PUMP HANDBOOK
GRUNDFOS Management A/S
Poul Due Jensens Vej 7
DK-8850 Bjerringbro
Tel: +45 87 50 14 00
www.grundfos.com
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96563258 1104
PUMP HANDBOOK
Copyright 2004 GRUNDFOS Management A/S. All rights reserved.
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may be reproduced in any form or by any means without prior written permission from
GRUNDFOS Management A/S.
Disclaimer
All reasonable care has been taken to ensure the accuracy of the contents of this material,
however GRUNDFOS Management A/S shall not be reliable for any loss whether direct,
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content of the material.
Foreword
The manufacturing industry places heavy demand on pumps, when it comes to
optimum operation, high reliability and low energy consumption. Therefore,
Grundfos has developed the Pump handbook, which in a simple manner deals
with various considerations when dimensioning pumps and pump systems.
We have elaborated a handbook for engineers and technicians who work with
design and installation of pumps and pump systems, containing answers to a
wide range of technical pump specific questions. The Pump handbook can either
be read from one end to the other or partly on specific topics.
The handbook is divided into 5 chapters which deal with different phases when
designing pump systems.
Throughout chapter 1 we make a general presentation of different pump types
and components. Here we also describe which precautions to adopt when dealing
with viscous liquids. Further, the most used materials as well as different types of
corrosion are presented here. The most important terminologies in connection
with reading the pump performance are presented in chapter 2. Chapter 3 deals
with system hydraulics and some of the most important factors to consider to obtain
optimum operation of the pump system. As it is often necessary to adjust the pump
performance by means of different adjustment methods, these are dealt with in
chapter 4. Chapter 5 describes the life cycle costs as energy consumption plays an
important role in today’s pumps and pump systems.
We sincerely hope that you will make use of The pump handbook and find it useful in
your daily work.
Segment Director Business Development Manager
Allan Skovgaard Claus Bærnholdt Nielsen
Chapter 1 Design of pumps and motors ......................7
Section 1.1 Pump construction ............................................................ 8
1.1.1 The centrifugal pump .................................................................. 8
1.1.2 Pump curves ................................................................................................9
1.1.3 Characteristics of the centrifugal pump ......................... 11
1.1.4 Most common end-suction and
in-line pump types .............................................................................. 12
1.1.5 Impeller types (axial forces) .......................................................14
1.1.6 Casing types (radial forces) ......................................................... 15
1.1.7 Single-stage pumps ........................................................................... 15
1.1.8 Multistage pumps ...............................................................................16
1.1.9 Long-coupled and close-coupled pumps ........................16
Section 1.2 Types of pumps ...................................................................17
1.2.1 Standard pumps .................................................................................... 17
1.2.2 Split-case pumps .................................................................................. 17
1.2.3 Hermetically sealed pumps ........................................................18
1.2.4 Sanitary pumps ..................................................................................... 20
1.2.5 Wastewater pumps ............................................................................21
1.2.6 Immersible pumps .............................................................................. 22
1.2.7 Borehole pumps ....................................................................................23
1.2.8 Positive displacement pumps ...................................................24
Section 1.3 Mechanical shaft seals ..................................................27
1.3.1 The mechanical shaft seal’s
components and function ............................................................29
1.3.2 Balanced and unbalanced shaft seals .............................. 30
1.3.3 Types of mechanical shaft seals .............................................31
1.3.4 Seal face material combinations ...........................................34
1.3.5 Factors affecting the seal performance ...........................36
Section 1.4 Motors .................................................................................... 39
1.4.1 Standards .................................................................................................... 40
1.4.2 Motor start-up .......................................................................................46
1.4.3 Voltage supply ........................................................................................47
1.4.4 Frequency converter ..........................................................................47
1.4.5 Motor protection ................................................................................. 49
Section 1.5 Liquids .......................................................................................53
1.5.1 Viscous liquids ........................................................................................54
1.5.2 Non-Newtonian liquids .................................................................. 55
1.5.3 The impact of viscous liquids on the
performance of a centrifugal pump .................................... 55
1.5.4 Selecting the right pump for a liquid
with antifreeze .......................................................................................56
1.5.5 Calculation example ..........................................................................58
1.5.6 Computer aided pump selection for dense and
viscous liquids .........................................................................................58
Section 1.6 Materials ................................................................................ 59
1.6.1 What is corrosion? .............................................................................. 60
1.6.2 Types of corrosion ................................................................................61
1.6.3 Metal and metal alloys ...................................................................65
1.6.4 Ceramics ....................................................................................................... 71
1.6.5 Plastics ............................................................................................................ 71
1.6.6 Rubber ............................................................................................................72
1.6.7 Coatings ........................................................................................................73
Chapter 2 Installation and performance
reading .............................................................................................................75
Section 2.1 Pump installation ............................................................76
2.1.1 New installation ....................................................................................76
2.1.2 Existing installation-replacement ........................................76
2.1.3 Pipe flow for single-pump installation ............................. 77
2.1.4 Limitation of noise and vibrations .......................................78
2.1.5 Sound level (L) .........................................................................................81
Section 2.2 Pump performance ........................................................ 83
