International Journal of Research In Science & Engineering
Volume: 1 Issue: 3
e-ISSN: 2394-8299
p-ISSN: 2394-8280
Pressure Drop in Slurry Flow
Shubham Grover1 , Rajat Jassi2
1
Undergraduate Student, Department of Chemical Engineering, Indian Institute of Technology Delhi,
New Delhi, India, [email protected]
2
Undergraduate Student, Department of Chemical Engineering, Indian Institute of Technology Delhi,
New Delhi, India, [email protected]
ABSTRACT
Over the years, slurry flow has been extensively investigated because of its wide scale application in transport of
materials in industries. Durand, Newitt, Wasp have explained their theories and models in this domain but they
are applicable in narrow ranges of conditions. There is still a need to find an optimal method for slurry
transportation that can be applied on a variety of slurries. The objective of this research is to experimentally
investigate the pressure drops for solid-liquid slurry flow in constant circular cross-section straight and helical
pipes of a finite pitch placed horizontally for homogeneous slurry flow regime. The sl urry used in the process is
calcium carbonate powder (d=56μm) mixed with water and the concentration is varied (0-7%). The results
obtained helped in understanding the difference in nature of flow i n straight and helical pipes and in comparing
the dependence of pressure gradient per unit length on velocity and concentration for the straight and helical
pipes. The concept of using the helical flow is to understand the effect of centrifugal forces on the pressure drop
and hence the friction factor. The pressure drop in slurry flow is influenced by particle -particle, particle-fluid,
fluid-wall and particle-wall interactions (R.C. Chen 1994). Size of the particles and the tendency to settle has a
strong impact on capital and operating cost.
Keywords: Pressure drop, Slurry, Helical pipe, Magnetic Pump, Concentration, Pitch, Flow rate, Velocity.
----------------------------------------------------------------------------------------------------------------------------1. INTRODUCTION
The pressure drop is considered the most important parameter in slurry flo w. Large amount of pump ing costs are
involved in transporting solid-liquid slurries in chemical industries. Efforts are made constantly to find the most
suitable comb ination of p ipe shape and particle size to minimise the energy losses incurred in flow. The pressure
drop in a flow depends upon the flow velocity and the interactions between the particle, fluid and the pipe wall. The
relationship between pressure gradient and mixture velocity is substantially different from that of a pure liquid flow.
Mud slurry is a mixture of a carry ing flu id and solid particles. In spiral p ipe, having a constant pitch in relation to
the axis, a swirling flow occurs when flu id flows in the pipe. Practically spiral pipes presents a very useful method
for preventing the holdup phenomenon in a transport pipeline fo r the hydraulic t ransport of mud slurry (Yanuar et al.
2009). Flow velocity generates turbulence which keeps particles suspended in the slurry. Mean velocity in a p ipeline
is a basic parameter characterizing pipeline flo w. It is defined as the bulk velocity, v, of a matter (solids, liquids or
mixtu res) obtained by dividing the volumetric flo w rate, Q, passing through a pipeline div ided by cross section of
the area, A.
There are two types of slurries: Settling slurries and non-settling slurries. For non-settling slurries the particle
diameter is less than 40 u m. Viscosity of a slurry is modified by particles. The flow becomes increasingly non newtonian as concentration increases. Settling slurries are identified as with particle d iameter greater than 40 u m.
They are further classified as small (suspensions) 40 μm ~ 200μm, mediu m (transition) 200 μm~2mm, large
(heterogeneous) 2 mm ~ 5 mm, very large (hetero) 5 mm >200 mm (Jeff Bermer 2008). The working particles of
CaCO3 used in this research is classified under the small settling suspensions with d=56um.
IJRISE| www.ijrise.org|[email protected] [50-54]
International Journal of Research In Science & Engineering
Volume: 1 Issue: 3
e-ISSN: 2394-8299
p-ISSN: 2394-8280
For a pure liquid, the pressure drop in a pipe depends on the flo w velocity. In straight pipes, for slurry flow the
patterns were classified by Tanaji Mali (2014) into the following four regimes:
Homogeneous flow: When the flow velocity is sufficiently high, there is a uniform distribution of solids about the
straight axis of the p ipe. In this regime, turbulent and other lift ing forces are capable of overcoming the net body
forces as well as the viscous resistance of the particles.
