Chapter 5 RELIEF SYSTEM SIZING

(expressed as- kg/m2s), is the average two-phase flow capacity per unit cross-sectional area of the vent-line. Methods for calculating G are given in Chapter 

If the reacting mixture is highly viscous,-contains solids or contains two separate liquid phases additional information is given in Chapter 10. 

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A starting point for relief sizing is therefore to determine the system types according to each of the above classifications. The appropriate Chapter (6, 7 or 8) can then be consulted for relief system sizing. 

Pressure relief of a runaway reaction is likely to be via a bursting disc or a safety valve, or a combination of both these items. Further information about these is given in Chapter 9. For relief system sizing, it is important to know the pressure at which a relief device will open. 

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). 

Measurement of data for relief system sizing is normally only done once the worst case conditions, or a very small number of worst case candidates, have been identified by small-scale screening tests (see Chapter 3). The calorimetry described in this Annex can be carried out in-house if a suitable calorimeter is available. Alternatively, there are a number of consultancies who will carry out the necessary measurements. Information on these can be obtained from the Institution of Chemical Engineers Jist of consultants. 

This suggests that, provided the reactor fills with a homogeneous two-phase mixture (inherently foamy fluid behaviour), the relief system size can be greatly reduced from the area of 0.0136 m2 calculated using the DIERS method (neglecting, mass loss during relief) given in Chapter 7.. ... 

The relief sizing methods described in Chapters 6, 7 and 8 make worst case assumptions about the vessel flow regime (see (1.) below) in terms of the,extent to which it causes two-phase flow to enter the relief system. It is therefore not necessary to know the vessel flow regime in order,to safety use these sizing methods. However, it may sometimes be desirable to determine it and calculate whether, or how much, two-phase relief would occur, because . , ... 

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. 

For the purposes of the Workbook, the worst case scenario is the credible combination of equipment failures and maloperations that gives rise to the largest calculated relief size compared with other credible scenarios. See Chapter 3. The worst case scenario is the basis for the relief system design. 

Relief valves are often seen to be undersized for the required relieving rate, due either to poor initial design or changes in the process conditions which occurred during design. The most common system problem is that the relief valve was adequately sized for blocked discharge but not sized for the flowrate that could occur as a result of a failure in the open position of an upstream control valve (i.e., gas blowby). See Chapter 13. 

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... 

Chapters 11-13 cover the selection and sizing of downstream disposal systems, reaction forces which require piping and vessel supports, maintenance, documentation and change management. Additional material is given in Annexes 1-8 and is referenced from the,text as required. This includes consideration of any safety factor to be applied to the calculated relief size. 

Fauske s method for vapour pressure systems (see A5 3) and-the sizing method for gassy systems (see Chapter 7) have been, used to do a very.approximate relief sizing. Alternatively for screening purposes, nomographs153 could be used. "... 

This section is concerned with calorimetry to determine the classification of the reacting system (as described in Chapter 4) so that appropriate relief sizing methods can be used. The measurement of data for relief sizing is described in A2.4 to A2.6 below, depending on the results of the classification of system type for relief sizing. 

This worked example calculates G for the tempered hybrid relief sizing worked example given in Chapter 8. It uses the mixing rule for hybrids and therefore shows example Omega calculations for vapour pressure, gassy and hybrid systems. 

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