2.2.1 Hydraulic terms .....................................................................................83
2.2.2 Electrical terms ...................................................................................... 90
2.2.3 Liquid properties ...................................................................................93
Table of Contents
Chapter 3 System hydraulic ....................................................95
Section 3.1 System characteristics .................................................96
3.1.1 Single resistances .................................................................................97
3.1.2 Closed and open systems ............................................................ 98
Section 3.2 Pumps connected in series and parallel
...................101
3.2.1 Pumps in parallel ................................................................................101
3.2.2 Pumps connected in series ....................................................... 103
Chapter 4 Performance adjustment
of pumps ..................................................................................................... 105
Section 4.1 Adjusting pump performance ..............................106
4.1.1 Throttle control ....................................................................................107
4.1.2 Bypass control .......................................................................................107
4.1.3 Modifying impeller diameter ................................................. 108
4.1.4 Speed control ........................................................................................ 108
4.1.5 Comparison of adjustment methods ...............................110
4.1.6 Overall efficiency of the pump system ........................... 111
4.1.7 Example: Relative power consumption
when the flow is reduced by 20% ........................................ 111
Section 4.2 Speed-controlled pump solutions .................... 114
4.2.1 Constant pressure control ..........................................................114
4.2.2 Constant temperature control ...............................................115
4.2.3 Constant differential pressure in a
circulating system ............................................................................. 115
4.2.4 Flow-compensated differential
pressure control ...................................................................................116
Section 4.3 Advantages of speed control .................................117
Section 4.4 Advantages of pumps with integrated
frequency converter ............................................................................... 118
4.4.1 Performance curves of speed-controlled
pumps ...........................................................................................................119
4.4.2 Speed-controlled pumps in different systems ........119
Section 4.5 Frequency converter .................................................... 122
4.5.1 Basic function and characteristics ......................................122
4.5.2 Components of the frequency converter .....................122
4.5.3 Special conditions regarding frequency
converters ................................................................................................124
Chapter 5 Life cycle costs calculation ....................... 127
Section 5.1 Life cycle costs equation ............................................ 128
5.1.1 Initial costs, purchase price (Cic) ........................................... 129
5.1.2 Installation and commissioning costs (Cin) ................ 129
5.1.3 Energy costs (Ce) ................................................................................. 130
5.1.4 Operating costs (Co) ......................................................................... 130
5.1.5 Environmental costs (C
env
) ......................................................... 130
5.1.6 Maintenance and repair costs (Cm) .....................................131
5.1.7 Downtime costs, loss of production (Cs) .......................131
5.1.8 Decommissioning and disposal costs (Co) ...................131
Section 5.2 Life cycle costs calculation
– an example ................................................................................................132
Appendix .........................................................................................................133
A) Notations and units .........................................................................134
B) Unit conversion tables ...................................................................135
C) SI-prefixes and Greek alphabet ............................................ 136
D) Vapour pressure and density of water at
different temperatures ................................................................. 137
E) Orifice ..........................................................................................................138
F) Change in static pressure due to change
in pipe diameter ................................................................................. 139
G) Nozzles ........................................................................................................140
H) Nomogram for head losses in
bends, valves, etc. ..............................................................................141
I) Pipe loss nomogram for clean water 20˚C ..................142
J) Periodical system ................................................................................143
K) Pump standards ................................................................................. 144
L) Viscosity for different liquids as a function
of liquid temperature ....................................................................145
Index ..................................................................................................................151
Chapter 1. Design of pumps and motors
Section 1.1: Pump construction
1.1.1 The centrifugal pump
1.1.2 Pump curves
1.1.3 Characteristics of the centrifugal pump
1.1.4 Most common end-suction and in-line
pump types
1.1.5 Impeller types (axial forces)
1.1.6 Casing types (radial forces)
1.1.7 Single-stage pumps
1.1.8 Multistage pumps
1.1.9 Long-coupled and close-coupled pumps
Section 1.2: Types of pumps
1.2.1 Standard pumps
1.2.2 Split-case pumps
1.2.3 Hermetically sealed pumps
1.2.4 Sanitary pumps
1.2.5 Wastewater pumps
1.2.6 Immersible pumps
1.2.7 Borehole pumps
1.2.8 Positive displacement pumps
Section 1.1
Pump construction
1.1.1 The centrifugal pump
In 1689 the physicist Denis Papin invented the centrifugal
pump and today this kind of pump is the most used around
the world. The centrifugal pump is built on a simple
principle: Liquid is led to the impeller hub and by means
of the centrifugal force it is flung towards the periphery of
the impellers.
The construction is fairly inexpensive, robust and simple
and its high speed makes it possible to connect the pump
directly to an asynchronous motor. The centrifugal pump
provides a steady liquid flow, and it can easily be throttled
without causing any damage to the pump.
Now let us have a look at figure 1.1.1, which shows the
liquid’s flow through the pump. The inlet of the pump
leads the liquid to the centre of the rotating impeller from
where it is flung towards the periphery. This construction
gives a high efficiency, and is suitable for handling pure
liquids. Pumps, which have to handle impure liquids, such
as wastewater pumps, are fitted with an impeller that is
constructed especially to avoid that objects get stocked
inside the pump, see section 1.2.5.
If a pressure difference occurs in the system while the
centrifugal pump is not running, liquid can still pass
through it due to its open design.
As you can tell from figure 1.1.2, the centrifugal pump can
be categorised in different groups: Radial flow pumps,
mixed flow pumps and axial flow pumps. Radial flow
pumps and mixed flow pumps are the most common
types used. Therefore, we will only concentrate on these
types of pumps on the following pages.
