Refinery compressor receives improvement boost08 June 2015

Rotor assembly on 4MW compressor Rotor assembly on 4MW compressor

A new rotor assembly on a 4MW compressor at a German refinery has provided significant improvements in efficiency

Large centrifugal impeller compressors are commonplace throughout the petrochemical sector, where they usually operate on a continuous duty basis. When a scheduled maintenance task at a refinery in Germany revealed the need for a new rotor assembly, working with specialists from Sulzer enabled the team responsible to improve the design and performance of the compressor without further interruption to service.

The project started when the maintenance engineers were completing a rotor change on a 4MW centrifugal compressor as part of a routine maintenance program. Once the rotor had been removed from service, a visual inspection showed damage to the impeller blades, with both cracks and heavy erosion in evidence.

Previous to the rotor being changed, issues had also been raised with the performance of the inter-coolers on each stage of the four stage compressor, which were becoming less efficient. With the original compressor having been built in the 1970's, the inter-coolers had reached the end of their design life and were due for replacement. A set of new, more efficient inter-coolers had therefore been purchased and were ready for installation.

The project was divided into two tasks: the repair of the rotor and the investigation into design improvements that could increase performance and efficiency. By selecting Sulzer, the client gained access to the extensive knowledge and expertise of its engineers with the Rotterdam Service Centre providing specialist skills for compressors. The project was managed by the Jänschwalde Service Centre to provide the client with coordinated feedback and progress reports.

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. The first task was to recreate the compressor, in its current configuration, as a 3D model and to use the latest software to generate a performance chart for the complete compressor, including the existing inter-coolers. The data generated from this model was then compared to the actual data from the compressor itself. In this way the model could be refined to ensure that the correct working parameters were being used.

Once the accuracy of the model was confirmed it was then possible to introduce design improvements to the impellers, the labyrinth seals, the inter-coolers, the bearings and the shaft itself. Each design change was simulated and evaluated in order to compare the performance of the alternative designs.

Since the impellers were already damaged, they would have to be replaced but modern design and manufacturing techniques would allow for much higher efficiency components to be installed that would also utilise the additional capacity of the new inter-coolers. Modifications to the vane geometry would ensure the optimum inlet flow angle which would also improve the mass flow rate (MFR).

Further improvements were also evaluated for the labyrinth seals, where the introduction of a polymer material would allow the seal clearance to be reduced which would reduce losses from the compressor. Each modification was assessed for its benefit to the performance of the compressor before being presented to the client.

The evaluations showed the most significant performance improvements would be as a result of optimizing the four impeller stages and installing the new coolers which would result in a 20% increase in MFR. Of this only 2-3% of improvement would be as a result of changing the inter-coolers, the majority came from redesigning the impellers. An additional 3-4% could also be gained through changing the labyrinth seal material and improving the clearances.

Mark Venables

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