Heterogeneous flow: With the decrease in slurry velocity, intensity of turbulence and lift forces also decrease which
creates a concentration gradient across the pipe cross section with a larger concentration of solids at the bottom.
When the velocity further decreases to the critical velocity, so me solids begin to move along the pipe bottom wh ich
can be called as a “bed load.” At this point, the pressure drop is usually minimum.
Saltati on flow: At further reduced velocity, the solids in the bed load starts to generate a stationary bed. Further
solids transported as suspended load decreases, and more amount of solid is transported as the bed load. The bed
motion is due to the movement of the uppermost particles tumbling over one another.
Stationary bed fl ow: The stationary bed further increases the apparent pipe frict ion factor, resulting in increased
pressure drop. Finally, at further reduced velocity, all solids stop moving. In this regime the slurry velocity is fu rther
reduced which leads to the lowermost particles of the bed being nearly stationary. Thus, the relat ionship between
shear stress and shear rate may be described by measuring the pressure drop gradient and the volumetric flow rate.
2. EXPERIMENTAL SET-UP
The set up consists of a feed tank having a slurry of (water + CaCO3 ). The particle size of the powder used in slurry
is 56 microns which falls in the category of settling slurries. So, to stop slurry sedimentation the slurry is
homogeneously agitated using an agitator. The slurry concentration is varied from 0 to 7% w/w. The slurry was
introduced to the straight pipe and helical pipe using 0.12 hp chemical pu mp at ro om temperature. The slurry flow
rate was controlled using ball valve in the main stream and a pin valve in bypass stream. The flo w rate was
measured by volu metric time analysis. The helical pipes were made of flexib le tubing with an inside diameter of
0.75 in. or 1.91cm wounded on a cylindrical core of D=22.3cm.
To min imize end effects on the pressure drop measurements, the pressure taps were located away fro m the pipe ends
by more than 50 t imes pipe rad ii. The pressure drop for the test section was measured using a digital mano meter.
Since the particle used has a diameter=56u m, it falls in the category of a settling slurry and needs an agitator, the
velocity was kept above the critical velocity (appro ximately 20cm) of the slurry. It was taken care that the flow
velocity is sufficient to have a homogenous slurry flow as defined above.
Number of loops
Core Diameter
Tube Diameter
Pitch
=3
Pipe diameter
= 1.91 cm
= 22.3 cm
Length of pipe between pressure tapings = 2.4 m
= 1.91 cm
= 30 cm
Fig-1: Experimental setups helical and straight pipe system respectively
IJRISE| www.ijrise.org|[email protected] [50-54]
International Journal of Research In Science & Engineering
Volume: 1 Issue: 3
e-ISSN: 2394-8299
p-ISSN: 2394-8280
2.1 PROCEDURE
After the flo w was developed readings were taken. It was taken into consideration that the slurry level in the holding
tank should be high enough to ensure that air was not entrained in the mixture entering the pipe. The joints were
made leak proof for minimising any loss of flow. The pump was chosen very carefully as the slurry consists of a
chemical slurry wh ich may corrode the iron casting of a normal pu mp. So we choose a non-corrosive centrifugal
chemical pump with polymer casting to develop the flow.
Pressure tapings may d isturb flow at the point where measurement is being taken. So the tapings should be made
carefully. Agitator is switched on regularly so that sedimentation may not take place and it was not switched on
continuously because it can disturb the flow. Bubble format ion at lo w flo w rate may develop irregularity, so flow
was kept above the minimum value.
3. EXPERIMENTAL RESULTS
Straight Pipe:
1.
2.
