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SET2008 th Internal Conference on Sustainable Energy Technologies; Seoul, Korea.
24-27 August, 2008
Effect of Water Temperature on Centrifugal Pumps
Performance under Cavitating and non-cavitating
Conditions
Ahmed A. S. Al-Arabi
Higher Institute of Engineering, Hoon Libya, bohmaid2000@yahoo.com
ABSTRACT: The effect of water temperature on performance and cavitation inception of a
centrifugal pump has been studied experimentally. A special test rig with a testing centrifugal
pump was constructed in the laboratory of fluid mechanics at Higher Institute of Engineering -
Hoon. The rig was designed so that the flow rate ratio, suction pressure, rotational speed and
water temperature could be varied independently.
The temperature and speed were varied from 15°C to 60°C, and from 1800 rpm to 2800
rpm respectively, while the ratio of flow rate to optimum flow rate was varied from 0.245
lit/sec to 0.767 lit/sec. The results showed that the pump head and pump efficiency increase
with the decrease of water temperature. The results showed that increasing water temperature
speeds up cavitation. The inception net positive suction head (NPSHi) was found to increase
with the increase of temperature up to a maximum value and then decreased again.
NOMENCLATURE
Patm The atmospheric pressure
Pv The vapour pressure at corresponding
temperature.
Pss The suction static pressure.
The specific weight of water
NPSH Net positive suction head
NPSHi Inception net positive suction head
1. INTRODUCTION
When operating centrifugal pumps at
temperatures higher than the ambient
temperature, especially in industrial and
pumps used for out door applications that
exposed to atmospheric temperature in the
hot climate zones. Therefore, more care
must be taken to avoid any troubles that may
occur due to higher fluid temperature. High-
temperature applications are becoming more
prevalent in the fluid handling industry .
Cavitation phenomenon in centrifugal
pumps is a basic problem for its effect of
head breakdown, increase in consumed
energy, erosion in pump impellers and
vibration. Pump cavitation is defined as the
formation of cavities on the surface of the
blade of pump impeller and the resulting
loss of contact between the impeller and the
water being pumped. It is believed, that the
water temperature plays a major role in the
cavitation inception, and pump performance.
Rudnev et.al., have studied the
effect of water temperature on cavitation
characteristics. The temperature range was
-C. The method is based on scaling
the change in cavitation characteristics as a
function of water temperature. The work
was carried out to solve out the problems
caused by cavitation on the pumps of
boiling water reactors. They had established
a correlation between vapor liquid ratio and
SET2008 th Internal Conference on Sustainable Energy Technologies; Seoul, Korea.
24-27 August, 2008
the temperature on logarithmic scale.
The boiling of liquid in the process of
cavitation is a thermal process and is
dependent on the liquid properties such as:
pressure, temperature, latent heat of
vaporization, viscosity and specific heat.
During cavitation conditions drop in the
pump head and efficiency is caused by the
appearance of vapor cavities in the lower
pressure zone that disrupt the dynamic
conditions during normal pump operation
when the flow is all liquid
Zika (1984)had studied the effects of
thermodynamic properties of incipient
cavitation (3% head drop) in centrifugal
pumps. The temperature range was varied
from 21oC to 148oC. He concluded that the
relationship between NPSH depression and
vapor pressure was a linear relationship. He
established a general relationship between
NPSH depression and latent heat for some
fluids. El-kadihad studied the
effect of hot water on the cavitating
centrifugal pumps. The temperature was
varied from 28ºC tooC, in order to obtain
the effect of water temperature on cavitation
inception and breakdown in centrifugal
pumps. He concluded that increasing the
water temperature speeds up to the
cavitation occurrence, and the maximum
and minimum values of Thoma cavitation
number are affected strongly by temperature
. Zika (1984) had studied the influence
of thermodynamics effect and their
correlation with the minimum NPSH
required for a cavitation free performance
of centrifugal pumps. The results were
found for different liquids and different
pumps. Zika has found that, using the NPSH
difference, the relation between NPSH
difference and vapor pressure is a straight
line, and small deviation was found due to
changing the pumps. Al-Arabi A. A. B.
and Selim S. M. A., (2007) built a
theoretical model to predict cavitation
inception in centrifugal pumps The model
includes the physical fluid parameters and
the real working phenomena at off-design
condition. The parameters considered in the
model were flow rate ratio, pump rational
speed, water temperature, thermodynamic
properties of water, nuclei and gas content,
relative velocity and incidence angle.
