Energy-Tech Magazine

As shown, the compressor system also includes an inlet filter, intercoolers after each stage, a discharge check valve and an aftercooler. The intercoolers are an extended surface, plate fin shell and tube type. Hot gas discharging from each stage flows through the shell and is cooled by water flowing in the tubes.

History
The compressor was installed and commissioned in 1995 and was put into continuous service. In 2002, the second stage impeller sustained foreign object damage and was replaced. In May 2009 the compressor was disassembled so that the impellers and diffusers could be inspected and cleaned, and some of the impeller clearances were adjusted to try to improve machine capacity.

There was no impeller damage or erosion. Other than these incidents, the machine ran trouble free until May 2011 when the Operations team reported that the compressor appeared to be short in capacity.

The overall and stage by stage machine performance was measured and analyzed. It was determined that there was a performance problem with the second

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stage. The head produced by this stage was significantly lower than the head predicted from the shop test curves. Head is the work (energy) produced by the compressor stage. At a given volume flow, a compressor stage will produce a specific amount of head. In thermodynamics, head is a function of the gas properties, inlet temperature and pressure ratio. With the operating conditions the subject compressor was actually running at, the calculated second head was approximately 10 percent lower than the head predicted from the original machinery shop test curves. Stages 1 and 3 matched fairly well with the shop test curves. It was not possible to determine the cause of the second stage deficiency and a plan was put together to shut the machine down and perform an internal inspection. Additional spare parts were ordered to address some possible causes to avoid having to shut down the machine more than one time or for an extended period.

Before the planned shutdown, the second stage vibration started increasing. A spectrum analysis showed that all the vibration was at a frequency corresponding to the pinion operating speed. It was concluded that this was most likely due to unbalance. The stage 1 and 3 vibrations were stable until the compressor suddenly tripped on high first stage vibration. A borosope inspection of the first stage impeller revealed that a piece was missing from one of the impeller inlet vanes. Besides the first stage impeller failure, the second stage impeller and inlet shroud were found to be heavily coated with white mineral deposits. This type of fouling is indicative of cooling water contamination and was determined to be due to a first stage intercooler tube leak. This fouling also explained the second stage performance problem. The LS rotor was repaired and several intercooler tubes were plugged. The compressor was eventually restarted and the performance was restored with low and stable rotor vibrations.

Discussion
A picture of the failed first stage impeller is shown in Figure 4. The location and size of the failure was consistent with past impeller failures that were associated with an impeller natural frequency that was excited by interaction with the number of diffuser vanes. Typically these failures showed up within weeks or months after initial start-up. In this case, the impeller operated for almost 16 years without a failure.