Pump sizing and selection

Learning Outcome 6 Ashraf Al Shalalfeh

Al Ain Men’s College 1
LO 6 - Sub Learning Outcomes:

• Sub-outcome 1:
 parameters Involved in Pump Selection.
 Types of Pumps: Positive displacement pumps, Kinetic pumps, and centrifugal
pumps.

• Sub-outcome 2:
 Net Positive suction Head, Suction Line details, and Discharge Line details.

• Sub-outcome 3:
 The System Resistance Curve.
 Pump Selection and the Operating point for the system.
6.1.1 Parameters Involved in Pump Selection:
When selecting a pump for a particular application, the following factors
must be considered:

I. The nature of the liquid to be pumped.

2. The required capacity (volume flow rate)

3. The conditions on the suction (inlet) side of the pump
4. The conditions on the discharge (outlet) side of the pump
5. The total head on the pump (the term ha from the energy equation)
6. The type of system to which the pump is delivering the fluid
7. The type of power source (electric motor, diesel engine,
steam turbine, etc.)
8. Space, weight, and position limitations
6.1.2: Parameters Involved in Pump Selection: Cont.
Pumps are typically classified as either positive-displacement or kinetic pumps. The table
below lists several kinds of each.

Table (1) : Classification of types of pumps.

Positive-displacement Pumps:
 is a type of pump that transfers the liquid through the action of gears, screws,
vanes, plunger, piston, or diaphragm. 

 A specific amount of the liquid is enclosed in the pump chamber; the piston or
plunger pressurizes the liquid inside the chamber.

 After this process, a certain quantity of liquid is discharged. PD pump is also referred
to as a constant volume pump due to its constant flow rate and speed.
1. Gear Pump:

 Gear pumps use the actions of rotating cogs or gears to transfer fluids. 

 The rotating element develops a liquid seal with the pump casing and creates suction at the pump
inlet.  Fluid, drawn into the pump, is enclosed within the cavities of its rotating gears and
transferred to the discharge. 

Figure (1) : Gear pump (External)

1. Gear Pump: Cont.

 There are two basic designs of gear pump: external and internal.

Figure (2) : Gear pump (External ”Left” & Internal

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“Right”)
1. Gear Pump: Demonstration Video

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 2. Vane Pumps:

 It is a self-priming positive displacement pump

providing constant flow at varying pressures.

 Operation is via a motor connected to a gearbox as

typically the maximum rpm is 900.

 The pump head contains a slotted rotor which contain

vanes.

 The vanes create segmented chambers within the pump

head, partitioning the pump head between the rotor
and outer casing which enable the vane pump to be
self-priming as the chambers operate similar to valves. 

Figure (3) : Vane Pump.

2. Vane Pump: Demonstration Video
 3. Piston Pumps:
 Piston pumps used for fluid transfer are classified as either:

• single-acting simplex or
• double-acting duplex types as

 In principle, these are similar to the fluid power piston pumps,

but they typically have a larger flow capacity and operate at
lower pressures.

 they are usually driven through a crank-type drive or swash

plate.

Figure (4) : Piston Pump (a): Single Acting, (b): Double Acting. Figure (5) : Piston Pump (Axial)
3. Piston Pumps (Swash Plate): Demonstration Video
 4. Lobe Pump:

 The pumping action of the rotary lobe pump principle is

generated by the counter rotation of two pumping elements
(rotors) within a chamber (rotor case).

 The rotors are located on shafts, which in turn are mounted

within an external gearbox and supported by bearings; the
timing gears are also located on the shafts.

 The timing gears transfer the energy from the drive shaft to
the driven shaft, synchronizing the rotors such that they
rotate without contact with each other.

Figure (6) : Lobe Pump

4. Lobe Pump: Demonstration Video
 5. Diaphragm Pumps :

 a reciprocating rod moves a flexible diaphragm within a cavity, alternately

discharging fluid as the rod moves to the left and drawing fluid in as it moves to the
right.

 One advantage of this type of pump is that only the diaphragm contacts the fluid,
eliminating contamination from the drive elements.

 The suction and discharge valves alternately open and close.

Figure (7) : Diaphragm Pump

Rotary Pump Performance:

 The below figure shows a typical set of

performance curves for rotary pumps such as gear,
vane, screw, and lobe pumps. It is a plot of
capacity, efficiency, and power versus discharge
pressure.

