Pumps Postlab

Analysis of Centrifugal Pump Performance Characteristics at Various Operating Configurations D.S. Corpuz, J.L. de Guzman and J.M. Golbin Department of Chemical Engineering, University of the Philippines-Diliman, Quezon City, Philippines

D.S. Corpuz, J.L. de Guzman and J.M. Golbin, 2008. Centrifugal pumps are one of the widely used pumps in industry because of their simple design and flexibility of application. The head they produced vary with flow rate and with geometry or configuration. This experiment analyzed their performance characteristics in single, series, and parallel configuration by making head vs. capacity plots. It was shown that the over-all head developed decreases as the capacity increases, and that the efficiency increases as the rotation speed increases. Keywords: parallel operation, power input, power output, pump capacity, pump efficiency, series and parallel operation, single operation, total head OBJECTIVES The primary objective of this experiment was to determine the performance characteristics of centrifugal pumps operating individually and in series and parallel configurations. This experiment was also done to investigate and establish the performance curves: i) Total head vs. Capacity of the pump at constant rotational speed. ii) Efficiency vs. Capacity at constant rotating speed.

Figure 1. Centrifugal Pump Source: http://www.cheresources.com/centrifugalpumps3.shtml

THEORETICAL BACKGROUND A pump aids the movement of liquids from one location to another. (Perry, 1997). Mechanical energy of the liquid, velocity, pressure and elevation are increased by using pumps. This mechanical energy is converted to fluid energy. (McCabe, 2001) Pumps can be further categorized to displacement and dynamic. Under dynamic pumps are centrifugal and special effect pumps. For centrifugal pump, it is subdivided into (1) axial flow, (2) mixed flow and radial flow and (3) peripheral.(Dickenson, 1988) For this study, an open impeller centrifugal pump is used. One of the types of pumps used in the industry is the centrifugal pump which falls under rotodynamic pump. It can generate high rotational velocities and convert the resultant, which is the kinetic energy, to pressure energy. The centrifugal action is the key in increasing the mechanical energy of the liquid. The liquid enters through the suction nozzle which is concentric with the axis of the impeller, a high-speed rotary element. The liquid spreads radially and comes in contact with the vanes. The liquid leaves the impeller at a higher velocity, passing between the vanes. Considering an ideal set-up, the spaces between the vanes are completely filled with the fluid in motion without cavitation. The volute, a spiral casing, then collects the fluid from the impeller. Here, the velocity head of the liquid is converted to pressure head. The impeller applies power to the liquid and the torque of the driveshaft transmits the power to the impeller. The liquid completely leaves the pump by passing through the discharge nozzle. If an ideal system is to be assumed, friction is negligible and the efficiency of the pump is 100%. It can deliver a definite discharge rate at every given developed head. It is also assumed that all liquid flowing across the periphery of the impeller is moving at the same speed. This results to the conclusion that the vanes are infinite, has zero thickness, and are infinitesimally apart. For multistage operation, the discharge in the discharge in the first stage provides the suction for the second and the same trend is followed for the succeeding pumps. This results to greater head developed compared to single pump operation. (McCabe, 2001)

Figure 2. Cross-sectional View of a Centrifugal Pump Source:http://www.cheresources.com/centrifugalpumps4.shtml Before operating the pump, priming is performed. It is done to displace the air which is present in the casing of the pump. Since it is airbound, it cannot function until the air present is replaced with liquid. It will not be able draw liquid upward from an initially empty suction line and force the liquid through may be influenced by the system used (e.g., single operation and multistage operation). Another problem that may be encountered in the operation of pumps is cavitation, which is the phenomena when the liquid flashes to vapor inside the pump. This can be avoided by the adjusting the pressure at the pump inlet in such a way that it exceeds the vapor pressure by a certain quantity. For multistage operation, the discharge in the discharge in the first stage provides the suction for the second and the same trend is followed for the succeeding pumps. This results to greater head developed compared to single pump operation. (McCabe, 2001) PROCEDURE Before performing the analysis of pumps with different set-ups, the system was subjected to priming. All of the valves were opened except the flow-regulating valve. This is done to ensure the system is free of air column which may result to erroneous values for pressure head as read from the height of the gauge. Using the adjustable counterweight, the motor stator is balanced prior to the experiment proper. The tank is filled with water until it coincides with the apex of the V notch, taking into consideration the surface tension effect. The point gauge in the stilling well is zeroed. The system is operated at a speed ranging from 1000 to 2000 rpm. The nut on the rear of the pressure gauge is removed and air is expelled. It is followed by

removing the nut on the rear of the vacuum gauge and filling the tube with water. Priming is concluded by shutting off the system.

