# Series & Parallel Pumps CVEN 3323

```CVEN 3323
Updated November 17, 2004
Single, Series, and Parallel Pumps
Theory
Introduction:
Whereas turbines convert fluid energy into mechanical energy, pumps convert mechanical
energy into fluid energy, increasing the energy possessed by the fluid. There are two main pump
types: positive displacement pumps—pistons, plungers, diaphragms, vanes, screws, lobes, which
have a fixed flow rate per stroke or revolution; and turbo-hydraulic or kinetic pumps—
centrifugal pumps, which convert fluid kinetic energy into static pressure energy.
Method of Operation
In 1730, Demour demonstrated that when rotating a T formation pipe with bottom end
submerged in a liquid and primed—loaded with the liquid, the centrifugal force lifts the liquid
and discharges it through the arms when the centrifugal force is greater than the gravitational
force. Today, the design of a centrifugal pump has the water entering the low-pressure center of
the impeller. The vanes then lead the water to the higher-pressure region to the casing. The
casing is designed with a gradually expanding spiral shape so that minimum loss occurs in the
transformation of kinetic energy to pressure energy. The pump receives the water at a low
velocity on the interior edge of the set of moving impeller vanes and discharges it from the outer
edge with kinetic energy sufficient to raise it to a desired height; and through the gradually
expanding spiral passage transforms the kinetic energy into pressure energy.
The &quot;Performance Characteristics&quot; of a pump at a fixed speed are represented by the following
graphical relationships:
Total Head (HP) versus Discharge (Q)
Power Input (P) versus Discharge (Q)
Efficiency ( %) versus Discharge (Q)
This lab examines the head/flow rate and efficiency/flow rate relationships. Pump characteristics
or performance curves are created when the head delivered by a pump (or pumps) is plotted
against the flow rate. These curves represent the behavior of a given size pump (or pumps)
operating at a given speed and are important tools in pump selection. Generally, the higher the
flow rate, the lower the head that the pump can contribute. Additionally, pumps are used to lift
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CVEN 3323
Updated November 17, 2004
water up or to increase the energy so that the water can travel farther. This lab determines the
head/flow characteristics of centrifugal pumps operating at a single speed: a single centrifugal
pump, two similar centrifugal pumps operating in parallel and in series.
Recall the total head is the difference between the total energy head at the outlet and the total
energy head at the inlet (neglecting the small differences in velocity heads). As shown be the
following equation:
 p 2 V22
  p1 V12



  HP  HL


z


z
2
1
 g 2 g
  g 2 g


 

where subscripts 1 and 2 refer to inlet and outlet sections. H p is the pressure head produced by
the pump and H L is the energy loss due to friction and pipe fittings. By conservation of mass, V1
= V2 if the pipe diameters are equal at the inlet and outlet sections.
The incoming pressure is read on the compound gauge at the pump inlet. After the water flows
through the pump, it travels up 0.8 meters (assuming the pump is placed on the floor) to the
manifold gauge. The pressure at the manifold is read on the manifold gauge. However, the
pressure head at the manifold is 0.8 meters less than the pressure head at the exit of the pump.
correction and subtracting the inlet pressure head. This should be approximately the same as
subtracting the inlet pressure head from the outlet pressure head (verify that this is the case for
the single pump).
= (outlet pressure/) – (inlet pressure/)
= [{(manifold pressure/+ datum correction} - (inlet pressure/)]
Single pump:
Qout
Qin
Qin =Qout
A
hpump = hwater
Pumps in Parallel:
When two or more similar pumps are connected in parallel, the head across each pump is the
same and the total flow rate is shared equally between the pumps, QP/n, where n is the number of
pumps in parallel. For identical pumps in parallel, the pressures at the two inlets and outlets are
identical and the maximum head the two pumps can deliver is no greater than for a single pump.
Theoretically, the flow rate is doubled, although in practice, this will not occur, due to losses in
the piping systems.
Total head (using outlet, not manifold) is determined the in the same manner as for the single
pump.
The theoretical curve for the parallel pump configuration is obtained from the single pump data
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CVEN 3323
Updated November 17, 2004
by multiplying the flow rate by two.
