Curtin University of Technology
Department of Mechanical Engineering
FLUID MECHANICS 433
LABORATORY REPORT 1
Pump Characteristic Curves
Prepared by : Nang The Truong
Student ID No. : 14392665
Date performed : 27th July 2012
Table of Contents I. Introduction ... 1 II. Objectives ... 1 III. Nomenclature ... 1 IV. Theory ... 2 1. Pump Curves ... 2 2. Similarity ... 2 V. Apparatus ... 3 VI. Procedures ... 3 VII. Results ... 5 1. Single pump ... 5 2. Parallel pumps ... 10 VIII. Discussions ... 11 IX. Conclusions ... 13
Appendix A – Pump curves calculations at pump operating speed 1200 rpm and zero flow rate ... 14
List of tables
Table 1 : Records for single pump experiment ... 5
Table 2 : Efficiencies of a single pump at various pump speeds and input powers .... 6
Table 3 : Efficiencies of a single pump at speeds of 1400 and 1800 rpm ... 7
Table 4 : Records for parallel pump experiment ... 10
Table 5 : Experimental Power input of pump #1 and pump #2 ... 10
Table 6 : Experimental Power output of pump #1 and pump #2 ... 10
List of Figures
Figure 1 : Laboratory apparatus ... 3 Figure 2 : Pump curves at various pump operating speeds and flow rates ... 8 Figure 3 : Efficiencies at various pump operating speeds and flow rates ... 9
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I.
Introduction
It is very common in engineering processes where fluid such air, water or oil are required to be delivered from one location to another. This can be typically achieved by pressure difference between the points. However, in a more complex system where fluid flows through various directions, pipes, junctions,etc. energy losses occurs due to friction along the pipe, valves, entrance and exists. In such cases, work needs to be added into the system to overcome these losses. For this purpose, pumps are introduced to do the job. Due to economic and design purposes, selecting the right pump is as significant as its role and also the purpose of this lab. Eventually, process of constructing a pump curve, application similarity laws and effects of pumps arranged in parrallel will be discussed.
II.
Objectives
Construct and analyse pump characteristic curves for a rotary dynamic pump operating at various speeds of 1200, 1600 and 2000 rpm.
Applying similarity law to determine the pump characteristic curves for operation at various speeds of 1400 and 1800 rpm.
III. Nomenclature
µ efficiency
Pin Input power to the motor [W]
Pout Hydraulic power output of the pump [W]
Δp Pressure increased [Pa]
Q Flow rate [m/s] or [l/s]
m Mass of counterweight [kg]
T Applied Torque [Nm]
ω Angular velocity [rad/s]
g Gravity acceleration [m/s2]
N Rotational speed [rpm]
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IV. Theory
1. Pump Curves
Efficiency of the pump at each operating point can be determined by the equation:
(1)
The hydraulic power output of the pump Pout and input power to the motor Pin can be obtained as below : (2) (3)
,where r = 0.235 is the length of lever arm . 2. Similarity
To determine flow rate, head and efficiency for pump operating at other speeds, three similarity laws can be applied :
For flow rate:
(4) For head (pressure increase):
(5)
For efficiency
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V.
Apparatus
Figure 1 : Laboratory apparatus
VI. Procedures
Single Pump
1. Valves were set so that only pump #2 was operating. Valve at pipe discharge to tank was fully close.
2. Motor started to drive pump #2 at operating speed of 1200 rpm.
3. Head losses Hs and Hd were recorded from manometer. Weights were added to the lever arm until the motor is balanced. Balance mass was also recorded. 4. The discharge valve was now fully open. The flow rate could be read and
recorded from the rotameter.
5. The flow rate was increased to 4 intermediate values so that head loss and balance mass were recorded in accordant with each set.
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Pumps in parallel
1. Valves were set so that pump #2 was operating at speed of 1600 rpm, and isolated from pump #1.
2. Valve on the suction of pump #1 was open. Pump #1 started to operate at the same speed as pump#2. Valve isolating pump #1 and pump $2 was open. 3. Discharge valve at the tank was fully open. Flow rate through pump #2 was
recorded using the rotameter. The total flow rate could be read from the mercury manometer. Head loss Hs and Hd were recorded from the
manometer. The masses required to balance each motor were also noted down.
