SLURRY ENGINEERING/PIPELINES
understanding of slurry pipeline flows allows for
the design of pipelines operating with a deposit
on the pipe invert. Designing for the formation of
a deposit at low flow rates allows the system to
operate at moderate velocities for high flow
rates, optimising system energy requirements
and maximising pipeline wear life.
The authors note that “tailings pipelines are
increasingly operated at higher solids
concentrations.” Thus it is more likely the
pipeline will operate in laminar flow due to the
higher tailings rheology. “Historically slurry
pipelines have been operated in turbulent flow
but we now know that laminar flow pipeline
operation is possible as there are several
operational laminar flow tailings pipelines. While
our understanding of laminar flow slurry
pipelines is not complete, recent research
provides guidance on how to approach the
design of these pipelines.”
The figure shows a typical design duty
envelope for a tailings delivery system as a
Specific energy consumption for different design approaches
Nominal flow
Maximum flow rate
Pipeline designed for deposit free
operation at lowest design flow rate
0.046 kW/t/km
0.067 kW/t/km
by about 35% compared with the conventionally
designed tailings pipeline.
The design of pipelines to deliberately operate
with a deposit requires that several measures are
considered to mitigate the risk of possible
pipeline blockage.
n The designer needs to be able to accurately
model the behaviour of tailings pipeline
operating with a deposit
n Steep slopes (>15%) should be avoided when
selecting the pipeline route. In cases where
such are unavoidable, smaller diameter pipe
should be considered in these sections to
avoid the formation of a deposit
n The design should limit
the amount of material
deposited onto the pipe
invert. In establishing this
limit, the authors suggest
considering the flow
conditions if there is a
process change which results
in rapid remobilisation (re-
suspension) of the deposited
material
n Reliable operation
requires that the system has
good flow rate control (i.e.
Pipeline designed for deposit free
operation at nominal design flow rate
0.030 kW/t/km
0.044 kW/t/km
accurate flow rate measurement and variable
speed pump drives)
n Ideally the pipeline should be equipped with
instrumentation to detect the height of
deposited material on the pipe invert.
New sensor technology has led to the
development of an instrument that provides
reliable indication of bed deposit height within a
pipeline. This technology is particularly helpful in
operating pipelines with a deposit and has
application in better understanding the operation
of laminar flow pipelines.
“Failure of tailings pipelines can result in
consequent damage to surrounding structures
and unacceptable spillage into the environment.
Modern pipeline leak detection technology
allows operators to better manage this risk by
ensuring safe and reliable operation of tailings
pipelines.”
Leak detection
Michael Twomey, one of the founders of Atmos
International, notes the importance of leak
detection that manages the changing densities of
slurry and thickened tailings in pipelines.
“Unlike the model-based leak detection
systems historically used on slurry pipelines, the
Atmos Pipe statistical volume balance system
Tailings pipeline operating envelope does not attempt to model
the composition and
function of solids throughput and solids
concentration by mass. “If the pipeline is
designed to operate without a deposit at the
lowest duty flow rate (4,480 m 3 /h), the operating
velocity at the highest duty flow rate (6,590
m 3 /h) will be about 1.5 times the deposition
velocity. Considering that the pipeline wear and
energy requirements are roughly proportional to
the square of operating velocity, this results in an
operationally inefficient system when the
pipeline is operated close to the maximum
design flow rate - which is where most systems
tend to operate after the post-startup
debottlenecking optimisations.” viscosity of the slurry or
tailings transported in a
If however, the design approach ensures the
pipeline operates without a deposit for the
nominal flow rate and accepts that a deposit will
form when operating at lower flow rates. “This
results in the selection of a larger diameter
pipeline, but there are significant benefits in
terms of reduced pumping and energy
requirements and lower pipeline wear.” The table
shows that the energy requirements are reduced
16 International Mining | DECEMBER 2018
pipeline. Atmos Pipe thus
avoids the high false alarm
rate and poor accuracy
displayed by real-time
transient models because
real-time transient models
cannot accurately model the
continually changing
composition and viscosity of
mineral compounds in a
Tailings pipelines are increasingly operated at
higher solids concentrations to help mining
operations conserve water and energy. As solids
concentrations increase, there is a greater
likelihood the pipeline will operate in laminar
flow due to the higher tailings rheology. The
tailings yield stress and slurry density largely
govern the transition from the laminar to
turbulent flow. As illustrated here, it is practical
to operate in the turbulent flow for only
moderate yield stresses (i.e. a Bingham yield
stress of 40 Pa requires that the pipeline is
operated above 4 m/s to remain in turbulent
flow)
slurry pipeline.
“Leak detection in slurry pipelines must
minimise false leak alarms because stopping the
flow could block a pipeline as the particles fall
out of suspension, causing expensive downtime.
Sometimes it is even necessary to cut the
pipeline to clear the blockage. Each shutdown
can cost hundreds of thousands in lost revenue.
The paramount importance of high-reliability was
the perfect opening for the statistical volume
balance leak detection to displace model-based