TITLE: CENTRIFUGAL PUMP PERFOMANCE CHARACTERISTICS
INTRODUCTION Pump A pump is a device used to move gases, liquids or slurries. A pump moves liquids or gases from lower pressure to higher pressure, and overcomes this difference in pressure by adding energy to the system (such as a water system). A gas pump is generally called a compressor, except in very low pressure-rise applications, such as in heating, ventilating, and air-conditioning, where the operative equipment consists of fans or blowers.
Centrifugal pump A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the velocity of a fluid. Centrifugal pumps are commonly used to move liquids through a piping system. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber, from where it exits into the downstream piping system. Centrifugal pumps are used for large discharge through smaller heads.
Figure 1.Basic element of a centrifugal pump
Centrifugal pump works
A centrifugal pump works by the conversion of the rotational kinetic energy, typically from an electric motor or turbine, to an increased static fluid pressure. This action is described by Bernoulli's principle. The rotation of the pump impeller imparts kinetic energy to the fluid as it is drawn in from the impeller eye (centre) and is forced outward through the impeller vanes to the periphery. As the fluid exits the impeller, the fluid kinetic energy (velocity) is then converted to (static) pressure due to the change in area the fluid experiences in the volute section. Typically the volute shape of the pump casing (increasing in volume), or the diffuser vanes (which serve to slow the fluid, converting to kinetic energy in to flow work) are responsible for the energy conversion. The energy conversion results in an increased pressure on the downstream side of the pump, causing flow.
Cavitations is the problems in the pump.It is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers etc.
Due to the general complexity of flow through a centrifugal pump ,the actual performance of the pump cannot be predicted on a completely theoretical basis .Actual pump performance is determine experimentally through test on the pump and the result are presented as pump performance curve .Performance characteristics for a given pump geometry and operating speed are usually given in the form of plots of head rise ,efficiency and power versus flow rate ( commonly referred as capacity).This information is most helpful to the engineer responsible for incorporating pump into a given pipe flow system.
THEORY-Performance characteristics Consider the experimental arrangement for determining the head rise (or pressure rise) gained by fluid flowing through a pump, as show schematically below.
Figure 2. Experimental Arrangement for Pump Performance Characteristics.
The pump head rise can be expressed using the energy equation as (
)
Typically the differences in elevation and velocities (between inlet and outlet) are also small so that
The power P, gained by the fluid is given by the equation
The pump overall efficiency η, is the ratio of power actually gained by the fluid to the shaft power supplied W shaft as given by the equation
Typical performance characteristic for a centrifugal pump of given size operating at a constant speed is shown below.
Figure 3. Typical Performance Characteristics of a centrifugal pump.
OBJECTIVES The objective of the experiment is to obtain performance characteristics (as described above) for variable speed centrifugal pump operating at different impeller speeds.
APPARATUS The experimental set-up consists of I. II.
Water-flow bench and centrifugal pump Instrumentation for data acquisition
Instrument Panel- Refer to the schematic diagram below I.
Speed Control to change pump speed. Pump speed can be varied over range Of 0-3000 rpm.
II. III.
Pump suction and delivery pressures Torque measurement.
Flow Measurement- Using “V” notch weir I.
Flow rate in the system can be measured relating the height of the water seen in the sight glass to graph and reading of the flow rate in liters per minute.
Preliminaries 1) We were supervised adequately. 2) The pump discharge and suction valves were turned to fully open and the motor speed turned to zero. 3) The electrical supply and motor drive switch are switched on. The motor control knob is adjusted slowly to around half way position. The system is ensured to have no leaking. All the gauges etc. are checked to reduce the motor speed back to zero and it is ready for testing.
Actual Experiment i) Suction valve is opened and the discharge valve is closed. The maximum speed N1 is selected by adjusting speed control to 50%. ii) Discharge valve is fully opened and water is allowed to circulate. The volume of flow indicator is noted and decides suitable increments in flow to give adequate sample points about 15 points between zero and maximum flow). iii) The discharge valve is closed (corresponding to no flow). When the measured readings are steady read all the measurements. iv) The discharge valve is opened slightly, to give the first increment in volume flow. When readings are steady, take the measurements. v) Step 4 is repeated. The final measurements corresponding to the valve being fully open. vi) The entire experiment is repeated at pump speed corresponding to speed control position of 75% and 100%.
