Fluid Structure Interaction and Moving Boundary Problems
81
Experimental investigation of the effect of
solid-mixture on the cavitation characteristic
of a centrifugal pump
A. Ladouani1 & A. Nemdili2
1
Department of Hydraulics, Laboratory LRTTFC,
Technical University of Oran, Oran, Algeria
2
Department of Marine Engineering,
Laboratory LRTTFC, Technical University of Oran, Oran, Algeria
Abstract
The purpose of this work is to study the effect of solid-mixture on the cavitation
characteristic of a centrifugal pump. However, pump tests under varied
volumetric concentrations of sandy clay are reported. The experimental results
show that:
- at low flow rates, NPSH curves are independent of the silt concentration,
- at high flow rates, NPSH curves of silt mixtures appear to be higher than
those of water,
- the both required and available NPSH curves show that cavitation flow rates
decrease with increasing concentration, indicating that cavitation accelerates
with the concentration.
Finally, based on the silt studied, it can be concluded that the cavitation NPSH
curve at high flow rates is not sensitive to gravitational forces, but only to the
viscous flows.
Keywords: centrifugal pump, solid-mixture, cavitation, NPSH curve, silt
concentration.
1
Introduction
In Algeria, silting of reservoirs has become a major drawback for all existing
dams, because it causes very high losses in capacity. Such losses are estimated at
20 million cubic meters per year for some dams (Errih [1]). Hydraulic dredging
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82 Fluid Structure Interaction and Moving Boundary Problems
of these reservoirs requires the use of efficient pumping systems in order to
increase the concentration of the silt dredged with minimum energy spending.
The use of centrifugal pumps in these systems poses a major difficulty as the
pump suction capacity is limited when the silt concentrations are high. This limit
which is expressed by the cavitation flow rate leads the user to apply the pump to
areas of flow rates. This is not economical and therefore studies must be carried
out in order to find data on the influence of concentration on NPSH curves. Since
silts are generally viscous, it is important to introduce the equation for the
suction characteristics of centrifugal pumps. In this study an experimental test rig
is used in order to generate data on the effect of silt concentration on NPSH
curves.
2
Characteristics of the silt
The silt samples used in this study are taken from the draining outlet at the dam
base. Table 1 shows its geotechnical characteristics. It is clay composed on very
thin particles and containing approximately 15% of fine sand.
Table 1: Geotechnical characteristics of the silt.
γa
(g/cm3)
2,67
W1
(%)
61
Wp
(%)
31,16
dso (µm)
dso (µm)
natural deflocculated
6
2
Also, note that samples taken closer to the dam wall are finer and more viscous
(Korso [2]). The silt has a Newtonian behaviour for a value of concentration
limit of up 7,2%. The shear stress is directly proportional to the shear strain i.e. :
τ = ηc ⋅ G
(1)
At higher values of this concentration limit, the silt has a non-Newtonian
behaviour and is of Bingham type. A shearing threshold is noted for each
concentration:
τ = τ o + ηc ⋅ G
(2)
Rheological measurements of the silt were produced using a rotative viscometer
for silt concentration values higher than the concentration limit.
Results are shown in Figure 1. It may be noted that for a fixed value of shear
strain, both the shear stress τ and the viscosity η increase with increasing
concentration.
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Table 2 shows measured values of dynamic viscosity and shear strain for the
four values of silt concentration.
20
18
Shear stress (Pa)
16
Cv = 17,5%
Cv = 15,0%
Cv = 12,0%
Cv = 10,0%
14
12
10
8
6
4
2
0
0
20
40
60
80
100
120
140
160
180
200
-1
Shear rate(s )
Figure 1: Shear stress versus shear strain.
Table 2: Rheological parameters of the silt.
N
1
2
3
4
3
Cv (%)
10
12,5
15
17,5
τo (Pa)
ηo (mPa.s)
0,7
3,6
6,0
13,4
0,04
0,0105
0,0189
0,0300
Test rig
Figure 2 shows the experimental rig used. The silt mixture is drawn up from the
mixing tank (1) by the centrifugal pump (11) via the suction pipe (3).
The mixture discharges into the channel (9) via the exit pipe (6). Valves (4)
and (7) are used to control the flow rate which is measured using the calibrated
tank (2). The mercury vacuumeter (5) and the mercury manometer (8) are used to
measure inlet and exit pressure of the pump respectively. A jet tranquilizer (10)
fixed at the bottom of the channel (9) prevents air intake into the mixing tank and
eventually into the pump.
4
NPSH measurement procedure
In order to draw the required NPSH curve, the available NPSH of the pumping
system is measured at first. In order to get the cavitation point, a pressure drop is
created at the suction pipe using the valve (4). The flow rate is then slowly
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84 Fluid Structure Interaction and Moving Boundary Problems
increased using the exit valve (7) until the following two conditions are observed
simultaneously:
9
7
2
10
6
8
5
1
1 Mixing tank
2 Calibrated tank
3 Suction conduite
4 Valve
4
11
3
5 Mercury vacuumeter
6 Exit pipe
7 Valve
8 Mercury manometer
9 Chanel
10 Jet tranquilizer
11 Centrifugal pump
Figure 2: Schematic diagram of the test rig.
a) stabilizing of the mercury level inside the vacuumeter. This corresponds to
the lowest pressure level at the pump entry.
b) a fall of the mercury level inside the manometer. This corresponds to the
start of the decrease of the pump manometric head. This decrease is usually
limited to 3% in industrial tests.
