Most leakage problems from pipe system breaches result in the formation of spray flash evaporation. In this study, a variety of experiments were carried out in the high temperature and high pressure (HTHP) steam-water test loop to study spray flash evaporation related to tube leakage problems. The focus of our study was on the flash evaporation from the highly superheated jet (the superheat degree can be up to 200 K) with a small injection rate. The temperature and relative humidity variations in the region of interest were measured in the experiment. As the transient relative humidity changes can enable better reflection of the characteristic of flash evaporation, non-dimensional relative humidity (NDRH) and the critical time of the relative humidity variation was proposed and used to analyze the flash evaporation. The effects of injection rate, injection direction, initial water temperature, and injection pressure were investigated. The experimental results showed that the increase of the injection rate and initial water temperature enhanced the flash evaporation. The corresponding critical time increased with the increase of spray angle. The injection pressure was found to result in better atomization and evaporation of the water on the premise that the injection pressure guaranteed complete flash evaporation. (C) 2017 Elsevier Ltd. All rights reserved.