Seal Basics
John Crane
Seal School
Handbook
John Crane
Training Center
Copyright© 2001 John Crane
Published by John Crane
6400 West Oakton St., Morton Grove, Illinois 60053 U.S.A.
All right to illustrations and text reserved by John Crane. This work may not be copied, reproduced, or
translated in whole or in part without written permission of John Crane, except for brief excerpts in
connection with reviews or scholarly analysis. Use with any form of information storage and retrieval,
electronic adaptation or whatever, computer software, or by similar or dissimilar methods now known or
developed in the future is also strictly forbidden without written permission of John Crane.
Printed in the United States of America
Table of Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
1.0 John Crane History . . . . . . . . . . . . . . . . . . . . . .3
1.1 John Crane Background . . . . . . . . . . . . . . .3
1.2 Classes of Sealing Technology . . . . . . . . . .3
2.0 Concept of Contacting Liquid Lubricated Seals .5
2.1 Packing . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
2.2 Seal Elements . . . . . . . . . . . . . . . . . . . . . . .7
2.3 Primary Sealing Elements . . . . . . . . . . . . . .9
2.4 Secondary Sealing Elements . . . . . . . . . . . .10
2.5 Drive and Load Elements . . . . . . . . . . . . . .12
2.6 Adaptive Hardware . . . . . . . . . . . . . . . . . . .13
3.0 Mechanical Seal Classification . . . . . . . . . . . . . .14
3.1 Classification by Design . . . . . . . . . . . . . . . .15
3.1.1 Pusher/Non-Pusher . . . . . . . . . . . . . .16
3.1.2 Balance . . . . . . . . . . . . . . . . . . . . . . .16
3.1.3 Face Pattern . . . . . . . . . . . . . . . . . . .18
3.1.4 Rotating Seal/Stationary Seal . . . . . .19
3.2 Classification by Arrangement . . . . . . . . . . .20
3.2.1 Single Seals . . . . . . . . . . . . . . . . . . . .20
3.2.2 Multiple Seals . . . . . . . . . . . . . . . . . . .21
3.2.3 Secondary Containment . . . . . . . . . . .22
4.0 Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
4.1 Heat Transfer . . . . . . . . . . . . . . . . . . . . . . .25
4.2 Flushing/Quenching . . . . . . . . . . . . . . . . . . .26
4.3 Piping Plans . . . . . . . . . . . . . . . . . . . . . . . .27
6.2 Bushings . . . . . . . . . . . . . . . . . . . . . . . . . . .37
6.3 Flow Meter . . . . . . . . . . . . . . . . . . . . . . . . .38
6.4 Heat Exchanger . . . . . . . . . . . . . . . . . . . . . .39
6.5 Pumping Ring . . . . . . . . . . . . . . . . . . . . . . .40
7.0 Sealing Systems . . . . . . . . . . . . . . . . . . . . . . .41
7.1 Seal Installation . . . . . . . . . . . . . . . . . . . . .41
7.1.1 Protecting the Seal Face . . . . . . . . . .41
7.1.2 General Rules . . . . . . . . . . . . . . . . . .41
7.1.3 Installing the Seal . . . . . . . . . . . . . . . .41
7.1.4 General Rules for Secondary Seals . .43
7.1.5 Pump Condition . . . . . . . . . . . . . . . .43
7.1.6 Shaft Straightness . . . . . . . . . . . . . . .43
7.1.7 Bearings . . . . . . . . . . . . . . . . . . . . . . .43
7.1.8 Couplings . . . . . . . . . . . . . . . . . . . . . .43
7.1.9 Unbalanced Impeller . . . . . . . . . . . . . .43
7.1.10 Pipe Strain . . . . . . . . . . . . . . . . . . . .44
7.1.11 Shaft Deflection & Cavitation . . . . . . .44
7.2 Pump Prep for Start up . . . . . . . . . . . . . . . .44
7.3 Pump in Operation and Shut Down . . . . . . .44
7.4 Troubleshooting . . . . . . . . . . . . . . . . . . . . . .45
7.5 Troubleshooting by Symptoms . . . . . . . . . . .48
8.0 Fluid Services . . . . . . . . . . . . . . . . . . . . . . . . . .51
9.0 Non-Contacting, Gas Lubricated Seals . . . . . . .54
9.1 Background . . . . . . . . . . . . . . . . . . . . . . . . .54
9.2 Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54
9.3 Compressors . . . . . . . . . . . . . . . . . . . . . . . .59
9.4 Mixers and Agitators . . . . . . . . . . . . . . . . . .61
5.0 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
10.0 Glossary of Terms . . . . . . . . . . . . . . . . . . . . . .62
6.0 Auxiliary Equipment . . . . . . . . . . . . . . . . . . . . . .36
6.1 Abrasive Separator . . . . . . . . . . . . . . . . . . .36
Seal Basics
1
Preface
This booklet provides a basic overview of the concepts associated with mechanical seals and sealing
systems. It is intended to be the starting point for sales representatives, inside support personnel, engineers
and customers to learn about mechanical seals. It covers the development of mechanical seals, seal types,
design features, sealing systems, fluids, and terms and definitions.
This booklet is based on the experience of sales representatives, engineers and technical trainers. A
mechanical seal is a technical product and a component of rotating equipment. It is "invisible" to the average
person. Explaining how and why it works can be difficult. The tendency is to become too technical or too
simplistic. Hopefully this booklet has found a middle ground and explains concepts and components with
enough technical detail to be helpful and not leave the reader mystified by complexity.
In addition to this booklet, John Crane has a variety of other technical training materials. For a complete
listing, please contact the John Crane Training Department at 847-967-2400 or 1-800 SEALING, John Crane
6400 W. Oakton, Morton Grove, Illinois 60053.
The information in this booklet is to be used as teaching material. This material should not be confused with
technical standards. For specific seal recommendations, readers are referred to their local John Crane sales
representative.
John Crane Training Center
September, 2001
2
Seal Basics
1.0 John Crane History
1.1 John Crane Background
For more than 80 years, John Crane Mechanical Seals has been a market leader in defining and redefining
the limits of sealing technology. Founded in 1917 as Crane Packing Company, it quickly established facilities
throughout the United States and in Canada and England.
Crane Packing Company moved from its Chicago location to its current Morton Grove location in 1951. The
complex is located on 26 acres of land and includes five buildings.
In its commitment to provide superior service to customers and to be the technological leader in the sealing
industry, John Crane continues to invest for growth. In 1998 the company acquired three sealing companies,
Sealol from EG&G, Safematic, and Flexibox. These companies expand John Crane's product lines and its
global presence.
The initial products of Crane Packing Company were packings and gasketing materials which continue in
today's John Crane product mix. The company established Rite-Pack® Packings, one of the most popular
packings available. They have broadened their offerings to include Live-Load Packing for valves. John
Crane produces enough footage of braided packing each year to reach to the moon and back.
The company invented the first automotive mechanical seal in 1939, and currently produces millions of seals
for American automotive companies and for the automotive aftermarket. In the early 1940's, John Crane
developed and introduced patented end face shaft seals under the John Crane brand label. The Type 1
elastomer bellows seals revolutionized sealing technology and it is still one of the most widely used seals in
industry.
Other types of patented mechanical seals for rotating equipment followed, creating an ongoing John Crane
customer service philosophy…the right seal for the right application. In the 1980's John Crane introduced
another breakthrough in sealing technology - the Type 28 non-contacting, gas lubricated seal designed for
centrifugal compressors. Its introduction revolutionized pipeline compressor stations from the mountains of
Western Canada to the deserts of the Middle East and the jungles of the Far East.
In the 1990's, John Crane applied its non-contacting technology to pumps handling liquids hazardous to the
environment. By applying this technology to a standard ANSI or API pump, a user can easily meet and
exceed the EPA's stringent regulations for hazardous emissions. This technology is successfully applied in
applications where the seal is operating near the boiling point of the fluid being sealed. Services such as
liquid nitrogen, argon, and oxygen found on the mobile tank trucks, are being sealed with non-contacting
metal bellows seals. Vaporizing hydrocarbons are also being sealed with this technology. Our new low
speed design handles conditions found on mixers and agitators. This area of application is significant since
liquid lubrication is eliminated and the highest purity of product in the vessel is maintained.
The Sealol acquisition strengthened the John Crane line of welded bellows seals. These seals provide
service to very low and very high temperature operations, allowing John Crane to offer the most extensive
line of mechanical seals in the industry.
With a nationwide salesforce and service centers geographically convenient to our customers, John Crane
continues to be committed to outstanding service and finding just the right solution for all sealing system
requirements.
1.2 Classes of Seal Technology
Emerging seal technologies are providing clear choices in sealing. Various plant services will require the
Seal Basics
3
application of these new technologies for emissions control, safety, and reliability. Sealing systems are now
available, based on the preferred method of lubrication to be used. These classes of seals are:
Contacting Liquid Lubricated Seals
!"Normally a single seal arrangement cooled and lubricated by the liquid being sealed.
!"Dual seal arranged to contain a pressurized barrier or non-pressurized buffer liquid. Normally,
this arrangement will be used on applications where the liquid being sealed is not a good
lubricating fluid for a seal and for emissions containment. These arrangements require an
auxiliary system for the circulation of barrier or buffer liquid.
Non-Contacting Gas Lubricated Seals
!"Dual non-contacting gas lubricates seals pressurized with an inert gas such as nitrogen.
!
Dual non-contacting gas lubricated seals used in an unpressurized arrangement where the
process liquid being sealed is allowed to flash to a gas at the seal face. This arrangement is
used on those liquids that represent a danger to the plant environment.
!"Single non-contacting seal can be used on vaporizing fluids that are not hazardous to the plant
environment.
Each of these solutions have been used on difficult applications to increase the Mean Time Between
Maintenance (MTBM), resulting in significant reduction in the operating and maintenance costs of the
equipment.
