<p>Steam traps play a critical role in industrial steam distribution systems, as in the case of a malfunction, the total cost becomes considerably high due to energy loss, along with increased maintenance costs due to leaking steam. This paper presents a lightweight and explainable inspection framework, referred to as Valve-Constrained Thermal Contrast Inspection (VC-TCI), for steam trap leakage detection using infrared thermography. The proposed approach is designed as a maintenance-oriented monitoring method and relies on a physically interpretable inlet–outlet temperature contrast rather than data-intensive learning models. Thermal images are first standardized via aspect-ratio-preserving resizing. The hot region is identified in the Hue–Saturation–Value (HSV) color space by applying a white-hot mask and a percentile-based threshold. In this process, the valve body is treated as the primary heat source, and the areas at the valve body are determined by mirroring areas of interest around the identified valve body to maximize geometric consistency and resistance to background effects. In each zone, thermal intensity is assessed using a top-10% bright score, while leaks are determined using threshold-based contrast decision rules. The model parameters and decision thresholds are optimized for recall to prioritize safety-critical maintenance. Experimental evaluation of labeled infrared thermal imagery indicates that the methodology can achieve 78.8% accuracy, an F1-score of 82.9%, and a recall rate of 1.0 for leaking steam traps (with no false negatives). The findings demonstrate that using Thermal Contrast Inspection within Valve Constraining is both computationally efficient and easy to interpret, making it straightforward to integrate into existing industrial maintenance procedures and thereby enhancing overall energy efficiency and operational reliability.</p>