Overall Head (m)

Several set-ups ups are required to be analyzed: series, parallel and single pump operation. For the series and parallel operations, the valves are adjusted as shown in the appendix. ix. Speed was set to 1800 rpm, 2000 rpm and 2200 rpm and four data points for flow rates were gathered for each trial. The discharge rate was regulated by slowly opening the flow-regulating regulating valve. The values for v notch head, discharge rate, suction head, head delivery head and the torque mass were directly obtained from the gauges and weights used. These quantities were recorded at incremental discharge flow rates. For the single operation, pump 1 and pump 2 were operated individually. The same values for speed were used. The valves are adjusted to fit the operation of the selected pump. The system is shut down every after operation, before proceeding to the next analysis.

Parallel Operation Overall Head vs Capacity 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5

1800 rpm 2000 rpm 2200 rpm

1.78E-03

1.98E-03

2.18E-03

2.38E-03

Capacity (m3/s)

Single Pump Operation (pump 1) Head vs Capacity

Some of the essential pump performance characteristics are pump capacity, head, power, and efficiency. Capacity is the mass rate of fluid through the pump (Perry, 1997).

Head (m)

RESULTS AND ANALYSIS

where Q = capacity he = h + kh h = measured head (meters) kh = 0.00085 meters Ce = 0.5765

6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

y = -3E+06x2 + 6547.x + 1.583 R² = 0.998

y = -4E+06x2 + 11167x - 1.761 R² = 0.999

best fit, 1800 rpm best fit, 2000 rpm best fit, 2200 rpm

y = 1E+06x2 - 7009x + 9.695 R² = 0.999

1.10E-03 1.30E-03 1.50E-03 1.70E-03 1.90E-03 Capacity (m3/s)

Head is the expression for the energy of the fluid due to its elevation above some reference point, its velocity and its pressure. The two fundamental pressure conditions of fluid are the static head, where fluids are at rest, and the dynamic head, where fluids are flowing. The total system head is the energy required to move this liquid at a given flow rate (Dufour & Nelson, 1993). Head (m)

Single Pump Operation (pump 2) Head vs Capacity

where Hm = manometric total head Hd = delivery head Hs = suction head

7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0

y = 77139x2 - 2691.x + 9.220 R² = 0.167

y = 1E+07x2 - 47693x + 46.55 R² = 0.997

y = -3E+07x2 + 93753x - 58.50 R² = 0.963

best fit, 1800 rpm best fit, 2000 rpm best fit, 2200 rpm

The total head versus capacity of a system are shown below. below For the stable overall head vs. capacity graph, the maximum head is at zero discharge. Head decreases as discharge increases. For the unstable graph, Hm initially increases from the value at zero discharge and falls with further increase in discharge (McCabe, 2001).

The graph of the actual developed head drops to zero as the flow rate increases to a certain value (zero-head head flow rate).

Series Operation Overall Head vs Capacity

It is customary to calculate the power output when arriving at the performance of the pump, that is

1.29E-03 1.49E-03 1.69E-03 1.89E-03 03 2.09E-03 Capacity (m3/s)

11

Overall Head (m)

10

The power input to the pump is greater than the power output because of the internal losses due to friction, leakage, etc.

9 8 7 6

1800 rpm 2000 rpm 2200 rpm

5 4 1.47E-03

1.97E-03 Capacity (m3/s)

2.47E-03

Efficiency is the ratio of fluid power wer to the total pow power consumed, that is

the sum of the discharges through each pump at the head concerned (Series/Parallel Centrifugal Pump Test Rig Instruction Manual). Figure 3 and 4 illustrate this.

Series Operation Overall Efficiency vs Capacity

Overall Efficiency

58 53 48 43

1800 rpm 2000 rpm 2200 rpm

38 33 1.47E-03

1.97E-03

2.47E-03

Figure 3. Pumps in Parallel Source: www.engineeringtoolbox.com

Capacity (m3/s)

Overall Efficiency

Parallel Operation Overall Efficiency vs Capacity 87 82 77 72 67 62 57 52 47 1.78E-03

Figure 4. Pumps in Series Source: www.engineeringtoolbox.com 1800 rpm 2000 rpm 2200 rpm 1.98E-03

Head loss factors include fluid friction in passages and channels and shock losses (the sudden change in direction of liquid leaving the impeller and joining the stream of the liquid flowing circumferentially around the casing. Friction is highest at the maximum flow rate (McCabe, 2001).

