Major rerate of a process air compressor during a plant shutdown

摘要

The process air for a Polyester Intermediates plant was provided by the main air compressor (20MW) and a smaller subsidiary compressor (4.5MW). The small compressor was reaching the point where further operation would require significant expenditure without any performance improvements. This led to the plant seeking an alternate source of additional process air. Alongside this the process conditions had changed over time such that the main compressor was running significantly away from its original design point. Different options were assessed and a decision taken to uprate the main process air compressor. The operating constraints meant that the modification had to be designed such that construction and testing could be completed within the three week plant shutdown. This paper describes the design and engineering applied through the project process to minimise the risk involved with such major changes. To ensure all aspects were considered the project team included personnel from the operator, the technical consultant, the detailed design contractor, the vendor, and the construction contractor. The major challenges were around producing a suitable design capable of being installed and tested within the shutdown. The requirement was for an increase in mass flow rate of 20%, which combined with a reduction in discharge pressure equated to a 30% increase in discharge volume. This necessitated a new HP compressor casing which had to be mounted onto the existing bearing housings, internal modifications to the LP compressor and a larger first stage intercooler. The reuse of the existing bearing housings meant that it was not possible to carry out conventional API testing of the new components at the vendor's works before installation.