 As pressure is increased, a slight decrease in

capacity into the center of the impeller and then
thrown outward by the vanes. Leaving the impeller,
the fluid passes through a spiral-shaped volute,
where it is gradually slowed, and causing part of
the kinetic energy to be converted to fluid pressure. Figure (8) : Performance Curve for a positive-displacement rotary
Pump.
 The pump shaft, bearings, seal, and the housing are
critical to efficient, reliable pump operation and
long life.
Figure (9) : Radial, open-type
 Figure (9) shows an open radial-type impeller impeller in the rear part of its
mounted in the pump case, oriented so that the pump case.
discharge port is to the left.
KINETIC PUMPS:

Kinetic pumps: add energy to the fluid by accelerating it through the action of a
rotating impeller. Figure (10) shows the basic configuration of a radial flow
centrifugal pump, the most common type of kinetic pumps.

Figure (10) : Centrifugal pump with drive motor on a mounting base.

Kinetic Pump Parts:

Figure (11) : Cutaway view of a centrifugal pump with an enclosed type

impeller.
Kinetic Pump Impeller Styles:

Figure (12) : Cutaway Three styles of impellers for kinetic pumps.

PERFORMANCE DATA FOR CENTRIFUGAL PUMPS:
 Because centrifugal pumps are not positive-displacement types, there is a strong dependency
between capacity and the pressure that must be developed by the pump.

 The typical rating curve plots the total head on the pump (ha) versus the capacity or discharge (Q), as
shown in Figure (13) below.

Figure (13) : Performance curve for a centrifugal pump total head versus capacity.
PERFORMANCE DATA FOR CENTRIFUGAL PUMPS: Cont.
 Total head (ha): it represents the amount of energy added to a unit weight of the fluid
as it passes through the pump.

 It is calculated from the general energy equation.

 Because there are large clearances between the rotating

impeller and the casing of the pump.

 This accounts for the decrease in capacity as the total head

increases. Indeed, at a cut-off head, the flow is stopped
completely when all of the energy input from the pump
goes to maintain the head.
Figure (13) : Performance curve for a
 Of course, the typical operating head is well below the cut- centrifugal pump total head versus capacity.
off head so that high capacity can be achieved.
PERFORMANCE DATA FOR CENTRIFUGAL PUMPS:
Cont.
 The efficiency and power required are
also important to the successful operation of
a pump.

 The beside Figure shows a more complete

performance rating of a pump,
superimposing head, efficiency, and power
curves and plotting all three versus capacity.

 Normal operation should be in the vicinity

of the peak of the efficiency curve, with peak
efficiencies in the range of 60-80 percent
being typical for centrifugal pumps..

Figure (14) : Centrifugal pump performance curves.

AFFINITY LAWS FOR CENTRIFUGAL PUMPS:

 Most centrifugal pumps can be operated at different speeds to obtain varying capacities.

 A given size of pump casing can accommodate impellers of differing diameters.

 The manner in which capacity, head, and power vary when either speed or impeller diameter is
varied. These relationships, called affinity laws.

 The symbol N refers to the rotational speed of the impeller, usually in revolutions per minute (r/min, or
rpm).

When speed varies:

𝑄1 𝑁1
a. Capacity varies directly with speed: =
𝑄2 𝑁2

( )
2
h𝑎 1 𝑁1
b. The total head capability varies with the square of the speed: =
h𝑎 2 𝑁2

( )
3
𝑃1 𝑁1
c. The power required by the pump varies with the cube of the speed: =
𝑃2 𝑁2
AFFINITY LAWS FOR CENTRIFUGAL PUMPS:

When impeller diameter varies:

𝑄 1 𝐷1
a. Capacity varies directly with impeller diameter: =
𝑄 2 𝐷2

( )
2
h𝑎 1 𝐷1
b. The total head varies with the square of the impeller diameter: =
h𝑎 2 𝐷2

( )
3
𝑃1 𝐷1
c. The power required by the pump varies with the cube of the impeller diameter: =
𝑃2 𝐷2
Example #1:

Assume that the pump for which the performance data are plotted in the figure (15) below, was operating at a
rotational speed of 1750 rpm and that the impeller diameter was 13 in. First determine the head that would
result in a capacity of 1500 gal/min and the power required to drive the pump. Then, compute the
performance at a speed of 1250 rpm.

Figure (15) : Centrifugal pump performance curves.

Example #1: Solution

From Figure (1), projecting upward from =1500 gal/min gives:

Total head=130 ft.=
Power required=50 hp=

When the speed is changed to 1250 rpm, the new performance can be computed by using the affinity laws:

Capacity: =1071 gal/min

Head: =66.3 ft.
Power: =18.2 hp
 MANUFACTURERS' DATA FOR CENTRIFUGAL PUMPS:
 Many manufacturers of centrifugal pumps for
industrial applications use a designation system that
provides useful data for the size of the pump.

 For example, a pump carry the designation, 2 X 3 – 10

could be explained as follows:

 The 2 X 3 - 10 centrifugal pump is one with a:

 2-in. discharge connection.

 3-in. suction connection.
 A casing that can accommodate an impeller diameter
of 10 in. or smaller.