Fro m the obtained graphs between pressure gradient per unit length versus velocity, it can be concluded
that the frictional losses at high velocity were h igher and consecutively higher pressure drop per unit length
is seen at high flow rate.
Increasing the concentration increases the pressure head loss may be due to increased viscosity.
Delta P/L vs Velocity (Straight Pipe)
9
8
delta P/L (mm Hg/m)
7
6
Concentration = 1%
5
Concentration = 3%
4
Concentration = 5%
3
Concentration = 7%
Concentration = 0%
2
1
0
20
30
40
50
60
70
Velocity (cm/s)
Fig-2: Dependence of pressure drop per unit length on velocity and concentration in straight pipe.
IJRISE| www.ijrise.org|[email protected] [50-54]
International Journal of Research In Science & Engineering
Volume: 1 Issue: 3
e-ISSN: 2394-8299
p-ISSN: 2394-8280
Helical Pipe:
1.
2.
Fro m the obtained graphs between pressure gradient per unit length versus velocity, it can be concluded
that the pressure head loss decreases with increasing flow rate or the velocity in p ipe. The reason for this
require further study of the nature of the fluid.
Increasing the concentration decreases the pressure head loss. This indicates towards the nature of flu id
becoming non-newtonian.
Delta P/L vs Velocity (Helical Pipe)
30
delta P/L (mmHg/m)
25
20
Concentration = 1%
Concentration = 3%
Concentration = 5%
15
Concentration = 7%
Concentration = 0%
10
5
0
25
30
35
40
45
50
55
60
Velocity (cm/s)
Fig-3: Dependence of pressure drop per unit length on velocity and concentration in helical pipe.
3. 1 CONCLUSION
While the helical pipe offers the advantage of no hold up but it also involves greater loss of pressure head per unit
length due to agitated and properly mixed flow that happens in a helical pipe while the particles can settle in a
straight pipe. Opposite dependency of pressure drop per unit length on velocity and concentration discussed above
may be attributed to the effect of a centrifugal force in a helical p ipe which is absent in a straight pipe. Centrifugal
force depends upon the density of the solution and the square of the velocity. The effect of the centrifugal force
causes the particles in the flow to be pushed to the outer side of pipe which increases the shear forces between the
pipe wall and the slurry and ultimately increases the frict ion factor which causes pressure drop per unit length in the
pipe. So, it can be concluded that energy losses in case of helical pipe is although greater than a straight pipe but as
we increase the flow rate, the losses decrease and at increased concentration and flow rate helical pipe may cause
lesser pressure drop per unit length than a straight pipe as can be inferred if the graph is ext rapolated. But the cost of
operating at high flo w rate is also higher so we have to find an efficient and economic balance betwee n the flow rate
and pressure head loss.
IJRISE| www.ijrise.org|[email protected] [50-54]
International Journal of Research In Science & Engineering
Volume: 1 Issue: 3
e-ISSN: 2394-8299
p-ISSN: 2394-8280
Delta P/L vs Velocity
30
Concentration = 1%
25
Concentration = 3%
delta P/L (mmHg/m)
Concentration = 5%
20
Concentration = 7%
Concentration = 0%
15
Concentration = 1%
10
Concentration = 3%
Concentration = 5%
5
Concentration = 7%
0
25
30
35
40
Velocity (cm/s)
45
50
55
60
Concentration = 0%
Fig-4: Dependence of pressure drop per unit length on velocity and concentration compared.
3.2 FUTURE SCOPE
The results obtained can be used to develop a new model and propose a theory on slurry flo w friction factor. The
topic can be further exp lored for different type of particles, with a particle size distribution of interest for different
shapes and materials of pipes to study the effect of particle-particle, particle-fluid, flu id-wall and particle-wall
interactions on slurry flow. The schemes and models developed can help in minimising the energy losses occurred
during slurry transport.
ACKNOWLEDGEMENT
We sincerely thank Dr. Sudip Pattnayak, IIT Delh i, for provid ing us an opportunity to work on this research project
and bestowing us with his guidance and financial assistance for acquiring all the equipment needed.
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