Page 80 - Handbook Of Multiphase Flow Assurance

Hydrodynamics of multiphase flow 75

The following images in Fig. 4.2 show the four slugs completely crossing the pipe
cross-section, moving from right to left, in a 6-in. multiphase flow loop donated to the
University of Tulsa with air and water. The images are frames from a video recorded for
seven seconds. The incomplete slugs and waves are not shown. The areas of churn at slug
fronts with high bubble entrainment and maximum energy dissipation are darker on these
images, whereas liquid without entrained bubbles is light and translucent. The longer slug
in the second image has two areas with bubble entrainment, which suggests this may be a
slug merged from two smaller slugs.
A video of slug flow in these frames is available on makogon.com/resources/6in-
chILOOP050101slugs.AVI and 6inchILOOP050101slugging.gif.
Images from this video may be used to train slug pattern recognition in machine learning.
Slug velocity may be determined from the video recorded in May of 2001.
Slug length and frequency are common parameters predicted by the multiphase flow cor-
relations. Slug length may be of a slightly greater interest from the system integrity stand-
point because knowing it helps one calculate the momentum of the moving slug and the force
with which the slug can impact on a pipe bend, a tee or an elbow in the flow path.
A simplified calculation for an average slug length is proposed as
2
L ft [] = ( D in [] )
Slug
where pipe inside diameter is in inches and average slug length is in feet, so a 10 in. inside
diameter pipe would produce 100 ft. or 30 m long slugs, on average. This may be used only as
a very rough approximation of the true slug length, just to find the order of magnitude for the
possible slugs in multiphase flow. The advantage is that this calculation is easy to remember
and thus use in the field or as needed. This proposed correlation is useful up to pipe diame-
ters of 14 in. which covers a large portion of installed multiphase pipes.
A comparison of software predictions with field data for slugging observed in a 12-in.
flowline is shown in Fig. 4.3 below (updated, from Fairhurst, 2002). The proposed slug length
correlation has a reasonable agreement with the slug lengths observed in the field.
Some examples of models for slug frequency are presented in Hill and Wood (1990) and
Shea et al. (2004).
The Hill-Wood model the slug frequency as function of mixture velocity and liquid height

U
.
.
f = 0 275 mixture 10 268 h film / D
slug
D
The Shea model of the hydrodynamic slug frequency as a function of liquid superficial ve-
locity and dimensionless slug length L expressed in pipe diameters was used in a commercial
transient software simulator.
068 U
.
f = SL
12 06
.
DL .
Application of correlations developed over the past decades for pressure drop and liquid
holdup should be chosen based on pipe diameter and inclination and target flow conditions
such as flow regime, viscosity.