Seals and sealing handbook

637 Pages·2014·46.778 MB·English

by  Robert K Flitney

SIXTH EDITION
SEALS AND SEALING
HANDBOOK
SIXTH EDITION
SEALS AND SEALING
HANDBOOK
ROBERT FLITNEY
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FOREWORD
Since you are consulting this book, you are probably in the process of selecting or
replacing a seal for a particular application. This can be a difficult task, since of all the
different kinds of machine elements, the seal exhibits the greatest variety. There are
many different types and classes of seals, e.g. static and dynamic, rotary and recipro-
cating, packing, mechanical (or face) seals, and lip seals. The sizes of seals range from
millimetre scale (for micro-bearings) to metre scale (for canal locks). The prices range
from a few dollars for an O-ring to hundreds of thousands of dollars for nuclear reactor
coolant-pump seals.
This large variety results from the very wide range of seal applications, e.g. in actu-
ators, pumps, motors, mixers and bearings, as well as from the particular industry in
which the seal is used, e.g. automotive, aerospace, construction, agriculture, hydraulics
and pneumatics.
If you make a poor choice in selecting your seal, the result can be failure, usually in
the form of excessive leakage but sometimes in other forms such as excessive heat gen-
eration. When a machine breaks down, the odds are pretty high that it is the seal that
has failed. The cost of failure can be much higher than just the price of the seal, since it
would include removal and installation costs, costs associated with downtime and pos-
sibly costs associated with liability. Even if a seal does not radically fail, a poorly chosen
seal can result in unnecessarily high energy costs, lower efficiency, unfavourable system
performance (e.g. of hydraulic systems) and environmental pollution.
The various demands on a fluid seal are quite stringent, and some are contradic-
tory. The leakage must be low or effectively zero, but a contradictory demand is low
friction and low wear rate. Further, the seal material must be compatible with the
sealed fluid, and the design must allow the seal to function properly over the range of
required operating conditions, e.g. pressures and temperatures.
To satisfy these demands, seals have evolved into effective machine elements. While
they may appear deceptively simple compared to other machine elements, their opera-
tion is actually quite complex. Mechanical seals are precision devices, which must be
carefully engineered to function properly. Polymeric lip seals, both rotary and recipro-
cating, rely on micro-scale surface characteristics to operate successfully and therefore
require careful and precise material formulation and processing.
In the approximately 40 years that I have been involved with fluid seals, I have seen
the design of mechanical seals progress from a purely empirical process with a lot of
‘black magic’ to a sophisticated design process, making use of complex computational
ix
x Foreword
tools and improved materials. While the development of design tools for polymeric lip
seals has not progressed as rapidly as those for mechanical seals, our knowledge of the
micro-scale processes that govern the operation of these seals has increasingly provided
guidance to lip-seal designers.
These developments have led to a large population of effective and robust seals.
This book provides a guide for you, the seal user, through this highly varied population.
Richard Salant
Georgia Power Distinguished Professor in
Mechanical Engineering, Georgia Institute of
Technology, Atlanta, GA
PREFACE
This book has set out to be a revision and update of the fifth edition. That edition was
a complete rewrite with new format, which attempted to provide the subject mat-
ter grouped into usable sections. I have been humbled to learn how many readers use
it on a regular basis. For this revision it therefore seems sensible to retain the overall
format. We have, I hope, corrected all the inevitable errors that crept in to the fifth edi-
tion, updated areas where there has been significant progress and included additional
experience that I have gained over the intervening 7 years.
Seals are everywhere. It is over 40 years since my introduction to a detailed study
of seals, working on a test rig to investigate hydraulic cylinder seals. And yet, the range
of seal types and the applications continue to expand. Over the last few years I have
assisted with a number of equipment developments where performance of the seals
was a critical path item. Seals are a vital component of virtually every area of our lives.
This includes our houses, domestic appliances and vehicles, while transport and virtu-
ally all industries are totally dependent on them for safe and reliable operation.