However, we will briefly present the positive displacement
pump in section 1.2.8.
The different demands on the centrifugal pump’s
performance, especially with regards to head, flow, and
installation, together with the demands for economical
operation, are only a few of the reasons why so many types
of pump exist. Figure 1.1.3 shows the different pump types
with regard to flow and pressure.
Fig. 1.1.2: Different kinds of centrifugal pumps
Fig. 1.1.1: The liquid’s flow through the pump
Radial flow pump
Mixed flow pump
Axial flow pump
Fig. 1.1.3: Flow and head for different types of centrifugal pumps
1 22446610
1
10
1
2
4
6
10
2
2
4
6
10
3
2
4
6
10
4
H
[m]
Q [m
3
/s]
2 4 6 10
2
2 4 6 10
3
2 4 6 10
4
2 4 6 10
5
Multistage radial
flow pumps
Single-stage radial
flow pumps
Mixed flow pumps
Axial flow pumps
8
1.1.2 Pump curves
Before we dig any further into the world of pump
construction and pump types, we will present the
basic characteristics of pump performance curves. The
performance of a centrifugal pump is shown by a set
of performance curves. The performance curves for a
centrifugal pump are shown in figure 1.1.4. Head, power
consumption, efficiency and NPSH are shown as a function
of the flow.
Normally, pump curves in data booklets only cover the
pump part. Therefore, the power consumption, the P2value, which is listed in the data booklets as well, only
covers the power going into the pump – see figure 1.1.4.
The same goes for the efficiency value, which only covers
the pump part (η = ηP).
In some pump types with integrated motor and possibly
integrated frequency converter, e.g. canned motor pumps
(see section 1.2.3), the power consumption curve and the
η-curve cover both the motor and the pump. In this case it
is the P1-value that has to be taken into account.
In general, pump curves are designed according to ISO
9906 Annex A, which specifies the tolerances of the
curves:
• Q +/- 9%,
• H +/-7%,
• P +9%
• -7%.
What follows is a brief presentation of the different pump
performance curves.
Head, the QH-curve
The QH-curve shows the head, which the pump is able
to perform at a given flow. Head is measured in meter
liquid column [mLC]; normally the unit meter [m] is
applied. The advantage of using the unit [m] as the unit
of measurement for a pump’s head is that the QH-curve is
not affected by the type of liquid the pump has to handle,
see section 2.2 for more information.
H
[m]
η
[%]
50
40
70
Efficiency
60
50
40
20
10
2
12
4
6
8
10
0
30
30
20
10
0
10
0
2
4
6
8
0 10 20 30 40 50 60 70
Q [m3/h]
P
2
[kW]
NP
SH
(m)
Power consumption
NP
SH
Fig. 1.1.4: Typical performance curves for a centrifugal
pump. Head, power consumption, efficiency and NPSH
are shown as a function of the flow
Fig. 1.1.5: The curves for power consumption and efficiency will
normally only cover the pump part of the unit – i.e. P2 and η
P
P
1
P
2
HM
3~
η
M η
P
Q
H
[m]
50
60
40
30
20
10
0
0 10 20 30 40 50 60 70 80
Q [m3/h]
Fig. 1.1.6: A typical QH-curve for a centrifugal pump;
low flow results in high head and high flow results
in low head
9
Efficiency, the η-curve
The efficiency is the relation between the supplied power
and the utilised amount of power. In the world of pumps,
the efficiency ηP is the relation between the power, which
the pump delivers to the water (PH) and the power input
to the shaft (P2 ):
where:
ρ is the density of the liquid in kg/m
3
,
g is the acceleration of gravity in m/s2,
Q is the flow in m3/s and H is the head in m.
For water at 20oC and with Q measured in m3/h and H in m,
the hydraulic power can be calculated as :
As it appears from the efficiency curve, the efficiency depends
on the duty point of the pump. Therefore, it is important to
select a pump, which fits the flow requirements and ensures
that the pump is working in the most efficient flow area.
Power consumption, the P2-curve
The relation between the power consumption of the pump
and the flow is shown in figure 1.1.8. The P2-curve of most
centrifugal pumps is similar to the one in figure 1.1.8 where
the P2 value increases when the flow increases.
NPSH-curve (Net Positive Suction Head)
The NPSH-value of a pump is the minimum absolute
pressure (see section 2.2.1) that has to be present at the
suction side of the pump to avoid cavitation.
The NPSH-value is measured in [m] and depends on the
flow; when the flow increases, the NPSH-value increases
as well; figure 1.1.9. For more information concerning
cavitation and NPSH, go to section 2.2.1.
50
60
70
80
40
30
20
10
0
0 10 20 30 40 50 60 70
Q [m3/h]
η
[%]
8
10
6
4
2
0
0 10 20 30 40 50 60 70
Q [m3/h]
P
2
[kW]
Fig. 1.1.7: The efficiency curve of a typical centrifugal pump
Fig. 1.1.8: The power consumption curve of a typical
centrifugal pump
10
0
2
4
6
8
0 10 20 30 40 50 60 7
0
Q [m3/h]
NPSH
[m]
η
p
=
PH
P
2
=
ρ . g . Q . H
P
2
Fig. 1.1.9: The NPSH-curve of a typical centrifugal pump
PH = 2.72
.
