Centrifugal Pumps

Centrifugal Pump CE 319W - Lab #4

Nathaniel R. Gant

Group Members: Gore, Pickens, Sexton, Wood Date Performed: February 6, 2012 Date Submitted: May 3, 2012 Submitted To: MAJ Dave Johnstone, Ph.D., P.E.

Virginia Military Institute Department of Civil and Environmental Engineering

Help Received: Cited on reference page.

INTRODUCTION AND THEORY One type of pump commonly used to transport water is the centrifugal pump. The centrifugal pump is a centrifugal fluid-machine that can transform mechanical energy into fluidpressure and kinetic energy (Rababa 2010). Figure 1 shows a 2D CAD drawing on a centrifugal pump with its components labeled. A centrifugal pump consists of a stationary pump casing and an impeller. The main function of the pump casing is to guide the liquid from the suction nozzle to the center of the impeller. The impeller creates a radial and rotary motion to the liquid, which results in increase in both the pressure and the kinetic energy and forcing it to the volute. The purpose of the volute is to collect the discharged liquid from the outside edge of the impeller at high velocity and gradually cause a reduction in fluid velocity by increasing the flow area. This converts the velocity head to static head. The fluid is then discharged from the pump through the

Volute

Figure 1. Centrifugal pump assembly. discharge nozzle (Podugu 2011). The flow rate and head of the pump are controlled to meet requirements set by engineers. In practice, control of the pump operation, by means of adjusting the rotational speed or the process characteristics, changes the pump operating point location or where the water needs additional energy for transportation (Ahonen 2011). In worst cases, a pump may operate with a low efficiency and be prone to cavitation, which is a well-known reason for pump failure. Cavitation is occurs when the static pressure of the pumped fluid falls below the vapor pressure limit of the fluid. This usually occurs in the suction inlet of the impeller, where the fluid velocity increases due to the smaller available area for the flow.

Vaporized fluid may move into the region of static pressure creating vapor bubbles that collapse resulting in pressure shocks (Ahonen 2011). These pressure shocks destroy the impeller and casing. Since cavitation minimizes pump performance, many solutions are sought to make pumps operate efficiently. When changes are considerable, or either the head or capacity requirement is too high for just one pump, two or more pumps are used in series or in parallel.

Pumps that operate in series have the same discharge. Pumps are used in a series system when substantial head changes take place without significant differences in the discharge. Pumps that operate in parallel have the same head. Parallel pumps are useful for systems with considerable discharge variations with no significant head changes (Gupta 2008). The current economic climate, escalating energy prices, and environmental concerns push steam plant operators in industry and public sectors to strive for new ways of cutting consumption and costs. Variable

speed pump technology can significantly cut operating costs in steam heating and process systems, says Oliver Brigginsha (Brigginshaw 2009). Nonetheless, the mode of operation of a pump depends on the system in which it operates. Relationships between pump discharge capacity, head, power, and efficiency are derived from actual test on a pump and are usually depicted graphically (Gupta 2008). The pump characteristic curve is a plot of head produced by a pump versus the flow rate the pump produces at a constant speed. The system characteristic curve is a plot of the output of the pump versus the total head losses. The intersection of these two curves reveals the actual pump-operating head and flow rate. The pump head (Ep) is calculated using the energy equation between the suction and discharge sides of the pump, given as: (1)

in which is the total head, is the pressure, is the average velocity, is the specific weight of water, is the gravitational constant, and is the elevation. The power passed on to the water by the pump (output energy) in watts is calculated using the equation: (2)

where is the specific weight, is the flow rate, and is the total energy across the pump. The mechanical power delivered to the pump (input energy) in watts is calculated using the equation: (3) where is the torque measured in Newton-meters and is the rotating speed measured in radians per second. The hydraulic efficiency of the pump is obtained from the two power values: (4) The objectives of this experiment were to determine characteristic and efficiency curves for a centrifugal pump and to observe the operation of two pumps operating in series and two pumps operating in parallel.

METHODOLOGY The apparatus of the experiment contained and Armfield centrifugal pump system, orifice flow meter (Poddymeter), motor torque measuring system, and a V-Notch weir flow meter. The system was first primed to insure that the small pressure tap lines connecting the pressure gages to the system were filled with water. If air remained in any lines would display an inaccurate pressure head at the tapping point. The pump was started assuring that the ball valves were opened or closed to observe singe pump, parallel, and series pump operation. The water level in the reservoir tank was checked to confirm that is was higher than the intake so the pump would draw water and not air from the tank. Once the water level was confirmed, the speed controller was set to zero. The power was turned on and the master speed dial was adjusted to half speed

for the single pump indicated by the tachometer. Each pump had five sets equally scattered over the flow range between zero flow and max flow. The following were recorded for each data set: the flow in m3/s as read on the meter, counterweight in kg required to balance the motor until the arm was horizontal, pump speed in rpm, pump suction pressures, and discharge pressures. Data was collect for a single pump at half speed, a single pump at max speed, two pumps in series at max speed, and two pumps in parallel at max speed. Then temperature of the water was recorded. The weir head and pressure recorded were plugged into Equation 1 to get pump head (Ep). The flow rate and total head was plugged into Equation 2 to find the output power for the pumps. Using the torque and mass, the input power was calculated using Equation 3. Using the output power and input power, the efficiency was calculated using Equation 4.