03 2.38E-03

2.18E-03

Capacity (m3/s)

Efficiency (m)

Single Pump Operation (pump 1) Efficiency vs Capacity 98 93 88 83 78 73 68 63 58 53 1.10E-03

1800 rpm 2000 rpm 2200 rpm

1.30E-03

1.50E-03

1.70E-03

1.90E-03

Capacity (m3/s)

Efficiency (m)

Single Pump Operation (pump 2) Efficiency vs Capacity 78 73 68 63 58 53 48 43 38 33 28 1.29E-03

1800 rpm 2000 rpm 2200 rpm 1.49E-03

1.69E-03

1.89E-03

CONCLUSIONS AND RECOMMENDATIONS

2.09E 2.09E-03

Capacity (m3/s)

For a series operation, the same discharge passes through both pumps but the developed heads supplements each other. Total developed head is calculated by simply adding the heads of each pump. For a parallel set-up, up, similar head across the pump is observed ed but it is to be noted that the individual discharges may be unlike unless the identical pumps are used. Total discharge is

Power lost happens because of (1) the effec effect of friction and shock losses i.e., conversion of mechanical energy to heat, (2) leakage – unavoidable reverse flow from the impeller discharge; reduces the volume of the actual discharge from the pump per unit of power expended, (3) disk friction – friction between the outer surface of the impeller and the liquid in the space between the impeller and the inside of the casing,, and (4) bearing losses – power required to overcome mechanical friction riction in th the bearing and seals of the pump (McCabe, 2001). Smoother impeller finishes will increase the efficiency of the pump. Generally, the head produced decreases as the amount of water pumped increases. The shape of the curve varies with pump’s specific ific speed and impeller design. Usually, the highest head is produced at zero discharge and it is called the shut-off off head. The efficiency of a pump steadily increases to a peak, and then declines as Q increases further (http://www.aces.edu). To improve the he performance characteristics of the pump, the pump system must be primed before use. The system must be completely air free ee so that reliable data can be obtained. Also, it is recommended that the experiment be performed well within acceptable operating ranges. The upper and lower limits are set by the allowable speed variations. The efficiency is sacrificed if the experiment is performed at extreme operating conditions. Although not seen in the experiment, pumps are more effici efficient at higher speeds. Improving the equipment such that readings such as torque and pressures are more accurately read would certainly improve the performance curves for the pumps. The pumps are already old. Also, the effect of temperature was not taken into effect. Installing temperature rature sensors would give more accurate data.

REFERENCES Dufour, J.W. and Nelson, W. E. (1993). Centrifugal Pump Sourcebook. USA: McGraw-Hill, Inc. pp. 7-8, 25 Foust, A.S. (1980). Principles of Unit Operations. Singapore: John Wiley & Sons (Asia) Pte Ltd. pp. 596 McCabe, W.L. (2001). Unit Operations of Chemical Engineering. Singapore: McGraw-Hill Book Co. pp. 195 http://www.aces.edu/dept/fisheries/education/ras/publications/fa cility_design/Selection%20of%20Centrifugal%20Pumping%20 Equipment.pdf - Retrieved March 6, 2008 Engineering Toolbox. http://www.engineeringtoolbox.com/centrifugal-pumpsd_54.html - Retrieved March 5, 2008 Centrifugal Pumps. http://www.tpub.com/content/doe/h1012v3/css/h1012v3_69.htm Retrieved March 6, 2008

Pumps in Series Rotational Speed

Flow Rate

Discharge (capacity)