Q . H [W]
10
Section 1.1
Pump construction
1.1.3 Characteristics of the
centrifugal pump
The centrifugal pump has several characteristics and in
this section, we will present the most important ones.
Later on in this chapter we will give a more thorough
description of the different pump types.
• The number of stages
Depending on the number of impellers in the pump, a
centrifugal pump can be either a single-stage pump or a
multistage pump.
• The position of the pump shaft
Single-stage and multistage pumps come with horizontal
or vertical pump shafts. These pumps are normally
designated horizontal or vertical pumps. For more
information, go to section 1.1.4.
• Single-suction or double-suction impellers
Depending on the construction of the impeller, a pump can
be fitted with either a single-suction impeller or a doublesuction impeller. For more information, go to section 1.1.5.
• Coupling of stages
The pump stages can be arranged in two different ways: in
series and in parallel, see figure 1.1.10.
• Construction of the pump casing
We distinguish between two types of pump casing: Volute
casing and return channel casing with guide vanes. For
more information, go to section 1.1.6.
Fig 1.1.10: Twin pump with parallel-coupled impellers
11
1.1.4 Most common end-suction and in-line pump types
End-suction pump = The liquid runs directly into the impeller. Inlet and outlet have a
90° angle. See section 1.1.9
In-line pump = The liquid runs directly through the pump in-line. The suction pipe and the discharge
pipe are placed opposite one another and can be mounted directly in the piping system
Split-case pump = Pump with an axially divided pump housing. See section 1.2.2
Horizontal pump = Pump with a horizontal pump shaft
Vertical pump = Pump with a vertical pump shaft
Single-stage pump = Pump with a single impeller. See section 1.1.7
Multistage pump = Pump with several series-coupled stages. See section 1.1.8
Long-coupled pump = Pump connected to the motor by means of a flexible coupling. The motor and
the pump have separate bearing constructions. See section 1.1.9
Close-coupled pump = A pump connected to the motor by means of a rigid coupling. See section 1.1.9
Horizontal
Close-coupled Close-coupled
End-suction
Single-stage
Long-coupled
Multistage
12
Section 1.1
Pump construction
13
Multistage
Horizontal / Vertical
Single-stage
Long-coupled Close-coupled
Close-coupled
In-line
Split-case
Single-stage
Long-coupled
Horizontal
14
1.1.5 Impeller types (axial forces)
A centrifugal pump generates pressure that exerts forces
on both stationary and rotating parts of the pump.
Pump parts are made to withstand these forces.
If axial and radial forces are not counterbalanced in the
pump, the forces have to be taken into consideration when
selecting the driving system for the pump (angular contact
bearings in the motor). In pumps fitted with single-suction
impeller, large axial forces may occur, figures 1.1.11 and
1.1.12. These forces are balanced in one of the following
ways:
• Mechanically by means of thrust bearings. These types
of bearings are specially designed to absorb the axial
forces from the impellers
• By means of balancing holes on the impeller,
see figure 1.1.13
• By means of throttle regulation from a seal ring
mounted on the back of the impellers, see figure 1.1.14
• Dynamic impact from the back of the impeller, see
figure 1.1.15
• The axial impact on the pump can be avoided by either
using double-suction impellers (see figure 1.1.16).
Fig. 1.1.11: Single-suction
impeller
Fig. 1.1.12: Standard pump with
single-suction impeller
Fig. 1.1.13: Balancing the axial forces
in a single-stage centrifugal pump
with balancing holes only
Fig. 1.1.14: Balancing the axial forces
in a single-stage centrifugal pump
with sealing gap at discharge side and
balancing holes
Fig. 1.1.15: Balancing the axial forces in
a single-stage centrifugal pump with
blades on the back of the impellers
Fig. 1.1.16: Balancing the axial
forces in a double-suction
impeller arrangement
Axial forces
Section 1.1
Pump construction
1.1.6 Casing types (radial forces)
Radial forces are a result of the static pressure in the
casing. Therefore, axial deflections may occur and lead
to interference between the impeller and the casing. The
magnitude and the direction of the radial force depend on
the flow rate and the head.
When designing the casing for the pump, it is possible
to control the hydraulic radial forces. Two casing types
are worth mentioning: the single-volute casing and the
double-volute casing. As you can tell from figure 1.1.18,
both casings are shaped as a volute. The difference between
them is, that the double-volute has an guide vane.
The single-volute pump is characterised by a symmetric
pressure in the volute at the optimum efficiency point,
which leads to zero radial load. At all other points,
the pressure around the impeller is not regular and
consequently a radial force is present.
As you can tell from figure 1.1.19, the double-volute casing
develops a constant low radial reaction force at any capacity.
Return channels (figure 1.1.20) are used in multistage pumps
and have the same basic function as volute casings. The
liquid is led from one impeller to the next and at the same
time, the rotation of water is reduced and the dynamic
pressure is transformed into static pessure. Because of the
return channel casing’s circular design, no radial forces are
present.
1.1.7 Single-stage pumps
Generally, single-stage pumps are used in applications,
which do not require a total head of more than 150 m.
Normally, single-stage pumps operate in the interval of
2-100 m.
Single-stage pumps are characterised by providing a low
head relative to the flow, see figure 1.1.3. The single-stage
pump comes in both a vertical and a horizontal design, see
figures 1.1.21 and 1.1.22.