4
Seal Basics
2.0 Concept of Contacting Liquid Lubricated Seals
2.1 Packing
The history of mechanical seals is relatively short in comparison to other mechanical sealing devices. The
first patents for mechanical seals began to appear in the early 1900's. Before the invention of mechanical
seals, packing was used for thousands of years to seal low speed rotating shafts.
The word "seal" is used to describe a variety of sealing devices. For example, gaskets, O-rings and lip seals
are frequently called "seals". In order to distinguish between various sealing devices, it may be more proper
to refer to a mechanical end face seal, but we will use the shorter "mechanical seal".
Mechanical seals can now be found in almost every industry. In aerospace they are used to seal propellant
pumps. In an automobile they are used in the water pump to seal the coolant. In petrochemical plants they
seal a variety of hazardous and nonFigure 1 - Fundamental Sealing Problem
hazardous liquids in pumps, mixers
and reactors. In many homes they are
used in the water well, washing
machine, dishwasher, and garbage
Vessel
disposal. Mechanical seals have
Process
Wall
Environment
become a part of our everyday life.
Fluid
Shaft
Leakage
Figure 2 - Packing
Process
Fluid
Vessel
Wall
Environment
Packing
Shaft
Leakage
Seal Basics
Before we can understand the
importance of mechanical seals to
industry, we need to consider the
fundamental problem of preventing
leakage around a rotating shaft that
enters a chamber, for example in a
pump. Figure 1 shows this situation.
An obvious way to reduce leakage is
to minimize the space or clearance
between the shaft and the pump
housing. If we make the clearance too
small, the shaft will rub against the
housing.
Figure 2 shows an improved situation.
The pump is modified so a
compressible material can be placed
between the shaft and the pump.
Because this material is "packed"
between the shaft and the pump, it is
called packing. The space between the
shaft and the pump is called the
stuffing box.
The use of compression packing can
be traced back thousands of years.
The first packing was made of flax and
tallow. Today different materials are
used but the basic concept has not
changed. The stuffing box is
completely filled with a resilient
material which is formed into rings.
5
These rings are mechanically compressed to reduce the leakage path.
Compression packing can be installed with a few simple tools. It is generally considered to have the lowest
initial cost for control of leakage. With packing, liquid enters the stuffing box area under hydraulic pressure.
This liquid flows under the rings of packing and eventually leaks out at the packing gland. If less leakage is
desired, the mechanic simply tightens the packing gland. Some visible leakage is required with packing. This
leakage provides cooling and lubrication.
Packing wears and compensation must be made for this loss of material. As wear occurs, the packing gland
is tightened. This adjustment must be made frequently and is truly arbitrary. Many packing failures are
caused by overheating and rapid wear as a result of overtightening the packing gland.
Packing has some obvious drawbacks as a sealing device. Constant packing adjustments and pump repacking consume maintenance resources. Downtime frequency is usually higher on packed pumps due to
corrosion problems, bearing failures and shaft sleeve replacement.
Incorrectly packed pumps can consume considerably more power than mechanically sealed pumps. In fact a
packed pump may use from 2-12% of its total power to overcome the friction of the packing compared to the
almost negligible amount of power used with mechanical seals.
In addition to the cost associated with constant packing adjustments are the costs associated with
housekeeping, product loss, and additional power consumption, Environmental Protection Agency (EPA)
regulations have mandated leakage amounts for commonly pumped fluids. Packing cannot comply with
these regulations and, therefore, mechanical seals provide an alternate approach.
In industrialized countries the majority of pumps have migrated to mechanical seals. There are still market
sectors that find packing a viable alternative for their pumps. A decision between choosing to use packing or
a mechanical seal can be made by reviewing the benefits and problems associated with each.
Table 1 - Packing and Mechanical Seal Comparison
PACKING
Advantage:
Inexpensive method of "sealing"
Installs easily
Disadvantage: Serves as a restricting device, not a seal
Requires leakage or it will burn
Will groove and wear shafts and sleeves
Loses product because of required leakage
Requires maintenance time for packing gland adjustment
Can damage and destroy equipment from leakage
Requires high horsepower load to overcome friction.
MECHANICAL SEAL
Advantage:
Will seal with practically invisible leakage
Can, in documented cases, run over 10 years without failure
Saves on product loss and energy usage
Requires minimal maintenance after initial installation
Provides better safety when sealing hazardous material
Costs less than packing over lifetime of the equipment
Disadvantage: Requires higher initial dollar investment
Requires a more complex installation than packing for component seals
6
Seal Basics
2.2 Seal Elements
The simplest possible mechanical end face seal consists of a shoulder on a rotating shaft which rubs
against a stationary case. This is shown in Figure 3. The shoulder provides one sealing face and the wall of
the pump the other. Hydraulic pressure will force fluid between these faces and provide a lubricating film but
the face separation must be kept very small to minimize leakage. Cooling is provided by the surrounding
liquid.
Figure 3 - Simple Seal
Process
Fluid
Vessel
Wall
Environment
Shaft
Leakage
Figure 4 shows a major shortcoming of this design. There will always be some shaft movement during
operation of the equipment. This movement may either decrease face separation, causing gross face
contact and wear, or increase face separation, causing increased leakage. Seal face leakage is governed by
many variables, but the dominant variable is face separation. A variation of a few microinches (millionths of
an inch) in the face separation can cause significant changes in leakage. Unfortunately, shaft movement can
amount to several thousandths of an inch.
Figure 4 - Simple Seal Shortcoming
Shaft Movement
Leakage
Seal Basics
7
A practical approach to overcoming shaft movement is to mount one of the seal faces in a flexible manner
so that it can move axially. Figure 5 shows an improved seal. The sealing shoulder on the shaft has been
removed and replaced with a component which is not rigidly attached to the shaft. This component is called
the primary ring. The "face" of the primary ring rubs against another component, the mating ring. Since the
primary ring and the shaft are two separate parts, an additional sealing device must be used to prevent
leakage between the shaft and the primary ring. The flexibly mounted primary ring can compensate for the
small variations in movement on the axial plane. It can also adjust for seal face wear. Of course, additional
components are required to preload the faces, transmit torque and provide ease of installation.
Figure 5 - Improved Seal
1. Mating Ring
2. Secondary Sealing Element
3. Primary Ring
2
3
1
Figure 6 shows a still more complete mechanical seal including a replaceable mating ring, "O-ring" gaskets,
springs, set screws and various other hardware. The design of these components may vary considerably
according to the required service for the seal. In addition, the assembled components themselves may be
arranged and oriented in various ways to meet the needs of various industries and pumping situations.
Figure 6 - Mechanical Seal
1. Mating Ring
2. O-Ring
3. Primary Ring
4. O-Ring
5. Retainer
6. Spring
7. Disc
8. Snap Ring
9. Set Screw
10. Anti-X-Ring
9
5
6
7
4 10
3
8
1
2
Simply put, a mechanical seal is
!"A wearable, stationary primary sealing element
!"A wearable, rotating primary sealing element
!"A secondary sealing element
!"A mechanical loading device to press the primary sealing elements together
!"Auxiliary components.
8
Seal Basics
2.3 Primary Sealing Elements
The primary ring, its secondary sealing elements, retainer and other components make up the "Head
Assembly" or the primary ring assembly. The mating ring and its secondary sealing elements, make up what
is referred to as the "Seat Assembly" or the mating ring assembly.
The primary ring is usually made out of carbon and the mating ring is made of a harder material, typically
tungsten carbide or silicon carbide. Both faces are lapped to a flatness so precise it must be measured in
light bands (a light band is equal to 11.6 millionths of an inch). John Crane has invested years of
engineering into the design of these faces. The material and design of these seal faces is often the critical
element in difficult duty services.
The basic function of the mating ring is to provide a smooth, flat surface for the primary ring to seal against.
To maintain its surface integrity, it must incorporate the following features: corrosion and wear resistance,
good thermal conductivity, stability, and have excellent surface finish quality. To prevent additional seal head
motion, the mating ring is usually mounted solidly to the stationary housing, forming as near as possible a
perfect perpendicular plane for the primary ring.
There are a few basic designs for mating rings. Figure 7 shows diagrams of the most common types.
Figure 7 - Mating Ring Designs
Square Cross Section
Floating “L”
O-Ring
Inverted L-Shape
Cup Mounted
Clamped-In
Square cross section design - This design houses the O-ring in the gland rather than the mating ring.
With this design, the mating ring can be further exposed to the cooling liquid in the seal chamber. This
design provides good thermal stability for applications where thermal distortions must be minimized.
This design can be fitted with an anti-rotational pin.
O-ring design - The O-ring is contained in a recess within the mating ring. It is mounted directly into the
gland plate that is attached to the housing face. This is a popular design that covers a wide range of
application requirements for general services. This design often incorporates an anti-rotational pin.
Cup mounted design - This design offers a low cost arrangement which has the distinct advantage of
being capable of sealing extremely rough surface finishes, often used in high volume production pumps.
Due to the insulating effect the elastomer ring creates, this design is limited to use on lower temperature
and lower pressure applications.
Floating L-shaped - This design is push fit into a housing and uses an O-ring as its secondary sealing
element. It is used on a wide range of general applications.
Inverted L-shaped - This mating ring is used where pressure reversal or maximum heat removal is
required. The L-shape has a balance step and an O-ring on the ID (inside diameter) matched to the
primary ring that allows the entire OD (outside diameter) of the mating ring to be exposed to the cooling
medium.
Seal Basics
9
Clamped-in design - This design clamps the mating ring between the pump casing and gland and uses
gaskets to prevent leakage. The clamped-in mating ring offers the widest range of temperature and
pressure limits since it is capable of being used with a wide range of gasket material and is essential
when using externally mounted seals.
Contacting liquid-lubricated seals have a fluid film between the primary ring and the mating ring. In many
instances, the fluid being pumped acts as the lubricant between the faces. When the rotating equipment is in
operation one face rotates with the shaft and one face is stationary. When the rotating equipment is
operating, there are forces (pressure) that work to push the faces together and there are opposing forces
(pressure) that work to push the faces apart.
