Mass Flow Rate to Be Discharged

The mass flow rate to be discharged following malfunctions depends on the process parameters, the nature of the malfunction and the pertinent boundary conditions. It is useful to determine these by a systematic safety analysis, for example a HAZOP study (vid. Sect. 9.1.2.3). In such an analysis special attention should be paid to identifying sequences leading to overpressure. Basically the following causes of overpressure can be distinguished (cf. [8])  

A substance flows from a higher-pressure system into a system designed for lower pressure. The mass flow rate to be relieved depends on the pressure difference. 

A substance is supplied to a system, for example by a pump or a compressor. The mass flow rate then depends on the capacity of the pump, respectively the compressor. If its stagnation pressure lies above the failure pressure of the system, its failure must be assumed. The mass flow rate to be relieved corresponds to the flow rate which results from the pressure difference after opening the relief device according to the characteristic of the pump or compressor. 

This includes mnaway reactions (vid. Chap. 3) due to cooling failure, wrong dosing, heat input from outside, too large quantities of catalyst etc. Three categories are distinguished  

The pressure rises along the vapour pressure curve of the reaction mixture due to an increase of reaction heat following a temperature rise. When the safety valve opens, vapour is removed from the system and the pressure drops. The system cools down due to expansion and the use of its energy for providing the latent energy to evaporate the liquid phase. 

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The necessary cross sectional area for discharge is calculated from Eq. (7.7) substituting m by the maximum mass flow rate to be discharged, thmax. The (smallest) cross sectional area F(m ) known as flow area is then obtained by isolating F in Eq. (7.7). 

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