Two-phase flow relief sizing

TWO-PHASE FLOW RELIEF SIZING FOR RUNAWAY REACTION ... 

Thus, the size of the relief deviee is signifieantly smaller than for two-phase flow. Sizing for all vapor relief will undoubtedly give an ineorreet result, and the reaetor would be severely tested during this runaway oeeurrenee. Table 12-7 gives the results of the VENT software program of Example 12-4. 

Sizing, safety relief, 436, 437-441 API liquid valve, 444 Balanced valves, 441 Conventional valves, 438 Critical back pressure, 440 Effects of two-phase flow, 437 Hydraulic expansion, 441 Rupture disks, 434 Sub-critical flow, 449 Slurry flow, process pipe, 142-147 Regimes, 143... 

Leung JC. Easily size relief devices and piping for two-phase flow. CEP, December 1996, pp 28-50. 

Figure 9-1 Required vent area as a function of overpressure for two-phase flow. The vent area is decreased appreciably as the overpressure increases. Data from J. C. Leung, Simplified Vent Sizing Equations for Emergency Relief Requirements in Reactors and Storage Vessels, AICHE Journal (1986), 32(10).

When a runaway reaction occurs within a reactor vessel, two-phase flow should be expected during the relief process. The vent sizing package (VSP) laboratory apparatus described in chapter 8 provides the much needed temperature and pressure rise data for relief area sizing. 

The first step in the relief sizing calculation for two-phase vents is to determine the mass flux through the relief. This is computed using Equation 4-104, representing choked two-phase flow through a hole ... 

This requires a relief device with a diameter of 0.176 m, a significantly smaller diameter than for two-phase flow. Thus, if the relief were sized assuming all vapor relief, the result would be physically incorrect and the reactor would be severely tested during this runaway event. 

As mentioned previously, two-phase flow discharges for fire scenarios are possible but not likely. To size the relief for fire and a single-vapor phase, use the heat input determined from Equations 9-36 to 9-38, and determine the vapor mass flow rate through the relief by dividing the heat input by the heat of vaporization of the liquid. This assumes that all the heat input from the fire is used to vaporize the liquid. The relief area is then determined using Equations 9-3 to 9-12. 

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... 

For tempered systems, the pressure relief system will almost always need to be bigger if two-phase flow occurs, and DIERS[1] recommended that two-phase relief should normally be assumed for vent sizing purposes using the type of hand calculation methods given in this Workbook. This is explained in 4.3.2, Subsection... 

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 . , ... 

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... 

The two-phase mass flow capacity per unit cross-sectional area, G, can be calculated using any applicable method for non-flashing two-phase flow (see Chapter 9). In order to minimise the relief size obtained, G should be evaluated at the maximum accumulated pressure, irrespective of the relief pressure. 

RELIEF SYSTEM SIZING FOR TEMPERED HYBRIDS WITH TWO-PHASE FLOW... 

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. 

BACKGROUND INFORMATION ON THE CHOICE OF TWO-PHASE FLOW MODEL FOR RELIEF SIZING... 

The simplified ERM is very convenient in that all the properties can be evaluated at the stagnation conditions in the reactor. The method may be appropriate for relief sizing for vapour pressure systems when flow is to be via a safety valve. Discharge coefficients for two-phase flow through safety valves are discussed in 9.7.1. -... 

A discussion of the different types of assumption that can be made in two-phase flow models is given in Chapter 9. DIERS[8] recommended the use of the homogeneous equilibrium model (HEM) for relief sizing, and so, preferably, a code which implements the HEM should be chosen. The model will need to incorporate sufficiently non-ideal modelling of physical properties and provision for multiple line diameters and potential choke points, as required by the application. 

In such cases, two-phase relief occurs only as a result of depressurisation (i.e. the pressure is falling) and so cannot overpressurise the reactor. Although the relief system can be sized for single-phase vapour flow, the design of any downstream disposal system must take account of the two-phase flow which will actually occur. 

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