Figure 6 (on page 8) shows the primary ring rotating with the shaft. Either the primary ring or the mating ring
maybe used as the rotating element. Seals with rotating primary rings are said to be rotating seals; seals
with stationary primary rings are said to be stationary seals.
The art and science of mechanical sealing is the tradeoff between leakage and wear. The primary leak path
is between the primary ring and the mating ring. Increasing the face separation increases the leakage. On
the other hand, when the seal faces rub together without lubrication, the resulting wear can lead to reduced
seal life.
2.4 Secondary Sealing Elements
The secondary sealing elements provide sealing between the primary ring and shaft (or housing) and the
mating ring and shaft (or housing), as in Figure 8. They are called secondary sealing elements because their
leakage path is secondary to the seal face leakage. Loading by hydraulic or mechanical force makes the
secondary seal tight in its confined area. The secondary sealing element for the mating ring is always static
axially, although it may be rotating. Secondary sealing elements for the primary ring are described as being
either pusher or non-pusher in the axial direction. The term pusher is applied to secondary seals that must
be pushed back and forth by the movement of the shaft or primary ring. A non-pusher secondary seal is a
static seal for the primary ring, as in Figure 9.
Figure 8 - Secondary Seals
Static
Dynamic
Figure 9 - Pusher Vs. Non-Pusher
10
Pusher
Non-Pusher
O-Ring secondary seal must slide along
shaft as seal face wears
Bellows secondary seal expands to
accommodate face wear. Bellows tail is
stationary against shaft.
Seal Basics
Figure 10 shows examples of pusher, non-pusher and static secondary seals. The pusher design may use
O-rings, wedges, etc. The non-pusher is always some sort of bellows with a static section. Mating rings use
various static gasketing and O-ring designs.
Figure 10 - Secondary Seals
Elastomeric
Bellows
V Rings
Half
Convolution
U Cup
PTFE
Bellows
Wedge
Welded Metal
Bellows
O-Ring
Encapsulated
O-Ring
Bellows - Seals that use a bellows as a secondary seal are all classified as non-pusher seals. Bellows are
typically referred to by their material of construction: elastomer bellows, PTFE bellows, or metal bellows.
Elastomeric bellows seals have a full or half convolution bellows. The full convolution offers the greatest
possible flexibility to the front section of the primary ring. The front section of this bellows has minimal
contact with the shaft or sleeve, thus minimizing wear and hang-up. The large tail section provides a
considerable sealing area to compensate for imperfections in the shaft.
PTFE or glass-filled PTFE bellows are designed for the extremes of corrosive environments. Due to the
requirements of flexibility, the convolutions in this bellows must be considerably larger in cross section than
the typical elastomeric bellows design.
Metal bellows designs can be used in high and low temperature applications. Due to its all-metal
construction, it offers considerable freedom of design. The mechanical closing force that is provided by the
spring on other seal designs is accomplished by stressing the metal bellows from free height in this design.
Wedges - Wedges are typically made of PTFE material. Special manufacturing fits are not a requirement for
wedges because of the shallow angle of contact between the wedge and the shaft. However, it does require
a polished surface for effective sealing.
O-rings - The O-ring is by far the simplest and most popular secondary sealing element. It has been used
successfully over a wide temperature range and in a variety of fluids. O-rings are considered to be selfenergizing seals and do not require mechanical preloading. This feature allows O-rings to be used in very
high pressures. O-rings are offered in a complete range of chemically resistant and general service
compounds. Buna-N, Neoprene, ethylene propylene, fluoroelastomers, and perfluoroelastomers are typical
materials selected for a variety of service conditions. Encapsulated O-rings are an attempt to obtain the
chemical resistance offered by PTFE materials and the flexibility of the elastomeric O-ring. Encapsulated Orings are usually recommended only for static sealing.
Seal Basics
11
2.5 Drive and Load Mechanisms
Drive Mechanisms
A drive mechanism is required because of the torque created between the seal faces. Both static and
dynamic drives are required. The static drive is only required to hold an axial position and transmit torque.
The dynamic drive must transmit torque and allow for the axial flexibility of the primary ring. Figure 11 shows
some variations of drive mechanisms.
Figure 11 - Drive Mechanisms
Dent Drive
Key Drive
Set Screw Drive
Pin Drive
Snap Ring Drive
Slot and Ear Drive
Elastomer Drive
Spring Drive
Dent drive - The dent drive is an effective drive generally utilized because of its simplicity, comparative
economy and it can be made with simple tooling.
Key drive - This is one of the more rugged forms of drive. In high pressure, comparatively large size units,
the ruggedness of this type of drive is in keeping with the balance of the sturdy features incorporated in
seals for high pressure applications.
Set screws - This simple method of driving takes advantage of a common hardware item. Set screws are
probably the most common type of static drive mechanisms. They are not recommended for dynamic drive
because the roughness of the threads can restrict axial flexibility of the primary ring.
Pins - Other positive drive designs include slotted pins, dowels or split roll pins. In the mating ring, they are
probably one of the simplest methods to eliminate the possibility of spinning.
Snap ring - Snap ring drives are sometimes utilized when axial space limitations prevent the use of other
types of drive. These are generally limited to light duty services.
Slot and ear - This is another of the more rugged methods of engaging metal to metal, especially when the
slots and mating ears are used in multiple forms. It is one of the best methods to assure positive
engagement between two parts that have relative axial motion between them. Its basic design also allows
for the high degree of flexibility generally incorporated in the primary ring.
12
Seal Basics
Elastomer drive - This is not a positive drive method and is generally limited to light duty seals. However, it
is extremely simple, economical and effective for some services.
Spring drive - Another effective drive mechanism for light duty seals is to incorporate the drive into the
springs. Care must be taken in this design as to the direction of rotation, corrosion rate, and spring rate.
Load Mechanisms
In every mechanical seal there is always a need for keeping the faces closed in the absence of hydraulic
pressure. Generally, a mechanical device in some form of a spring is used.
Single spring - A single spring seal has the advantage of a comparatively heavy cross section coil which
can withstand a higher degree of corrosion. Another advantage is that single springs do not get clogged by
viscous liquids. The disadvantage of a single spring is that it does not provide uniform load to the faces.
Also, centrifugal forces may tend to unwind the coils. Single springs also tend to require more axial space
and a specific spring size is required for each seal size.
Multiple springs - Multiple springs are usually smaller than single springs and provide a more uniform load
at the faces. The same spring size can be used with many seal sizes by simply changing the number of
springs that are used. Multiple springs resist unwinding from centrifugal force to a much higher degree than
a single coil spring since the forces act differently. The most obvious disadvantage of small springs is the
small cross section wire. This makes the smaller springs subject to corrosion and clogging.
Wave spring - This type of spring is simply described as a washer into which waves have been formed to
provide a given amount of mechanical loading. The main reason for using this type of spring is that it
requires even less axial space than the multiple spring design. On the other hand, special tooling must be
made for best manufacturing results. Further, the tempering required on this design, limits materials to those
which are not as corrosion resistant as the high grade stainless and Hastelloy™ groups. When using wave
springs, a greater change in loading for a given deflection must be tolerated. That is, a great deal of force
loss or force gain, with comparatively small axial movement, must be expected.
Metal bellows - A metal bellows is actually a combination of a spring and secondary sealing element.
Formed bellows may be used to reduce the quantity of welding. However, a formed bellows has a much
higher spring rate than a welded bellows. The bellows thickness must be selected for resistance to pressure
without an excessive spring rate. The welding technique and the bellows shape must be selected for
maximum fatigue life.
2.6 Adaptive Hardware
These are the component pieces that complete the seal. They include items like the sleeve, collar, and
gland plate. A sleeve is a cylinder that fits over a shaft. Its original purpose was to provide easy repair and
to prevent damage to the shaft associated with packing use. Now they are used with mechanical seals to
provide a step down for hydraulic balancing and/or for easy assembly as with a cartridge. A standard sleeve
can be used and locked in place with a locking collar that is set screwed to the shaft with a static O-ring
sealing on the shaft. The gland plate (end plate) holds the non-rotating assembly of a mechanical seal and
connects it to the seal chamber. The gland plate can be drilled and tapped to provide access for flushing
and quenching.
Mechanical seals have evolved into cartridge designs. This means that the primary seal assembly, the
mating ring assembly and the adaptive hardware are put together in a single package. The primary ring seal
assembly is pre-loaded and does not have to be set at the time of installation. Cartridge seals make seal
installation an easier process.
Hastelloy is a registered trademark of Haynes International, Inc.
Seal Basics
13
3.0 Mechanical Seal Classification
People often use the word seal to mean many different things. Before we can begin to classify mechanical
seals, we must first identify the components of a sealing system. Figure 12 illustrates a system that contains
mechanical seals along with adaptive hardware and auxiliary equipment. In this figure the mechanical seals
are used in a cartridge assembly.
Figure 12 - Sealing System
Figure 13 shows the relationship of these system components to the system. The seal assembly contains
the mechanical seals and the adaptive hardware that is necessary to fit a commercially produced
mechanical seal into a variety of equipment. The assembly sometimes includes auxiliary equipment such as
throat bushings and pumping rings. Adaptive hardware includes various sleeves, collars, and glands.
Adaptive hardware is an important aspect of system design because many pumps do not have standardized
dimensions.
Figure 13 - Sealing System
Sealing System
Auxiliary
Equipment
Seal
Assembly
Seals
14
Adaptive
Hardware
Seal Basics
Auxiliary equipment is especially recognizable when multiple seals are used but even simple single systems
use piping and orifices as part of the flush system. Auxiliary equipment includes piping, flow controls,
reservoirs, heat exchangers, pumping rings, gages, and alarms - even pumps! Such equipment may be
packaged as a unit.
Mechanical seals themselves may be classified by their design features and the arrangement of those
features. The design category includes the details and features incorporated into a single primary
ring/mating ring pair. The Arrangement category includes the number, orientation and combination of the
primary ring/mating ring pair. Figure 14 illustrates the classification of mechanical seals.
