Hydraulic Conveying Association[1]

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Hydraulic Conveying Association of South Africa

4thOne Day Seminar on Hydraulic Transport in the Mining Industry

7 April 2003, Indaba Conference Centre

How to Reduce Pipe Friction in Slurry Flows

Dr Nigel Heywood

Aspen Technology, Harwell, UK

Abstract

This paper describes three main methods for reducing the frictional pressure loss, andtherefore the pump discharge pressure requirement, when transporting viscous, often non-

Newtonian, slurries and pastes in pipelines. The first method reduces the degree of

flocculation of the particles in the slurry using suitable chemical additives, and thereby

reduces the slurry viscosity. The second method makes use of boundary liquid (such as water,

oil, or polymer solution) which is injected at comparatively small flowrates into the pipe

downstream from the pump to form a lubricating annulus adjacent to the pipe wall. The third

method involves gas injection into the pipe downstream from the pump to form a slug flow

pattern which results in substantial frictional pressure loss reductions for shear-thinning, non-

Newtonian slurry, initially flowing in the laminar flow regime prior to gas injection.

1. Introduction

Many new developments have taken place over the last few years in slurry handling practice

(Heywood, 1999), and particularly in slurry pipeline technologies (Heywood & Alderman,

2003). One example of this is the increased tendency for slurries and pastes containing very

fine particles to be pumped at increasingly high concentrations. These slurries tend to be

pseudo-homogeneous and do not segregate readily under gravity forces. They are often highly

viscous and exhibit highly shear-thinning, non-Newtonian flow properties. Figure 1 indicates

the typical consistency of these slurry types.

Figure 1: Viscous, Fine Particle Slurry Discharging from a Pipe


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In addition, it is also possible to pump relatively low moisture, unsaturated (compressible)

filter and centrifuge cakes (Figure 2) using both reciprocating positive displacement pumps

and some classes of rotary pump using bridge breakers and single or double intermeshing,

contra-rotating augers at the base of feed hoppers.

Figure 2: Unsaturated Sewage Sludge from a Compression Cake Filter

In order to pump both classes of slurry (either saturated paste or unsaturated cake) high pump

discharge pressures are frequently required. These discharge pressures result in:

High pumping power consumption

High pump differential pressures, leading to pump slippage and higher wear rates in some

pump types

More expensive, thicker-walled pipe

Less flexibility to pump slurry at higher, more viscous solids concentrations

There are various techniques to reduce the frictional pressure loss in pipe flow, which is oftenthe major contribution to the pump discharge pressure. This paper describes three of these

methods:

1. Reduction in the level of particle flocculation through the use of additives to reduce the

zeta potential on particle surfaces;

2. Injection of water or some other liquid (such as oil or aqueous polymer solution) into the

pump discharge pipe using an appropriate injection ring, thus creating a lubricating

annulus in the pipe for the viscous slurry;

3. Injection of air (or inert gas) into the pump discharge pipe to reduce the proportion of theinner pipe wall wetted by viscous slurry.


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2. Use of Addi tives to Disperse/Deflocculate Fine Slurry partic les

Adding soluble ionic compounds to flocculated slurries (i.e. slurries where a significant

proportion, say 5 to 10%, of solids are below approximately 5 m) can result in substantial

pressure drop reductions in pipe flow in the laminar flow regime. However, the effect is

usually not discernible in the turbulent regime. The ionic compounds disperse the particles,

thereby breaking up the flocculated particle network that gives rise to higher shear stresses inpipe flow. In order to disperse particles, the charge on their surfaces needs to be of the same

sign, and the higher the charge the greater the repulsive forces between particles.

It is only relatively recently that the reduction in pressure drop has been related to the degree

of deflocculation in quantitative terms. Several studies (Horsley et al, 1980 & 1982a, b) have

been undertaken in which the zeta potential on particle surfaces, defined as the potential in the

electrochemical double layer at the interface between a particle moving in an electric field and

the surrounding liquid, has been measured and correlated with the pressure drop reduction.

