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Large centrifugal impeller compressors are commonplace throughout the petrochemical sector, where they usually operate on a continuous-duty basis. Rigorous maintenance schedules and condition monitoring are employed to ensure that reliability and efficiency are maintained. Continuous service is also ensured by maintaining a spare rotor assembly for each compressor which allows any repairs to be completed without any impact on the refining process.
Case in point: Inter-coolers on each stage of a 4 MW four-stage centrifugal compressor in a refinery in Germany were becoming less efficient. With the original compressor having been built in the 1970s, the intercoolers had reached end of life. A new set of inter-coolers were purchased and ready for installation in the next maintenance window.
Once the rotor had been removed, the maintenance staff noticed cracks and heavy erosion on the impeller blades. Repairs were coordinated through Sulzer’s Jänschwalde Service Center, as well as actions to improve the design and performance of the compressor. The project was divided into two tasks: the repair of the rotor and investigation into design improvements that could increase performance and efficiency (Figure 1).
The most obvious hurdle to improving the design of the rotor was the lack of access to the stationary section of the compressor, since it was still in operation with the replacement rotor inside. A scan of the rotor shaft was done to digitize it into a 3D CAD model to recreate the compressor in its current configuration, as well as a performance chart for the compressor and inter-coolers.
The data generated was compared to actual data from the compressor to refine the model with the correct working parameters. Once accuracy was confirmed, it was possible to introduce design improvements to the impellers, the labyrinth seals, the inter-coolers, the bearings and the shaft (Figure 2).
Each design change was simulated and evaluated. Modifications to vane geometry were aimed at achieving the optimum inlet flow and better mass flow rate (MFR) (Figure 3). A polymer material was proposed for use on the labyrinth seals to allow clearances to be reduced which would in turn reduce losses from the compressor.
These evaluations showed a 20% gain in MFR would come from optimizing the four impeller stages and installing new coolers, with less than 3% of the improvement from changing the inter-coolers. An additional 3% could also be gained through changing the labyrinth seal material and improving the clearances. The refinery reviewed the potential gains available and overall cost and elected to focus on changes to the impellers.
Whereas the original impeller was a welded design, the new components comprised a solid piece of chrome molybdenum vanadium steel (Figure 4). This was manufactured using electrical discharge machining (EDM) to provide a stronger, more durable component. Each impeller was spin tested before being mounted onto a new rotor shaft. In accordance with American Petroleum Institute (API) specifications, the spin test speed was carried out at 115% of maximum continuous speed.
The shaft was low-speed-balanced before each impeller stage was installed and then again once the assembly was complete. The operating speed of this rotor is over 8,800 rpm and the balancing was carried out at rated speed as well as at 10% overspeed. A process known as “trowalizing” refined the surface finish of all surfaces. It involves immersing the component in abrasive media, with a vibrating action being applied to the container. Gradually the component is polished by the media for a near-mirror finish.
The completed rotor assembly was delivered to the refinery ready for installation during the next scheduled maintenance period, along with the new inter-coolers.