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Branch Line Fatigue

The Industry Challenge:

Vibrations induced in piping systems due to flow and pump pressure cycling can cause small diameter branch lines to be susceptible to fatigue failures.

These failures are typically located in the welded fitting that adjoins the branch line to the main line. Maintenance personnel monitor these connections to identify cracks in the welds that could lead to leaks or ruptures of the branch line connection.

Predicting the rate of crack growth and the critical crack size are required to decide when the connection must be repaired or replaced. Ultimately, the operator’s objective is to design a branch line connection that is not susceptible to fatigue failure under normal operating conditions.

How We Help:

C-FER constructed a branch line fatigue testing machine that subjected four independent branch line connections to cyclic bending load while exposed to internal pressure.

A servo-hydraulic control system enabled full control of the amplitude and frequency of the vibrations applied to each branch.

Crack development in the connection weld was assessed intermittently to correlate crack growth rate to cycling history.

If a branch line eventually failed, it was isolated from the specimen and the test of the remaining branches continued, usually until all four branches had failed.

The information from these tests were used to develop criteria for deciding when a damaged connection needs to be replaced and to predict the remaining life of a connection once a crack is observed.

The same testing setup can be used to assess the performance of different connection options to help identify fittings and welding procedures that eliminate the chance of branch line fatigue under normal operating conditions.

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