Larsen & Toubro Limited (L&T) is one of the leading engineering, procurement and construction (EPC) players in the refinery and power sectors in Asia. The Technology and Innovation Centre of L&T at Baroda has been using CFX TASCflow extensively for flow and combustion applications to evaluate the reliability of in-house-designed equipment as well as critical out-sourced equipment.

The software has played an important role in understanding, designing and troubleshooting of equipment. Recently, CFX-TASCflow was used to troubleshoot a centrifugal fan used in a coal-dust environment. The fan was accumulating dust unevenly on the impeller vanes, causing severe vibration problems and leading to frequent shutdowns. On visual inspection, it was found that dust was collecting near the tip of the fan vanes, which might be due to the sticky nature of the dust and the high curvature of the vanes near the tip.

1. The original impeller;
2. A slight modification of the original impeller with a plate welded near the tip to reduce particle build-up;
3. A new impeller of shorter size and with less curved vanes, offered by the vendor as a replacement if Option 2 did not work, and
4. A radial straight impeller, since dust accumulation for this type is very low.

CFX-TASCflow was chosen for the study because of its ability to solve rotating machinery problems with relative ease and to produce reliable solutions. The computational grids for the inlet, impeller and volute were generated with CFX-Build and then imported into CFX-TASCflow. The problem was set-up using CFX-TASCflow's multiple frames of reference, and its particle tracking capabilities were used to simulate the coal dust. Collection of dust on different impellers was then evaluated by calculating the percentage of particles impacting the vanes based on a fixed particle size distribution at the inlet

The analysis revealed that the modified impeller (Option 2) could achieve half the dust accumulation compared to the original design. For the new impeller (Option 3), the results showed this value to be one third of that for the original impeller. The radial straight vane impeller (Option 4) showed no particle build-up, but a much lower efficiency.

These findings were used to finalize the decision to go with the second option and modify the impeller by welding a plate near the tip of the vane. The fan has since been reported to perform satisfactorily with fewer problems. The study helped in the decision to use the modified impeller fan. Meanwhile, the new impeller, provided as an alternative by the fan supplier, has been kept as a standby, thus saving capital cost.

Centrifugal fan geometry and computational grid.
Image courtesy of Technology & Innovation Centre, Larsen & Toubro Limited, Baroda, India.

Relative (in the impeller) and absolute (in the diffuser) velocity vectors in the fan.
Image courtesy of Technology & Innovation Centre, Larsen & Toubro Limited, Baroda, India.

Particle trajectories and build-up profile for the original impeller. The build-up of particles on the vanes occurs in the area highlighted by the black oval.
Image courtesy of Technology & Innovation Centre, Larsen & Toubro Limited, Baroda, India.