This white paper presents a comprehensive study utilizing Computational Fluid Dynamics (CFD) to predict transient pressure fluctuations in an HP steam turbine bypass valve. The bypass system allows the steam generator and turbine to operate independently during transient modes such as startup and shutdown. The bypass valve consists of cages and diffuser sections with multiple columns of holes. Pressure fluctuations during transit through these holes can result in vibration issues and potential failures. Conducting a transient CFD analysis would aid in evaluating pressure pulsations over time. However, performing a complete valve geometry simulation with transient CFD is computationally expensive due to the requirement of very low time step (in the order of 10-7 s) and sufficient duration to capture notable pressure variations. To address this, an initial study was conducted using a representative geometry comprising a pattern of holes in a diffuser section of the bypass valve. The transient simulation focuses on studying pressure variations at selected probe points located upstream and downstream of the holes. Inlet and outlet pressure conditions were approximately determined, and Fast Fourier Transform (FFT) plots were extracted to comprehend the frequencies and amplitudes of corresponding pressure fluctuations oscillations.