RESULTS These results from the experiment reveal the differences among pump operations in regards to single pumps, pumps in series, and parallel pumps. Table 1 consists of the data recorded from the readings of the meters when the pump was running a single pump at half speed. The pressure at the suction point did not have much of a change from the zero point to the highest point. However, the discharges were affected considerably.

Table 1: Trial-reading data for a single pump operating at half speed Pump 1 Suction Discharge (Kpa) (Kpa)

Pump 2 Suction Discharge (Kpa) (Kpa)

Trials

Weir Head (h)

Mass

Torque

(#)

(mm)

1

0

-24.6

16.5

-

-

(kg)

(rpm)

0.6

150

2

33.2

-27.0

11.0

-

3

39.3

-28.0

8.0

-

-

0.6

150

-

0.6

150

4

45.5

-30.5

2.0

5

47.5

-31.0

0.0

-

-

0.6

150

-

-

0.6

150

6

51.1

-33.5

-4.0

-

-

0.6

150

Table 2 shows the trail reading data for a single pump operating at max speed. Table 2 reveals that the speed had a tremendous impact on the discharge pressures for a single pump operation. Table 3 shows the trail reading data for two pumps operating in series at max speed.

When the pumps were set up in series, the flow rates were made the same in each pipe. Since one flow ran into the next, this would have doubled the head at the discharge for a majority of flow Table 2: Trial-reading data for a single pump operating at max speed.

Pump 1 Suction Discharge (Kpa) (Kpa) -3.00 150

Trials

Weir Head (h)

(#) 1

(mm) 0

2

60.5

-32.0

3

66.8

4

71.1

Pump 2 Suction Discharge (Kpa) (Kpa) -

Mass

Torque

(kg) 1.2

(rpm) 291

110

-

-

1.2

291

-35.0

102

-

-

1.2

291

-39.0

98.0

-

-

1.2

291

5

73.5

-41.0

90.0

-

-

1.2

291

6

81.7

-58.0

58.0

-

-

1.2

291

rate configurations.

In Table 3, the second discharge pressure seems to be double for four out of the six trails.

Table 3: Trial-reading data for two pumps operating in series at max speed.

Trials

Weir Head (h)

(#)

Pump 1

Pump 2

Mass

Torque

(Kpa)

(kg)

(rpm)

140

270

1.2

291

95.0

210

1.2

291

79.0

190

1.2

291

30.0

120

1.2

291

50.0

88.0

10.0

1.2

291

22.0

-31.0

29.0

1.2

291

Suction

Discharge

Suction

Discharge

(mm)

(Kpa)

(Kpa)

(Kpa)

1

0

-12.0

142

2

65.0

-33.0

110

3

71.4

-39.0

97.0

4

84.0

-56.0

64.0

5

88.0

-57.5

6

93.4

-73.5

Table 4 consists of trial-reading data for two pumps operating in parallel at max speed.

For the parallel configuration, the pressures at the first suction point and second suction point seemed to be close for each trail.

Table 4: Trial-reading data for two pumps operating in parallel at max speed.

Pump 1 Suction Discharge (Kpa) (Kpa)

Pump 2 Suction Discharge (Kpa) (Kpa)

Trials

Weir Head h

Mass

Torque

(#)

(mm)

1

0

-22.0

137

-28.0

104

(kg)

(rpm)

1.2

291

2

58.9

-28.0

122

-34.0

3

60.7

-32.0

117

-34.0

92.5

1.2

291

88.0

1.2

291

4

76.9

-28.5

121

5

83.6

-31.0

118

-39.0

89.0

1.2

291

-40.0

87.0

1.2

291

6

99.6

-38.5

98.0

-50.0

52.0

1.2

291

Using the Table 1, calculations were performed to determine the vales in Table 5. Table 5 shows the flow rate, flow meter head, pump head, output power, input power, and efficiency for a single pump at half speed. Primarily, the flow rate, efficiency, and pump head were determined so the graphs could be created to compare the data.

Table 5: Trial calculation data for a single pump operating at half speed.

