2021 RETA Breeze March-April

CASE STUDY

SMALL BORE PIPING FAILURE INVESTIGATION Report by Peter A. Lepschat

A cold storage facility has experienced numerous, and in some cases repeated, failures of steel pipe nipples on some of their screw compressor packages. These failures seem to have coincided with the installation of Varible Frequency Drives on two of the compressor packages in July of 2019. No failures of the same fittings have occured to date on the compressors that Vibration analysis and alignment checks have been performed on the compressors in question. Based on measured vibration at the test points, the VFDs were configured to skip several speeds between the minimum and maximum frequencies to avoid operation under conditions that caused harmonic vibration within the equipment. remained configured as single speed units.

This Pipe nipple is constructed of ¼ inch IPS schedule 80 steel material with cut NPT threads. Upon first examination the failure appears to be a fatigue/stress crack originating at the root of a thread. It is very likely that the failed thread root is at the first or outermost thread that was not fully engaged in the female fitting thread. Failures such as this are

At of the time of my visit, I observed that the vibration of these units seemed to be extremely low when palpated at various locations. My first question was if any of the failed parts had been retained, and if so could I examine them. One failed part was available, below are some photographs of it:

Background

Investigation

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