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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.