Figure 14 - Classification of Mechanical Seals
Sealing System
Seal
Assembly
Auxiliary
Equipment
Adaptive
Hardware
Seals
Design
Arrangement/Type
Balance ratio
Face Pattern
Springs
Secondary seals
Drive
Type
Single
Multiple
3.1 Classification by Design
All mechanical seals contain both rotating elements and stationary elements. A variety of design options are
necessary to provide effective sealing solutions.
Figure 15 - Classification by Design
Mechanical Seals
Design Features
Arrangement/Types
Pusher/Non-Pusher
Balance
Face Pattern
Rotating Seal/Stationary Seal
Single Seals
Multiple Seals
Secondary Containment Seals
The Design classification considers the details which enter into the features of these components. Some
examples of these features are balance ratio, face pattern, springs, secondary sealing elements, and drive
mechanism. In general, these design features are not completely independent, that is, emphasis of a
particular feature may also influence other features. For example, selection of a particular secondary sealing
element may influence the shape of the primary ring.
Seal Basics
15
3.1.1 Pusher/Non-Pusher
This is the broadest classification. It is determined by the secondary sealing element used in the seal,
as in Figure 16.
Figure 16 - Pusher vs. Non-Pusher Mechanical Seals
Pusher
O-Ring secondary seal must slide along
shaft to compensate for face wear and
misalignment.
Non-Pusher
Bellows secondary seal expands to
compensate for face wear and misalignment.
Bellows tail is stationary against shaft
Pusher Seals - With pusher type seals, the secondary sealing element is "pushed" axially back and
forth along the outside diameter of the shaft or sleeve. During this "pushing", the secondary sealing
element is in constant contact with the shaft. Two common types of secondary sealing elements used in
pusher seals are O-rings and wedges. In these seals the constant axial "pushing" of the secondary
sealing element coupled with any abrasives in the process fluid can cause wear to the shaft or sleeve
under the contact area. Another potential concern associated with pusher seals is "hang-up". This
occurs as dissolved solids, products of corrosion, and seal fluid oxidation or decomposition, accumulate
and restrict the required axial movement of the secondary sealing element. This causes the seal to lose
its flexibility. It can no longer compensate for face wear and misalignment.
Non-Pusher Seals - The non-pusher secondary sealing elements include bellows constructed out of
elastomeric compounds, PTFE or, metal. The non-pusher secondary sealing elements are not pushed
along the shaft to compensate for the face wear and equipment misalignment, but form a static joint
between the tail end of the bellows and the shaft for elastomeric bellows. The convolution of the
bellows, which does not contact the shaft OD, compensates for the axial movement. The advantage to
this type of seal is that there is no possibility of shaft wear, since the secondary sealing element is free
to float above the shaft. The non-pusher secondary seal forms a static joint on the shaft.
3.1.2 Balance
Balance is an additional way to classify mechanical seals. Seals can be balanced or unbalanced. The
specific sealing or pumping application determines whether the seal needs to be balanced or
unbalanced. Balance in seals is actually a ratio. The balance ratio is defined as the ratio of the hydraulic
closing area to the hydraulic opening area. This ratio is customarily expressed in a percentage. Figure
17 (following page) illustrates the concept of balance ratio. In a seal, hydraulic pressure acts on the
back of the primary ring. The resulting force pushes the faces together. This force is called the closing
force and this area the closing area. Similarly, pressure between the seal faces creates an opening force
which tends to separate the faces. Therefore, the face area is called the opening area. The balance
ratio is simply the ratio of the closing area to the opening area.
16
Seal Basics
Figure 17 - Balance Ratio
AO
AC
Balance Ratio =
Closing Area
Opening Area
As shown in Figure 18, the area above the seal face outside diameter is disregarded when the closing area
is computed. This area is not considered because the pressure is the same all around it. Consequently the
contribution of the resultant of the hydraulic forces on this area is zero.
Figure 18 - An Unbalanced Seal
AC
AO
FC
When the closing area is reduced, the closing force is reduced proportionally. This feature can be used to an
advantage when designing a seal. However, for a seal shape such as shown in Figure 17, the closing area
will always be greater than the opening area. In order to make the closing area less than the opening area,
the shape can be changed as shown in Figure 19.
Figure 19 - A Balanced Seal
AC
AO
The seal in Figure 18 is an unbalanced seal. Its balance ratio is greater than 100% because of the
Seal Basics
17
necessary clearance underneath the mating ring. Typical balance ratios for unbalanced seals range from
120% to 150%. The seal shown in Figure 19 is a balanced seal. Its balance ratio is less than 100%. The
balance ratio of balanced seals is typically from 65% to 90%.
The distinction between balanced seals and unbalanced seals is simply that balanced seals have a
balance ratio less than 100%. A general rule is to use a balanced seal when pressures are above 200
psi in the seal chamber. Each application should be reviewed on an individual basis because the seal
faces are affected not only by pressure, but also by rotational speed, temperature, and the properties of
the liquid being sealed. A more precise way to determine balance requirements is with the use of a PV
(pressure velocity) chart for seal faces which considers the relative materials and rotational speed.
To avoid confusion, one should always think and speak in terms of balance ratio rather than balance.
Seals with a high balance ratio have greater seal face loads than seals with a lesser balance ratio.
The concept of balance ratio in a metal bellows seal is not as obvious as shown in Figure 20. Metal
bellows are considered to be inherently balanced and do not need a step in the shaft. Most standard
metal bellows seals are balanced to approximately 80%. The balance ratio increases with pressure. At
very high pressures the balance ratio may be greater than 100%.
Figure 20 - Metal Bellows Seal
Metal bellows seals may be “inherently” balanced at low pressures.
Balance ratio increases at higher pressures.
Balance Ratio < 1
Zero Pressure
Balance Line
3.1.3 Face Pattern
The most common seal face design is a plain, flat surface but there are many special treatments
designed for specific applications. Figure 21 (next page) shows some of the more common face
treatments. In general, face treatments are a means of modifying the pressure distribution between the
seal faces. The most common objective is to increase the opening force and thereby reduce the
magnitude of the mechanical contact. Face treatments may be considered to produce hydrostatic or
hydrodynamic forces. Hydrostatic forces do not depend on the rotational speed, whereas hydrodynamic
forces vary with the rotational speed.
The simplest face treatment is a plain face that is not flat. This design produces forces that are primarily
hydrostatic. An example is a face that is lapped so it tends to touch the mating ring at the inside
diameter. This means that the leakage path is converging. Although this is a simple concept, it is actually
difficult to lap the required taper with the desired accuracy. Too little taper will reduce the lubrication to
the faces causing them to run hot. Too much taper will cause gross separation of the faces with resulting
high leakage.
18
Seal Basics
Figure 21 also shows another type of hydrostatic face treatment - the hydropadded face. Hydropads are
recesses that are machined into one face - usually the face with the softer material. After machining the
recesses, the remainder of the face is lapped flat. The size, quantity and location of the hydropads is
such that the faces still rub, therefore, leakage is low. The depth of the hydropads is sufficient to allow
for long life in spite of the rubbing. Hydropads actually show some slight hydrodynamic effects,
especially in viscous liquids.
Figure 21 - Seal Face Treatments
Hydrostatic
Hydrodynamic
Plain, Flat
Hydropads
Plain, Tapered
Spiral Grooves
Spiral grooves produce hydrodynamic forces and generally are used for non-contacting operation.
Spiral grooves also produce a pumping effect. The grooves may be oriented so pumping is either with
or against the pressure differential. With a suitable design, the pumping effect can overcome the
hydrostatic leakage effect.
3.1.4 Rotating Seals/Stationary Seals
Although we have shown the primary ring to be rotating with the shaft, either the primary ring or the
mating ring may be used as the rotating element (Figure 22). Seals with rotating primary rings are said
to be rotating seals; seals with stationary primary rings are said to be stationary seals. Because the
springs are always associated with primary rings, sometimes the distinction is made as rotating springs
versus stationary springs.
Figure 22 - Rotating and Stationary Seals
Rotating Seal
Rotating
Stationary Seal
Stationary
Advantages
Less radial and axial space requirements
Lower cost
Seal Basics
Rotating
Stationary
Advantages
Higher speed capability
Tolerates misalignment better
Better cooling
19
For convenience, rotating seals are used in most equipment. Pump shafts are already made of
comparatively high grade material and manufactured to close tolerances. This makes pump shafts well
suited for rotating seal applications. Assembly of rotating seals can generally be done directly on the
shaft or by using relatively simple sleeves.
Stationary seals have some advantages over rotating seals. In small, mass produced seals for modest
services, the entire seal may be placed in a package which minimizes shaft and housing requirements
for the equipment. Stationary seals are also used to advantage in large sizes or at high rotational
speeds. Above 5,000 to 6,000 fpm, a rotating primary ring, which is flexible, by definition may require
dynamic balancing (for rotational imbalance) in order to operate in a stable mode. A stationary primary
ring does not require this balancing.
3.2 Classification by Arrangement
Classification by arrangement describes how the primary ring/mating ring pair is placed in the equipment.
Classification by arrangement includes both single and multiple seals. For multiple seals it also describes
the type of pressurization used between the seals. Sometimes people also classify multiple seals with terms
like 'back-to-back', 'face-to-face', 'series', etc. These terms are not universally defined and, therefore, will not
be discussed in this manual.
3.2.1 Single seals - Most equipment has a single seal. In this category the seal is mounted either in
the process fluid (an 'inside' mount), or outside the process fluid (an 'outside' mount). Inside mounting
(Figure 23) is the most common design because it is easier to cool and generally can handle higher
pressures. Some single seals have a secondary containment device located in the gland plate. These
devices can be a close clearance bushing, a lip seal, or a mechanical seal designed for emission
containment. When a bushing is used, it is intended to restrict leakage in the event of major failure of
the mechanical seal.
Figure 23 - Single, Inside Mounted Seal
Outside mounted seals (Figure 24) have minimal contact with the process fluid. This feature can be an
advantage when handling very corrosive fluids. Outside mounted seals are more difficult to cool and
generally have limited pressure capacity.