The zeta potential is a measure of the repulsive forces acting on the particles and thus is

useful in measuring the dispersing effects of chemical additives.

Figure 3 shows some typical head loss-slurry velocity data (Horsley & Reizes, 1980) for the

flow of a 43% by volume sand slurry (37% by weight of particles less than 10 m) at different

pH levels in the range 6.2 to 10.8; the pH was controlled by the addition of nitric acid and

sodium hydroxide. The changes in pressure drop in the laminar regime were observed to be

greater as the sand concentration was raised.

Figure 3: Effect of pH and Zeta Potential on 43% Volume Concentration Sand Slurry

(d50=17 m) in Pipe Flow (Horsley & Reizes, 1980)


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Figure 4 shows a plot of head loss against zeta potential. This illustrates that the higher the

zeta potential (i.e. the larger the negative value) the lower the pressure drop.

Figure 4: Effect of Zeta Potential on Head Loss Gradient for 33% and 43% Volume

Concentrations of Sand Slurry (Horsley & Reizes, 1980)

The effects of chemical additives have been studied (Sikorski et al, 1982)) on a number of

mineral slurries including drilling muds (thinners such as sodium acid pyrophosphate and

sodium hexametaphosphate), phosphate rock slurries (caustic soda), limestone cement feed

(combination of sodium tripolyphosphate and sodium carbonate), and coal slurry (sodium

tripolyphosphate, sodium dioctyl sulphosuccinate and sodium carbonate).

Various alkaline additives have been used to modify the flow properties of coal-water slurries

(Shook & Nurkoski, 1977 & Teckchandani & Shook, 1982). Several thinners have been

investigated (McInnes, 2002) at different concentrations to reduce the viscosity of chalk

slurry, so facilitating long distance pumping, and additives have been used to reduce pumping

energy requirements for flyash waste slurries from power stations (Thomas & Sobota, 2002).

The level of dispersion of a flocculated slurry, and therefore indirectly its viscosity level, can

now be assessed on-line through zeta potential using commercially-available instrumentation.

Off-line zeta potential measurements have been possible for some time. With the on-line

instrument, an electroacoustic sensor (Hunter, 1998 & 2001) applies high-frequency

alternating voltage pulses across the slurry. This causes the slurry particles to oscillate at avelocity that depends on their size and electric charge (or zeta potential). This particle motion

generates sound waves, which in turn depend on particle size and zeta potential. The input

voltage pulse is applied across two flat, parallel platinum electrodes that are in contact with

the slurry.

The slurry flows vertically upwards through a polyphenylenesulphide (PPS) spacer in the

sensor. One of the platinum electrodes is coated onto a rectangular glass delay line, at the

opposite end of which is mounted a thin ultrasonic transducer. The gated amplifier produces a

pulse of high frequency alternating voltage of about 3 s duration, which is applied across the

parallel electrodes. The resulting high frequency sound wave produced by the motion of the

slurry particles in the alternating electric field travels down the glass delay line and is detectedby the ultrasonic transducer.


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The measured electrical signal, called the ESA, contains information about the particle size

and charge, and is the primary measurement made by the electroacoustic sensor. The ESA

measurement is repeated a number of times at a range of frequencies from 1 MHz to 20 MHz.

The Fourier Transform of these signals is referred to as the ESA frequency spectrum. From

this spectrum, the particle size distribution and the zeta potential are ultimately calculated.

3. Injecting Boundary Layer Liquid into Pipe

Very highly concentrated slurries can now be pumped using either progressive cavity or

reciprocating piston pumps (Zey, 1999) by force-feeding the slurry into the pump body using

various auger designs and bridge breakers at the base of wedge-shaped hoppers. Even

unsaturated solid/liquid/air mixtures such as compressed filter or centrifuge cakes can be

pumped in this way. To reduce the frictional pressure loss in the pump discharge pipe, it is

now common for many pump manufacturers to offer boundary layer injection facilities

whereby a liquid is injected at three or four points through the pipe wall to generate an

annulus that helps to lubricate the flow. The liquid injected is often water but greater

reductions in frictional pressure loss can be achieved using aqueous polymer solution, waste

or heating oil or polyelectrolytes. Because the central slurry core is so concentrated andviscous, mixing between the annular wall layer of injection liquid and the slurry can be

minimal so the friction reduction effect is maintained over a significant pipe length.

