Due to the penetration of variable renewable energy (VRE) sources into electricity supply grids, conventional coal fired power plants need to operate with greater flexibility while remaining reliable and conserving the lifetime of components. Thick-sectioned components are prone to thermal fatigue cracking as a result of through-wall temperature gradients. These temperature gradients can be significantly amplified during quenching when components at high temperature are unintentionally exposed to colder liquid or steam. Such quench events are known to occur during two-shift operation of a large once-through coal fired tower type boiler. The purpose of this study is to develop and demonstrate a model that can be used to determine the root cause and magnitude of quenching. The model is developed using the least level of detail to make it readily usable by power plant engineers. Two different approaches are used. A liquid tracking model (LTM) was developed from first principles that approximates the liquid level in the superheater as a function of time. The model is presented and verified by comparison with real-plant data. The second approach was to configure a model in flownex, which is a commercially available software package. The LTM model with eight control volumes provided better steam temperature results and was able to simulate the correct superheater pressure behavior without solving the momentum equation. The models proved that a separator overflow was the cause of quenching for this particular case study.