Design Guideline for Small Diameter Branch Connections

Thu, April 14, 2011

Vibration of piping due to mechanical excitation is present in nearly any environment where small diameter branch connections (BCs) are used. In initial piping designs, small diameter connections are often overlooked and not considered. However, these smaller connections are just as susceptible to fatigue failures as the larger piping which is carefully designed. Several industry standards mention high level considerations for BCs, but there is a lack of recommendations for the smaller diameter connections, especially related to the length of the connection and allowable weight. This Project generated a guideline which addresses the design of small diameter BCs in order to minimize the likelihood of failures due to fatigue. 

The guideline is laid out for selection of branches based on operational frequency (speed) and configuration type. The design philosophy of this guideline is to reduce risk by placing the lowest mechanical natural frequency of the branch connection above the frequencies of most significant excitation occurring at the base of the branch line. The guideline does not attempt to predict actual vibration amplitudes. In addition to the frequency based criteria, a secondary non-resonant stress criterion was also included. It is possible to generate dynamic stress (and strain) in the branch connection if there is significant vibration in the mainline even if the mechanical response of the branch line is not excited. This project is a joint research effort with the Gas Machinery Research Council and the reporting is made available to both organizations.

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