Failure Analysis of Fractured 2 Inch Drain Line

FEA Model Showing Drain Line


Failure Analysis of Fractured 2 Inch Drain Line

Insulation damage occurred on a 48-inch diameter steam turbine crossaround pipe at the Cook Nuclear Plant in Bridgman, Michigan. In addition, there was fracture of a 2-inch drain line connected to an 8-inch drip leg near the turbine end of the pipe. Fracture of the 2-inch pipe occurred at a socket weld connection. The damage was found after a turbine trip.

To further investigate this incident, the plant owners sought vibration analysis of the insulated steam pipe, drip leg and attached drain line. We were requested to perform an evaluation of the design including the vertically-acting spring supports.

Two analyses were performed. The first analysis was a determination of natural modes of vibrations of the piping – frequencies of vibration and corresponding mode shapes were obtained. In the second analysis, harmonic excitations of the pipe end connected to the steam turbine were investigated. The excitations were applied in a direction perpendicular to the turbine axis.

The following methodology and assumptions were applied.
(1) The 48-inch diameter pipe is comprised of rolled carbon steel plate. The 8-inch drip leg and attached 2-inch drain line were assumed to be a 3xx-series Stainless Steel.
(2) The Elastic Modulus of all piping was assumed to be 29×106 psi
(3) One end of the 48-inch cross-around steam pipe attaches to the steam turbine and the other attaches to a Reheater/Moisture Separator Unit. At the Reheater end, the pipe was assumed to be completely fixed.
(4) All damage leading to failure of the 2-inch drain line was assumed to be caused by stresses induced in this pipe by vibrations from turbine imbalance.

Modal and harmonic analyses of piping between the steam turbine and separator have indicated that natural frequencies of vibration exist in the range of startup and shutdown speeds of the turbine. Such modes may be excited during startup and shutdown events, particularly if an imbalance is present in the turbine.


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