For two-phase flow

The pressure drop for gas—Hquid flow is deterrnined by the Lockhart-MartineUi method. It is assumed that the AP for two-phase flow is proportional to that of the single phase times a function of the single-phase pressure drop ratio P. 

In the macroscopic heat-transfer term of equation 9, the first group in brackets represents the usual Dittus-Boelter equation for heat-transfer coefficients. The second bracket is the ratio of frictional pressure drop per unit length for two-phase flow to that for Hquid phase alone. The Prandd-number function is an empirical correction term. The final bracket is the ratio of the binary macroscopic heat-transfer coefficient to the heat-transfer coefficient that would be calculated for a pure fluid with properties identical to those of the fluid mixture. This term is built on the postulate that mass transfer does not affect the boiling mechanism itself but does affect the driving force. 

Pressure drop during condensation inside horizontal tubes can be computed by using the correlations for two-phase flow given in Sec. 6 and neglec ting the pressure recoveiy due to deceleration of the flow. 

The shape of the coohng and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell-sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

Two-phase multiplier, pressure drop for two-phase flow... 

For two-phase flow, the phase contraction coefficients Cqc. nd Col relate the area of each phase Ac and A at the vena contracta to the known area of the orifice Ay. Thus ... 

A further generahzation for two-phase flow as suggested by Tan-gren et al. (1949) is to use the generalized value of k as ... 

Never use inclined piping for two-phase flow in a process plant. This is particularly true for reboiler return piping. Use only horizontal or vertical runs. 

Slug flow must be avoided in all two-phase applications. The designer must be alert for two-phase flow developing in a system. In one case, absorber liquid going to a lower pressure stripper produced a two-phase mixture. The absorber stream entered the stripper in a line that was elled down onto the stripper tray. The two-phase mixture beat out a section of trays. A /4-in. protection plate was provided and this had a hole cut in it in two years. 

Kern, Robert, C. E. Refresher—Piping Design for Two-Phase Flow, C/ em, Engr., June 23, 1975, p. 145. Simpson, L. L., Sizing Piping for Process Plants, Chem. Engr., June 17, 1968, p. 192 (Vertical downflow example p. 203). 

For two-phase flow through pipes, an overall dimensionless dis-eharge eoeffieient, /, is applied. Equation 12-11 is referred to as the equilibrium rate model (ERM) for low-quality ehoked flow. Leung [28] indieated that Equation 12-11 be multiplied by a faetor of 0.9 to bring the value in line with the elassie homogeneous equilibrium model (HEM). Equation 12-11 then beeomes... 

ICI reeommends a safety faetor of 1-2 on flow or area. The safety faetor assoeiated with the inaeeuraeies of the flow ealeulation will depend on the method used, the phase nature of the flow, and the pipe frietion. For two-phase flow, use a safety faetor of 2 to aeeount for frietion or statie head. 

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. 

There are no precise formulas for calculating orifice area for two-phase flow. The common convention is to calculate the area required for the gas flow as if there were no liquid present and the area required for the liquid flow as if there were no gas present. The two areas are then added to approximate the area required for two-phase flow. 

As indicated in the previous section, infonnation on liquid emissions for a variety of conditions is available in the literature, including equations for two phase flow "°. Key equations for liquid and two-phase discliarges liave been adopted from CCPS and provided below ,... 

Figure 2-42. Estimating pressure drop in uphiil sections of pipeline for two-phase flow. By permission, O. Flanigan, Oil and Gas Journal, Mar. 10, 1958, p. 132.

Kern, Robert, Flow to Size Process Piping for Two-Phase Flow, Hydrocarbon Processing, Oct. 1969, p. 105. 

Kern, Robert, Piping Design For Two-Phase Flow, Chem. Eng., June 23, 1975, p. 145. 

First, K. E. and Huff, J. E., Design Charts for Two-Phase Flow in Emergency Pressure Relief. Systems, Plant/Operations Progress, 8 (1), 40-54, January 1989. 

For steam distillation columns, it is desirable to sparge tbe steam uniformly to all reboiler tubes. Because tbis provides full tube length for two-phase flow, thermal circulation is permitted. 

This calculation is not accurate, because it does not account for two-phase flow. 

Weekman and Myers (W2) examined the fluid-flow characteristics of cocurrent downward flow of gas and liquid. The pulsing effect first noted by Larkins et al. was also observed in this work. Pressure-drop data could be correlated satisfactorily by a relation similar to those used for two-phase flow in pipes. Surface-active agents were observed to have a pronounced influence upon flow regime transition and pressure drop. 

A momentum balance for the flow of a two-phase fluid through a horizontal pipe and an energy balance may be written in an expanded form of that applicable to single-phase fluid flow. These equations for two-phase flow cannot be used in practice since the individual phase velocities and local densities are not known. Some simplification is possible if it... 

Chisholm, D. Inti. Jl. Heat and Mass Transfer 10 (1967) 1767. A theoretical basis for the Lockharl-Martinelli correlation for two-phase flow. 

A PTpf Pressure drop for two-phase flow over length l of pipe N/m2 ML T ... 

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