Combined cycle plants in cold climates experience low circulating water inlet temperatures during winter months. Low circulating water inlet temperatures combined with partial bypass steam flow to the condenser results in extremely low condenser pressures and high steam velocities. Improper design, control & operation of desuperheating valve and improper drainage of bypass header lines can lead to pockets of wet steam in the bypass steam. High steam velocities combined with wet steam pockets of varying quality can cause flow-induced vibration and tube failures. This paper examines the performance of a condenser in bypass mode for varying condenser pressures, bypass steam flow rates, support plate spacing, and moisture pockets with varying quality. Actual and critical steam velocities are calculated. Condenser operating points prone to flow-induced vibration and associated tube failures are predicted. Recommendations on safeguards to eliminate flow induced vibration and resulting tube failures are discussed.

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