Some dual seals have a secondary containment device located in the gland plate. These devices can be
a close clearance bushing, a lip seal, or a mechanical seal designed for emission containment. When a
bushing is used, it is intended to restrict leakage in the event of major failure of both mechanical seals.
Figure 24 - Single, Outside Mounted Seal
20
Seal Basics
3.2.2 Multiple seals - Multiple seals are used when one or more of the following criteria cannot be
achieved by using a single seal design:
!"Emission control of process fluid to the environment under normal operating conditions.
!"Emission of process fluid under upset conditions or failure of the primary seal.
!"Acceptable life (time between repairs). Generally this condition exists with very high sealing
pressures and/or fouling process fluids.
For the remainder of this section, we will describe seal designs that use two mechanical seals as 'dual'
seals. In some very difficult services, three or more seals may be used. However, these applications are
rare. The detailed arrangements of primary and mating rings depend on the application. The orientation
of these pieces should not be used to describe the sealing system.
Figure 25 - Dual Unpressurized Seal
Process
Pressure
Buffer
Atmosphere
Dual Unpressurized seals - As shown in Figure 25, when the pressure between the two seals is lower
than the process pressure, the seal design is called a 'Dual Unpressurized' (DU) design. This design
was traditionally called a 'Tandem Seal'. The fluid between the two seals is called 'Buffer Fluid'. Most DU
seals have the following characteristics:
!"Both seals must be capable of handling full process pressure and temperature while achieving
normal seal life (run length). This feature will not be present when DU seals are used in very
high pressure applications. In these situations the pressure is 'staged'. The pressure between
the seals is usually about one-half of the process pressure.
!"A means of routing normal leakage of the primary (inboard) seal to a collection system without
allowing leakage to the atmosphere.
!"No leakage of buffer fluid to the process fluid.
Some of the advantages and disadvantages of DU seals are shown below:
Advantages:
!"Can provide redundancy if primary (inboard) seal fails
!"Does not require external pressurization system for buffer fluid supply
!"Individual seal leakage is easily identified
Disadvantages:
!"Secondary (outboard ) seal design must include low pressure normal operation and high
pressure if primary seal fails
!"Cooling of this seal is often inadequate
!"Buffer fluid will be contaminated by the process fluid. Regular monitoring and replacement
of the buffer fluid is required to achieve satisfactory seal life.
Seal Basics
21
Dual Pressurized Seals - As shown in Figure 26, when the pressure between the two seals is higher
than the process pressure, the seal design is called a 'Dual Pressurized' (DP) design. This design was
traditionally called a 'Double Seal'. The fluid between the two seals is called 'Barrier Fluid'. DP seals are
used in the most demanding applications. Some of the advantages and disadvantages of DP seals are
as follows:
Advantages:
!"Barrier fluid is not contaminated by the process.
!"Conditions for each seal are consistent for all operating modes.
Disadvantages:
!"Pressurization system required for barrier fluid. This feature often leads to more complex
and costly systems.
!"Small amounts of barrier fluid will leak into the process.
Figure 26 - Dual Pressurized Seal
Barrier
Process
Pressure
Atmosphere
3.2.3 Secondary Containment
A secondary containment device is a means of containing and controlling the primary seal leakage from
a mechanical seal. In contrast to a dual mechanical seal, which operates in a buffer or barrier fluid, a
secondary containment device operates primarily in the leakage from the process seal. There are many
different types of secondary containment devices. Simple and complex bushings, packing, lip seals and
even mechanical seals can be used to provide secondary containment. Leakage rates for the various
secondary sealing devices can vary by several orders of magnitude. Selection of the secondary
containment device and system will depend on the level of leakage to atmosphere that is considered
acceptable as well as performance requirements for normal operation, upsets and in the event of
process seal failure.
By definition, the secondary containment device does not necessarily have the performance or rating of
the primary seal; however, it may be able to temporarily tolerate seal chamber pressure and fluid in the
event of a failure of the primary seal. Large clearance devices like fixed bushings have the highest
leakage rates, floating bushings with reduced clearance are much better. Floating, segmented bushings
have still lower leakage rates. Lip seals have very low leakage rates at low pressures. Gas lubricated
mechanical seals - both contacting and non-contacting - may also be used as secondary containment
devices. Careful consideration should be given to the system that is used with these various devices.
There are two basic types of secondary containment systems: un-piped and piped. Most secondary
containment systems use piping to connect the interspace between the secondary containment device
and the primary seal to a disposal area such as a flare system. In this approach, the secondary
containment device routes a portion of the leakage to the disposal area. Sometimes secondary
containment devices are used without connecting to the disposal area, that is unpiped. Some users
22
Seal Basics
prefer to connect the secondary containment device to a disposal system but isolate the secondary
containment device with valves in the event of a primary seal failure.
The simplest system, un-piped, is essentially no system at all. Leakage from the primary seal is
contained by the secondary containment device within its performance capabilities. If the secondary
containment device provides a tight seal then there is a pressure buildup. But since the secondary
containment device usually has a higher leakage rate than the primary seal, there is usually no pressure
buildup and all leakage from the primary seal escapes readily through the secondary containment
device.
If the secondary containment device is piped to a disposal system, most of the leakage from the primary
seal may be routed to the disposal area, but there is always some leakage from the secondary
containment device. If the secondary containment device is isolated from the disposal area, all leakage
occurs at the secondary containment device.
Secondary containment seals may also be used as a means of controlling a quench fluid for the primary
seal. Typical quench fluids are steam and water.
In the event of a primary seal failure, there are several scenarios with respect to leakage from the
secondary containment device. If the secondary containment device is piped to a disposal system, most
leakage from the primary seal is routed to the disposal system. There will always be some leakage from
the secondary containment device but because pressures are usually low for piped systems, the
secondary sealing device leakage rate is also low. With an un-piped or blocked in secondary
containment device, there will be a pressure build up in the sealing interspace. The degree of this
pressure build up is a function of the leakage rate from the (failed) primary seal and the leakage rate of
the secondary containment device to atmosphere. This means that leakage is reduced but the point of
escape is still at the seal gland.
Many end users prefer that the secondary sealing device use a different principle from the primary seal.
For example, the primary seal might be a mechanical seal (with radial leakage path) but the secondary
containment device may be a floating segmented bushing that uses an axial leakage path. This
preference is based on the feeling that any upsets or mechanical problems affecting the primary seal will
have a less significant effect on the secondary containment device.
Still another variation on secondary containment is to add a purge to a piped up secondary containment
system. Typically, low pressure nitrogen is supplied to the seal interspace. This purge then flows to the
disposal system and carries leakage from the primary seal along with it. When a mechanical seal is
used as the secondary containment device, the performance of the combination can approach that of a
dual unpressurized arrangement.
Seal Basics
23
4.0 Lubrication
In the operating mode, mechanical seals have a microscopic film between the lapped sealing faces acting
as a lubricant. The film is either the process fluid or an external flush injection fluid. The principle of
establishing this fluid film is essential to all seal designs. Most mechanical seals are designed to operate in
liquids. These seals require a liquid film that separates the faces. A magnified cross section of the sealing
surfaces show high and low points (Figure 27). The lubricant should be thick enough to keep the high points
from touching and thin enough to carry away heat from the surfaces (Figure 28). However, there are
mechanical seals specifically engineered for dry running or gas service.
Figure 27 - The Purpose of Lubrication
Lubrication Purpose:
! Separate surfaces
!"Prevent contact of high surface points
!"Reduce friction and heat generation
Magnified Surface A
Separation
Heat
Lubricant
Magnified Surface B
Figure 28 - Seal Face Lubrication
Pump Housing
Gland Plate
Process Fluid
Primary Ring
Mating Ring
Invisible Leakage:
Fluid Evaporates Before Reaching Atmosphere
If the liquid film is too thin, it will not separate the surface high points, causing them to contact and shear off
and ultimately become contaminants. As the film thickness increases, leakage also increases. Too much
heat will distort, crack, deteriorate, or burn the sealing surfaces (Figure 29 ).
Figure 29 - Dry Running Seal faces
Pump Housing
Gland Plate
No Fluid
Dry Running
Primary Ring
24
Mating Ring
Seal Basics
4.1 Heat Transfer
Mechanical seals generate heat as the primary ring and mating ring "rub together". Heat is also generated
through viscous shear of the fluid film. For low viscosity fluids, most heat generation is by rubbing. In this
case the coefficient of rubbing friction for the primary ring and mating ring pair determines the heat
generation. For viscous fluids there may be no rubbing but viscous shearing may generate about the same
amount of heat as if rubbing occurred. An important difference is that the wear rate is likely to be much
greater when low viscosity fluids are between the seal faces (Figure 30).
Figure 30 - Heat Generation
Primary
Ring
Mating
Ring
Heat transfer plays an important role in seal performance. Both conduction and convection heat transfer are
significant in mechanical seals. Conduction heat transfer is the process of heat transfer through solids.
Convection heat transfer is the transfer of heat from the solid to the surrounding liquid or gas (Figure 31).
Figure 31 - Heat Transfer
Since heat generation takes place in the sealing interface, the first heat transfer process is through the
primary ring and mating ring. The thermal conductivity of these materials is very important. Materials like
silicon carbide and tungsten carbide have relatively high thermal conductivity. Materials like alumina
(ceramic) and carbon graphite have a much lower thermal conductivity.
Convection transfers heat from the primary ring and mating ring to the surrounding fluid. Convection heat
transfer is usually evaluated as having three components: a convection (or film) coefficient, wetted area, and
temperature difference. The convection coefficient is the combined effects of fluid properties, rotational
speed and seal chamber design. Low viscosity and high speeds promote high convection coefficients. It is
very important to have enough wetted area, so heat transfer takes place without excessive temperature
increase in the mating ring and primary ring.