For theoretical parallel pump curve, plot: Hp(single pump) vs. 2*Q(single pump)
Parallel Pumps:
n= 2 pumps
QA
QT
hpumpA = hpumpB =hwater
QA + QB = QTotal
QB
Since the head loss across the parallel pumps is equal, the pump curve derived for each should be
the same.
For the experimental parallel pump curves, plot: Hp(pump i) vs. Q where i = 1 or 2
Pumps in Series:
When two or more similar pumps are connected in series, the same flow rate passes through each
pump and under goes a head boost of total head divided by number of pumps, HP/n. Therefore,
the series configuration of two identical pipes provides a pump characteristic of twice the head as
for a single pump. For series pumps, the total head can be computed as follows:
Total Head: HP = (pressure head at pump 2 outlet - pressure head at pump 1 inlet)
= [(outlet 2 pressure/) – (inlet 1 pressure/)]
For the experimental series pump curve, plot: Hp vs. Q
The theoretical curve for the series pump configuration is obtained from the single pump data by
multiplying the head by two. This doubled head is plotted with the measured flow rate.
To get the theoretical series pump curve, plot: 2*HP(single pump) vs. Q(single pump)
Series pumps:
n = 2 pumps
Qout
Qin
A
B
Qin = Qout = Qtotal
hpumpA + hpumpB =hwater
Pump Efficiency
For a pump, the efficiency is defined as
 = Po/Pi
where Po = power out from the pump = power imparted to the fluid
= *Q*Hp = [N/m3]*[m3/s]*[m] = [N-m/s] = [J/s] = [W]
Pi = power input to the pump shaft = power output from the motor = [W]
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CVEN 3323
Updated November 17, 2004
Output power is determined experimentally. Input power should be given in the manufacturer’s
specifications for the pump. For the pumps used in this lab, Pi = 0.37 kW = 370 W.
An important objective when selecting a pump for an engineering system is maximizing the
efficiency for the desired flow conditions.
Experimental Procedure
Purpose:
Determine the head/flow characteristics of centrifugal pumps operating at a single speed: a single
centrifugal pump, two similar centrifugal pumps operating in parallel and in series.
Equipment:
Hydraulics bench, two auxiliary centrifugal pumps, discharge manifold, appropriate hoses and
NOTE:
For this lab, it is important to emphasize the need to turn off the pumps before any hoses
are disconnected. If other experiments are being performed at the same time, it can be difficult
to hear the motor(s).
Procedure:
1. Straddle the manifold block across the channel of the bench. Make sure the outlet is pouring
into the stilling basin for correct measurement of flow rate.
2. The auxiliary pump should be placed on the floor, which is important for the datum
correction, and connected to the bench with the appropriate hoses as follows:
3. First set up for the SINGLE PUMP:
i. The single pump receives its water directly from the sump. The sump drain outlet is
located next to the built-in bench pump. A hose should extend between the sump and
the auxiliary pump. The hose is attached to both the sump outlet and the auxiliary
pump inlet. Next, a hose continues from the auxiliary pump to the manifold. Make sure
manifold outlet drains down into hydraulic bench for water collection. Figure 1 shows
the proper piping connections.
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CVEN 3323
Updated November 17, 2004
Figure 1. Single Pump Operation
ii. After the connections are made open the sump drain valve. Close the discharge control
valve on the manifold; turn on the auxiliary pump, then completely open the discharge
control valve. Turn on only the auxiliary pump, not the hydraulic bench. Water should
flow out the manifold. If water is not emitted, check that:
a) the auxiliary pump is plugged in;
b) the sump valve is open;
c) the discharge manifold control valve is open;
d) the sump has sufficient water.
iii. Slowly close the flow control knob to obtain manifold pressure readings.
iv. Record inlet pressure (compound gauge) and outlet pressure (pump) using gauges.
Record the flow rate using the graduated cylinder and stopwatch.
v. If the sump drain valve is not fully open, inlet pressure will fall low enough to where
cavitation can occur. Cavitation will be marked by a loud noise and a sharp decrease in
pump performance. To avoid cavitation, make sure sump drain valve is fully open.
vi. Turn off pump motor after all readings have been taken.
