Abstract: Fluidelastic instability (FEI) is widely recognized as the most destructive flow-induced vibration (FIV) mechanism in tube bundles. FEI has been linked to a number of steam generator heat transfer tube failures in pressurized water reactors, which can result in leaks, expensive shutdowns, and even the irreversible decommissioning of nuclear plants. Because it controls the strength of fluid-structure interactions in tube arrays, the pitch-to-diameter ratio (P/d, pitch ratio) is one of the most important factors affecting FEI. Nevertheless, not enough attention has been paid to the systematic effects of varying pitch ratios on transverse FEI in typical triangle tube arrays. The goal of this study is to clarify this influence and to provide a prediction framework that can be readily applied in nuclear engineering practice. The study expands on prior experiments of triangular tube arrays with a fixed pitch ratio of 1.3, increasing the scope to pitch ratios ranging from 1.3 to 1.6. A semi-analytical tube-in-channel model was used, which reduces the complex three-dimensional flow to one-dimensional channels while keeping crucial dynamics. To obtain model parameters, computational fluid dynamics (CFD) simulations using dynamic mesh techniques were run. In these simulations, a central elastic tube was subjected to predetermined harmonic oscillations, and the ensuing transient flow fields were recorded over several vibration cycles. Thousands of velocity contour images were collected and processed using image-based algorithms to calculate essential metrics such fluctuating flow channel area, phase lag, steady-state area terms, and steady-state velocity terms. These parameters were then fitted into explicit mathematical functions of flow velocity and spatial position. In order to calculate the unsteady fluid forces operating on the tubes and forecast their vibrational response, the fitted equations were incorporated into the time-domain semi-analytical FEI model. Correlating mass-damping characteristics with critical velocities produced stability maps. Strong agreement is found when model predictions were compared to published experimental datasets, demonstrating the precision and resilience of the proposed approach. The findings show that pitch ratio significantly affects FEI thresholds. Larger pitch ratios increase the threshold and improve system stability, while smaller ratios drastically lower the critical velocity, suggesting higher instability concerns. Furthermore, the model captures the extra destabilization brought on by tube-tube coupling and effectively distinguishes between instability behavior in single flexible tubes and numerous flexible tubes. These results are in line with physical predictions of inter-tube flow interactions, which show that bigger gaps weaken couplings while smaller gaps amplify flow disturbances. This study’s main contribution is to increase the applicability of semi-analytical prediction models to systematically take pitch ratio fluctuations into account. The approach provides a practical alternative for computationally costly full-scale fluid-structure interaction simulations by enabling quick and effective evaluation of instability thresholds. Crucially, the findings offer engineering techniques and theoretical underpinnings for enhancing the safety and economic efficiency of nuclear power plants, reducing the likelihood of FEI-induced failure, and optimizing the design of steam generator tube arrays.