Vnotch Head

Pump 1

Pump 2

Pump 1

Suction Head

Delivery Head

Suction Head

Delivery Head

Pump 2

Overall Torque Mass

Total Head

N

Q

h

Q

Hs1

Hd1

Hs2

Hd2

Hm1

Hm2

HmT

rpm

L/min

mm

m3/s

m

m

m

m

m

m

m

g

1800

170

71

1.88E-03

1

0.75

-1.5

2

0.17

3.92

4.08

2287.18

160

68.4

1.72E-03

0.9

1.8

-0.75

3

1.25

4.10

5.34

2110.18

150

66.4

1.60E-03

0.8

2.1

0.3

4.5

1.60

4.50

6.10

2103.18

140

64.5

1.49E-03

0.75

2.7

1

5.5

2.21

4.76

6.97

2006.18

200

75

2.16E-03

1.2

1.5

-2

2.75

0.85

5.30

6.14

2948.18

190

73

2.02E-03

1.1

2

-1

3.5

1.38

4.98

6.36

2845.18

180

70.8

1.87E-03

1

2.5

0

5

1.91

5.41

7.32

2693.18

170

69.3

1.77E-03

1

3

1

6

2.37

5.37

7.74

2615.18

225

79.1

2.46E-03

1.4

1

-2.5

3

0.31

6.21

6.52

3666.77

215

77.6

2.35E-03

1.25

2.5

-1.5

4.5

1.90

6.65

8.54

3518.27

205

76

2.23E-03

1.25

3

-0.5

6

2.33

7.08

9.42

3341.27

190

74

2.09E-03

1.1

7.01

10.42

3000.77

2000

2200

Pump 1

Rotational Speed

Torque

N rpm 1800

2000

2200

4 Pump 2

1

7.5

3.41

Pump 1

Pump 2

Combined

Power Output

Power Input

Power Output

Power Input

Efficiency

T Nm 5.61

Po1 W 3.07

Pi1 W 110.68

Po2 W 72.36

Pi2 W 110.68

2.78

65.38

34.08

5.17

21.00

102.11

69.02

102.11

20.56

67.59

44.08

5.16

25.04

101.77

70.45

101.77

24.60

69.22

46.91

4.92

32.20

97.08

69.37

97.08

33.17

71.46

52.31

7.23

17.89

158.52

112.04

158.52

11.29

70.68

40.98

6.97

27.26

152.98

98.50

152.98

17.82

64.39

41.10

6.60

35.05

144.81

99.27

144.81

24.21

68.56

46.38

6.41

41.23

140.61

93.44

140.61

29.32

66.45

47.89

8.99

7.48

216.87

149.86

216.87

3.45

69.10

36.28

8.63

43.64

208.09

152.96

208.09

20.97

73.51

47.24

8.19

50.99

197.62

154.83

197.62

25.80

78.35

52.08

7.36

69.80

177.48

143.48

177.48

39.33

80.84

60.09

η1

η2

ηT

M

Pumps in Parallel Pump 1 Rotational Speed

Flow Rate

Vnotch Head

Discharge

N

Q

h

Q

rpm

L/min

mm

1800

200

74.1

190

73.8

180

71.7

170 2000

2200

Pump 2

Overall

Suction Head

Delivery Head

Hs1

Hd1

Hs2

Hd2

m3/s

m

m

m

m

m

m

m

2.09E-03

0.7

4

-1.5

4

3.81

6.01

4.91

2.07E-03

0.65

4.2

-1.5

4.2

4.05

6.20

5.13

1.93E-03

0.65

4.2

-1.3

4.5

3.99

6.24

5.11

69.7

1.80E-03

0.65

4.2

-1.1

4.5

3.93

5.98

4.95

245

78.6

2.42E-03

0.75

5.5

-1.8

5.2

5.44

7.69

6.56

215

77.5

2.34E-03

0.7

5.5

-1.5

5.5

5.44

7.64

6.54

205

74.6

2.13E-03

0.7

5.8

-1.5

5.8

5.63

7.83

6.73

190

73.5

2.05E-03

0.65

5.9

-1.5

5.8

5.74

7.79

6.77

255

79.9

2.52E-03

0.8

7

-2

6.8

6.95

9.55

8.25

240

79.6

2.50E-03

0.75

7

-1.8

7

6.98

9.53

8.26

230

78.2

2.39E-03

0.75

7.2

-1.5

7.2

7.12

9.37

8.25

220

77.4

2.33E-03

0.75

7.2

-1.5

7.2 Pump 1

7.09 Pump 2

9.34 Overall

8.21

Rotational Speed

Torque Mass

Torque

N rpm

M g

1800

2200

Pump 2

Delivery Head

Pump 1

2000

Pump 1

Suction Head

Pump 2

Total Head Hm1

Hm2

HmT

Total Q

Power Output

Power Input

Power Output

Power Input

Efficiency

T Nm

Po1 W

Pi1 W

Po2 W

Pi2 W

η1

1613.09

3.95

78.32

178.06

123.50

178.06

43.99

69.36

53.83

4.19E-03

1562.09

3.83