Seal Basics
25
4.2 Flushing/Quenching
For most seal designs, all generated heat enters the fluid surrounding the seal. This means that the fluid
tends to increase in temperature. Unless the surrounding fluid is cooled or continuously replaced, it may
become too hot for reliable seal performance. Most seals operate with continuous replacement of the
surrounding liquid. This replacement process is called the seal flush. Figure 32 illustrates this process.
Flush rates are selected so the fluid properties and average temperature remain satisfactory after removing
all generated heat. Using this approach, a typical rule is the average temperature rise should be less than
5°F / 2.8°C for light hydrocarbons, 15°F / 8.4°C for water and 30°F / 16.8°C for oils. A simpler rule is the
seal flush rate should be about one gpm per inch of shaft size (1.5 lpm per cm of shaft size) for water and
non-flashing services but twice that rate for flashing services. It is important to note that all these rules give
about the same results for the majority of seal applications. Most of the time, the actual flush rate is greater
than this guideline. Even so, many seal problems are related to the seal flush rate, design, and location. It is
important to apply the proper flush rate and system for each application.
Figure 32 - Flush Injection
Quench - A quench is a neutral fluid that is introduced on the atmospheric side of a seal to retard formation
of solids that may interfere with seal movement. It is also used to dilute any fluid that may have leaked
across the seal faces, cool the fluid coming across the faces and help control the temperature of the seal.
Figure 33 - Quench Injection
Quench
26
Seal Basics
4.3 Piping Plans
Following are the most popular of the industry-recognized plans from API (American Petroleum Institute)
and ANSI (American National Standard Institute).
API Plan 11 (ANSI Plan 7311)
This plan uses a recirculation line from the pump case discharge through an orifice to the seal chamber
(Figure 34). This is the most common piping plan for single seals and it is also frequently used for the
inboard seal in multiple seal arrangements. This Plan is commonly referred to as "by-pass from discharge".
Figure 34 - API Plan 11
Plan 11 is sometimes used when a single seal is running in a product that is near its vapor pressure and
cooling is not practical. Vaporization can be suppressed by installing a close-clearance throat bushing (less
than .012 inches of diametrical clearance) to increase seal chamber pressure. While this is not as efficient
as cooling for vapor suppression, it is much less expensive. It sometimes can be advantageous to take the
recirculating flush from downstream of the discharge check valve. When this is done, the flush to the seal
will be maintained in case of pump suction loss. This makes the seal more forgiving to pump upsets,
particularly if a close-clearance throat bushing is used.
Some of the potential problems that can arise when using this plan include:
!"Not effective when the seal chamber is at or near discharge pressure.
!"Thermosensitive or viscous products can set up in the flush line, particularly when the pump is
down. This can be prevented by heat tracing and insulating the piping to keep the pumped fluid at
the proper temperature or by flushing out the piping when the pump is down.
!"When this plan is used in a heavy slurry, the flush line can plug. API Plan 32 (injection from an
outside source) is usually recommended for these applications.
!"If there is a large pressure differential between the seal chamber and pump discharge, an orifice or
a series of orifices must be installed to break down the pressure. In order to minimize the chance of
plugging, the smallest orifice diameter that should be used is 0.125 inches.
!"Erosion of the seal parts can occur if the flush enters the seal chamber with excessive velocity. This
can be prevented by using tangential flush ports and installing the orifice(s) as far from the seal
chamber as possible.
API Plan 13 (ANSI Plan 7313)
This plan (Figure 35) is for some vertical pumps. The seal will see full discharge pressure under normal
arrangement. Because of this arrangement, there is not the pressure differential that allows a Plan 11 to
work. In this plan, product is routed from the seal chamber back to the pump suction to provide cooling for
the seal and to vent air or vapors. Plan 13 is also used in high-head pumps where Plan 11 will not work.
Seal Basics
27
This plan will not work well in low-head pumps due to the low pressure differential between the seal
chamber and the pump suction. Calculating the required flush flow rate and then calculating the required
orifice will determine the suitability of the service for Plan 13.
Figure 35 - API Plan 13
API Plan 14 (ANSI Plan 7314)
This plan (Figure 36) is a common flush plan for vertical pumps. It is a recirculation from pump discharge
through a flow control orifice (when required) to the seal and back to the suction nozzle. The orifice must be
sized in accordance with the throat bushing and the return line. This plan is similar to Plan 11. The
difference is that the flow back to the suction side will evacuate vapors that may collect in the seal chamber.
This plan is recommended for light hydrocarbon service. It can be a common plan for vertical pumps, where
the seal chamber is subject to pump discharge pressure. On vertical pumps the return to suction line should
extend 12 inches minimum above the "FO" connection to ensure proper venting of vapors within the seal
chamber.
Figure 36 - API Plan 14
28
Seal Basics
API Plan 23 (ANSI Plan 7323)
Plan 23 (Figure 37) is generally recommended for hot water services, like boiler feed water and many
hydrocarbon services. It is the standard selection for hot water above180°F / 80°C. Water has very low
lubricity above 180°F / 80°C and can cause high seal face wear This plan is also effective in many
hydrocarbon and chemical services. In these services it is necessary to cool the fluid to establish the
required margin between fluid vapor pressure (at the seal chamber temperature) and the seal chamber
pressure. In a Plan 23, the cooler only removes seal face-generated heat plus heat soak from the process.
In API Plan 23 a throat bushing isolates the product in the seal chamber from that in the impeller area of a
pump. A pumping ring circulates seal chamber fluid through a heat exchanger and back to the seal
chamber. The heat exchanger only cools the fluid that the seal is operating in and this fluid does not enter
the process. High energy efficiency is a result. As in any piping plan, Plan 23 requires a high point vent.
Plan 23 should not be used in applications with solids.
Figure 37 - API Plan 23
API Plan 31 (ANSI Plan 7331)
This plan (Figure 38) is recommended only for services containing solids with a specific gravity two times or
more than that of the process fluid. Water service to remove sand or pipe slag is a typical use for this plan.
In API Plan 31, product is routed from the discharge of the pump to a cyclone separator. Solid particles are
centrifuged from the stream and routed back to suction. The clean seal flush is routed from the cyclone
separator into the flush connection on the gland plate. Throat bushings are required when a Plan 31 is
specified. In a Plan 31 it is difficult to obtain the desired pressure differential required for the cyclone
separator. Use should be restricted only to those services where absolutely required.
Figure 38 - API Plan 31
Seal Basics
29
API Plan 32 (ANSI Plan 7332)
This plan (Figure 39), sometimes called an external or foreign flush, is utilized when the process fluid is not
suitable for proper seal environmental control. This is the case when pumping hazardous, corrosive, dirty, or
fluids devoid of the required lubricating and cooling properties for the application at hand. An outside source
of a clean flush that is compatible with the process is injected into the gland port. The outside flush source is
injected at a pressure higher than at the throat of the seal chamber to ensure the flush will flow under the
throat bushing preventing the process fluid from entering the sealing area. Dilution of the product must be
tolerated and flow rate (gpm) and pressure (psi) of the injected flush must be determined. Note that care
should be exercised in choosing a proper source in system to eliminate possible vaporization in the seal
flush.
Figure 39 - API Plan 32
API Plan 52 (ANSI Plan 7352)
This plan (Figure 40) provides circulation of a buffer fluid in a closed loop arrangement between the seal
chamber and an unpressurized external supply tank. It is used in conjunction with multiple seal
arrangements to isolate the pumped product from the atmosphere or extend service life by providing a better
environment for the seal. It is commonly used in flashing applications as well as with fluids that change state
when exposed to atmosphere.
The buffer fluid will lubricate the outboard seal, while the inboard seal is lubricated by the pumped product.
To provide a driving force for the buffer fluid to flow across the outer seal, it can be advantageous to
maintain a pressure of 5-10 psig on the buffer fluid. In this plan, any emissions of the pumped product past
the inner seal will migrate to the buffer fluid. If the pumped product is immiscible (incapable of being mixed)
with the barrier fluid, or has a higher vapor pressure, these emissions can safely be vented to the flare or
any other vapor recovery system.
By using a low leakage inner seal, zero emissions of the pumped product to atmosphere can be achieved.
If the pumped product is miscible with the buffer fluid or has a lower vapor pressure, the buffer fluid must be
changed on a regular basis to prevent contamination and to provide near zero emissions of the pumped
product to the atmosphere. Some form of forced circulation, such as a pumping ring, should be used
whenever possible. Thermal convection is sometimes used, however, flow is easily interrupted.
30
Seal Basics
Figure 40 - API Plan 52
Because installation of this system is critical to its success, keep in mind the following general rules when
installing an API Plan 52 with a pumping ring:
!"The distance from the bottom of the reservoir to the centerline of the shaft should be no less than
18 inches.
!"The horizontal distance from the reservoir to the seal should be as short as possible, with a
maximum of three feet.
!"The size of the reservoir is generally determined by the shaft size. As a general rule, it should be
one gallon per inch of shaft diameter, with a two gallon minimum. However if following API 682, the
reservoir should be sized to contain a minimum of five gallons of liquid.
!"The minimum buffer fluid level in the reservoir should be maintained at least one inch above the
return line fitting. This ensures a fluid packing system so the pumping ring will only need to
overcome line losses.
API Plan 53 (ANSI Plan 7353)
Plan 53 (Figure 41) is used in services where no product leakage to atmosphere can be tolerated, such as
toxic or carcinogenic fluids. This plan system consists of a dual mechanical seal with a barrier fluid between
them. The barrier fluid is contained in a seal reservoir which is pressurized to a higher pressure (20-25 psi)
than the seal chamber. Inner seal leakage will be barrier fluid leakage into the product. There will always be
some leakage.
Plan 53 can be chosen over Plan 52 for abrasive, dirty or polymerizing products which damage the seal
faces or cause problems with the buffer fluid system if Plan 52 were used. There are two problems with Plan
53 that must be considered. First, there will always be some leakage of barrier fluid into the product, so the
product must be able to accommodate a small amount of contamination from the barrier fluid. Second, Plan
53 is dependent on having the seal reservoir pressure maintained at the pressure above the seal chamber.
