Analysis of Threaded Connection Reliability for Compression Application

The objective of this paper is the investigation and evaluation of the performance and life expectancy of a national pipe thread tapered thread (NPT) threaded connections as applied to a particular type of reciprocating compressor unit. The study focused on the assessment of the levels of excitation forces acting on the threaded connections and the resulting stress. These forces and stresses were determined from field vibration measurement results applied to a mechanical model of the system. As an extensive literature search did not yield satisfactory data for specific fatigue properties of NPT connections, we applied a first-principles approach to the problem and calculated the stress concentration factors from a detailed finite element study of the thread engagement under axial and circumferential loads. The results and details are reported here. Stress levels determined by this methodology were compared to the allowable fatigue limits for the piping and valves materials and a life expectancy was calculated. The same methodology can be applied to other situations involving threaded connections.