considering the expense involved in Plant down time, maintenance costs for removal and fitting of a spare Aerzen Blower and the subsequent repair costs for the damaged Blower it was recommend that Thermal Imaging of the ducting be carried out on a monthly basis.
If not already fitted a PD indication should be fitted across the filter system.