If the seal reservoir pressure drops, the system will begin to operate like a Plan 52, or unpressurized dual
seal, which does not offer the same level of sealing integrity. Specifically, the inner seal leakage direction will
be reversed and the barrier fluid will over time become contaminated with the process fluid. The problems
that result include possible seal failure.
Seal Basics
31
Figure 41 - API Plan 53
API Plan 54 (ANSI Plan 7354)
Plan 54 (Figure 42) systems are also pressurized dual seal systems. The inboard seal leakage is barrier
fluid into the pumped product. In a Plan 54 the barrier fluid supplied to the seal is a cool clean product from
an external source. The supply pressure of this product is 20+ psi greater than the pressure the inner seal is
sealing against. Because of this, some of the barrier fluid leaks into the process. This arrangement should
never be used where the barrier fluid pressure is less than the sealed pressure. If this happens, the failure
of one inner seal could contaminate the entire external system and cause additional seal failures.
Plan 54 is frequently used in services where the pumped fluid is hot, contaminated with solids or both. If
Plan 54 is specified, consider carefully the reliability of the external fluid source. When the source is
interrupted or contaminated, the resulting seal failure can be very expensive. A properly engineered barrier
fluid system is complex and often expensive. When these systems are properly engineered and operated,
they provide the most reliable systems.
Figure 42 - API Plan 54
32
Seal Basics
API Plan 62 (ANSI Plan 7362)
API Plan 62 (Figure 43) provides for an external quench to be delivered through a gland connection to the
low pressure side of the seal to the seal's ID. The quench fluid is used for cooling, heating, cleaning, and
isolation from air depending on the pumped product. The quench fluid is contained using a throttle bushing,
lip seal, auxiliary packing or an auxiliary sealing device.
Figure 43 - API Plan 62
A quench is most frequently used in applications where the pumped product changes state when exposed to
the atmosphere. For example, steam is frequently used to prevent coke formation in hydrocarbon
applications exceeding 350°F / 177°C. A steam or water quench can prevent buildup of crystalline deposits
in caustic applications. Water is used to cleanse the atmospheric side of a single seal in a variety of other
applications. A quench can provide the additional benefit of helping to cool the pumped product to prevent
flashing, or to warm the pumped product to keep it from setting up.
The quench should be regulated so that there is minimal amount of flow to the atmospheric side of the seal.
The most common methods of regulation are needle valves and pressure regulators. Needle valves are
generally preferred because they are typically more reliable than pressure regulators at low pressures (less
than 5 psi / .3 bar) .
Problems that can be encountered when using a steam quench include:
!"If the bearings are exposed to the quench, they can be damaged. This can be prevented by
reducing the quench flow or installing an auxiliary sealing device.
!"A wet steam quench should not be used, because it can enter the gland as a liquid and vaporize
near the seal faces, causing the faces to pop open.
!"The quench should be turned on before the pump is started, particularly when steam is being used
in a high temperature application. If it is turned on after the pump is started, a slug of water could
damage the seal.
!"The steam quench should be on anytime there is a process in the pump.
The needle valve will sometimes plug during operation. If this occurs, precautions must be taken to prevent
the seal from being hit with a slug of cold water when unplugging the line. This can be accomplished by
installing a block valve and blowdown on the line downstream of the needle valve.
Plans not shown include:
Plan 01 - Integral (internal) recirculation from pump discharge to seal.
Plan 02 - Dead-ended seal chamber with no circulation to flushed fluid.
Plan 21 - Recirculation from pump discharge through cooler to seal.
Plan 41 - Recirculation from pump discharge through cyclone separator delivering clean fluid
through cooler to seal.
Plan 61 - Tapped connections for the purchaser's use, typically used for steam, gas, or water to
an auxiliary device.
** Plans 12 & 22 - were dropped from API 610, 8th Edition.
Seal Basics
33
5.0 Materials
Table 2 - Common Seal Materials
Secondary
Seal
Primary
Ring
Chemloy
Bronze
Chloroprene
Carbon (Metal Filled)
Ethylene Propylene
Carbon (Resin Filled)
Fluorocarbon
Carbon (Resin Coked)
Graphite
Chemloy
Hastelloy
Phenolic
Inconel
Polytetrafluoroethylene
Nitrile
Silicon Carbide
Perfluoroelastomer
Siliconized Graphite
Polytetrafluoroethylene
Tungsten Carbide
Silicone
Stainless Steel
Tetrafluoroethylene
Various Fibers
Hardware
Mating
Ring
Brass
Hastelloy
Inconel
Monel
Stainless Steel
Titanium
Alumina
Cast iron
Ni-Resist
Silicon Carbide
Stellite
Tungsten Carbide
Spring
Hastelloy
Inconel
Monel
Stainless Steel
Titanium
Table 3 - Face Material Comparison
34
Conductivity
Chemical
Wear
Cost
Phenolic
Poor
Good
Good
Very Low
Carbon
Poor
Good
Good
Low
NiResist
Good
Good
Good
Low
Alumina (Ceramic)
Poor
Good
Good
Low
Tungsten Carbide
Excellent
Excellent
Excellent
High
Silicon Carbide
Best
Best
Best
High
Seal Basics
Graph 1 - Temperature Ranges for Common Secondary Seal Materials
700°F
371°C
700°F
600°F
200°F
121°C
250°F
0°F
-54°C
-65°F
204°C
204°C
400°F
204°C
400°F
149°C
300°F
-45°C
-50°F
316°C
287°C
550°F
287°C
550°F
400°F
371°C
93°C
-29°C
-20°F
-40°C
-40°F
-29°C
-20°F
-18°C
-200°F
-129°C
-212°C
-350°F
-400°F
Nitrile
EPDM
FKM
FFKM
Aflas®
PTFE
-240°C
-400°F
-240°C
Graphite
Aflas is a registered trademark of Asahi Glass Co. Ltd.
Seal Basics
35
6.0 Auxiliary Equipment
The following pages will briefly explain auxiliary equipment associated with mechanical seal environmental
control. Selection of the correct size unit is critical to the successful operation of a mechanical seal and
should be determined prior to installation. More detailed information can be obtained in the auxiliary
equipment's specification sheets or through engineering.
6.1 Abrasive Separator
Abrasives within a fluid can cause damage to a mechanical seal's faces and other components and in turn
cause premature seal failure. To ensure that proper seal lubrication is achieved, abrasives must be removed
from the fluid stream. One method is to insert a cyclone separator into the piping arrangement to the seal
(Figure 44). This piping arrangement is API Plan 31
Figure 44 - Abrasive Separator
Clean Fluid to Seal
Feed
From Discharge
Abrasives to Suction
The separator works in the following manner. The abrasive laden fluid is piped from the discharge or high
pressure side of the equipment into the feed tap of the cyclone separator. The abrasive fluid enters the cone
shaped cavity of the separator tangentially and is spiraled downward by centrifugal force. The particles in
the fluid, being of greater density, remain close to the tapered wall of the cone and course downward out the
discharge outlet at the bottom of the separator and back to suction or the low pressure side of the
equipment. The clean liquid is forced up through the outlet port located at the top of the separator and is
then piped to the seal for proper face lubrication. The factors effecting efficiency of the separator are
pressure differential, the particles specific gravity, viscosity of the fluid, and size of abrasives.
!"The velocity of the fluid entering the separator is dependent on the pressure differential between the
fluid feed and the outlet port and discharge outlet. The pressure differential between the pump
discharge and the seal chamber should be adequate enough to ensure proper flow through the
separator. A minimum pressure differential of 20 psi / 1.4 bar, between the feed and overflow, should
36
Seal Basics
·
be maintained. Maximum differential pressure should never exceed published curves.
!"The particles to be separated must have a specific gravity greater than that of the fluid. A quick
field test is to take a fluid sample in a clear container, shake it up and let it sit for ten minutes.
If separation occurs within that time frame it is safe to assume a separator will prove effective.
!"The abrasive separator will be effective on sand, small iron oxide particles, and other debris.
Substances that are dissolved in the liquid cannot be taken out by a separator.
A cyclone separator does not remove abrasive particles from the fluid stream the way a filter does. It acts as
a "classifier" dividing particles by size. The advantage the separator has over a filter is that it does not retain
the abrasive particles and thus avoids clogging. Separators can be manufactured in various metallic and
non-metallic materials.
6.2 Bushings
Throat bushings - Using a clean fluid injected into the seal chamber is considered one of the best
methods to ensure the successful operation of a mechanical seal. The flushing fluid is injected over the seal
faces at a higher pressure than that of the process fluid at the throat of the seal chamber. This causes the
cooling fluid to migrate under the clearance at the throat of the seal chamber. This reduces the possibility of
the process fluid contaminating the seal chamber. However, this requires a considerable amount of fluid
unless some provision is made to reduce the loss of the flushing fluid.
Figure 45 - Throat Bushing
Throat Bushing
Placing a throat bushing at the bottom of the seal chamber provides three advantages for the seal: seal
chamber pressure can be controlled; seal chamber fluid can be isolated; and the flow in or out of the seal
chamber can be controlled. The bushing is placed between the mechanical seal and the impeller at the
bottom of the seal chamber. The bushing forms a close clearance around the shaft. Bushings can be made
out of various materials and should not mar the shaft or spark upon contact with the rotating shaft. Typical
materials include carbon, PTFE, and bronze.
Seal Basics
37
Figure 46 - Floating Segmental Bushing
1. Garter Spring
2. Segmental Bushing
3. Spring
4. O-Ring
5. Follower Plate
6. Cap screw
1
2
3
6
4
5
Note:
To obtain the best results, it is
recommended that the area under
the bushing be hard coated.
Note
Floating Segmental Bushing - This bushing, a type of secondary containment, is placed at the outboard
end of a mechanical seal gland plate. It is positively retained within the gland plate and prevents blowout in
the event of a major failure. It forms a close clearance around the shaft restricting the leakage from the
primary seal to the atmosphere. The gland plate often incorporates a vent port allowing escaping vapors to
be disposed of and a drain port allowing the escaping liquids to drain out. This bushing is also used with a
quench system to restrict the quench fluid from leaking to the atmosphere. The bushings can be arranged in
a fixed design or floating designs.