Flow Meter Head (H)

Pump Head (Ep)

Output Power

Input Power

Trials

Flow Rate (Q)

Efficiency

(#)

(m3/s)

(m)

(m)

(W)

(W)

(%)

1

0

0.00085

4.1896

0.0012

23.114

0.005%

2

0.00029

0.03405

3.8835

11.0979

23.114

48.0%

3

0.00044

0.04015

3.6924

15.9309

23.114

68.9%

4

0.00063

0.04635

3.3594

20.7543

23.114

89.8%

5

0.00070

0.04835

3.2175

22.0913

23.114

95.6%

6

0.00084

0.05195

3.0894

25.3834

23.114

109.8%

Using the Table 2, calculations were performed to determine the vales in Table 6. Table 6 shows the flow rate, flow meter head, pump head, output power, input power, and efficiency for a single pump at max speed.

Table 6: Trial calculation data for a single pump operating at max speed.

Trials

Flow Rate (Q)

Flow Meter Head (H)

Pump Head (Ep)

Output Power

Input Power

Efficiency

(#)

(m3/s)

(m)

(m)

(W)

(W)

(%)

1

0

0.00085

15.5963

0.0044

89.683

0.005%

2

0.00127

0.06135

14.6639

182.6012

89.683

203.6%

3

0.00162

0.06765

14.2732

226.9394

89.683

253.0%

4

0.00189

0.07195

14.3843

266.7993

89.683

297.5%

5

0.00205

0.07435

13.8474

278.7975

89.683

310.9%

6

0.00267

0.08255

12.6576

331.0268

89.683

369.1%

Using the Table 3, calculations were performed to determine the vales in Table 7. Table 7 shows the flow rate, flow meter head, pump head, output power, input power, and efficiency for two pumps operating in series at max speed.

Table 7: Trial calculation data for two pumps operating in series at max speed.

Trials

Flow Rate (Q)

Flow Meter Head (H)

Pump Head (Ep)

Output Power

Input Power

Efficiency

(#)

(m3/s)

(m)

(m)

(W)

(W)

(%)

1

0

0.00085

15.6983

0.0044

89.683

0.005%

2

0.00152

0.06585

14.8460

220.6568

89.683

246.0%

3

0.00191

0.07225

14.2912

267.8437

89.683

298.7%

4

0.00286

0.08485

13.1880

369.4271

89.683

411.9%

5

0.00320

0.08885

12.1614

382.2470

89.683

426.2%

6

0.00371

0.09425

11.3510

413.4798

89.683

461.0%

Using the Table 4, calculations were performed to determine the vales in Table 8. Table 8 shows the flow rate, flow meter head, pump head, output power, input power, and efficiency for two pumps operating in parallel at max speed.

Table 8: Trial calculation data for two pumps operating in parallel at max speed.

Trials

Flow Rate (Q)

Flow Meter Head (H)

Pump Head (Ep)

Output Power

Input Power

Efficiency

(#)

(m3/s)

(m)

(m)

(W)

(W)

(%)

1

0

0.00085

16.2080

0.0046

89.683

0.005%

2

0.00119

0.05975

15.4560

180.1607

89.683

200.9%

3

0.00128

0.06155

15.3805

193.0900

89.683

215.3%

4

0.00230

0.07775

15.8569

357.0162

89.683

398.1%

5

0.00282

0.08445

16.1219

446.3073

89.683

497.6%

6

0.00435

0.10045

16.1366

689.2940

89.683

768.6%

Figure 2 is a graph of the pump head versus flow rate from the single pump system when it was set at half speed and max speed. Looking at the graph, the pump performed in a similar trend for both speeds, which was as flow rate increased the head decreased. However, due to the affinity laws, head is proportional to the square of shafts speed and flow rate is directly proportional to shaft speed. This means the full speed pump produced a head four times the head of the half speed pump for any given flow rate.

Half Speed

Full Speed

18 16 14

Pump Head (m)

12 10

8 6 4 2 0 0

0.0005

0.001

0.0015

0.002

0.0025

0.003

Flow Rate (m3/s)

Figure 2. Graph of pump head versus flow rate for the single pump trials.

Figure 3 is a graph of the efficiencies versus flow rate for the single pump, series pumps, and parallel pumps at max speed. These systems show are similar trend as well, which was as flow rated increase the efficiency increased. From this graph, one can determine that the two pump parallel system had the highest efficiency at max speed and the single pump lowest efficiency at max speed.

Single

Series

Parallel

Power (Single)

Power (Series)

Power (Parallel)

900% 800%

Efficiency

700% 600% 500% 400% 300% 200% 100% 0% 0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

0.005

Flow Rate (m3/s)

Figure 3. Graph of efficiency versus flow rate for max speed trials.