Figure (16) : Composite rating chart for a line of

centrifugal pumps.
MANUFACTURERS' DATA FOR CENTRIFUGAL PUMPS: Cont.
Effect of Impeller Size
 Beside figure shows how the performance
of the sample 2 X 3 - 10 centrifugal pump
varies as the size of the impeller varies.

 Shown are the capacity-versus-head curves

for five different sizes of impellers from 6
in. to 10 in. within the same casing.

Figure (17) : Illustration of pump performance for

different impeller diameters. Performance chart for a
2 X 3 - 10 centrifugal pump at 3500 rpm.
MANUFACTURERS' DATA FOR CENTRIFUGAL PUMPS: Cont.
Effect of Speed:
Beside Figure shows the performance of the same 2 X 3 – 10 pump operating at 1750 rpm (a typical
operating speed for a four-pole AC motor) instead of 3500 rpm.

Figure (18) : Pump performance for a 2 X 3 - 10 centrifugal pump operating at 1750 rpm
& 3500 rpm respectively.
MANUFACTURERS' DATA FOR CENTRIFUGAL PUMPS: Cont.
Required Power:
Figure (19) is the same as Figure (17), except that the curves showing the power required to drive the pump at
3500 rpm have been added.

Figure (19) : Illustration of power performance for different impeller diameters with power required.
Performance chart for a 2 X 3 - 10 centrifugal pump at 3500 rpm.
MANUFACTURERS' DATA FOR CENTRIFUGAL PUMPS: Cont.
Efficiency:
Figure (20) is the same as Figure (17), except that curves of constant efficiency have been added..

Figure (20) : Illustration of power performance for different impeller diameters with
efficiency. Performance chart for a 2 X 3 - IO centrifugal pump at 3500 rpm.
MANUFACTURERS' DATA FOR CENTRIFUGAL PUMPS: Cont.
Net Positive Suction Head Required: (NPSHR)
 Net positive suction head required (NPSHR) is an important factor to consider in applying a pump.
(NPSHR) is related to the pressure at the inlet to the pump.
 A low (NPSHR) is desirable.
 after locating a point in the chart for a particular set of total head and capacity, (NPSHR) read from the
given set of curves as shown the figure below.

Figure (20) : Illustration of pump performance for different impeller diameters with net positive
suction head required. Performance chart for a 2 X 3 - 10 centrifugal pump at 3500 rpm.
Net Positive Suction Head Required: (NPSHR) Cont.

Figure (21) : Complete pump performance chart for a 2 X 3 - 10 centrifugal pump at

3500 rpm. 33
Example #2:

A centrifugal pump must deliver at least 200 gal/min of water at a total head of 300 ft
of water. Specify a suitable pump. List its performance characteristics.

Solution:

One possible solution can be found from Figure (21):

 The 2 x 3 - 10 pump with a 9-in impeller will deliver approximately 229 gal/min at
300 ft. of head.
 At this operating point, the efficiency would be 58 %, near the maximum for this
type of pump. Approximately 30 hp would be required. The (NPSHR)at the suction
inlet to the pump is approximately 8.8 ft of water.

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Additional Performance Charts:

Figure (22) : Model TE-6 centrifugal pump.

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Net Positive Suction Head :

The basic issues of NPSH are:

 Preventing a condition called cavitation, because of its extreme detrimental effects

on the pump.

 The effect of the vapor pressure of the fluid being pumped on the onset of
cavitation.

 The piping system design considerations that affect NPSH.

 The (NPSHR)for the selected pump must be satisfied.

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Cavitation & Vapor Pressure:

Cavitation:
When the suction pressure at the pump inlet is too low, vapor bubbles form in the
fluid in a manner similar to boiling.

Vapor Pressure:
The fluid property that determines the conditions under which vapor bubbles form in
a fluid is its vapor pressure (Pvp) typically reported as an absolute pressure in the
units of kPa absolute or psi(absolute).

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Additional Performance Charts:

Table (2) : Vapor pressure and vapor pressure head of water.

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NPSHA & NPSHR :

For the net positive suction head calculations, the vapor pressure head (hvp)rather
than the basic vapor pressure (Pvp)

Where:
 NPSHA : available Net Positive Suction Head.
M = NPSHA - NPSHR
 NPSHR : Required Net Positive Suction Head.

 In design problems in this handout, we call for a minimum of 10 % margin. That is:

NPSHA > 1.1NPSHR

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NPSHA:

eqn. (1)

Sign Convention for :

 If the pump is below the reservoir, is positive.

 If the pump is above the reservoir, is negative.

=Vapor pressure (absolute) of the liquid at the pumping temperature.

=Vapor pressure head of the liquid at the pumping temperature, expressed in meters or feet of the liquid

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NPSHA: Cont.