Figure 5 shows a liquid injection arrangement supplied by Schwing America for their

reciprocating piston pumps, and Figure 6 shows how a lubricating annulus is formed in the

pipe.

Figure 5: The Boundary Layer Injection Method used by Schwing Pumps


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Figure 6 : Details of the Schwing Pumps Boundary Layer Injection Method

Figure 7 shows the extent of the reduction in pump discharge pressure for a progressive cavitypump which is pumping sewage sludge at various solids concentrations, and using water at

two different concentrations of polymer solution. It can be seen that a 0.5% polymer solution

injected at 20 litre/h into the pipe carrying sludge at 2 m3/h (1:100 flowrate ratio) gives rise to

the greatest reduction in discharge pressure from 13 bar down to about 1.8 bar. Other

conditions also give substantial reductions in discharge pressure.

Figure 7: Reductions in Pump Discharge Pressure for Sewage Sludge using Alternative

Boundary Layer Liquids (Courtesy, Seepex Pumps)

The nature of the boundary layer fluid appears to have a significant effect on the pressure drop

reduction on the pipe wall. Various liquids are used by Putzmeister Pumps depending on the

application (Zey, 1999). Table 1 from Putzmeister shows the various liquids they use.


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Table 1 : Boundary Layer Fluids used by Putzmeister Pumps

Boundary layer fluid Advantages Disadvantages Pressure Reduction

Water Cheap Mixes with

slurry/sludge

20-50%

Heating oil/waste oil Greater cost

benefit when usedas combination

aid

Used only in

incineration plant

25-50%

Polyelectrolytes High efficiency Mixing station may

be required

50-75%

PMLC Extremely

efficient

Does not mix

with slurry

Mixing station

required

70-90%

Figure 8 shows the substantial effect that either water injection or other special lubricant canhave on the pressure gradient (Zey, 1999).

Figure 8: Putzmeister Pumps Data for Pressure Gradient Reduction

for the Flow of 37 to 47% Sewage Sludge


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4. Injecting Air Downstream From Pump

It has been known for several decades that injecting air (or another gas) into a shear-thinning

slurry in laminar pipe flow will result in a reduction in frictional pressure loss. No such effect

occurs for the laminar flow of a Newtonian slurry, or for the turbulent flow of a slurry having

any specific laminar flow property. Figure 9 shows some typical data; the drag ratio in this

figure is defined as the ratio of frictional pressure gradient along the pipe with gas injection tothat with no gas injection, at constant superficial slurry velocity, i.e., constant slurry flowrate.

Figure 9: Typical Reductions in Frictional Pressure Loss for the Horizontal Pipeflow of

Shear-thinning Slurry at Various Superfical Slurry Velocities (Farooqi et al, 1980)

The friction reduction effect occurs in both horizontal (Heywood & Richardson, 1978, and

Farooqi et al, 1980) and vertical (Heywood & Charles, 1980, and Khatib & Richardson, 1984)

slurry pipe flows, and there is the additional advantage of reduced static head in vertical pipe

flows. Figure 10 shows how the gas can distribute itself within the slurry in both pipe

orientations, giving rise to six identifiable flow patterns in horizontal pipe flow, and four in

vertical pipe flow. In horizontal pipeflow, frictional pressure loss reduction occurs for

practical purposes mainly in bubble and plug or slug flow (intermittent flow), but it can occur

in stratified flow at low slurry flowrates (Heywood & Charles, 1979).

For a given flowrate combination of gas and slurry in a pipe, it is possible to predict the

resultant flow pattern. This information is often presented in the form of a flow pattern map.An example of one developed for gas and non-Newtonian slurry flow in a horizontal pipe is

given in Figure 11.