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VII. Results
1. Single pump
Table 1 : Records for single pump experiment
Pump speed, N [rpm] Flowrate, Q [l/s] Hs [mmHg ] Hd [mm Hg] Pressure increase, ΔP [Pa] Balance mass, m [g] 1200 0 930 1183 33754 330 0.8 936 1177 32153 350 1.2 941 1171 30686 375 2.5 976 1138 21613 440 2.65 982 1132 20012 440 1600 0 848 1265 55634 450 1 852 1259 54300 580 1.8 868 1243 50031 600 2.7 896 1216 42693 650 4 957 1155 26416 700 2000 0 730 1376 86187 600 1 743 1368 83385 700 1.9 766 1345 77248 865 3.6 827 1285 61105 975 5.2 942 1171 30552 1040
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Table 2 : Efficiencies of a single pump at various pump speeds and input powers
Pump speed, N [rpm]
Hydraulic power output , Pout [W]
Input Power,
Pin [W] Efficiency = Pout/Pin
1200 0 96 0 26 101 25 37 109 34 54 127 42 53 127 42 1600 0 130 0 54 168 32 90 174 52 115 188 61 106 203 52 2000 0 174 0 83 203 41 147 251 59 220 282 78 159 301 53
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Table 3 : Efficiencies of a single pump at speeds of 1400 and 1800 rpm
Pump speed, N [rpm] Flowrate, Q [l/s] Pressure increase,
ΔP [Pa] Efficiency = Pout/Pin
1400 0.00 45943 0 0.93 43764 25 1.40 41767 34 2.92 29418 42 3.09 27239 42 1800 0.00 75947 0 1.20 72345 25 1.80 69043 34 3.75 48630 42 3.98 45028 42
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Figure 2 : Pump curves at various pump operating speeds and flow rates
0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000 0 1 2 3 4 5 6 Pr e ssur e in cr e ase, P [Pa] Flowrate , Q [l/s] 1200rpm 1600rpm 2000rpm
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Figure 3 : Efficiencies at various pump operating speeds and flow rates
0 10 20 30 40 50 60 70 80 90 0 1 2 3 4 5 6 Eff ic ie n cy Flowrate, Q [l/s] 1200rpm 1600rpm 2000rpm
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2. Parallel pumps
Nominating speed of pump #1 and pump #2: 1600 rpm
Table 4 : Records for parallel pump experiment
Total Flow rate, Q [l/s] Pump #1 Flowrate, Q [l/s] Pump #2 Flowrate, Q [l/s] Hs [mmH g] Hd [mmHg ] Pump #1 Mass, m [g] Pump #2 Mass, m [g] ΔP, [Pa] 0 0 0 814 1400 500 400 78182 3 2.8 0.2 836 1279 650 430 59103 4 3.2 0.8 845 1265 750 500 56035 6 3.6 2.4 885 1230 790 630 46029 7.5 3.9 3.6 943 1173 850 680 30686
Table 5 : Experimental Power input of pump #1 and pump #2
Power input P#1, [W] Power input P#2, [W]
193 155
251 166
290 193
305 243
328 263
Table 6 : Experimental Power output of pump #1 and pump #2
Power output P#1, [W] Power output P#2, [W]
0 0
165 12
179 45
166 110
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Assuming total flow rate is evenly distributed between the pumps . Theoretical flow rates for each pump are therefore half total flow rates. In addition, both pumps are assumed to operate on the pump curve shown in figure 1. Applying flow rates from table 7 in conjunction with figure 1 for operating speed at 1600 rpm, the pressure increase ∆P, [Pa] can be obtained and tabulated as below
Table 7 : Theoretical pump #1 and #2 flow rates and increased pressures
Total Flowrate, Q [l/s]
Pump #1 & #2 Flowrate, Q [l/s] Delta P, [Pa] Power output 0 0 56000 0 3 1.5 52000 78 4 2 48300 97 6 3 38500 116 7.5 3.75 29500 111
VIII. Discussions
Figure 1 shows that, for a single pump at all operating speeds, the pressure increase drops down non-linearly as the flow rate increases. At a particular flow rate, the pressure increase at higher operating speed will also be larger than that at lower operating speed. In fact, if not considering losses due to pipe roughness, losses within the pump and valves, elbow, entrance and exist of pipe system, etc. the theoretical curve can be shown as linear. These losses will cause the
difference between the two curves. The effects of these losses on pump performance are clearer via operating efficiency as shown in figure 2. Rotary dynamic pump performance is relatively low which is less than 80%, specifically at low flow rate and operating speed. The efficiency reaches its maximum point at some particular value of the flow rate and then falls with a continued increase in the flow rate. These points are typically considered as normal or design flow rate or capacity for the pump. The points on various curves corresponding to the maximum efficiency are called as the best efficiency points (BEP). Apparently, in
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selecting a pump for a particular application, it is desirable to have the pump operate near its maximum efficiency.
