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An Experimental Investigation on Characteristics of a Slurry
Pump
F. NI, W. J. VLASBLOM, A. ZWARTBOL
ABSTRACT: The presence of very fine solid particles, such as
silt, clay, in a flow tends to
produce favourable effect on pump performance. However, there is
little experimentalevidence on performance of a slurry pump
transporting mixtures composed of two or more
sand fractions that differ in size. Effects of various sand
sizes and distributions on thecharacteristics of a slurry pump are
reported in this paper. Three sorts of narrowly gradedsands and
their mixtures were tested in the laboratory DN150mm pump loop of
the Delft
University of Technology, The Netherlands. The investigation
reveals that sand size strongly
affects the pump performance and adding a fraction of the fine
sand to a coarse-slurry cansignificantly improve the pump
performance.
1. EXPERIMENTS
From September 1998 to March 1999 laboratory experiments were
carried out. The pump,driven by a 164 kW MAN diesel engine, has a
centrifugal impeller of 400mm diameter,100mm width at discharge and
3 logarithmic vanes. A data acquisition system supported byLabview
provides on-line observation of all measured quantities in volts
and records the data
on a computer disk. The data are collected at the frequency of 1
Hz for 30 seconds. Three
sorts of narrowly graded sands with specific density of 2.65
were used: Fine sand (0.10-0.15mm, d50 =0.123mm), Medium sand
(0.2-0.5mm, d50=0.372mm) and Coarse sand (1.4-
2.0mm, d50=1.840mm). Pump tests on the narrowly graded sands
were carried out within tworanges of delivered volumetric
concentration: Cvd =12-35% for the fine sand, and Cvd =6-42% for
the medium and coarse sands. Eight sand mixtures were investigated.
The total
concentration of the mixtures ranged from Cvd =25% to 45%. The
experiments were
executed at 800 rpm, 1000 rpm and 1200 rpm for each
concentration. Pump performancecurves were measured for clear water
before and after each sand or mixture test to investigatethe effect
of wear. This is taken into account in data processing.
2. EFFECT OF SAND SIZE ON PUMP PERFORMANCE
A dimensionless plot is a useful presentation of pump
characteristics and is used in this paper.
Dimensionless head H*is defined as H
*= gH/(
2D
2) and dimensionless flow rate as Q
*=
Q/( *D3), where g denotes the gravity acceleration in m/s
2, H either the head developed in
slurry service Hm, measured in height of slurry, or the head
developed in water service Hw,measured in height of water, Q flow
rate in m
3/s, pump speed in radians per second, and D
impeller diameter in meter. Fig.1 clearly indicates that sand
size has a strong influence on theperformance of the slurry pump.
Reductions in head and efficiency increase with sand size
and delivered volumetric concentration. The reductions are
rather small for the fine sand butvery significant if the medium or
coarse particles are pumped. Fig.1 also reveals that for all
the three sorts of sand the solids effect or the absolute drop
in head Hw-Hmor in efficiency
Ew-Emat the same concentration is approximately independent of
flow rate, and as a resultthe flow rate at the best efficiency
point remains unchanged.
F. NI, Prof., Dr., Faculty of Mechanical and Electrical
Engineering, Hohai University, Changzhou, 213022,Jiangsu, P.R.
China. Tel/Fax +86 519 5120010. E-mail [email protected](o),
[email protected](h)
W.J.VLASBLOM, Prof. ir. and A. ZWARTBOL, Ing, Chair of Dredging
Technology, Delft University of
Technology, The Netherlands. Tel +31 15 2783973. Fax +31 15
2781397.
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0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0 0.01 0.02
water 15/09
water 30/09
13-16% 23/09
23-25% 24/09
34-35% 28/09
H* (Dimensionless head)
(Dimensionless
flow rate)Q*
0
10
20
30
40
50
60
70
80
0 0.01 0.02
water 15/09
water 30/09
13-16% 23/09
23-25% 24/09
34-35% 28/09
E [%] (Efficienc
(Dimensionless
flow rate)Q*
(a) Fine sand, d50=0.123 mm
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0 0.01 0.02
water 30/09
water 09/10
12-14% 02/10
25-27% 06/10
34-35% 07/10
44-45% 09/10
H* (Dimensionless head)
(Dimensionless
flow rate)Q*
0
10
20
30
40
50
60
70
80
0 0.01 0.02
water 30/09
water 09/10
12-14% 02/10
25-27% 06/10
34-35% 07/10
44-45% 09/10
E [%] (Efficienc
(Dimensionless
flow rate)Q*
(b) Medium sand, d50=0.372 mm
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0 0.01 0.02
water 09/03
water 11/03
6-7% 10/03
17-19% 10/03
30% 10/03
42% 10/03
H* (Dimensionless head)
(Dimensionless
flow rate)Q*
0
10
20
30
40
50
60
70
80
0 0.01 0.02
water 09/03
water 11/03
6-7% 10/03
17-19% 10/03
30% 10/03
42% 10/03
E [%] (Efficiency)
(Dimensionless
flow rate)Q*
(c) Coarse sand, d50=1.840 mm
Fig.1 Dimensionless head and efficiency as functions of
dimensionless flow rate.
