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Worn Out Rotors Call for Thorough Exhaust Fan Replacement

July 8, 2018
The restoration effort for an industrial fan that’s critical to emissions control required both experience and design expertise

Exhausting large volumes of industrial-grade particulate is, well, exhausting for heavy-duty machinery designed for that task. Consider the exhaust fans used to draw emissions from combustions processes, like smelting or melting, or high-temperature sintering of iron fines. ThyssenKrupp Steel Europe AG operates the largest steelmaking complex in Europe at Duisburg, Germany. After completing a program to reduce dust emissions there, signs of wear were discovered during a regular, biannual equipment inspection of the rotor of the sinter exhaust fan. In order to ensure continued functionality, the rotor was briefly replaced with a unit available in storage, which took five days for a team of 20 to complete. Then, an industrial fan specialist was commissioned to produce a new rotor.  For TLT-Turbo GmbH, one challenge in this project was that the undamaged shaft still had to be retained, and the rotor reconstructed around it. 

At the ThyssenKrupp Steel Europe sinter plant, on the Rhine River at Schwelgern, iron ore and other substances are processed into sinter and prepared for the production of pig iron. Over 30,000 metric tons of sinter are produced on three conveyor belts every day, which of course creates dust. Multiple process units continuously filter this dust so as to reduce emission volumes as much as possible. To this end, an electrical filtration unit was initially installed directly behind the sintering unit, which is followed by sinter exhaust fans. In order to improve air pollution control even further, ThyssenKrupp invested an additional €46 million (est. $54 million) in a modern tissue filter for sinter belt III, which was installed following the sinter exhaust fans. These systems are able to filter out 99.99% of the dust, according to TLT-Turbo. 

“While installing this hose filtration unit onto belt III it was necessary that we curb the sinter unit’s output,” explained engineer Erwin Unger, product specialist in Aftermarket Service at TLT-Turbo GmbH, and head of the project at
ThyssenKrupp Steel Europe. “The result was that the output of the sinter exhaust fan had to be reduced as well. This happens by closing the inlet vane control.” However, during this process the free spaces between the individual inlet vane control blades get smaller, while the exhaust gas speed increases. This results in the gas spinning faster than the rotor itself. Then, the sinter dust migrates from the top to the bottom of the rotor blades. 

Routine check reveals signs of wear — In normal operation, the sinter dust accumulates only on the top of the rotor blades. Because these are coated with a special, anti-wear substance, the dust is effectively harmless. However, the bottoms of the rotor blades are uncoated, and made from relatively soft steel. On the Mohs hardness scale, sinter dust is in the same category as diamond. The material is one of the hardest substances in the world. This also explains the signs of wear on the rotor, which were discovered during a routine check of the fan of belt III despite the low quantity (approximately 50 mg/m3) of gas. 

“Some of the fastening screws for wear protection were heavily eroded. And the round iron at the opening to the shovels showed massive signs of wear,” said Unger, the project head. Sinter dust also created holes in the middle plate of the rotor below the blades, approximately 300 mm long and 30-40 mm wide.

Reconstructing of the rotor, retaining the shaft —  In order for the sinter unit to be able to resume operations as quickly as possible, ThyssenKrupp Steel Europe dismantled the rotor with help from TLT-Turbo installers and replaced it with a substitute rotor. The old rotor, fabricated in 1970 by TLT-Turbo GmbH’s predecessor company, was “burned off” the hub after nearly 50 years of service. 

Then, the shaft and hub were shipped about 170 miles to TLT-Turbo in Bad Hersfeld, where a rotor would be developed and produced based on the old design. 

One particular challenge was that the shaft had to be retained. “The construction of a new shaft would have cost about €150,000 more, but this investment wasn’t necessary because the old one was still completely intact,” said Unger. Still, this posed other challenges for TLT-Turbo. First, the new rotor had to be fitted to the old shaft, which is not a standard process even for expert designers and suppliers in industrial fans. Second, the 47-year-old rotor also exhibited a rather complex structure, the reproduction of which required considerable expertise and experience. 

On-time replacement —  “Our team of engineers tailored the new rotor to the current standards. The shaft also was adjusted for further use and then combined with the rotor. In order to optimally protect the rotor against sinter dust, it was treated with anti-wear protection that has to be replaced only every 10 years. “The work on the rotor was particularly challenging for our new balancing bench, because the rotor puts about 35 mt on the scale,” Unger reported. Despite all these challenges, the rotor was constructed, finished, and delivered on time, and is being stored at a ThyssenKrupp Steel Europe warehouse so that they can react quickly to similar incidents, and replace the damaged rotor.

Installing the replacement rotor stored in the warehouse also required extensive planning, and a team of 20 workers to complete. “Despite the size of the warehouse, the space is used quite effectively, so installing the new rotor demanded good preparation. That means, we had to devise a plan in advance by which parts would be placed properly, so that everything could proceed smoothly and without any major loss of time,” explained Unger. 

Dismantling the replacement rotor and commissioning the new rotor was estimated to take four to five days. The TLT-Turbo team worked around the clock with 10-man shifts to meet that deadline. “Our team is very experienced and often finishes their work hours before the predicted time of completion. This time was no different,” said Unger. Thus, the fan unit could resume regular operation with the usual output, on schedule. After the work was completed ThyssenKrupp Steel Europe was satisfied with the completed work, and with the course of the project and the quality of the new rotor. 


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