4. Next we’ll set up for the SERIES PUMPS.
i. Close the sump drain valve. Disconnect the hose from the manifold. Connect this hose
to the inlet of the second pump. Attach a hose from the outlet of the second pump to the
manifold.
ii. After the connections are made, close the discharge control valve, turn on both pumps,
and then completely open the discharge control valve. Water should flow out the
manifold.
iii. Slowly close the flow control knob to obtain manifold pressure readings.
iv. Using the appropriate gages, record the outlet pressure for pump 2 and the inlet pressure
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CVEN 3323
Updated November 17, 2004
for pump 1. Record the flow rate using the volumetric tank and a stopwatch.
v. Turn off both pumps and close the sump drain valve when all readings have been taken.
5. Finally, set up for the PARALLEL PUMPS.
i. Disconnect all hoses, being careful not to spill water everywhere. Re-connect two
auxiliary pumps in a parallel configuration as shown in instructed by the TA.
ii. After the connections are made open the sump drain valve. Turn on only pump 1. Water
should flow out the manifold. Next, turn off pump 1 and turn on pump 2. Again, water
should flow out the manifold. Now the pumps should be primed. Close the discharge
control valve; turn on both pumps, then completely open the discharge control valve.
Check outlet pressure gages on pumps to ensure equal flow is moving through each.
iii. Slowly close the flow control knob to obtain manifold pressure readings.
iv. Record inlet and outlet pressures for both pumps using the gauges. Record the flow rate
using the volumetric tank and a stopwatch. Turn off both pumps, close sump drain valve,
and disconnect hoses. Return pumps and hoses to ITLL module room.
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CVEN 3323
Updated November 17, 2004
Data Tables
Table 1. Single Pump
Manifold
Pressure
Outlet
Pressure
Inlet
Pressure
Height
Correction
Units
[psi]
[m]
1
6
0.8
2
9
0.8
3
12
0.8
4
15
0.8
5
18
0.8
6
21
0.8
Total
Manifold
Total
Outlet
Volume
[s]
Table 2. Series Pumps
Manifold
Pressure
Units
[psi]
1
9
2
15
3
21
4
27
5
33
6
39
7
45
8
51
9
57
Pump 2
Pump 1
Outlet
Pressure
Inlet
Pressure
Total
Outlet
Volume
Time
[s]
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Time
Flow
Rate
Flow
Rate
CVEN 3323
Updated November 17, 2004
Table 3. Parallel Pumps
Manifold
Pressure
Units
[psi]
1
14
2
16
3
18
4
20
5
22
Pump 1
Outlet
Pressure
Pump 1
Inlet
Pressure
Pump 2
Outlet
Pressure
Pump 2
Inlet
Pressure
Total
Outlet 1
Total
Outlet 2
Volume
Time
[s]
Table 4. Single Pump Efficiency
Pi
Po


Units
[W]
[W]
[W/W]
[%]
1
370
2
370
3
370
4
370
5
370
6
370
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Flow
Rate
CVEN 3323
Updated November 17, 2004
Results and Questions
1. Fill in the tables on the data sheet for each pump configuration. USE CONSISTENT UNITS. For
the single pump, discuss the relationship between the total head determined by the outlet pressure
gage and the total head determined by the manifold pressure + datum correction. Explain any
differences.
2. Plot the Total Head Outlet vs. Flow Rate for the single pump, parallel pumps, and series pumps on
the same graph. (Graph 1)
3. For the parallel pumps, plot the experimental and theoretical Total Head vs. Flow Rate curves on
the same graph. (Graph 2)
4. Comment on the flow rates obtained on Graph 2. What are some reasons that flow rates do not
increase in proportion to the number of pumps used?
5. For the series pumps, plot the experimental and theoretical Total Head vs. Flow Rate curves on the
same graph. (Graph 3)
6. Does the head from the experimental series pump graph in Graph 3 increase exactly by a factor of
two? Discuss.
7. For the single pump, plot both the Total Head Outlet vs. Flow Rate and the Efficiency (as percent)
vs. Flow Rate on the same graph. (Graph 4)
8. From this graph (Graph 4), estimate the desired operating flow rate for the single pump.
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