Annex 3 LEVEL SWELL CALCULATIONS

Where it is uncertain whether thle system is inherently foamy, it is recommended that the worst case assumption is used (see 4.3.2(1)). For tempered systems, the worst case will be inherent foaminessl Where tempered systems are not inherently foamy, the level swell calculations described in this Annex may lead to a reduction in calculated relief system size. For untempered systems, the worst case is vapour/ liquid disengagement causing reduced mass loss from the reactor during relief. In this case, dynamic simulation (see A3.4) may be needed to take account of level swell in relief sizing. 

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Further information on level swell calculations and the determination of vessel flow regimes is given in Annex 3. 

Figure A3.2 illustrates terminology used in level swell calculations. Annex 10 gives...

In order to use the sizing method, the reactor, void fraction, aD, at which total vapour/ liquid disengagement is expected at the maximum accumulated pressure, must first be evaluated. This may be done by level swell calculation (see Annex 3) or by small-scale experiment with the same vapour superficial velocity as will occur at plant-scale (see Annex 2). Equation (A5.7)can then be used to find the relief area ... 

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 ... 

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