Relief sizing using Leungs method

In order to calculate a relief flow area the two-phase mass flow rate per unit area, G, needs to be calculated. Since relief is to be via a safety valve, friction can be neglected and.the equilibrium rate model (ERM) can be used to calculate G (see 9.4.2) .  

The difference between the two methods may be due to deviations from the applicability of the Clausius-Cjapeyron equation (for the alternative method). Therefore, G has been taken from equation (9.3) (the average value of G of 2738 kg/m2s between the relief pressure and maximum accumulated pressure) for sizing purposes. However, this must be,multiplied by the discharge coefficient of the safety 

The required relief flow area can now be calculated using equation (5.1)  

H G Fisher et al., Emergency Relief System Design Using DIERS 

J C Leung, Simplified Vent Sizing Methods Incorporating Two-phase Flow , International Symposium on Runaway Reactions and Pressure Relief Design, 200-236, AlChE, 1995, ISBN 0-8169-0676-9 

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Where the Leung methods are inapplicable, a detailed computer simulation can be used to sizing the relief system (see Annex 4). In such cases, care should be taken that the computer code models all necessary features of the relieving runaway reaction. Therefore computer simulations are best carried out by competent specialists. 

WORKED EXAMPLE OF RELIEF SYSTEM SIZING FOR A VAPOUR PRESSURE SYSTEM USING LEUNG S METHOD... 

The relief, size should also be calculated using Leung s method for vapour pressure systems (see 6.3) and the larger of the vapour pressure system and tempered hybrid system relief sizes should be used121. 

Once it has been checked that the method is applicable (see above), it can be used for relief system sizing. Leung s method is[2,3,6] ... 

If Leung s method is to be used for pressure relief system sizing (see 6.3), then the temperature and rate of heat evolution at the relief pressure and at the maximum accumulated pressure are needed. A possible test protocol to obtain this information from a single test is as follows (see Figure A2.11) ... 

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