They are often treated as a trivial commodity product, but the costs and con-
sequences of a failure can be substantial. One failed fitting in the water system of a
house can cause a lot of damage. In our vehicles they can be costly to replace, and, in
a hydraulic braking system, fundamental to our safety. In many industries reliable seals
are equally fundamental to reliable operation of the plant and containment of fluids.
The costs of seal replacement on process plant can be very high and the implications
of sudden failure can be substantial in terms of lost product, cleanup and pollution.
Unreliable seals can be a rapid cause of loss of reputation to a manufacturer.
Whether it is a domestic appliance, road vehicle, earth-moving equipment or industrial
plant, frequent seal replacement and the inconvenience of clearing up leakage can soon
make the customer start looking elsewhere. For this reason nearly all engineers require
some appreciation of seal selection and application and this book sets out to provide an
unbiased overview of the potential options.
During the last 20–30 years our understanding of seal function and progress with
new materials have seen quite relentless progress. Our understanding of how seals
operate and the ability to optimize the design have progressed substantially. This has
been accompanied by a steady flow of new or improved materials that can be used to
improve seal performance. It is interesting to note that since the previous edition there
have been quite significant developments in the research on both rotary and recipro-
cating polymeric seals, and the avid reader will be able to find reference to key areas of
this work.
xi
xii Preface
There are individual chapters for rotary, reciprocating and static seals, so that it is
relatively easy to explore the potential variants available for an application. It has been
another aim when preparing this book to provide a balance of suitable information as
a basis for initial seal selection across all the potential seal types available in each appli-
cation area. This is intentional as I find that in many cases I deal with, the engineers
involved may have a lot of experience of one seal type for an application, but may have
never considered, or even been aware of, potential alternatives. Alternatively they may
have contact with one supplier who only offers a limited range of products. Hopefully
the book will also help the reader to carefully question overenthusiastic sales staff, some
of whom appear to have limited comprehension of the challenges involved with a new
application.
Almost every sealing application is some sort of challenge. At high temperature,
pressure or speed, there is the technical challenge of reducing leakage and friction or
increasing life. With apparently simple applications the challenge can be ever-present
demands to reduce cost and improve reliability. Designs that will operate at wider tol-
erance bands or are easier to assemble may then be required. You will find that design
for assembly occurs frequently throughout this book. And then, when it seems that a
reliable solution has been found, the seal supplier changes the manufacturing or supply
route and quality issues appear.
Working with seals may seem extremely specialist, and indeed it does require spe-
cialist knowledge of the design criteria, the materials and the limitations of an individ-
ual seal arrangement. But it is also important for the seal specialist to have a very broad
understanding of engineering including manufacturing techniques and machinery
operation. I have seen many ‘good ideas’ fail because at least one of two vital criteria
has been overlooked: the requirement for assembly in a real working environment and
appreciation of the fact that machinery does not run at constant conditions forever; in
the real world it stops and restarts.
The preparation of this book has been supported by many of the friends that I
have been fortunate to make across the sealing industry and the organizations that
have helped are acknowledged separately. My thanks go to all who have contributed
to making this book as up to date and relevant as possible. My special thanks are also
due to my wife, Christine, without whose unstinting support it would never have
happened.
Robert Flitney
ACKNOWLEDGEMENTS
This book would not have been possible without the assistance of many colleagues
from across all sectors of the fluid-sealing industry.
The assistance of the following organizations is gratefully acknowledged:
AESseal
AMG Intellifast
Bal Seal Engineering
BHR Group
Compressor Products International
DuPont
European Sealing Association
Flexitallic
Flowserve
Freudenberg Sealing Technologies
Garlock
Hallite
Henkel Loctite
HillSeals
IMA, Universität Stuttgart
James Walker
John Crane
SM Seals
Solvay Specialty Polymers
Taylor Hobson
Trelleborg Sealing Solutions
xiii
CHAPTER ONE
Introduction
1.1 PURPOSE OF THIS BOOK
This book sets out to introduce the subject of sealing technology and provide
the reader with an understanding of the issues involved and sufficient information,
or sources of information, to assist with achieving reliable and effective sealing when
designing new equipment or as a solution to a maintenance or performance problem.
