Page 160 - Handbook Of Multiphase Flow Assurance

156 5. Flow restrictions and blockages in operations

Each company has to perform their own research if they intend not to use the recom-
mended safe limit.

Liquid with solids (hydrate, corrosion products, scale)
We propose several methods for calculation of shear stress in multiphase flow with solids
such as gas hydrates as shown in Fig. 5.29. Shear affects performance of antiagglomerant
chemicals in two ways: (1) it breaks up water droplets into size of few microns (see size esti-
mate in Autoclave section) and (2) it keeps hydrate agglomerates dispersed (shear-degraded).
Hence shear stress should be consistent across test methods. The following cases are illus-
trated: pipeline fluid wall shear stress, Flowloop fluid wall shear stress, Flowwheel fluid wall
shear stress, Rocking cell—shear from rolling ball and Autoclave impeller energy dissipation.
The example shows that turbulent shear stress can be maintained equal in field operating
conditions and in laboratory tests.
The formulas used above are:
[
[
[
V ms]
3
3
τw _ lamPa] = 8 µ Pas] [ / / D m [] = 32 µ Pas]   3 /s  / π D m ( )
Q m
[
3

2
2
τw _ turPa] = 05 f ρ kgm   V   m 2 /s  
.
/

V = average velocity = Q/A, flowrate divided by the cross-sectional area, μ = viscosity,
ρ = density.
These are simplified calculations and use friction factor f = 0.316/Re 0.25 (Blasius smooth
pipe) for consistency because most tests as in a rocking cell have smooth surfaces.
0.5
Effective diameter for the rocking cell calculation is used as = (4 × A ANNULUS /π) where
annulus area is opening between test tube and ball.
Dissipation energy for the Autoclave calculation is = power per unit mass = Power/ρ
Vol_Liq.
Turbulent microscale for water droplets breakup by the Autoclave impeller.
3
_
η = (kinematicviscosity / dissipation energy ) . 025
_
according to the Kolmogorov (1941) turbulent energy dissipation.
Paraffin wax
Introduction
Paraffin wax deposition is one of the most common flow assurance issues. Petroleum flu-
ids by their nature commonly contain straight molecular chain or normal paraffins. Normal
paraffins deposits are known as macrocrystalline wax. Such deposits contain normal paraf-
fins, branched paraffins which co-deposit with normal paraffins, occluded precipitated as-
phaltenes, resins, sand, oil and other solids which may be present such as rust, scale.
Normal paraffins crystallize at higher temperatures because they have straight molecular
chains, which get easily organized into crystals as illustrated in Fig. 5.30.