RESULT AND DATA ANALYSIS Pump Performance Curves (N=1439 rpm@ ω =150.69 rad/s@50%) 250.00
45.00 40.00
200.00
35.00 30.00
150.00
25.00 20.00
100.00
15.00 10.00
50.00
5.00 0.00
0.00 0
0.5
1 Head
1.5 Efficiency
2
2.5
BHP
Pump Performance Curves (N=2178 rpm@ ω =228.08 rad/s@75%) 50.00
700.00
45.00
600.00
40.00 35.00
500.00
30.00
400.00
25.00 20.00
300.00
15.00
200.00
10.00
100.00
5.00 0.00
0.00 0.00
0.50
1.00
1.50 Head
2.00 Efficiency
2.50 BHP
3.00
3.50
Pump Performance Curves (N=2945 rpm@ ω =308.40 rad/s@100%) 60.00
1600.00
50.00
1400.00 1200.00
40.00
1000.00
30.00
800.00 20.00
600.00
10.00
400.00
0.00
200.00 0.00
1.00
2.00
3.00
4.00
-10.00
5.00 0.00
Head
Efficiency
BHP
From the pump performance curve, we can see that the maximum volume flow rate through a pump occurs when its net head is zero, H=0; this flow rate is called the pump’s free delivery. The free delivery condition is achieved when there is no flow restriction at the pump inlet or outlet or when there is no load on the pump. At this operating point, Q is large, but H is zero and the pump’s efficiency is zero because the pump is doing no useful work. At the other extreme, the shutoff head is the net head that occurs when the volume flow rate is zero, Q=0, and is achieved when the outlet port of the pump is blocked off. Under this condition, H is large but Q is zero; the pump’s efficiency is again zero because the pump is doing no useful work. Between these two extremes, from shutoff to free delivery, the pump’s net head may increase from its shutoff value somewhat as the flow rate increases, but H must eventually decrease to zero as the volume flow rate increases to its free delivery value. The pump’s efficiency will reaches its maximum value somewhere between the shutoff condition and the free delivery condition; this operating point of maximum efficiency is appropriately called the best efficiency point (BEP). The pump performance curves change with rotational speed as shown above from the three different rotational speeds (N = 50%, 75%, 100%) that we do in the experiment. It is
important to realize that for steady conditions, a pump can operate only along its performance curve.
Sample Calculation
For N = 1439 rpm,
(
)
ω = 150.69 rad/s-1
Torque = 1.1 Nm
= (150.69 rad/s-1) ( 1.1 Nm) = 165.76 Watt
Capacity, Q Q = 90 L/min = 90 L/min / (1000L) (60s) = 1.5 x 10-3 m3/s
(
)
= (41.36 +0.98) kPa/ (1000 kgm-3) (9.81 ms-2) = 4.32 m
= (1000 kgm-3) (9.81 ms-2) (1.5 x 10-3 m3/s)( 4.32 m) = 63.52 Watt
= = (63.52 Watt) / (165.76 Watt) = 0.383 x 100% = 38.3%
TEST 1 PUMP SPEED.N : 1439 rpm @ 50% ω : 150.69 rad/s Water head, h
Efficiency,η
m3/s (10-3)
(m)
(%)
0
0
5.87
0
120.55
45
0.75
5.57
34.0
3
150.69
70
1.17
4.92
37.5
4
165.76
90
1.50
4.32
38.3
5
180.83
105
1.75
3.81
36.2
6
180.83
120
2.00
3.21
34.8
7
195.90
130
2.17
2.61
28.3
8
195.90
135
2.25
2.11
23.7
No
BHP,
Volume flow rate
(Watt)
l/min
1
75.35
2
TEST 2 PUMP SPEED.N : 2178 rpm @ 75% ω : 228.08 rad/s Water head, h
Efficiency,η
m3/s (10-3)
(m)
(%)
0
0
13.85
0
433.35
95
1.58
12.54
45.0
3
501.78
123
2.05
11.54
46.2
4
547.39
135
2.25
10.53
42.5
5
570.20
163
2.72
9.33
43.6
6
593.01
173
2.88
8.32
39.7
7
615.82
183
3.05
7.12
34.6
8
638.62
200
3.33
6.01
30.8
9
638.62
210
3.50
4.71
25.3
No
BHP,
Volume flow rate
(Nm)
l/min
1
228.08
2
TEST 3 PUMP SPEED.N : 2945 rpm @ 100% ω : 308.40 rad/s Water head, h
Efficiency,η
m3/s (10-3)
(m)
(%)
0
0
0
0
1017.72
118
1.97
24.19
45.9
3
1171.92
158
2.63
22.68
50.0
4
1233.60
178
2.97
20.88
49.3
5
1326.12
205
3.42
19.17
48.4
6
1356.96
218
3.63
17.46
45.9
7
1418.64
243
4.05
15.75
44.1
8
1449.48
248
4.13
14.04
39.3
9
1480.32
260
4.33
12.13
34.8
10
1511.16
273
4.55
10.33
30.5
No
BHP,
Volume flow rate
(Nm)
l/min
1
493.44
2