It is intended for design and development engineers from the many industries that
use seals, and the operations and maintenance engineers involved with the plant and
machinery once it is in use. The reader should find information that will help to select
an appropriate seal arrangement for a new design, optimize a current design, or if nec-
essary explore alternative sealing methods in the search for a more reliable or cost-
effective solution.
Even a relatively simple looking seal, such as the humble O-ring, depends on some
quite unique properties of the elastomer material from which it is manufactured to
enable it to function reliably. Failure to understand the properties of the material and
adhere to some well-developed design rules can rapidly cause problems. New materials
have dramatically extended the potential application areas with respect to temperature
range and fluid compatibility. But, these materials often necessitate specific design rules
to accommodate their characteristics.
Dynamic seals have to be optimized to provide both minimum leakage and mini-
mum friction and wear, often mutually exclusive tribological objectives. A better
appreciation of the factors involved should permit a more informed dialogue with
potential seal suppliers. Practically all seals, either static or dynamic, depend on the
properties of the materials from which they are constructed and the interaction of the
mating surfaces to achieve a reliable seal with minimum leakage. Again, developments
in materials and design of the seals have progressively extended the performance of
dynamic seals in most application areas.
User demand and legislation restricting emissions are creating requirements for
tighter control of leakage, longer seal life and reduced life-cycle costs. By introducing
a wide variety of seal designs and methods together with details of the materials, it is
the intention of this book to provide information on methods used in many different
industries that may then promote healthy debate on the potential options leading to an
informed selection of a cost-effective and appropriate seal arrangement.
Seals and Sealing Handbook. © 22001144 Elsevier Ltd.
ISBN: 978-0-08-099416-1 All rights reserved. 1
2 Seals and Sealing Handbook
1.2 WHAT DOES IT COVER?
The following chapters cover seals for static, rotating and reciprocating appli-
cations in turn. Each of these chapters also includes guidance on the selection of an
appropriate seal for individual applications. The specific properties of materials used
for seals or sealing systems are covered in a separate chapter. The Failure Guide sets
out typical symptoms that may be observed with failed seals and the factors to con-
sider, many of which involve attention to the fluid system rather than the seal. Each
industry and technology has some specific terminology, standards and key organiza-
tions. Information on these areas, together with an explanation of surface texture ter-
minologies, is included in Chapter 7. Compliance with emission regulations is a fact
of life for industrial plant. The European Sealing Association (ESA) has made a major
contribution to this area and the BAT Guidance Note prepared by the association
members is provided as an appendix, together with information on the position with
respect to European ATEX regulations for operation of mechanical seals in explosive
atmospheres.
Static seals are discussed in Chapter 2, and this starts by explaining the use of elas-
tomer O-rings and then extends to alternative materials and seal sections. The relative
merits of the many potential elastomer, plastic and metal seals are discussed together
with key design criteria. The basics of gasketed joints and the various material options
are discussed together with the fundamental differences between gasketed and sealed
flanges. A number of the specialized flange sealing techniques are also covered.
Static seals are far more numerous than dynamic seals. They have therefore been
the focus of considerable attention with respect to production engineering for volume
components. A variety of techniques are presented for consideration that integrate the
seal or allow it to be formed or moulded in place.
Chapter 3 discusses rotary seals. The rotary seal produced in the highest volume
is undoubtedly the humble lip seal. Although they have been manufactured for many
years, a thorough understanding of their operation is relatively recent and still devel-
oping. The basic operation, key design features and the many potential variants that
may be considered are discussed together with developments of plastic seals that can
be used to extend the performance envelope. Exclusion of contaminant is often just as
important as the sealing of lubricant, so this aspect is also discussed. Alternative designs
of elastomer and plastic seals, some of which can be used at high pressures in special-
ized applications, are covered.
Mechanical seals cover a very wide market and range of applications from domes-
tic white goods to turbo machinery. The basic designs, key design features and areas of
application are discussed.