A quantitative analysis can be obtained from Table 1, where the
head reduction factor Rhis
defined as Rh= (Hw- Hm)/Hw and the efficiency reduction factor
as Re= (Ew- Em)/Ew. For thefine and medium sands Re Rhholds at
concentrations up to 35%. This is different from thegeneral
conclusion stated in Wilson (1997) in which a concentration of
about 20% isconsidered as the turning-point concentration, above
which the efficiency reduction factor Reis more pronounced than the
head reduction factor Rh (Sellgren and Vappling, 1986). Withinmost
of the range of concentrations of coarse sand tested, the pump
efficiency drops muchfaster than the head as the concentration
increases. At concentration of 42% the efficiency
reduction factor and the head reduction factor reach 59% and
40%, respectively.
Table 1. Average performance reduction factors and power
ratioFine sand (d50=0.123 mm) Medium sand (d 50=0.372 mm) Coarse
sand (d 50=1.840 mm)
Cvd(%)
Rh(%)
Re(%)
Pr Cvd(%)
Rh(%)
Re(%)
Pr Cvd(%)
Rh(%)
Re(%)
Pr
13-16 1.7 4.0 1.03 12-14 5.6 4.7 0.99 6-7 6.8 6.8 1.00
23-25 4.3 4.0 1.00 25-27 10.1 9.3 0.99 17-19 19.0 22.3 1.04
34-35 6.8 6.8 1.00 34-35 12.7 12.0 0.99 30 28.3 40.7 1.21
44-45 20.7 23.4 1.03 42 39.8 58.6 1.45
Dimensionless power P*as a function of dimensionless flow rate
Q
*is plotted on Fig.2. The
dimensionless power is defined as P*
= P/( *3 *D
5), where P denotes the power input to
the pump in kW, the density of water or slurry in kg/m3, and D
the same parameters as
defined in the above section. Fig.2 shows that for either the
medium sand or the fine sand
only one curve results for water and all slurry densities,
except that data points for
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concentration of 44-45% of the medium sand are located a little
higher than the curve. Thismeans that the power ratio, defined as
Pr = Pm/(Sm*Pw), where Pm and Pw are powerrequirements for slurry
service and water service, respectively, and Sm the slurry
relative
density, remains unity (see Table 1). In other words, the power
consumption at the same flow
rate increases directly with the relative density of the slurry.
For the coarse sand, however, Pr
> 1 holds at moderate and high concentrations. At 42% (Sm
=1.7), the average powerrequirement Pmfor slurry service of the
coarse sand is 1.45 times larger than (Sm*Pw) and 2.5
times larger than Pwthe power requirement for clear water (Table
1).
5.00E-07
1.50E-06
2.50E-06
3.50E-06
0 0.01 0.02
water 15/09
water 30/09
13-16% 23/09
23-25% 24/09
34-35% 28/09
P* (Dimensionless power)
(Dimensionless
flow rate)Q*
5.00E-07
1.50E-06
2.50E-06
3.50E-06
0 0.01 0.02
water 30/09
water 09/10
12-14% 02/10
25-27% 06/10
34-35% 07/10
44-45% 09/10
P* (Dimensionless power)
(Dimensionless
flow rate)Q*
5.00E-07
1.50E-06
2.50E-06
3.50E-06
0 0.01 0.02
water 09/03
water 11/03
6-7% 10/03
17-19% 10/03
30% 10/03
42% 10/03
P* (Dimensionless power)
(Dimensionless
flow rate)Q*
(a) Fine sand (b) Medium sand (c) Coarse sand
Fig.2 Dimensionless power as a function of dimensionless flow
rate.
3. EFFECT OF PARTICLE SIZE DISTRIBUTION ON PUMP PERFORMANCE
By adding the fine sand into the medium sand or the coarse sand,
we tested eight mixtures ofthem to find the influence of particle
size distribution on the pump performance. Fig.3(a)
shows the result of one mixture of the medium sand with the fine
sand. By adding 10% of thefine sand into 25% of the medium sand, we
get the round black points for the mixture.
