Impeller Overspeed Test

Obviously this test applies only to the centrifugal compressor. The manufacturer normally performs this test whether specified or not. Since the user does have an interest in the test, some discussion of the parameters is in order. API 617 mandates the test, which includes the requirement to spin the impeller at a speed of 115% of the maximum continuous speed for a duration of one minute.

The tests are performed in a special overspeed chamber, which is evacuated to a minimum level to minimize windage pumping of the impeller and to control the temperature rise in the impeller.

The overspeed test is a form of proof test, in that it looks for a failure. In this case, it is a permanent deformation. Not all permanent deformations are failures. In fact, manufacturers have autofrettaged high tip speed impellers for decades. The fourth edition of API 617 was somewhat misleading as it identified the bore as the only critical area. On process compressors, particularly with a hardness limit due to suspected H2S contamination, the cover disc (front shroud) eye on high specific speed impellers will be the more likely candidate for a permanent deformation. The semiopen impellers would have the bore critical, but are less frequently used for process applications.

This lest is better left to the vendor for interpretation unless the user representative is quite experienced in the design of centrifugal impellers. The vendor can be asked to identify the critical areas prior to the test if there is time to check his analysis. Actually, the liability to the vendor is much too high for him to risk an overstress failure just to pass a test. Unfortunately, failures come in the unexpected situations, where the only hedge is "hind sight."

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  • t
    What is overspeed test of compressor?
    3 years ago

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