DISCUSSION: From the experiment, the best operating range for the pump is from 1439 rpm . This is base on the characteristics of the pump at that instant speed. From our analysis, the average efficiency is equal to 0.383%. While for brake horsepower the average value is 165.76 Watt and the head is 4.32 m. From the three average of that value, it is found that the flow rate, Q is between the ranges of 0 to 273 (liter/minutes). Therefore due to this factor, we have chosen that the pump will operate efficiently between this range compare to the other two speeds. The efficiency of the pump tends to fall because of the increasing frictional losses in the long blade passages of these narrow impellers. Since impeller blade have finite thickness, an impellers with too many blades leads to passage losses due to excessive flow and large skin friction drag. Also, the efficiencies depend upon the design of the machine, shape of the blade and, hence, the capacity handled.
CONCLUSION : From the experiment, the characteristics of this pump which is the efficiency, brake horsepower and the head can be defined by using a different speed of pump. Thus, the objective of this experiment is achieved in order to define any centrifugal pump characteristics. It also can be concluded that the main objectives of this experiment have been achieved successfully. We can also determine all the parameters required. However, it is to be noted that, some of the value of readings obtained may be inaccurate due to certain errors that occurred during the experiment i.e. parallax error and human error. Yet it still leads and produced the expected results accordingly and as required.
DISCUSSION:
For this experiment, we have follow the procedure in order to obtain the performance characteristic of the pump by using variable speed operating centrifugal pump at three impeller speeds. The pump performance characteristic, which is pressure (head) jump, power requirement, flow rate influence and pump speed influence. Below is the obtain data result for the experiment.
Water Head Speed
(m)
(rpm)
Output Power
Volume Flow Rate
Efficiency
(W)
(m3/s)
(%)
Start
End
N1=1439
5.87
2.11
46.47
2.25
23.7
N2=2178
13.85
4.71
161.70
3.5
25.3
N3=2945
25.90
10.33
460.91
4.55
30.5
Based on the table above, we can see the overall data result obtained from the experiment from three different speed. In these three speeds, all value for water head pump from the start to the end of the experiment is decreased. This is because due to starting point of the discharge pressure where we slowly decreased the amount of pressure in certain gap, and as expected it effect the value of water head result.
For output power and volume flow rate obtain from the experiment, we can see the increased number of value in volume flow rate due to water height is increasing in the matter of time. For output power, it also can be put as increasing but if we see the data obtain from the experiment, we can see the inclined and declined number of data. The output power is affected due to the water head value, where if the water head is decreased, the output will be decreased. By comparing with these three speeds, we can say the more speed is occurred in the experiment, the bigger number of output power will produced.
For the efficiency, the number of the effectiveness of the experiment data is increasing due to the increasing value of the speed. In this part, we can see that we may have some error occurred in this experiment where our overall efficiency is low which is in 20-30% range. We can say the bigger number of efficiency, the bigger effectiveness of the operated machine. To get the better efficiency, the rate of power output and input must be bigger.
Other error that may occur during the experiment is parallax error when taking the data reading of water height. Also it happens in pressure gauge for inlet pressure. And when the adjusting the discharge water pressure, the machine cause other apparatus, such as the discharge pipe is vibrating extremely hard. This may be due to the material of pipe which it cannot absorb the vibration, and a steel pipe is recommended in this situation.
CONCLUSION: From the nut shell, we can say that we have achieve our objective, which is to obtain performance characteristics for variable speed centrifugal pump operating in three different impeller speeds. Based on the graph obtain from the data result, we can see the entire graph are inclined at first then slowly it started to declined. Based on these three speed performance, we can see the difference of parameter calculated to achieving our objective.