2. EXPERIMENTAL OPERATION
The general arrangement of the test rig is
indicated in a schematic diagram shown in
figure 1. The flow system consists of 1 hp
centrifugal pump using DC current and
maximum rotational speed of 3000 rpm, the
flow orifice meter, pressure measuring
devices, suction and delivery pipelines,
speed control unit, and valves. The
temperature has been measured by a
thermometer, ranging from -50ºC to 110ºC
with an error of ± 1ºC. The temperature has
been checked continuously at different
points in the system such as at the tank
suction point, at the pump inlet, and at the
end of the delivery line, in order to avoid
any deviation in temperature measurements.
The maximum allowed deviation in
temperature was ± 2°C. The accuracy of
suction and delivery pressure gauges was ±
0.02 bar. The heat was supplied by two
electrical heaters of 1000 Watt each.
3. EXPERIMENTAL PROCEDURE
After assembling the system and
connecting all the measuring devices, the
driving pump is operated via the speed
inverter, at three different operating speeds
1500, 2300, and 2800 rpm. The temperature
had been changed from 15
pump performance test and cavitation test
had been carried out at each temperature,
had been carried out at each temperature,
and at different values of flow rate ratios.
SET2008 th Internal Conference on Sustainable Energy Technologies; Seoul, Korea.
24-27 August, 2008
Figure 1 Construction of test rig
The cavitation test on the pump has been
carried out by keeping the pump running at
the required speed, temperature, and flow
rate ratio, and then reducing the inlet
pressure step by step until the inception
condition occurred. At each step, the flow
rate was adjusted through the delivery valve,
then the inlet pressure further reduced until
developed cavitation and fall off head and
efficiency was noticed. At each setting of
inlet pressure and inception condition, the
measurements of suction and discharge
pressures, flow rate, and input power were
recorded.
The NPSH at each condition was calculated
using the following equation:
ssvatm PPP
NPSH
4. RESULTS AND DISCUSSION
The effect of water temperature on
the pump head and NPSHi was studied
experimentally at different flow rates and
different rotational speeds. Figures (2
show the relation between the pump head
and NPSH at different water temperatures,
while Figures 6 and 7 show the variation of
NPSHi with water temperature. The water
temperature was varied from 15°C to 60°C.
From Figures (2-5) it can be seen that the
head of the pump is maintained nearly at
constant value from the maximum NPSH
down to inception condition and close to the
breakdown of the pump head.
- Centrifugal pump.
- Suction pipeline.
- Suction control valve.
- Suction pressure gauge.
- Delivery pressure gauge.
- Delivery pressure gauge.
- Orifice meter.
- Mercury U-manometer.
- Delivery pipeline.
- Calibration valve.
- Calibration line.
- Non-return valve.
- Electrical heaters.
- Main water tank.
- Speed control unit.
- Electric motor.
- Thermometer.
SET2008 th Internal Conference on Sustainable Energy Technologies; Seoul, Korea.
24-27 August, 2008
.
Figure 2: Variation of pump head with NPSH at different water temperature
Figure : Variation of pump head with NPSH at different water temperature
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
H ( m )
NPSHi ( m )
Q = 0.245 lit/sec
N = 1800 rpm
T = 15 oC
T = 30 oC
T = 45 oC
T = oC
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
NPSH ( m )
H ( m )
Q = 0.318 lit/sec
N = 1800 rpm
T = 15 oC
T = 30 oC
T = 45 oC
T = 60 oC
SET2008 th Internal Conference on Sustainable Energy Technologies; Seoul, Korea.