15
Q/Qopt1.0
Volute casing
Double-volute
casing
Radial force
Fig. 1.1.18: Single-volute casing Double-volute casing
Fig. 1.1.22: Vertical single-stage
in-line close-coupled pump
Fig. 1.1.21: Horizontal single-stage
end-suction close-coupled pump
Radial forces
Fig. 1.1.17: Single-suction
impeller
Fig. 1.1.19: Radial force for single- and double-volute casing
Fig. 1.1.20: Vertical multistage
in-line pump with return
channel casing
Return channel
Fig. 1.1.25: Long-coupled pump
with basic coupling
Fig. 1.1.26: Long-coupled pump with spacer coupling
16
1.1.8 Multistage pumps
Multistage pumps are used in installations where a high
head is needed. Several stages are connected in series and
the flow is guided from the outlet of one stage to the inlet
of the next. The final head that a multistage pump can
deliver is equal to the sum of pressure each of the stages
can provide.
The advantage of multistage pumps is that they provide
high head relative to the flow. Like the single-stage pump,
the multistage pump is available in both a vertical and a
horizontal version, see figures 1.1.23 and 1.1.24.
1.1.9 Long-coupled and close-coupled
pumps
Long-coupled pumps
Long-coupled pumps are pumps with a flexible coupling
that connects the pump and the motor. This kind of
coupling is available either as a basic coupling or as a
spacer coupling.
If the pump is connected to the motor by a basic coupling,
it is necessary to dismount the motor when the pump
needs service. Therefore, it is necessary to align the pump
upon mounting, see figure 1.1.25.
On the other hand, if the pump is fitted with a spacer
coupling, it is possible to service the pump without
dismounting the motor. Alignment is thus not an issue,
see figure 1.1.26.
Close-coupled pumps
Close-coupled pumps can be constructed in the following
two ways: Either the pump has the impeller mounted
directly on the extended motor shaft or the pump has a
standard motor and a rigid or a spacer coupling, see figures
1.1.27 and 1.1.28.
Fig. 1.1.24: Horizontal multistage
end-suction pump
Fig. 1.1.23: Vertical
multistage in-line pump
Fig. 1.1.27: Close-coupled pump with
rigid coupling
Basic coupling
type
Long-coupled
pump with
flexible coupling
Close-coupled
pump with
rigid coupling
Spacer coupling
(option)
Fig. 1.1.28: Different coupling types
Section 1.1
Pump construction
17
Fig. 1.2.1: Long-coupled standard pump
Fig. 1.2.2: Bare shaft standard pump
Fig. 1.2.3: Long-coupled split-case pump
Fig. 1.2.4: Split-case pump
with double-suction impeller
1.2.1 Standard pumps
Few international standards deal with centrifugal pumps.
In fact, many countries have their own standards, which
more or less overlap one another. A standard pump is
a pump that complies with official regulations as to for
example the pump’s duty point. What follows, are a couple
of examples of international standards for pumps:
• EN 733 (DIN 24255) applies to end-suction centrifugal
pumps, also known as standard water pumps with a
rated pressure (PN) of 10 bar.
• EN 22858 (ISO 2858) applies to centrifugal pumps, also
known as standard chemical pumps with a rated
pressure (PN) of 16 bar, see appendix K.
The standards mentioned above cover the installation
dimensions and the duty points of the different pump
types. As to the hydraulic parts of these pump types, they
vary according to the manufacturer – thus, no international
standards are set for these parts.
Pumps, which are designed according to standards, provide
the end-user with advantages with regard to service, spare
parts and maintenance.
1.2.2 Split-case pumps
A split-case pump is a pump with the pump housing
divided axially into two parts. Figure 1.2.4 shows a singlestage split-case pump with a double-suction impeller.
The double-inlet construction eliminates the axial
forces and ensures a longer life span of the bearings.
Usually, split-case pumps have a rather high efficiency,
are easy to service and have a wide performance range.
Section 1.2
Types of pumps
Section 1.2
Types of pumps
18
1.2.3 Hermetically sealed pumps
It comes as no surprise that a pump’s shaft lead-in has to be
sealed. Usually, this is done by means of a mechanical shaft
seal, see figure 1.2.5. The disadvantage of the mechanical
shaft seal is its poor properties when it comes to handling
of toxic and aggressive liquids, which consequently leads
to leakage. These problems can to some extent be solved
by using a double mechanical shaft seal. Another solution
to these problems is to use a hermetically sealed pump.
We distinguish between two types of hermetically sealed
pumps: Canned motor pumps and magnetic-driven pumps.
In the following two sections, you can find additional
information about these pumps.
Canned motor pumps
A canned motor pump is a hermetically sealed pump with
the motor and pump integrated in one unit without a seal,
see figures 1.2.6 and 1.2.7. The pumped liquid is allowed to
enter the rotor chamber that is separated from the stator
by a thin rotor can. The rotor can serves as a hermetically
sealed barrier between the liquid and the motor. Chemical
pumps are made of materials, e.g. plastics or stainless
steel that can withstand aggressive liquids.
The most common canned motor pump type is the
circulator pump. This type of pump is typically used in
heating circuits because the construction provides low
noise and maintenance-free operation.
