Seals and Sealing Handbook

Robert K. Flitney

• Publisher: Butterworth-Heinemann
• Year: 2014
• ISBN:
  9780080994130
• Edition: 6
• Topic: Physical Sciences
• Subtopic: Fluid Mechanics

Seals and Sealing Handbook, Sixth Edition provides comprehensive coverage of sealing technology, bringing together information on all aspects of this area to enable you to make the right sealing choice. This includes detailed coverage on the seals applicable to static, rotary and reciprocating applications, the best materials to use in your sealing systems, and the legislature and regulations that may impact your sealing choices.Updated in line with current trends this updated reference provides the theory necessary for you to select the most appropriate seals for the job and with its 'Failure Guide', the factors to consider should anything go wrong. Building on the practical, stepped approach of its predecessor, Seals and Sealing Handbook, 6th Edition remains an essential reference for any engineer or designer who uses seals in their work.

• A comprehensive reference covering a broad range of seal types for all situations, to ensure that you are able to select the most appropriate seal for any given task
• Includes supporting case studies and a unique 'Failure Guide' to help you troubleshoot if things go wrong
• New edition includes the most up-to-date information on sealing technology, making it an essential reference for anyone who uses seals in their work

Chapter One

Introduction

This chapter provides an introduction to the following chapters. It introduces the wide range of seal designs, many different applications and multi-disciplinary approach that is often required to achieve optimum sealing.

Keywords

Introduction; background; technical disciplines

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 necessary 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 minimum 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.

1.2 What Does it Cover?

The following chapters cover seals for static, rotating and reciprocating applications 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 consider, many of which involve attention to the fluid system rather than the seal. Each industry and technology has some specific terminology, standards and key organizations. Information on these areas, together with an explanation of surface texture terminologies, 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 elastomer 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 developing. 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 specialized applications, are covered.

Mechanical seals cover a very wide market and range of applications from domestic white goods to turbo machinery. The basic designs, key design features and areas of application are discussed.

Compression packing is still a relevant contender in many applications, so the key features and modern material options are presented.

A wide range of other seal types is also used in rotating machinery, particularly in high-speed turbo machinery. As efforts are made to increase machine efficiency and reduce emissions, both seal designs and new materials are continually being investigated. Current developments for a number of seal types, such as labyrinth, honeycomb, leaf seals, brush seals and viscoseals, are summarized.

O-rings are used as reciprocating seals in some areas. They do not provide the optimum seal and present certain hazards. Chapter 4 discusses these problems and introduces many specialist designs of elastomer or plastic seal that may be used to improve performance. The reasons for this are explained, together with summary charts to guide readers to the most appropriate style for a given application area.

Piston rings are a very commonly used form of reciprocating seal. They fall into quite distinct application areas covering internal-combustion engines and compressors. Although dry compressors have been in use for many years, there is an increasing requirement for ‘bone-dry’ gas compression, which has further stretched material development. Clearance seals must again be considered as compression packings.

In sealing technology extensive use is made of specialized materials, and these are discussed in Chapter 5. Elastomers have quite unique properties that make them especially useful as seals. Much of the development of high-temperature and chemically resistant elastomers has been prompted by the demand of the seal industry customers. The plastics used are also developed specifically for seal applications, as are certain carbons. The important considerations relevant to material selection are discussed, especially the importance of considering the entire range of fluid constituents.

The Failure Guide, Chapter 6, presents key areas to investigate and a range of specific examples across the seal types discussed in the earlier chapters. It will normally have to be used in conjunction with the relevant technical section to ensure that the important criteria for the individual seal type are considered.

Chapter 7 includes a range of general information. A comprehensive glossary for much of the terminology used across the sealing industry is provided. There is a summary of some of the key standards across the seal types and numerous industry sectors, together with contact details for standard organizations. An introduction to surface texture measurement is also provided and the most frequently used texture parameters that the reader may come across.

The two appendices have been contributed by the ESA. These provide valuable additional information, especially for seal applications in the process industries. The Best Available Technology Guidance Note is a very comprehensive guide to the performance and cost effectiveness of mechanical seals, compression packing and gaskets in the process industry. It is reproduced in full in Appendix 1.

The European Union ATEX Regulations have created much discussion and raised a number of questions. A position statement compiled by the mechanical seal members of ESA is provided in Appendix 2.

1.3 What Applications Does it Cover?

The use of seals in our modern world is very wide ranging. In fact it can be a challenge to think of anything in our daily lives that does not involve some form of sealing. We are surrounded by them in our homes: from food containers, body-care products and pharmaceutical containers, to water-system and glazing seals. All forms of mechanized transport and any manufacturing process involve a bewildering array of seals. Utilities are totally reliant on seals to either contain water or gas within the distribution network or exclude water and contamination from electrical and electronic services.

As with any technical area, life does not stand still. Advances in analytical and measurement techniques have provided a much improved understanding of the behaviour of seals and the critical parameters to provide improved performance. When combined with advances in material technology, this has led to quite dramatic improvements in seal performance during the last 30–40 years.

The impetus for this change has been created by a number of factors, including demands for higher reliability, emissions legislation together with a realization of the costs associated with product loss, and demands to reduce manufacturing costs while still improving reliability, all combined with a change of corporate culture which has pushed responsibility further down the supply chain.

1.4 What Disciplines are Involved?

The design, manufacture and application of seals involve a number of disciplines:

Materials, covering the entire range of elastomers, plastics, ceramics and metals. Many materials, especially elastomers, plastics and ceramics, are developed specifically for the manufacture of seals. Many seal designs involve high strains, and often plastic and metal static seals depend on plastic flow to provide an effective seal. The stress/strain properties, fluid compatibility and corrosion effects can be critical to success. Very often either minor constituents of the sealed fluid or impurities in a material can be a source of severe problems.

Tribology – all dynamic seals involve friction, lubrication and wear. There is seldom the luxury of selecting the lubricant; it is most often the fluid to be sealed. For many seals, instead of optimizing the lubricant and lubricant film for reliability, the amount of lubricant is minimized to meet leakage requirements. Dry or boundary lubrication is involved in an increasing number of seal applications. Even static seals can be subject to small repeated movements with pressure and temperature cycles that can cause fretting damage and wear.

Manufacturing technology impacts on both the seal manufacture and seal design. Considerable effort can be involved in the development of manufacturing techniques for new elastomers and plastics. Seal designs may be limited by factors such as the moulding of elastomers, geometry and characteristics of ceramics, and production facilities for specialized plastics.

Production engineering of the equipment to be sealed is equally important. Seals can require specially prepared surfaces machined to close tolerances and installation of seals with...