Leung’s method

These systems are ealled tempering (i.e., to prevent temperature rise after venting) systems beeause there is suffieient latent heat available to remove the heat of reaetion and to temper the reaetion at the set pressure. The vent requirements for sueh systems are estimated from the Leung s Method [31,32] ... 

The answer obtained from the Leung s method should not be signifieantly smaller than that of Fauske s method. 

Using the Leung s method and assuming L/D = 0 and F = 1.0, two-phase mass flux from Equation 12-14 gives... 

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

LEUNG S METHOD FOR RELIEF SYSTEM SIZING FOR VAPOUR PRESSURE SYSTEMS "... 

Leung s method is given in 6.3.2 below. The method.is an approximate solution to the differential mass and energy balances for the reactor during relief and takes account of both emptying via the relief system and the tempering effect of vapour production due to relief. The method makes use of adiabatic experimental data for the rate of heat release from the runaway reaction (see Annex 2). Nomenclature is given in Annex 10. 

Leung s method (as given in equation (6.5) below) is applicable if all the above assumptions are true. Assumption (d) above, regarding the use of an average rate of heat release, tends to be the most limiting in terms of the maximum difference that can be allowed between the relief pressure and the maximum pressure. The absolute overpressure (often referred to simply as the "overpressure") has been sometimes used to characterise this. This is given by ... 

If.equation (6.2) is used, then comparison with dynamic simulation131 suggested that Leung s method would increasingly oversize at absolute overpressures above 50%. Provided the rate of temperature rise due to the runaway continues to increase at high overpressures, the arithmetic mean (equation 6.2) overestimates the true average q. 

An alternative criterion to equation (6.1) for the applicability of Leung s method at high overpressures is given by CCPS(4] ... 

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] ... 

Alternatively, the mean of G (calculated at the relief pressure) and G (calculated at the maximum accumulated pressure) can be used. Since Leung s method assumes that the reactor contents are homogeneous during relief, the consistent assumption for the vapour fraction at inlet to the relief system (needed to calculate G) is that it is the same as the average for the reactor, i.e. at the inlet to the relief system the void fraction is given by ... 

A worked example of.the use of Leung s method is given in 6.5 below. 

Alternative version of Leung s method (and associated applicability). 

If Leung s method is inapplicable due to the presence of external heating, then alternative hand calculation methods are given in Annex 5 or a computer simulation could be used (see Annex 4). In either case, the thermal data should be obtained in a small-scale test which also simulates the external heat input. 

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

Size using Leung s method for tempered hybrids (see 8.3.1), if applicable... 

This is related to Leung s method for vapour pressure systems (see 6.3) and shares the same assumptions and conditions of applicability as are detailed in 6.3.1. Additionally, the method assumes the following . 

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) ... 

Leung s method for relief system sizing for tempered hybrid systems (see 8.3.1) requires the ratio of the vapour pressure to the total pressure, P P. This is approximately given by ... 

This calculated relief area is very close to the 0.0378 m2 calculated using Leung s method in 6.5. 

The version of Fauske s method given in A5.3.2 below is simplified by assuming that two-phase relief occurs until the reactor is empty (a safe assumption for vapour pressure systems). This makes the results directly comparable with those from Leung s method (in 6.3). The full version of Fauske s method14,51 allows account to be taken of vapour/ liquid disengagement before the vessel has emptied. This method is given in A5.3.4 below. 

As the safety valve has a de-rated discharge coefficient of 0.87, then A=0.0383/0.87 = 0.044 m2. In this case, Fauske s method gives a slightly higher relief area than the 0.0378 m2 given by Leung s method. 

This method14 is for vapour pressure systems. It solves the material and energy balance equations, on which Leung s method (see 6.3) is based, in a step-wise manner. The method shares most of the conditions of applicability of Leung s method (see 6.3.1) but does not require values of parameters to be constant over the pressure range from the relief pressure to the maximum pressure. Indeed, systems in which there is a discontinuity in rate of reaction or physical properties can be handled by this method, provided that the step boundaries are chosen such that any discontinuity coincides with a boundary (see, for example, Figure A5.1).. 

This method131 is a modification of Leung s method for vapour pressure systems (see... 

This method181 is designed for use in cases where there is simultaneous runaway reaction and external heating, such as external fire. It is for vapour pressure systems, and makes use of the same assumptions as in Leung s method (see 6.3) with the same conditions of applicability.. ... 

The method uses Leung s method (equation (6.5) or (6.9)), but with a modified value of the heat release rate per unit mass, q, to account for the additional external heat input. This modified value of q is given by ... 

Worked example of Leung s method modified for external heating... 

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