DISCUSSION: From the experiment, we conducted three tests which are at pump speed of 50%, 75% and 100%. The test requires us to record the values for water height, torque, inlet pressure and discharge pressure. For each test, we are required to plot graph head pump vs flow rate, graph
efficiency vs flow rate and graph shaft power vs flow rate. From the graph, we can see clearly the relationship between flow rate and head pump, efficiency and shaft power.
Below are the important details for 3 different speeds of pump.
For the pump speed, N1 = 1439 rpm:
The efficiency is proportional to the flow rate and it is increase until the maximum value of 38.30%.
The water head of pump is decrease slowly from 5.87 m to 2.11 m.
For the pump speed, N2 = 2178 rpm:
The efficiency is proportional to the flow rate and it is increase until the maximum value of 46.2o%.
The water head of pump is decrease slowly from 13.85m to 4.71m
For the pump speed N3 = 2945 rpm:
The efficiency is proportional to the flow rate and it is increase until the maximum value of 49.30%.
The water head of pump is decrease slowly from 25.90 m to 10.33 m.
From our inspection, it is found that the overall efficiencies of centrifugal pumps are high, of the order of 90% in the range. They tend to fall of rapidly at lower type numbers, mainly because of the increased frictional losses in the long inter blade passages of these narrow impellers.
Also, the efficiencies depend upon the size of the machine and, hence, the capacity handled. The larger the machine, the higher is the efficiency.
CONCLUSION: From the experiment, we can conclude that the best performance of this centrifugal pump to operate is at speed N1 = 1439 rpm and flow rate, Q between the range of 0 to 130 (liter/minutes). By doing this experiment also, all the characteristics of this pump which is efficiency, break horsepower and the head can be defined by using a different speed of pump, and thus the objective of this experiment is achieved already. Finally, we can conclude that this experiment is success by achieve the objective.
DISCUSSION: According to my observation, it is known that the centrifugal pump is a type of turbo machines which has two main components;
an impeller attached to a rotating shaft
stationary casing which cover the impeller
The impeller consists of a few blades arranged in a pattern around the shaft. Theoretically, both pressure and absolute velocity are increased as the fluid flows from the eye to the periphery of the blades. But the results obtained shows quite a great result and meet the objectives besides all the difficulties I had experienced during the experiment as to be compared with typical performance characteristics for a centrifugal pump, it is not that far. Maybe there are several mistakes have been made s during the experiment such example by taking inaccurate data and etc. This happen because it takes some times for the water in the basin to achieve its steady state as the measurement meter can’t be read as long as the water still churning. Otherwise maybe the machine itself is quite old and the efficiency of the equipment has dropped a little bit. While doing the experiment, I have taken the data for several times in order to ensure that the data is accurate by taking average data for all readings. The best operating range for the 3 different speed controls are determined from the graph at the highest pump efficiency that is the best condition for the pump to operate
CONCLUSION: As a conclusion, what I get here is that I know what a centrifugal pump is and how it functioned. I also learned new knowledge on the performance characteristics which is very useful for me as a future mechanical engineer in order for me to select an appropriate pump for several operations, which depends largely on the brake horsepower, the efficiency, and also the pump head varies with the capacity or the flow rate. All these need to be considered in selecting an ideal pump.
DISCUSSION: There are major and minor losses in the piping system for example friction in the pipe surfaces, elevation of the piping system, vibration of the centrifugal pump, valves, cavitations occur at the impeller blade, etc. For a fluid entering a circular pipe at a uniform velocity. Because of the no-slip condition, the fluid particle in the layer in contact with the surface of the pipe comes to complete. This layer also causes the fluid particle in adjacent layers to slow down gradually as a result of friction.