Fig. 1.2.5: Example of a standard pump with mechanical shaft seal
Liquid
Atmosphere
Seal
Fig. 1.2.7: Circulator pump
with canned motor
Motor can
Fig. 1.2.6: Chemical pump with canned motor
Motor can
Magnetic-driven pumps
In recent years, magnetic-driven pumps have become
increasingly popular for transferring aggressive and toxic
liquids.
As shown in figure 1.2.8, the magnetic-driven pump is
made of two groups of magnets; an inner magnet and
an outer magnet. A non-magnetizable can separate these
two groups. The can serves as a hermetically sealed barrier
between the liquid and the atmosphere. As it appears
from figure 1.2.9, the outer magnet is connected to the
pump drive and the inner magnet is connected to the
pump shaft. Hereby the torque from the pump drive is
transmitted to the pump shaft. The pumped liquid serves
as lubricant for the bearings in the pump. Therefore,
sufficient venting is crucial for the bearings.
19
Fig. 1.2.8: Construction of magnetic drive
Fig. 1.2.9: Magnetic-driven multistage pump
Can
Inner magnets
Outer magnets
Can
Inner magnets
Outer magnets
20
Fig. 1.2.10: Sanitary pump
Fig.1.2.11: Sanitary self-priming side-channel pump
1.2.4 Sanitary pumps
Sanitary pumps are mainly used in the food, beverage,
pharmaceutical and bio-technological industries where it
is important that the pumped liquid is handled in a gentle
manner and that the pumps are easy to clean.
In order to meet process requirements in these industries,
the pumps have to have a surface roughness between
3.2 and 0.4 µm Ra. This can be best achieved by using
forged or
deep-drawn rolled stainless steel as materials
of construction,
see figure 1.2.12. These materials have a
compact pore-free surface finish that can be easily worked
up to meet the various surface finish requirements.
The main features of a sanitary pump are ease of cleaning
and ease of maintenance.
The leading manufacturers of sanitary pumps have
designed their products to meet the following standards:
EHEDG –
European Hygienic Equipment Design Group
QHD – Qualified Hygienic Design
3-A – Sanitary Standards:
3A0/3A1: Industrial/Hygienic Standard
Ra ≤ 3.2 µm
3A2: Sterile Standard
Ra ≤ 0.8 µm
3A3: Sterile Standard
Ra ≤ 0.4 µm
Sand casting
Precision casting
Rolled steel
Fig.1.2.12: Roughness of material surfaces
Section 1.2
Types of pumps
1.2.5 Wastewater pumps
A wastewater pump is an enclosed unit with a pump
and a motor. Due to this construction the wastewater
pump is suitable for submersible installation in pits.
In submersible installations with auto-coupling systems
double rails are normally used. The auto-coupling system
facilitates maintenance, repair and replacement of the
pump. Because of the construction of the pump, it is
not necessary to enter the pit to carry out service. In
fact, it is possible to connect and disconnect the pump
automatically from the outside of the pit. Wastewater
pumps can also be installed dry like conventional pumps
in vertical or horizontal installations. Likewise this type of
installation provides easy maintenance and repair like it
provides uninterrupted operation of the pump in case of
flooding of the dry pit, see figure 1.2.14.
Normally, wastewater pumps have to be able to handle large
particles. Therefore they are fitted with special
impellers that
make it possible to avoid blockage and clogging. Different
types of impellers exist; single-channel impellers, doublechannel impellers three and four-channel impellers and
vortex impellers.
Figure 1.2.15 shows the different designs of
these impellers.
Wastewater pumps usually come with a dry motor, which
is IP68 protected (for more information on IP-classes,
go to section 1.4.1). Motor and pump have a common
extended shaft with a double mechanical shaft seal system
in an intermediate oil chamber, see figure 1.2.13.
Wastewater pumps are able to operate either intermittently
or continuously depending on the installation in question.
Fig. 1.2.14: Wastewater pump for dry installations
Fig. 1.2.15: Impeller types for wastewater
Vortex
impeller
Single-channel
impeller
Double-channel
impeller
21
Fig.1.2.13: Detail of a sewage pump
for wet installations
1.2.6 Immersible pumps
An immersible pump is a pump type where the pump part
is immersed in the pumped liquid and the motor is kept
dry. Normally, immersible pumps are mounted on top of
or in the wall of tanks or containers. Immersible pumps
are for example used in the machine tool industry
for
example in spark machine tools, grinding machines,
machining
centres and cooling units or in other industrial
applications involving tanks or containers, such as
industrial washing and filtering systems.
Pumps for machine tools can be divided in two groups:
Pumps for the clean side of the filter and pumps for the
dirty side of the filter. Pumps with closed impellers are
normally used for the clean side of the filter, because they
provide a high efficiency and a high pressure if necessary.
Pumps with open or semi-open impellers are normally
used for the dirty side of the filter, because they can
handle metal chips and particles.
Fig. 1.2.16: Immersible pump
22
Section 1.2
Types of pumps
1.2.7 Borehole pumps
Two types of borehole pumps exist: The submerged
borehole pump type with a submersible motor, and the
deep well pump with a dry motor, which is connected
to the pump by a long shaft. These pumps are normally
used in connection with water supply and irrigation. Both
pump types are made to be installed in deep and narrow
boreholes and have thus a reduced diameter, which makes
them longer than other pump types, see figure 1.2.17.
The borehole pumps are specially designed to be
submerged in a liquid and are thus fitted with a
submersible motor, which is IP68 protected. The pump
comes in both a single-stage and a multistage version (the
multistage version being the most common one), and is
fitted with a non-return valve in the pump head.
