Transfer Line Critical Velocity
By definition, critical velocity is the maximum velocity that a compressible fluid (a gas or a two-phase mixture) can attain through a pipe of constant cross-sectional area. When critical velocity is reached, downstream conditions will not affect the fluid flow rate.
The ideal location for reaching critical velocity is at the exit of the transfer line (entrance to the column). However, transfer lines should be sized large enough so that at the design feed rate, reaching critical velocity does not significantly increase the furnace outlet temperature.
The recommended method for calculating two-phase critical velocity in transfer lines is shown below. The method is derived from the theoretical velocity of a compressible wave through a homogeneous two-phase fluid.
D @ 0.333 (Ratio of Vapor Critical Velocity to Liquid Critical Velocity)
G = Total Mass Velocity, lb/hr – ft2
P = Pressure, psia
rL = Liquid Density, lb/ft3
rTP = Homogeneous two-phase density
(from Equation 400-5)
rv = Vapor Density
VbTP = Homogeneous two-phase Bulk Velocity, ft/sec
VcTP = Homogeneous two-phase Critical Velocity, ft/sec
Vv = Newton’s formula for vapor critical velocity
(from Equation 400-4)
x = Wt Fraction Vapor