For friction, elevation and vibration losses, we cannot avoid these losses but we can minimize it, but we can prevent the cavitations by minimize the performance parameter which is called as the required net positive suction head (NPSHrequired) to give us a better result and increase the efficiency. For a minor losses, the fluid in typical piping system passes through various fitting valves, bends, elbows, tees, inlet, exit, enlargement, and contractions in addition to the pipes. These components interrupt the smooth flow of the fluid and cause additional losses because of the flow separation and mixing they induce. In a typical system with long pipes, these losses are minor compared to the total head loss in the pipes (the major losses) and are called minor losses. Although this is generally true, in some cases the minor losses may be greater than the major losses. This is the case, for example in system with several turns and valves in a short distance. The head loss introduced by a completely open valve, for example, may be negligible. But a partially closed valve may cause the largest head loss in the system, as evidenced by the drop in the flow rate. Flow through valves and a fitting is very complex, and a theoretical analysis is generally not plausible. Therefore, minor losses are determined experimentally, usually by the manufactures of the components. There is also an error that cause from the reading that we taken while undergo the experiment and to minimize this error, we can do some action to overcome this problems. The actions that we must consider during the experiment are:
1. The apparatus must be check carefully to make sure it is in a good condition before carrying the experiment because it may effects the results of the experiment.
2. The reading at the pump suction and delivery pressure gauges must be read with more carefully to avoid any error.
3. The reading should be taken several times and the average value should be taken to know the average reading that more accurate.
4. The eyes must sharp and perpendicular to the scale of the water level.
CONCLUSION: From the experiment, we can conclude that the performance characteristics of a centrifugal pump from the theory are a little bit similar with the result from the experimental value. The different value from the theory is because of some errors and losses in the system and also a little bit error in reading process.
DISCUSSION: This experiment has been carried out to differentiate and identify the properties of pump by different speed. Three tests have been carried out to identify the properties of the pump at different speed. For the pump speed, Test 1, N1 = 1439rpm, the efficiency increase as the flow rate increase until the maximum value of effeciency of 38.3% and at the flow rate of 1.5 m3/s. The head of pump decrease slowly from 5.87m to 2.11m as the valve is opened. The brake horsepower (bhp) is increase proportionally to the flow rate from 75.35W to 195.90W. For Test 2, the pump speed, N2 = 2178rpm, the efficiency is proportional to the flow rate and it increases until the maximum value of 46.2% and at the flow rate of 2.05 m3/s. The reduction of head pump slowly decrease from 13.85m to 4.71m as the water flow rate increases until maximum flow rate
of 3.5 m3/s. The slope for the brake horsepower graph increases as the flow rate increase to maximum value. The maximum bhp is 638.62W, and the best bhp with the highest effeciency for this operating speed of the pump is 501.78W. In Test 3, the pump speed used is N3 = 2945rpm. The effeciency is proportional to the flow rate and it is increase until the maximum value of 50.0%. The head of pump decrease slowly from 25.90m to 10.33m. The brake horsepower is proportional to the flow rate and it is increased from 493.44 W to 1511.16W. The effeciencies of the pump depends on the size of the machine and, hence, the capacity handled. The larger the machine, the higher is the efficiency. From our observation, overall, for the case of motor speed with the percentage with 50% and 75% the condition of the result is almost the same. The readings are different but the condition is almost the same. In the case of the torque, the values keep increasing for conditions, 50% and 75%. As for the average pump speed also, both conditions stated the same result, which keep on increasing. For Test 1, the reading for Q is from 0 to 2.25 m3/s and for Test 2 the value of Q is from 0 to 3.50 m3/s. As for the efficiency, we can see that in both conditions it increase first but later on it decreased because of the flow rates. In the other hand, as for the bhp, the same thing happens also because of the flow rates where the valve is fully opened causing less pressure. In all cases, 50%, 75%, and 100% motor speed, the values of the head keeps on decreasing. CONCLUSION As the conclusion from this experiment, the objective of the experiment has been achieved. From the experiment, we know that the principal dependent variables in the pump are the head ( h ), brake horsepower (bhp) and the efficiency (). The efficiency depends upon the size of machine and hence, the capacity handled. The larger the machine, the higher is the efficiency. As the speed of pump increases, the torque (T) increases, followed by the brake horse power (bhp) and also the volume flow rate (Q). Since the pump speed increase, the head value of the pump will also increase.
REFERENCES 1. Bruce R. Munson, D. F. (2006.). Fundementals of Fluid Mechanics, 5th Edition . Asia: John Wiley & Sons. 2. Frank M. White. (2003). Fluid Mechanics, 5th Edition. New York, USA: Mc Graw Hill. 3. John F. Douglas, J. M. (2001). Fluid Mechanics, 4th Edition. Scotland: Pearson Prentice Hall. 4. Fluid Dinamics Handbook By; R. D. Blevins, Van Nostrand Reinhold Co., New York,1984.