Operation of the pump at zero discharge should also be considered. From table 1, the head developed at zero discharge is called the shutoff head. It represents the rise in pressure head across the pump with the discharge valve closed. Since there is no flow with the valve closed, the related efficiency is zero as shown in table 2. The input power to the pump is simply dissipated as heat. Although centrifugal pumps can be operated for short periods of time with the discharge valve closed, damage will occur due to overheating and large mechanical stress with any extended operation with the valve closed.
Due to time and cost, pump characteristics are not always determined
experimentally. Performance of the same pump operating at different speeds can be predicted by using similarity laws. Shown in table 3 are characteristics of the pump at operating speed at 1400 and 1800 rpm. The behaviour and performance are as per discussions above. However, assumptions should be considered carefully when applying these laws. The similarity law shown as equations (4), (5), and (6) are based on the condition that, as the impeller diameter is changed, all other important geometric variables are properly scaled to maintain geometric similarity. This is not normally the case in practice due to difficulties associated with manufacturing the pumps. The effects of viscosity and surface roughness have also been neglected. It has been found that the effect of these on efficiency as pump size decreases due to smaller clearances and blade size. Lastly, it should also be aware that the similarity laws will not be very accurate if tests on a model pump with water are used to predict the performance of a prototype pump with a highly viscous fluid, such as oil, because at much smaller Reynolds
number associated with the oil flow, the fluid physics involved is different from the higher Reynolds number flow associated with water.
In practice,it is common to see a combination of pumps arranged in series or parallel to provide addition head or flow. Theoretically, the pump flow rate is assumed to be evenly distributed when the pumps are arranged in parallel. However, at relatively low flow rate as at less than 4 [l/s], this is not the case.
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Pump #1 does a majority of work comparing with pump #2. Nonetheless, the total flow rate had been increased as opposed to that from the operation of a single pump at the same head. For a single pump system at operating speed of 1600 rpm and ∆P of 30 KPa, the total flow rate was about 3.6 l/s, whereas it was 7.5 l/s for a parallel pump system.
IX. Conclusions
Pump curves had been constructed for a single pump operating at three different speeds of 1200, 1600 and 2000 rpm. Similarity laws were introduced to
determine theoretical pump characteristics at other speeds of 1400 and 1600 rpm. Finally, effect of pumps arranged in parrallel were also analysised.
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Appendix A – Pump curves calculations at pump operating speed
1200 rpm and zero flow rate
Given Hg = 13600 [kg/m3]
For Hs = 930 [mmHg], Hd = 1183 [mmHg], pressure increase is obtained as:
[ ]
From equation (2), output power of the pump is determined as :
[ ]
From equation (3), input power to the motor is calculated as :
(
) [ ]
Pump efficiency can be obtained using equation (1) :
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Appendix B – Similarity laws
Given pump operating speed of 1400 rpm. From equation (4), flow rate is calculated as :
( ) ( )
From equation (5), the pressure increase is :
( ) [ ]
