In some cases, the possibility exists for a volatile material (e.g., light hydrocarbon, or water) to enter into direct contact with a relatively hot fluid (e.g. heat transfer oil, or process fluid). This direct contact results in rapid vaporization of the volatile stream, which, in turn, may create substantial overpressure. Some of the equipment configurations in which this contingency is possible include heat exchangers (in which tube rupture may bring the hot and volatile fluids into direct contact), and water or steam connections to process vessels (when a single valve separates the process from the utility fluid).
If the flow rate of volatile material and the amount of heat present in the hot fluid are known, the required relief flow rate may be calculated assuming vaporization to be instantaneous upon contact of the two fluids. However, these input data for the calculation are often unknown. Furthermore, the generation of vapor volume may be so rapid that the response time of a relief device is too long to supply effective relief flow. Therefore, it is essential that proper design and maintenance procedures be used to eliminate the possibility of mixing such streams.