Figure 4 is a graph of the total pump head versus the flow rate for the single pump, series pumps, and parallel pumps at max speed. An obvious difference in performance is shown in Figure 4 because each pump did not create the same head for the same flow rate. The series pump doubled the head of the single pump. The parallel doubled the flow rate of the single pump.

Single

Series

Parallel

Poly. (Single)

Poly. (Series)

Linear (Parallel)

Pump Head (m)

17 16 15 14 13 12 11 10 0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

Flow Rate (m3/s)

Figure 4: Graph of pump head versus flow rate for max speed trials.

DISCUSSION The results from this experiment show a relationship among each pump systems’ performance. From the pump versus flow graphs, it is clear to see that the pumps followed a trend, which was as flow rate increased the total pump head decreased. Out of all the systems, the parallel pump system had the highest flow rate when the pump was set in max speed and master speed controller was turned to its max, which was .00435 m3/s. The parallel pump also created the highest output energy as well as efficiency. The max output energy was 689.3 watts and it ran at a max efficiency of 786.6%. The single pump system set at half speed had a max flow rate of .00084 m3/s, a max output energy of 25.4 watts, and a max efficiency of 109.8%.

The single pump system set at max speed had a max flow rate of .00267 m3/s, a max output energy of 413.5 watts and a max efficiency of 416.0%. The series pump system set at max speed had a max flow rate of .00371 m3/s, a max output energy of 331.0 watts and a max efficiency of 369.1%. The most efficient system for this pump based off the result is two pumps in parallel. In the single pump system, when the pump was set to half speed, the head values ranged from 3.14.2 m. However, when the single pump was set to max speed, the head values ranged from 12.715.6 m. This reveals that the speed of the motor and torque affects head produced in a pump system. This is reasonable because the greater the speed of the motor, the greater the force created by the impeller. This causes a greater force pushing the water in a certain amount of time; simultaneously, the more power is produced. One real life application of centrifugal pump is for sewage plant systems. Since water from a sewer needs to be transported to a treatment plant, pumps are be used to push water to the plant as shown in Figure 5.

Figure 5. Centrifugal pump transporting sewage water.

Not only does the pump need to be functional, an engineer designs a pump to generate power to keep the flow of water continuous. If not, the waste in the water can cause in the pipe to erode and block the flow of the water. Not to mention, cavitation can occur. The reliability and performance of any centrifugal pump system can be directly affected by its dynamic characteristics (Podugu 2011). An important characteristic of the head versus flow curve is the best efficiency point. At the best efficiency point, the pump operates most cost-effectively both in terms of energy efficiency and maintenance considerations (Khin 2008).

CONCULSION For a given pump at a given speed, there are definite relationships among pump discharge capacity, head, power, and efficiency. By performing tests on a pump, the pump characteristics can be compared such as, pumping head versus discharge, brake horsepower versus discharge, efficiency versus discharge (Gupta 2008). It is important to understand that as there is an increase in head in a pump, the capacity, or flow rate, decreases. Most manufactures supply performance curves for buyers. In addition, since some pump casings have impellers of different sizes, manufacturers display the curve for each impeller size on the same graph. This makes it easier to compare the behavior between pump sizes as well as reveal the most efficient option. In today’s society of competitive and sophisticated technology, a centrifugal pump is more widely used than any other applications because the advantages of following factors are effect on the centrifugal pump. The cost if low, efficiency is high, discharge is uniform, discharge is continuous, installation is easy, maintenance is easy, and it can run at high speeds without separation of flow (Khin 2008).

REFERENCES Ahonen, T., Tamminen, J., Ahola, J., and Kestil, J. (2011). "Novel method for detecting cavitation in centrifugal pump with frequency converter." Insight: Non-Destructive Testing & Condition Monitoring, 53(8), 439-449.

Brigginshaw, O. (2009). "Change is good." TCE: The Chemical Engineer, (821), 41-42.

Gupta, R. S. (2008). "Pressue Flow Systems: Pipes and Pumps." Hydrology and Hydraulic Systems, Waveland Press, INC., Long Grove, Illinois, 659.

Khin, C. T., Mya, M. K., and Khin, M. A. (2008). "Design and Performance Analysis of Centrifugal Pump." Proceedings of World Academy of Science: Engineering & Technology, 48 422-429.

Podugu, R., Kumar, J. S., murthy, B. V. R., and Kumar, N. S. (2011). "A modal approach for vibration analysis and condition monitoring of a centrifugal pump." International Journal of Engineering Science & Technology, 3(8), 6335-6344.

Rababa, K. S. (2010). "Full-scale Investigations of Axial Force of Large Centrifugal Pumps."

European Journal of Scientific Research, 47(4), 586-594.