Definition of the Relief Scenario

The key factor of success in the design of emergency pressure relief systems lies in a good understanding of the behavior of the reaction under relief conditions. The first point in this context is the cause of pressure increase. This may be the vapor pressure of the reaction mass, the so-called tempered system. Pressure increase may also be due to gas release by a reaction, the so-called gassy system. There are also cases where the pressure stems from both vapor pressure and gas release, the so-called hybrid system, which may or may not be tempered. 

One must be aware of the fact that when the set pressure of the protection device is reached, the pressure increase does not stop immediately, but continues to increase to the maximum pressure before decreasing. These two pressure levels, set pressure and maximum pressure, have to be defined during the design procedure. The design can be for two different scenarios, the physical scenario where no chemical reaction is involved and the chemical scenario where a chemical reaction determines the behavior of the system. 

There are many different physical scenarios to be considered. In the first category, we find scenarios that result from gas compression, such as by liquid transfer into a closed reactor or gas inlet from a line connected to the reactor. With such scenarios, two-phase flow is unlikely to occur. Other common physical scenarios are linked to unwanted heating of the reactor contents, either by fire or by inadvertently heating of the reactor by the heating system. In this case, two-phase flow may occur. 

For chemical scenarios, the kinetic behavior of the reaction, the temperature and pressure increase rate must be known under runaway conditions in the interval between set pressure and maximum pressure. This implies a good knowledge of the thermo-chemical properties of the reaction mass. The required data are traditionally obtained from adiabatic calorimetric experiments [22, 25, 26]. Nevertheless, other calorimetric methods, especially dynamic DSC or Calvet experiments evaluated using the isoconversional approach, can also provide these data with accuracy and an excellent reliability for the temperature increase rate [27], as well as for the pressure increase [28, 29]. 

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