82.41

175.59

126.12

175.59

46.94

71.83

56.77

4.15E-03

1512.09

3.71

75.47

173.17

118.06

173.17

43.58

68.18

53.17

3.86E-03

η2

ηT m3/s

1461.09

3.58

69.37

170.70

105.56

170.70

40.64

61.84

49.05

3.60E-03

1940.59

4.76

129.19

204.34

182.65

204.34

63.22

89.38

74.06

4.85E-03

1889.59

4.63

124.85

201.60

175.33

201.60

61.93

86.97

72.34

4.68E-03

1766.09

4.33

117.58

194.96

163.51

194.96

60.31

83.87

70.16

4.26E-03

1706.09

4.18

115.60

191.73

156.86

191.73

60.29

81.81

69.42

4.10E-03

2440.68

5.98

171.86

244.35

236.18

244.35

70.33

96.66

81.42

5.05E-03

2407.18

5.90

171.15

242.37

233.66

242.37

70.62

96.40

81.52

5.00E-03

2236.18

5.48

167.05

232.26

219.83

232.26

71.92

94.65

81.74

4.78E-03

2054.68

5.04

162.10

221.52

213.55

221.52

73.17

96.40

83.20

4.66E-03

Pump 1 Rotational Speed N rpm 1800

2000

2200

Flow Rate

Vnotch Head

Discharge

Suction Head

Delivery Head

Total Head

Torque Mass

Torque

Power Output

Power Input

Q L/min 140 130 120 110 160 150 140 130 180 170 160 150

h mm 65 63.4 61.2 57.6 68.9 67.8 64.9 61.6 72.5 71.9 69.4 64.9

Q m3/s 1.52E-03 1.42E-03 1.31E-03 1.12E-03 1.75E-03 1.68E-03 1.51E-03 1.33E-03 1.98E-03 1.94E-03 1.78E-03 1.51E-03

Hs m 0.8 0.8 0.8 0.75 0.9 0.9 0.8 0.75 1.1 1.1 1 0.9

Hd m 2.5 2.8 3.3 4 3.5 3.8 4.5 5 4.2 4.5 5.2 6

Hm m 1.97 2.24 2.70 3.40 2.96 3.23 3.97 4.46 3.56 3.84 4.57 5.37

M g 1088.5 1039.5 993 942 1336.09 1285.09 1208.09 1105.09 1562.09 1562.09 1461.09 1398.59

T Nm 2.67 2.55 2.43 2.31 3.28 3.15 2.96 2.71 3.83 3.83 3.58 3.43

Po W 29.26 31.27 34.58 37.48 50.76 53.28 58.72 57.99 69.29 73.25 79.85 79.44

Pi W 52.67 50.30 48.05 45.58 71.84 69.10 64.96 59.42 92.39 92.39 86.41 82.72

Flow Rate

Vnotch Head

Discharge

Suction Head

Delivery Head

Total Head

Torque Mass

Torque

Power Output

Power Input

Efficiency η1

Efficiency η1 55.54 62.16 71.95 82.22 70.66 77.11 90.40 97.60 75.00 79.28 92.40 96.03

Pump 2 Rotational Speed N

Q

h

Q

Hs

Hd

Hm

M

T

Po

Pi

rpm

L/min

mm

m3/s

m

m

m

g

Nm

W

W

1800

160

66.9

1.63E-03

-2.5

0.15

2.96

1101.59

2.70

47.22

153.31

30.80

150

66.4

1.60E-03

-2.5

0.17

2.97

1101.59

2.70

46.49

153.31

30.33

140

64.5

1.49E-03

-2.3

2.2

4.76

1004.09

2.46

69.37

148.59

46.69

130

61.2

1.31E-03

-2.1

2.8

5.10

1039.5

2.55

65.31

150.30

43.45

175

70.7

1.86E-03

-3

2

5.41

1382.59

3.39

98.87

174.34

56.71

160

65.2

1.53E-03

-3

2.5

5.77

1331.59

3.26

86.43

171.60

50.37

150

63.9

1.45E-03

-2.5

3.55

6.30

1234.09

3.03

89.70

166.35

53.92

2000

2200

140

63.9

1.45E-03

-2

4

6.25

1156.09

2.83

88.99

162.16

54.88

200

75.4

2.19E-03

-4

2.5

7.06

1738.59

4.26

151.35

202.83

74.62

190

71.9

1.94E-03

-3.5

2.8

6.74

1691.09

4.15

128.52

200.02

64.26

180

69.4

1.78E-03

-3.2

3.5

7.07

1609.59

3.95

123.51

195.20

63.27

170

66.9

1.63E-03

-2

4.5

6.81

1461.09

3.58

108.64

186.41

58.28

Schematic Diagram for Parallel Flow

M, H2O

M, H2O

RPM

M, H2O

M, H2O L/S

C

D

B PUMP 1

PUMP 2

A

A

1

To V

Notch

Schematic Diagram for Series Flow

M, H2O

M, H2O

RPM

M, H 2O

M, H 2O L/S

C

D

B PUMP 1

A

To V

PUMP 2

A