Required relief rate

Relief sizing for single-phase relief can be done assuming steady-state, i.e. the gas/ vapour needs to be removed by the pressure relief system at the rate at which it is generated by the reaction. A required relief rate, Wg, can be specified, and the relief system can be sized to give a capacity which exceeds it. If the capacity is expressed as mass capacity per unit flow area, Gg, then the required relief area can be obtained from  

The calculation of the required relief rate, Wgf is described in A6.2 below, and the calculation of the single-phase relief mass flow capacity per unit area, Gg, is described in A6.3 below.  

Data about the rate of the runaway reaction, (characterised by the adiabatic rate of temperature rise, dT/dt, and the rate of permanent gas generation, QG, as appropriate), is best obtained experimentally. Methods are given in Annex 2. The relief system sizing method will depend on whether the system is tempered or untempered (see 4.2 and Annex 2). 

The required relief rate for tempered systems is given by  

The first term is the vaporisation rate and the second term is the rate of production of permanent gas by the reaction The second term is only required for tempered hybrids 

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Required Relief Rate The required relief rate is the venting rate required to remove the volume being generated within the protected equipment when the equipment is at its highest allowed pressure ... 

Tj = temperature eorresponding to relief aetuation, K X = mass fraetion of vapor at inlet Vq = speeifie volume of gas, m /kg Vl = speeifie volume of liquid, m /kg W = required relief rate, kg/s X = latent heat, J/kg... 

Required Relief Rate The required relief rate is the vent rate W... 

Most of the relief sizing equations given in Chapters 6-8 yield the two-phase required relief rate, W. The two-phase mass flow capacity per unit area, G, is then needed in order to obtain the required relief area. Chapter 9 contains important background information about two-phase flow, and calculation methods for G. Some system types are special cases involving highly viscous (laminar) flow, solids and/or... 

The Chapters indicated by Figure 5.1 give methods for calculating an average two-phase required relief rate W (expressed as kg/s) The required relief system area, A, is calculated from - ... 

The logic given in Figure 5.1 can be used to check that this section is the correct one for relief system sizing for any particular case. As explained in Chapter 5, the required relief rate, W, should first be calculated using the methods described in this Chapter. A two-phase mass flow capacity per unit area, G, should then be calculated using the methods described in Chapter 9 (or Chapter 10 in special cases). The required relief flow area can then be calculated using equation (5.1). 

Calculate required relief rate, W, using, t Leung s method (see 6.3)... 

The required, relief rate will be calculated using. Leung s method for tempered hybrids ... 

The relief sizing methods detailed in Chapters 6-8 (and most methods in Annexes 4 and 5) yield an average two-phase required relief rate, W. In order to calculate the required relief flow area, A, using equation (5.1), the two-phase mass flow capacity per unit cross-sectional area of the relief system, G, is needed. This Chapter is concerned with methods for the, calculation of G. 

Before using the method, the void fraction at disengagement must be evaluated at conditions corresponding to the maximum accumulated pressure during relief. This can be done by level swell calculation (see A3.3) or possibly by a small-scale experiment that uses depressurisation to achieve the same vapour superficial velocity as in the full-scale reactor during relief (see Annex 2). The required relief rate can then be calculated fromt31 ... 

The relief system capacity should be calculated at the same pressure as was the required relief rate in A6.2 above, so that they can be compared. 

The required relief rate will first be calculated using equation (A6.2). It can be seen from Table A6.2, that both dT/dt and QG are considerably lower at the relief pressure than at the maximum accumulated pressure. The calculation will therefore be performed at the relief pressure since this should give the smallest relief size. Operation of the relief system will then prevent the pressure rising to the. maximum accumulated pressure, for which a larger relief system would be needed. 

Due to the variety of blocked outlet situations, different methods were used to determine the required relief rates for different types of equipment. The quantity of the material to be relieved was generally determined at the relieving conditions (i.e. the MAWP plus the code-allowable overpressure) based on the capacity of upstream pressure sources or duty of process heaters. 

This is a very particular application where we have the potential for overpressure due to loss of overhead condensing or reflux failure. In the event the cooling medium in the condenser is lost, additional vapour may be present at the top of the column. This additional vapour may require pressure relief. In a typical distillation system, a cooling failure also results in a loss of reflux within a short period of time (typically about 15 minutes). API RP 521 states that the required relief rates before and after loss of reflux should be considered. The Berwanger audit method encompassed both of these calculations, as it was not intuitive, which case would require the larger required relief rate. 

W = required relief rate, kg/s X = mass fraction of vapor at inlet a = void fraction in vessel... 

Location of relief valves, bursting disks, and m or vents should be carefully reviewed. Normally, these should be at the top of superatmospheric columns (or in their overhead systems). Conversely, the vacuum-breaking device should normally be at the bottom of the column. This is important for avoiding damage to trays and for achieving the required relief rates. Detailed discussion is in Sec. 9.5. 

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