Process Modeling

A leading Power Provider was evaluating the performance of their air compression train, which had been plagued by acoustic resonance in the discharge piping of an intermediate stage of compression. A field performance test was conducted to determine if there was an aerodynamic instability in the compressor, or whether one of the intermediate stages of compression was experiencing surge. 

The purpose of the field testing was to isolate the characteristics of the acoustic signature in the piping and identify the root cause. As the investigation proceeded, the scope changed based on the findings. The investigation into piping vibration and acoustic resonance evolved into an evaluation of centrifugal and axial compressor performance.

The analysis was done using a combination of hand calculations and computer modeling, using industry standard software (IPSEpro) and generally accepted approximations where instrumented equipment data was not available. Using OEM compressor design data for Head and flow curves, the operating performance of the entire compressor train was measured, corrected to design conditions, and compared to OEM performance. The operating data revealed that the flow through the first section of the intermediate compressor was too low, and this was creating a rotating stall in one of the intermediate stages. The rotating stall was stimulating a resonant frequency of the structural piping that resulted in an acoustic signature that was detrimental to the piping and intercooler connected to the intermediate stage of compression. Testing was conducted to measure the compressor performance and quantify whether the operating points of each stage of compression were operating as designed. It was determined that the first section of the Intermediate Compressor was low on performance and flow. A field test was conducted by manually opening a blow off valve to increase the air flow through that section of compression. By increasing the flow rate, the compressor efficiency increased, and the acoustic resonance was eliminated. This provided a temporary solution to allow the compressor to operate at its design point. It was later found that the upstream stage of axial compression was underperforming, and thus robbing the Intermediate Compressor of flow. By opening the blowoff valve manually, the volumetric flow through the Intermediate Compressor increased and alleviated the acoustic instability. Although this operation has a performance related impact to the overall train and plant efficiency, it permitted temporary operation of the train to support power generation. A long-term solution was developed to overhaul the preceding stage Low Pressure axial compressor to increase the inlet flow to the intermediate compressor and provide stable operation.