Figure (23) : Vapor Pump suction-line details and definitions of terms for computing NPSH.
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Example #3:

Solution:

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Moody's diagram:

Figure (24) : Moody's diagram 43

Example #3: Solution Cont.

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Example #3: Solution Cont.

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SUCTION LINE DETAILS:
 The suction line refers to all parts of the flow system from the source of the fluid to
the inlet of the pump.
 special conditions may require auxiliary devices, such as:
1. highly recommended to install a pressure gauge in the suction line near the pump to monitor the condition of the fluid and
to detect the tendency for cavitation to develop.

2. A strainer should be installed either at the inlet or elsewhere in the suction piping to keep debris out of the pump and out
of the process to which the fluid is to be delivered.

3. the pipe size for the suction line should, should be larger than the inlet connection on the pump to reduce flow velocity
and friction losses.

4. Pipe alignment should eliminate the possibility of forming air bubbles or air pockets in the suction line because this will
cause the pump to lose capacity and possibly
to lose prime.

5. If a reducer is required, it should be of the eccentric type

DISCHARGE LINE DETAILS:
 In general, the discharge line should be as short and direct as possible to minimize the head on
the pump.
 Elbows should be of the standard or long-radius type if possible.
 Pipe size should be chosen according to velocity or allowable friction losses.
 The discharge line should contain a valve close to the pump to allow service or pump replacement.
 A pressure relief valve will protect the pump and other equipment in case of a blockage of the flow
or accidental shut-off of a valve.
 A check valve prevents flow back through the pump when it is not running and it should be placed
between the shut-off valve and the pump.

Figure (25) : Discharge line details.

THE SYSTEM RESISTANCE CURVE:

The operating point of a pump is defined as:

the volume flow rate the pump will deliver when installed in a given system and working against a
particular total head.

The piping system includes several elements valves, elbows, process elements, and connecting
straight lengths of pipe. The pump must accomplish the following tasks:

1. Elevate the fluid from a lower tank or other source to an upper tank or destination point.
2. Increase the pressure of the fluid from the source point to the destination point.
3. Overcome the resistance caused by pipe friction, valves, and fittings.
4. Overcome the resistance caused by processing elements as described in Section 13.11.
5. Supply energy related to the operation of flow control valves that inherently cause changes to the
system head to achieve the desired flow rates.
THE SYSTEM RESISTANCE CURVE: Cont.

The first two items in the previous list are components of the static head, h0, for the system, where
the name refers to the fact that the pump must overcome these resistances before any fluid begins to
move, that is, the fluid is static. The static head h0, is defined as:

eqn. (2)

As soon as fluid starts to flow through the pipes, valves, fittings, and processing elements of the
system, more head is developed because of the energy losses that occur. Recall that the energy
losses are proportional to the velocity head in the pipes ( v2 /2g)
THE SYSTEM RESISTANCE CURVE: Cont.

This causes the characteristic shape of a system resistance curve (SRC), sometimes
called a second degree curve, as shown in Figure (26) below:

Figure (26) : General shape of the system resistance curve (SRC)

for a pumped fluid flow system.
Example #4:

Figure (27) : General System for Example Problem #4. 51

Example #4: Cont.
Solution:

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Example #4: Cont.
Solution:

53
Example #4: Cont.
Solution:

Figure (28) : General

Spreadsheet calculation for the
total head on the pump at the
desired operating point for
Example Problem #4.

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Example #4: Cont.
Solution:

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Example #4: Cont.
Solution:

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PUMP SELECTION AND THE OPERATING POINT FOR THE SYSTEM:

Guidelines for Pump Selection:

Given the desired operating point for the system with the desired flow rate and the expected
total head on the pump:

1. Seek a pump with high efficiency at the design point and one for which the operating point is
near the best efficiency point (BEP) for the pump.

2. For the selected pump, specify the model designation, speed, impeller size, and the sizes for
the suction and discharge ports.

3. At the actual operating point, determine the power required, the actual volume flow rate
delivered, efficiency, and the NPSHR. Also, check the type of pump, mounting requirements, and
types and sizes for the suction and discharge lines to ensure that they are compatible
with the intended installation.
PUMP SELECTION AND THE OPERATING POINT FOR THE SYSTEM:

Guidelines for Pump Selection: Cont.

4. Compute the NPSHA for the system, using Eq. (1).

5. Ensure that NPSHA > 1.10 NPSHR for all

expected operating conditions.

Figure (29) : Generic illustration of the operating point

for a pump in a fluid flow system.
Example #5:

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Example #5: Cont.
Solution:

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Example #5: Cont.
Solution:

Figure (30) : Operating point Operating point for Example Problem #5.
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Example #5: Cont.
Solution:

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Example #5: Cont.
Solution:

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Example #5: Cont.
Solution:

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