At these conditions, flow rate and mercury level readings are taken. Values of
entry geometrical head Ha and suction pressure loss Ja, may be calculated and
substituted in eqn. (3) below:
NPSH avail . = NPSH req . = 10 − ( Ha + Ja )
5
(3)
NPSH results
Figures 3 and 4 show NPSH results for two rotational speeds of 1400 and 1800
rpm respectively and for different silt concentration. Results for water are shown
for comparison. It may be noted that for low flow rates below 11 l/s, NPSH is
independent of silt concentration. For the speed of 1800 rpm and for flow rates
values above 11 l/s, NPSH increases with the increase of silt concentration.
This result was not observed for the lower speed due to the limitation of the
test rig.
Further tests were carried out with a silt concentration of 17,5%. It was not
noted that at this silt concentration, silt became very viscous. Measurements
showed that NPSH increases slightly and the manometric head decreases. These
results are not shown here since measurements could not be done accurately. It
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85
was deduced that the centrifugal pump used could not work accurately at
concentrations higher than a value between 15% and 17%.
10
: Water
: Cv = 5% and γm = 1,08
: Cv= 10% and γm = 1,16
: Cv= 15% and γm = 1,23
at N = 1400 Rev/min
9
8
NPSH (m)
7
6
5
4
3
2
1
0
0
5
10
15
20
25
Flow (l/s)
Figure 3: Effect of silt concentration on NPSH for a pump speed of 1400 rpm.
10
Water
Cv = 5%
Cv = 10%
Cv = 15%
at N = 1800 Rev/min
9
8
NPSH (m)
7
6
5
4
3
2
1
0
0
2
4
6
8
10
12
14
16
18
20
22
24
Flow (l/s)
Figure 4: Effect of silt concentration on NPSH for a pump speed of 1800 rpm.
Effect of silt concentration on NPSH is caused by the increase of the shear
stresses between the silt and the rotor at higher concentration and higher shear
strain. Shear stresses cause further pressure loss between the pump entry point
and the cavitation point supposed between the rotor blades. The appearance of
increases of NPSH for higher flow rates indicates that concentration effects are
due to viscous forces only but not to gravitational forces.
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86 Fluid Structure Interaction and Moving Boundary Problems
5.1 Effect of silt concentration on cavitation flow rate
Cavitation flow rate in a pumping system is determined by taking the
intersection between the pressure loss characteristic of the suction pipe and the
required NPSH curve. It was shown in Khaldi [3] that pressure loss in a pipe
decreases with increasing silt concentration. Hence the available NPSH also
decreases with increasing silt concentration.
Figure 5 shows the effect of silt concentration on the cavitation flow rate, Qc,
for two values of Cv equal to 0% and 15%. It is to note that Qc decreases due to
the increase of the required and the available NPSH.
10
Water
Cv = 15%
9
Water
NPSHavailable
8
NPSH (m)
7
6
Cv = 15%
5
4
3
2
1
0
0
5
10
Qc2 15
Qc1 20
25
Flow (l/s)
Figure 5: Effect of silt concentration on the cavitation flow rate Qc.
25
Head (m.c.mixture)
20
15
10
: Water
: Cv = 15%
at N = 1800 Rev/min
5
0
0
5
10
15
20
25
Flow (l/s)
Figure 6: Effect of silt concentration on the pump characteristic.
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87
5.2 Effect of silt concentration on the pump characteristic
Figure 6 shows the effect of silt concentration on the pump characteristic for
values of Cv equal to 0% and 15%. It is to note that for a value of flow rate, the
pump manometric head decreases with the increase of the concentration.
This result was shown in Ladouani [4] for silts with very fine solid particles of
variable concentration. These decreases in the pump characteristic may be
attributed to the higher shear stresses between the rotor blades and the fluid,
which cause further pressure loss in the flow and therefore cause the drop of the
manometric head.
6
Conclusions
Cavitation in centrifugal pumps discharging silt is accelerated due to increase of
silt concentration and flow rate leading to lower cavitation flow rates. This is
attributed to the higher shear stresses between the fluid and the rotor blades.
Cavitation and pump characteristics are related to the rheological behaviour of
the fluid. Further tests are required in order to establish correlations for different
fluids by varying fluid viscosity, pump dimensions and rotational speed.
References
[1] Errih, M., Problem of siltation of small reservoirs in Algeria, Proceedings of
the 7th Int. Conference on Transport and Sedimentation of Solid Particles,
Poland, 1992.
[2] Korso, K., Contribution à l’analyse du processus de la sédimentation dans
les barrages, Thèse de Magister, USTO, Algeria, 1986.
[3] Khaldi, A., Drag reduction in hydraulic transport of silt, Proceedings of the
7th Int. Conference on Transport and Sedimentation of Solid Particles,
Poland, 1992.
[4] Ladouani, A., Etude de la cavitation des pompes centrifuges débitant des
mixtures solides-liquides, Thèse de Magister, USTO, Algeria, 1986.
[5] Ladouani, A. & Nemdili, A., Experimental study of centrifugal pump when
handling industrial oils, Proceedings of the XXIst IAHR Symposium Hydraulic Machinery and Systems, September 9–12, Lausanne, Switzerland,
Volume I, ISBN 3-85545-865-0, pp. 183–190, 2002.
[6] Ladouani, A. & Nemdili, A., Experimental study of effects of polymer
additives on the performance of a centrifugal pump, Proceedings of the 6th
International Conference on Hydraulic Machinery and Hydrodynamics,
Timisoara, Romania, October 21–22, 2004.
[7] Ladouani, A. & Nemdili, A., Optimal method of selecting pumping systems
for viscous fluids, Proceedings of the 6th International Conference on
Hydraulic Machinery and Hydrodynamics, Timisoara, Romania, October
21–22, 2004.
[8] Ladouani, A. & Nemdili, A., Synthesis study on the silt pumping of dams,
Proceedings of the 13th International Seminar on Hydropower Plants, 24–26
November, Vienna, Austria, 2004.
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