Although the total concentration of the mixture is higher than
the medium sand only, the
efficiency reduction for mixture service is smaller than for the
medium sand only. A directimprovement of about 3.0% on the
efficiency reduction was obtained. Fig.3(b) shows the
result of one mixture of the coarse sand with the fine sand. A
considerable improvement onpump performance was observed by adding
10% of the fine sand into 25% of the coarse sand.The direct
improvement reaches more than 20%! From the figure one can conclude
two
points: (1)the fine sand can improve the pump performance,
though it is not very finecompared with particles of silt or clay,
and (2)the degree of the influence of adding the fine
sand into the medium sand is different from that of adding the
fine sand into the coarse sand.The fine sand addition to the coarse
sand can improve the pump performance significantly.
0%
5%
10%
15%
20%
0 10 20 30 40 50
24-25%
Medium sand
13-16% Fine
sand
25% Medium
sand + 10%
Fine sand
Re (Efficiency reduction factor)
Cvd [%]
0%
10%
20%
30%
40%
0 10 20 30 40 50
21-25%
Coarse sand
13-16% Fine
sand
25% Coarse
sand + 10%
Fine sand
Re (Efficiency reduction factor)
Cvd [%]
(a) 25% Medium sand plus 10% Fine sand (b) 25% Coarse sand plus
10% Fine sand
Fig.3 Efficiency reduction factor for the mixtures.
Fig.4 presents the flow behaviour of the medium sand and the
coarse sand, respectively, in
the horizontal DN150 mm pipe. The hydraulic gradient Im is
defined as head loss (m water)
over 1 m length of pipe, and Vm the mean slurry velocity in the
horizontal pipe. The figure
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flow stratification under changing velocity Vm while the
medium-sand slurry behavesregularly with less stratification.
Although flow velocities in a pipeline are much smaller
thanvelocities in a pump, it is believed that slurry flow through
pump passages between vanes
may experience similar stratification due to impeller rotation
instead of gravitational
sedimentation in a horizontal pipe. Earlier results have shown
that the effects of
concentration, particle size and fine particle content on
characteristics of a slurry pump havegreat similarities to those on
flow behaviour in a pipeline (Wilson 1997). Adding the fine
sand to a coarse-slurry may increase the capability of the
carrier liquid, formed by water and
the fine sand, to carry coarse particles. As a result, the flow
stratification and the coarse-particle trajectory deviation from
the liquid streamline due to inertia in a pump decrease.
Since the coarse sand has larger inertia than the medium sand
and exhibits stronger flowstratification, the favourable effect due
to the fine sand on head reduction factor andefficiency factor
becomes more significant.
0
0.1
0.2
0.3
0.4
0 1 2 3 4 5 6 7 8 9
water
11-13%
25-26%
Vm [m/s]
Im (Hydraulic gradient)
0
0.1
0.2
0.3
0.4
0 1 2 3 4 5 6 7 8 9
water
13-16%
23-24%
Vm [m/s]
Im (Hydraulic gradient)
(a) Medium sand, d50=0.372 mm (b) Coarse sand, d50=1.840 mm
Fig.4 Hydraulic gradient versus mean slurry velocity in the
horizontal 150 mm pipe
4. CONCLUSIONS
1.
Sand size has a strong influence on characteristics of the
slurry pump. For the fine andmedium sand the head reduction equals
to the efficiency reduction until Cvd=35%. Thisis different from
the existing conclusion. While for the coarse sand, the
turning-pointconcentration is only 15%, above which the efficiency
drops faster than the head.
2. Particle size distribution is very important in determining
slurry pump characteristics. Amixture composed of broader grading
solids exhibits smaller resistance in a slurry pump.
The fine sand can lessen the flow stratification and thus
improve the performance.3. A significant improvement on pump
performance was observed by adding the fine sand
to the coarse sand. This is because the coarse sand slurry
exhibits stronger stratification.
ACKNOWLEDGEMENT
The study is supported by the MOE Supporting Program for
National University KeyTeachers and the MOE Visiting Scholars
Research Fund (P.R. China).
REFERENCES
1. Sellgren, A. and Vappling, L. (1986). Effects of highly
concentrated slurries on the performance ofcentrifugal pumps.
Proceedings International Symposium on Slurry Flows, FED Vol. 38,
ASME,
USA, pp.143-148.2.
Wilson, K.C., Addie, G.R. Sellgren, A. and Clift, R. (1997).
Slurry Transport Using Centrifugal Pumps,
Blackie Academic and Professional.
3.
Ni, F. and Matousek, V. (1999). Flow of aqueous mixture of sand
composed of fractions of different