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Figure 10: Flow Patterns for Horizontal and Vertical Gas/Slurry Pipe Flow

Figure 11: Flow Pattern Map for Gas/Slurry Flows in Horizontal Pipes

(Chhabra & Richardson, 1984)

Friction reduction is greater the more shear-thinning the slurry and a maximum effect occurs

when the combined gas and slurry pipe flows nears the laminar-turbulent transition for the

slurry flow. In this way, the maximum effect can be reliably predicted from a knowledge of

the slurry rheological properties, the pipe diameter and the gas and slurry flowates. Figure 12

shows the maximum reduction in frictional pressure drop as a function of the pipe Reynolds

number for slurry flow alone.


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There are many advantages to using air injection:

(1) reduced discharge pressure requirement for a slurry pump for a given slurry flowrate

though a given discharge pipe length;

(2) increased capacity of an existing pipeline carrying a given slurry while retaining the same

pump system;(3) extension of an existing pipe run while maintaining the same discharge pressure;

(4) application of an existing pump and pipeline combination to more viscous, shear-thinning

slurry, while maintaining the same discharge pressure.

(5) reduced pump differential pressure, and therefore reduced slippage in some pump types,

with a corresponding reduction in pump wear.

Surprisingly, the advantages of using air injection have still largely been overlooked by

industry, although a Polish sugar factory takes full advantage of the technology when

pumping waste molasses (Dziubinski & Fidos, 1992). Numerous examples of highly viscous,

shear-thinning slurries being pumped through pipework occur in industry, e.g., red mud waste

from alumina production, pulverised fuel ash slurry from power stations, titanium dioxideslurry, chalk and clay slurries, etc. Many of these operations could benefit from the

appropriate application of air injection.

5. Conclusions

This paper has described three alternative methods to reduce pipe friction for highly viscous,

often non-Newtonian slurries. In addition to frequent economic advantages to be had by

employing one of these methods, other benefits also accrue. These include lower wear rates

and therefore maintenance costs for pumps, and reduced instances of pipe blockages. These

methods also increase the flexibility of existing pipeline systems when there is a requirement

to increase slurry concentrations, and therefore slurry viscosities. It is recommended that each

of these three methods is considered whenever an existing pipeline needs upgrading, or a new

pipeline system is to be designed.

References

Chhabra, R.P. & Richardson, J.F. (1984) Prediction of flow pattern for the co-current flow of

gas and non-Newtonian liquid in horizontal Pipes, Can J Chem Engng, Vol 62, Page 449-454.

Dziubinski, M. & Fidos, H. (1992) An industrial installation for two-phase transportation of

carbonation mud,Zuckerindustrie, Vol. 117, No.8, Pages 631-633.

Farooqi, S.I. & Richardson, J.F. (1982) Horizontal flow of air and liquid (Newtonian and Non-

Newtonian) in a smooth pipe. 2. Average pressure drop Trans I Chem E, Vol 60, Pages 323-333.

Farooqi, S.I., Heywood, N.I & Richardson, J.F. (1980) Drag reduction by air injection for

highly shear-thinning suspensions in horizontal pipeflow, Trans I Chem E, Vol 58, Pages 16-27.

Heywood, N.I. (1999) Stop your slurries from stirring up trouble. Chemical Engineering

Progress, September, Pages 21-41.

Heywood N. I. & Richardson, J.F. (1978) Head loss reduction by gas injection for highly shear-thinning suspensions in horizontal pipe flow. In : Proc Hydrotransport 5, Paper C1. Organised

by BHRA Fluid Engineering, UK.


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Heywood, N. I. & Charles, M.E. (1979) The stratified flow of gas and non-Newtonian liquid

in horizontal pipes.Int J Multiphase Flow, Vol 5, Pages 341-352.

Heywood, N. I. & Charles, M E (1980) Effects of gas injection on the vertical pipeflow of

fine slurry. In: Proc of Hydrotransport 7, Paper F5. Organised by BHRA Fluid Engineering,

UK.

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Chemical Engineering Progress, April, Pages 100-107.

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concentrations by weight, with and without additives. In : Proc. of Hydrotransport 8. Paper

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