24-27 August, 2008
Figure : Variation of pump head with NPSH at different water temperature
Figure : Variation of pump head with NPSH at different water temperature
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
NPSH ( m )
H ( m )
Q = 0.409 lit/sec
N = 00 rpm
T = 15 oC
T = 30 oC
T = 45 oC
T = 60 oC
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
NPSH ( m )
H ( m )
Q = 0.587 lit/sec
N = 2800 rpm
T = 15 oC
T = 30 oC
T = 45 oC
T = 60 oC
SET2008 th Internal Conference on Sustainable Energy Technologies; Seoul, Korea.
24-27 August, 2008
Figure 6: Variation of NPSHi with water temperature at constant flow rate
Figure 7: Variation of NPSHi with water temperature at constant flow rate
Water temperature
(oC )
010 20 30 40 50 60 70 80
2.00
3.00
4.00
5.00
6.00
7.00
8.00
NPSHi ( m )
Q = 0.40 lit/sec
N = 1800 rpm
N = 2300 rpm
N = 2800 rpm
Temperature ( oC )
NPSHi ( m )
Q = 0.35 lit/sec
N = 1800 rpm
N = 2300 rpm
N = 2800 rpm
010 20 30 40 50 60 70 80
2.00
3.00
4.00
5.00
6.00
7.00
8.00
SET2008 th Internal Conference on Sustainable Energy Technologies; Seoul, Korea.
24-27 August, 2008
For further reduction in NPSH the pump
head reduced rapidly and the performance
breakdown occurred. The results show that
the pump head decreases with the increase
of water temperature. This drop occurs
mainly due to the increase of vapour
pressure value, which in turn reduces the
value of NPSH, and then the cavitation will
appear earlier. From these Figures it can
also be seen that there is interference
between the points of break-down
conditions with respect to variation of water
temperature, and the reason may be due to
some factors, such as the tensile strength,
static pressure, vapour pressure, the number
of bubbles, the bubble volume and the gas
content. Figures and show the variation
of NPSHi with water temperature These
Figures show that the NPSHi increases with
the increase of water temperature till reaches
its maximum value, then started decreasing
with the increase of water temperature. This
occurs mainly due to that at low temperature
values, the effect of suction pressure is
stronger than the effect of vapour pressure,
while at higher temperature values the effect
of vapour pressure becomes stronger.
5. CONCLUSION
Based on the experimental results
obtained for different water temperatures,
pump flow rate ratios and pump speeds, the
following important conclusions can be
drawn:
The pump head decreases with
increasing water temperature.
For all temperatures test at various flow
rate ratios and pump speeds, it was
found that the inception net positive
suction head (NPSHi) increased as the
temperature increases reaching its
maximum value at nearly 30°C then
decreased with increasing temperature
It was observed that the maximum
NPSHi was likely independent of the
flow rate ratio and speeds.
Increasing water temperature
accelerates cavitation occurrence.
REFRENCES
Kevorkov L. R. 1975, Analysis of
influence of scale factors on
similarity of pump cavitation
characteristics when pumped water
temperature is varied. Russian Engineering
Journal, , Vol. . pp9- Russia.
&
Kevorkow L. R., 1978, The effect of
properties of the pumped fluid on
cavitation in centrifugal pumps. Fluid
Mechanics- Soviet Research., Vol.7, No.3
May-June.
Dorota Z. Haman, Forrest T. Izuno
and Allen G. Smajstrla. 1994, Pumps for
Florida Irrigation and Drainage ,
University of Florida. January, USA.
Zika V. J., 1984, Correlation of
cavitating centrifugal pumps. ASME
Journal of Fluid Engineering, Vol. 106,
June, pp. 141-
El-kadi M. A., 2001, Cavitation in
centrifugal pumps handling hot water
Engineering Research Journal, Helwan
University., Vol.77, October, p.p. 200-,
Egypt.
Zika V.T., 1984, Thermodynamics of
incipient cavitation in centrifugal pumps.
ASME Journal of Fluid Engineering,
December, pp. 161-
Al-Arabi A. A. B. & Selim S. M. A.
A theoretical model to predict
cavitation in centrifugal pumps. Proceeding
of th International Conference on Heat
Transfer, Fluid Mechanics and
Thermodynamics. July, Sun City, South
Africa.