Today, the deep well pump has been more or less replaced
by the submerged pump type. The long shaft of the deep
well pump is a drawback, which makes it difficult to
install and carry out service. Because the deep well pump
motor is air-cooled, the pump is often used in industrial
applications to pump hot water from open tanks. The
submersible pump cannot handle as high temperatures
because the motor is submerged in the liquid, which has
to cool it.
Fig. 1.2.17: Submersible pump
23
1.2.8 Positive displacement pumps
The positive displacement pump provides an approximate
constant flow at fixed speed, despite changes in the
counterpressure. Two main types of positive displacement
pumps exist:
• Rotary pumps
• Reciprocating pumps
The difference in performance between a centrifugal
pump, a rotary pump and a reciprocating is
illustrated
to the right, figure 1.2.18.
Depending on which of these
pumps you are dealing with, a small change in the pump’s
counterpressure results in differences in the flow.
The flow of a centrifugal pump will change considerably,
the flow of a rotary pump will change a little, while the
flow of a reciprocating pump will hardly change at all.
But, why is there a difference between the pump curves
for reciprocating pumps and rotary pumps? The actual
seal face surface is larger for rotary pumps than for
reciprocating pumps. So, even though the two pumps are
designed with the same tolerances, the gap loss of the
rotary pump is larger.
The pumps are typically designed with the finest tolerances
possible to obtain the highest possible efficiency and
suction capability. However, in some cases, it is necessary
to increase the tolerances, for example when the pumps
have to handle highly viscous liquids, liquids containing
particles and liquids of high temperature.
Positive displacement pumps are pulsate, meaning that
their volume flow within a cycle is not constant.
The variation in flow and speed leads to pressure fluctuations
due to resistance in the pipe system and in valves.
Q
H
H
23
1
3
2 1
Fig. 1.2.19: Classification of positive displacement pumps
Simplex
Duplex
Simplex
Duplex
Triplex
Multiplex
Fig. 1.2.18: Typical relation between
flow and head for 3 different pump types:
1) Centrifugal pumps
2) Rotary pumps
3) Reciprocating pumps
24
Section 1.2
Types of pumps
Reciprocating
Rotary
Plunger
Diaphragm
Steam Double-acting
Power
Single-acting
Double-acting
Gear
Lobe
Circumferential piston
Screw
Vane
Piston
Flexible member
Screw
Single rotor
Multiple rotor
Positive
displacement
pumps
+
Dosing pumps
The dosing pump belongs to the positive displacement
pump family and is typically of the diaphragm type. Diaphragm
pumps are leakage-free, because the diaphragm forms
a seal between the liquid and the surroundings.
The diaphragm pump is fitted with two non-return valves
– one on the suction side and one on the discharge side
of the pump. In connection with smaller diaphragm pumps,
the diaphragm is activated by the connecting rod, which is
connected to an electromagnet. Thereby the coil receives
the exact amount of strokes needed, see figure 1.2.21.
In connection with larger diaphragm pumps the
diaphragm is typically mounted on the connecting
rod,
which is activated by a camshaft. The camshaft is turned by
means of a standard asynchronous motor, see figure 1.2.22.
The flow of a diaphragm pump is adjusted by either
changing the stroke length and/or the frequency of the
strokes. If it is necessary to enlarge the operating area,
frequency converters can be connected to the larger
diaphragm pumps, see figure 1.2.22.
Yet, another kind of diaphragm pump exists. In this case,
the diaphragm is activated by means of an excentrically
driven connecting rod powered by a stepper motor or a
synchronous motor, figures 1.2.20 and 1.2.23. By using a
stepper motor drive the pump’s dynamic area is increased
and its
accuracy is improved considerably. With this
construction
it is no longer necessary to adjust the pump’s
stroke length because the connection rod is mounted
directly on the diaphragm. The result is optimised suction
conditions and excellent operation features.
So therefore, it is simple to control both the suction
side and the discharge side of the pump. Compared to
traditional electromagnetic-driven diaphragm pumps
which provide powerful pulsations, stepper motor-driven
diaphragm pumps make it possible to get a much more
steady dosage of additive.
Fig.1.2.21: Solenoid spring return
1.2.22: Cam-drive spring return
1.2.23: Crank drive
Fig. 1.2.20: Dosing pump
+
25
Chapter 1. Design of pumps and motors
Section 1.3: Mechanical shaft seals
1.3.1 The mechanical shaft seal’s components
and function
1.3.2 Balanced and unbalanced shaft seals
1.3.3 Types of mechanical shaft seals
1.3.4 Seal face material combinations
1.3.5 Factors affecting the seal performance
Section 1.3
Mechanical shaft seals
From the middle of the 1950s mechanical shaft seals
gained ground in favour of the traditional sealing method
- the stuffing box. Compared to stuffing boxes, mechanical shaft seals provide the following advantages:
• They keep tight at smaller displacements and vibrations
in the shaft
• They do not require any adjustment
• Seal faces provide a small amount of friction and thus,
minimise the power loss
• The shaft does not slide against any of the seal’s
components and thus, is not damaged because of
wear (reduced repair costs).
The mechanical shaft seal is the part of a pump that
separates the liquid from the atmosphere. In figure 1.3.1
you can see a couple of examples where the mechanical
shaft seal is mounted in different types of pumps.
