Dryer Agitator Mechanical Assessment
Dryer Agitator Mechanical Assessment
A customer had been operating several conical dryers for 20 years, producing pharmaceutical intermediates. These machines, which Delta did not manufacture, were equipped with double mechanical seal helix agitators. The agitators never encountered major problems during operation.
However, in recent months, due to market conditions, production changed. As a result, the dryers’ agitators unexpectedly began having trouble – the blades bent, cracks appeared on the shafts, and the machines required more frequent maintenance on their mechanical seals.
Evidently, the new product generated too much stress for the agitators. Over a machine’s long-lasting operational life, it can often encounter conditions not previously considered during its design phase. Such a situation, as was the case for this customer, can occur when the machine is used with different products and processes, resulting in unexpected ruptures.
We conducted an audit to gather information from the customer about the products and dryers (both from older documentation and in-person visits), such as:
- product features like bulk density, repose angle, and stickiness
- agitator main features (rotation speed, electric motor power, etc.)
- impeller shape
The second step involved generating a 3D model of the existing agitators, including the shaft and blades. We designed each component in the model to match precisely with the machines. We then used FEM tools to convert the 3D model into a mesh for mechanical stress analysis.
Our next step was imposing appropriate constraints on the mesh, simulating what was happening with regard to the torque by:
- Applying torque generated by the existing motor/gearbox system to the top of the shaft, where the gearbox is coupled; and
- Stopping the blades artificially to simulate the presence of the product.
This procedure was repeated several times, with changes to the applied torque (pushing the limit to the max for the driving system before rupture) and focusing the analysis on a different agitator blade every time.
Initial Analysis Results
The simulation output indicated the stress (in N/mm²) created at each point on the agitator. It showed that:
- The existing shaft was highly under-dimensioned and could only support the nominal torque generated by the driving system.
- The smallest blade, which once supported all the torque, presented some critical issues near the connection to the shaft. At these spots, material stress was near the yield strength.
In comparing old and new product features, the latter had a higher density and repose angle, which indicated that the product required more torque.
With further analysis, it was deduced that:
- The first product required a torque that was lower than what the agitator was capable of handling.
- The second product, being denser and stickier (with a higher angle of repose), generated a stopping torque lower than what the driving system was able to generate, but higher than what had been considered for the blades’ sizes and shaft at the time of the machines’ construction.
To resolve the issue, the technical team proposed several solutions that had different timeframes, costs, and impacts on the process. This allowed the customer’s technical team to weigh the options and select the solution that best met their needs:
- Force the frequency converter to stop feeding the agitators’ electric motors when the torque exceeds a defined limit. This is a quick solution (requiring limited resources) that did not solve the problem, but allowed the customer’s production to continue to operate, even if the produced quantity was reduced.
- Replace the impellers, keeping the driving systems as they are. This is a longer-term solution that corrected only a part of the issue.
- Completely replace the agitators. While solving the current issue, there is a possibility that new problems would present after replacing the agitators, which ultimately could lead to having to fully replacing the machines.
By delivering a deep and detailed analysis of the conical dryers’ agitators’ status quo, Delta provided easy tools for the customer’s technical team to decide how to proceed to resolve the issue.