Page 242 - Fluid mechanics, heat transfer, and mass transfer

220 TWO-PHASE FLOW SYSTEMS

duction, pulverized coal ash at power plants, titanium . What is the recommended minimum transport velo-
dioxide, chalk and clay slurries, and so on. city for upflow of slurries to be used for design
. Name the methods to prevent settling of particulates in a purposes?
slurry pipeline. & About twice the particle settling velocity.
& Using a larger diameter line to lower pressure drop . “Turbid water flows at a greater volume under the same
will cause settling of the solids and slug flow. This head than clear water.” True/False?
leads to higher pressure drop, abrasion, and excessive & True. A small percentage of fine particles in water
mechanical stress on the pipeline. In addition, slime actually reduces the friction head loss. Apparently, in
may form on the interior of the pipe and abrasion can a solution with a very small percentage of flowing
reduce wall roughness over time. solids, the particles act as guide vanes and reduce the
& Maintaining velocities over critical values to pre- water turbulence.
vent settling of particulates, as in the case of dilute . Are sewage sludges Newtonian or non-Newtonian?
phase pneumatic conveyance of dry particulates, is & Sludges below 3% solids are usually near Newtonian.
the normal method. This approach does not prevent
& Above 3% of solids content, thixotropic or pseudo-
settling and blockages that are frequent in slurry
plastic behavior starts appearing.
piping as there can be variations in flow velocities
& In one test, sludge with 9% solids flowing at
along the piping for reasons of nonuniformity of
30.5 cm/s (1 ft/s) pipe velocity gave friction losses
particle sizes, valves, fittings, and other obstructions
of almost 10 times those of water, while the same
in the piping, liquid property changes (density and
sludge had only 3 times the friction loss of water at
viscosity) due to temperature fluctuations along the
90 cm/s (3 ft/s).
line, and so on.
. For vertical flow of fast settling solid–liquid
& As with pneumatic conveyors, operating too close to
mixtures (slurries), what is the thumb rule of slip
the saltation velocity can be problematic, a safety
velocity?
factor of at least 25% over the minimum critical
& Slip velocity is roughly same as terminal settling
velocity should be applied. The saltation velocity is
determined through an empirical relationship based velocity of the solids.
on the solids loading, particle size, and physical . What are the two types of slurry pipelines?
properties of the system. & Brute Force: These are usually short, high-velocity
& While the saltation velocity for the largest particle in systems that operate at relatively low solids concen-
the suspension is usually used, particle size distri- trations and usually carry relatively large particles.
bution may be more important. For instance, a slurry Most dredging and tailings pipelines are in this
with a large percentage of very fine particles may category.
behave as a single larger particle and overshadow the & Conventional: These are generally well-designed
effect of a few large particles. The fine particles may systems in which particle size distribution and solids
cluster and cause the larger particles to settle at an concentration are closely controlled in order to main-
even higher rate, which would require a higher slurry tain an economical yet stable operating velocity.
velocity. These systems usually have a high solids concentra-
& One technology involves swirl inducement in the tion. The Black Mesa coal pipeline in Arizona and the
flow at strategic points along the piping, which causes Samarco iron ore pipeline in Brazil are examples of
rotational flow along the pipe axis, thereby lifting the conventional slurry systems.
particulates concentrated in the lower part of the pipe . What are the factors that determine the minimum
to the upper part creating more homogeneous flow transport velocity in a slurry pipeline?
pattern. Aspen technologies are used in computer & Particle size distribution, particle shape, concentra-
simulation of these techniques. tion of solids, and the resulting slurry rheological
& Swirl inducement is claimed to operate the piping at properties.
reduced average flow velocities to prevent settling, . What are the typical operating velocities in a slurry
lower pumping power and pipe erosion, and reduce pipeline?
settling at pipe bends and fittings. It has to be realized
& Conventional long-distance slurry pipeline systems
that the cost of maintaining pressures required to
generally operate at 1.5–2 m/s. If the slurry concen-
induce swirl is understandably higher. Efforts are
tration is properly matched to the particle size dis-
being aimed at improving efficient swirl inducing
tribution, velocities in this range will result in a stable
piping technologies.
operating system that may be easily restarted after