The majority of mechanical shaft seals are made
according to the European standard EN 12756.
Before choosing a shaft seal, there are certain things
you need to know about the liquid and thus the
seal’s resistance to the liquid:
• Determine the type of liquid
• Determine the pressure that the shaft seal is exposed to
• Determine the speed that the shaft seal is exposed to
• Determine the built-in dimensions
On the following pages we will present how a mechanical shaft
seal works, the different types of seal, which kind of materials
mechanical shaft seals are made of and which factors that
affect the mechanical shaft seal’s performance.
28
Fig. 1.3.1: Pumps with mechanical shaft seals
1.3.1 The mechanical shaft seal’s
components and function
The mechanical shaft seal is made of two main components:
a rotating part and a stationary part; and consists of the parts
listed in figure 1.3.2. Figure 1.3.3 shows where the different
parts are placed in the seal.
• The stationary part of the seal is fixed in the pump
housing. The rotating part of the seal is fixed on the
pump shaft and rotates when the pump operates.
• The two primary seal faces are pushed against each other
by the spring and the liquid pressure. During operation
a liquid film is produced in the narrow gap between the
two seal faces. This film evaporates before it enters the
atmosphere, making the mechanical shaft seal liquid tight,
see figure 1.3.4.
• Secondary seals prevent leakage from occurring
between the assembly and the shaft.
• The spring presses the seal faces together mechanically.
• The spring retainer transmits torque from the shaft to
the seal. In connection with mechanical bellows shaft
seals, torque is transferred directly through the bellows.
Seal gap
During operation the liquid forms a lubricating film
between the seal faces. This lubricating film consists of a
hydrostatic and a hydrodynamic film.
• The hydrostatic element is generated by the pumped
liquid which is forced into the gap between the seal faces.
•
The hydrodynamic lubricating film is created by
pressure generated by the shaft’s rotation.
Fig. 1.3.4: Mechanical shaft seal in operation
Lubrication film
Liquid force
Spring force
Vapour
Evaporation
begins
Fig. 1.3.3: Main components of the
mechanical shaft seal
Rotating part
Stationary part
Shaft
Primary seal
Secondary seal
Primary seal
Secondary seal
Spring
Spring retainer
Mechanical shaft seal Designation
Seal face (primary seal)
Secondary seal
Spring
Spring retainer (torque transmission
)
Seat (seal faces, primary seal)
Static seal (secondary seal
)
Rotating par
t
Stationary par
t
Fig. 1.3.2:
The mechanical shaft seal’s components
29
1.3.2 Balanced and unbalanced shaft seals
To obtain an acceptable face pressure between the
primary seal faces, two kind of seal types exist: a balanced
shaft seal and an unbalanced shaft seal.
Balanced shaft seal
Figure 1.3.6 shows a balanced shaft seal indicating where
the forces interact on the seal.
Unbalanced shaft seal
Figure 1.3.7 shows an unbalanced shaft seal indicating
where the forces interact on the seal.
Several different forces have an axial impact on the seal
faces. The spring force and the hydraulic force from the
pumped liquid press the seal together while the force from
the lubricating film in the seal gap counteracts this. In
connection with high liquid pressure, the hydraulic forces
can be so powerful that the lubricant in the seal gap cannot
counteract the contact between the seal faces. Because the
hydraulic force is proportionate to the area that the liquid
pressure affects, the axial impact can only be reduced by
obtaining a reduction of the pressure-loaded area.
The thickness of the lubricating film depends on the pump
speed, the liquid temperature, the viscosity of the liquid
and the axial forces of the mechanical shaft seal. The
liquid is continuously changed in the seal gap because of
• evaporation of the liquid to the atmosphere
• the liquid’s circular movement
Figure 1.3.5 shows the optimum ratio between fine
lubrication properties and limited leakage. As you can tell,
the optimum ratio is when the lubricating film covers the
entire seal gap, except for a very narrow evaporation
zone
close to the atmospheric side of the mechanical shaft seal.
Leakage due to deposits on the seal faces is often observed.
When using coolant agents, deposits are built up quickly by
the evaporation at the atmosphere side of the seal. When
the
liquid evaporates in the evaporation zone, microscopic
solids
in the liquid remain in the seal gap as deposits
creating wear.
These deposits are seen in connection with most types
of liquid. But when the pumped liquid has a tendency
to crystallise, it can become a problem. The best way to
prevent wear is to select seal faces made
of hard material,
such as tungsten carbide (WC) or silicon carbide (SiC).
The narrow seal gap between these materials (app. 0.3 µm
Ra) minimises the risk of solids entering the seal gap and
thereby minimises the amount of deposits building up.
Pressure
LiquidPump pressure
Stationary
seal face
Rotating
seal face
Vapour Atmosphere
Entrance
in seal
Exit into
atmospher
e
Start of
ev
aporation
1 atm
Fig. 1.3.6: Interaction of
forces on the balanced
shaft seal
Fig. 1.3.7: Interaction of
forces on the unbalanced
shaft seal
A
Spring forces
Hydraulic forces
Contact area of seal faces
B
A B
Hydraulic forces
Contact area of seal faces
Fig. 1.3.5: Optimum ratio between fine lubrication
properties and limited leakage
30
Section 1.3
Mechanical shaft seals
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