6.3 Flow Meter
Maintaining the required amount of flush to a mechanical seal is vital to the successful operation of that
seal. A flow meter can be used by itself or in conjunction with other mechanical seal environmental controls.
The purpose of the flow meter is to monitor the desired flow to the seal chamber. The flow meter monitors
any leakage within the sealing line, leakage to or from the process, and plugging or restricted flow of the
coolant source. Metering information will help determine where the source of the fluid flow disturbance is, so
maintenance can perform the required corrective action. Flow meters can be equipped with alarm units to
alert an operator of any potential problems with the sealing water system, prior to its damaging the
mechanical seal.
Flow meters are commonly used to monitor quench systems and dual seal barrier/buffer fluid flows. They
can also be used to maintain proper flow to single seals. With the quench system, the external flow will enter
the flow meter and a regulated amount will be sent to the quench seal chamber on the atmospheric side of
the seal, and a line sent out to a drain.
38
Seal Basics
Figure 47 - Heat Exchanger
6.4 Heat Exchanger
The main function of a heat exchanger is to cool seal flush. High temperatures can cause damage to seal
components. A heat exchanger circulates hot process fluid through coils within its shell to cool it. Since the
process fluid remains isolated from the cooling medium, there is no chance for contamination of the coolant
(usually water) or barrier/flush fluid.
The hot process fluid is directed from either the high pressure side of the equipment, usually discharge, as
in Plan 21 and 41, or circulated out of the seal chamber to the heat exchanger, as in Plan 23. The hot
process fluid flows through an internal, one piece coil housed within the exchanger shell. The cooled
process fluid exits the exchanger and is piped back to the seal chamber for seal face lubrication.
When using the by-pass from discharge piping plans (Plans 21 and 41), care must be taken to ensure that
the pressure differential between the exchanger inlet and discharge pressure is not excessive. A throat
bushing in the seal chamber or piping orifice can be utilized to help control the product flow. When product
flow is coming out of the seal chamber to the exchanger (Plan 23) a pumping ring is commonly used to
generate the required product flow through the exchanger.
In selecting the proper size heat exchanger to cool the flushing fluid on piping plans using a product
recirculation from the pump case (Plans 21 and 41), data on the desired flush rate, desired temperature of
the flush stream, the product's specific gravity, and operating process temperature will be required. With a
piping plan utilizing the product circulated out of the seal chamber (Plan 23), there is a confined amount of
process fluid being cooled. Therefore, the heat exchanger will only be required to remove the heat that is
generated by the seal faces and heat soak absorbed from the pump. Graphs and calculations are available
on the heat exchanger specification sheets for determining cooling water requirements and pressure drop.
6.5 Pumping Rings
A pumping ring is an internal pumping device (Figure 48, page 40) used in various loop circulation systems
to promote the fluid flow around the mechanical seal. They typically are used in conjunction with a pressure
reservoir in dual seal arrangements to make up a complete and efficient cooling system with minimal parts.
The pumping is produced by either axial flow and/or radial flow devices.
Axial Flow Pumping Rings - This pumping ring is usually positioned on the outside diameter of the
outboard seal in dual seal configurations. The successful operation of the pumping ring is not dependent on
an external pump because the positive flow is provided by the rotation of the shaft. The fluid in the seal
chamber is pumped from one end of the ring to the other. The inlet and outlet ports should not be directly
over the ring, as this will reduce its pumping performance. The pumping rate will also be diminished, as a
general rule, as the clearance between the seal chamber bore and the pumping ring is increased.
Radial Flow Pumping Rings - There are three basic types of radial flow pumping rings including vanes,
slots and the rotating element of the seal. The radial flow pumping ring uses vanes that act like an enclosed
Seal Basics
39
Figure 48 - Pumping Rings
Radial Flow
Drilled Vane Holes
Axial Flow
Radial Flow
Paddle Wheel
centrifugal pump impeller except the vanes are straight, drilled holes. The flow to the outlet port is radially
directed through the drilled vane holes. Pumping performance will be maximized when accompanied with a
tangential outlet port.
The radial pumping ring uses slots that act like an enclosed centrifugal pump impeller except the vanes are
straight slots. This design is also known as a "paddle wheel" since it functions in a similar fashion to one. It
operates similar to the drilled vane rings and also performs better when the outlet port is tangentially drilled.
This design typically pumps less than similar sized drilled vane pumping rings, and typically is used when
both radial and axial space are limited.
Radial flow can also be achieved simply by the rotating element of the mechanical seal. Obviously, the
pumping effect is less than the previously discussed designs, but may be enough to suit certain applications.
The performance will be increased when the clearance between the rotating element and the outlet port is
decreased, and the rotating element is placed directly under a tangential outlet port.
40
Seal Basics
7.0 Sealing Systems
A mechanical seal does not work in isolation. The previous chapters have referenced processes and pieces
of equipment that are part of what can be called the sealing system. This chapter will focus on what
happens to the mechanical seal when it is put into operation in a pump.
7.l Seal Installation
If a mechanical seal fails soon after installation, it is usually caused by improper start up procedures or
installation errors. Some common examples of installation errors include:
!"Mistake in assembly. Some examples: installing the seal backwards; omitting seal parts; not
installing anti-rotating pins in stationary mating rings; or not tightening or over-tightening set screws.
A lack of understanding of how seals work may contribute to seal failure at installation.
!"Damage to secondary seals. Cuts and damage to O-rings and gaskets; damage to the shaft or
gland surfaces; or improper shaft finishes or dimensions.
!"Damage to faces. Dirt, distortion, scratches or chips caused by handling, or improper clamping.
!"Improper spring tension. Incorrect positioning of the seal. Spring tension not only holds the faces
together, but in many seals loads the secondary seal to the shaft. Too much compression or too little
can cause early failure.
!"Misalignment. Certain alignments are critical for successful seal operation. The main objective is to
ensure that both the stationary and rotating faces run perpendicular to the shaft. Misalignment
usually shortens seal life rather than causing immediate failure.
The use of cartridge seals reduces most seal failure due to installation errors.
7.1.1 Protecting the Seal Faces
New seal faces are lapped flat to tolerances of one to three Helium Light Bands (one light band is equal
to 11.6 millionths of an inch). Even very stiff materials, such as ceramic, will warp enough to leak if they
are over-stressed by clamping. It is important to use clamping surfaces that are flat and smooth, or
gaskets on both sides of the clamped surface. Four gland bolts should be used if at all possible. Glands
clamped with only two bolts have a tendency to affect the alignment of the seal faces. On a horizontalsplit case pump, the seal chamber faces of each half of the pump should be machined after they are
together to ensure a precise sealing surface.
7.1.2 General Rules
!"If a seal face has been used, repaired or re-lapped, check for flatness before reusing it.
!"Keep the faces covered or protected until they are installed in the pump.
!"Keep your hands and the work area as clean as possible
!"When using clamped mating rings, make sure clamping surfaces are smooth, flat and clean.
!"Use clean cloths if you push on a seal face during installation.
!"To lightly polish or clean a face, use an alcohol based solvent and a lint-free tissue.
!"Where possible, handle seals on their outside diameters. Faces should be cleaned if contact
occurs.
!"When using a new shaft sleeve, chamfer the end to ensure no damage to the secondary seal
on installation.
7.1.3 Installing the Seal
A significant amount of skill is required to correctly install a mechanical seal. Following is a brief
description of the steps used to install a component seal. For more detailed information, contact the
John Crane Training Department. John Crane offers a seal installation training school where the
participants do hands-on work with pumps and mechanical seals. These instructions are only a guide.
Your John Crane sales representative is available for consultation on seal installation.
Seal Basics
41
The following is a brief summary of instructions for installing a component seal.
1. Color the shaft or sleeve at the face of the seal chamber with machinist bluing. Manually turn the
shaft and scribe a line at the seal chamber face in the colored section using a straight edge held
across the seal chamber face. This line is represented by the letter "B" in Figures 49 and 50. This
line is the point of reference from which the seal is set in its proper position.
2. Remove the impeller and seal chamber, exposing the shaft or sleeve and the seal installation
reference line "B".
3. If the stationary mating ring extends into the seal chamber, as shown in Figure 49, measure the
distance and label this dimension "X". This dimension is added to the working height "A" to find the
distance "D" from seal chamber line "B" to reference line "C" to the back of the seal set position. All
gaskets should be in place for this measurement.
Example: Figure 49
Seal working height "A" = 1-1/2 inches
Mating ring pilot length "X" = 1/4 inch
Seal set dimension "D" from seal chamber "B" = 1-3/4 inches
4. Scribe the shaft or sleeve with line "C" 1-3/4 inches from the seal chamber line "B". The back of the
seal head is installed and attached to the shaft or sleeve at line "C".
Figure 49 - Calculating Seal Installation Position - Piloted Mating Ring
A = Seal Working Height
B = Seal Chamber Face
C = Seal Set Line
D = Installation Reference
X = Mating Ring Pilot
D
X
C
B
D=A+X
A
Example: Figure 50
Seal operating length "A" = 2 inches
Faces meet outside box "X" = 1/2 inch
Seal set dimension "D" from seal chamber face "B" = 1-1/2 inch
Scribe the shaft or sleeve with line "C" 1-1/2 inches from the seal chamber line "B". The back of the
primary ring assembly is installed and attached to shaft or sleeve at line "C".
Figure 50 - Calculating Seal Installation Position - Mating Ring Recessed
A = Seal Working Height
B = Seal Chamber Face
C = Seal Set Line
D = Installation Reference
X = Distance Outside Seal
Chamber
D
X
C
B
A
D=A-X
42
Seal Basics
Summary of steps
1. Mark the seal chamber face position on the shaft or sleeve.
2. Determine the operating length of the seal.
3. Determine the distance from the seal chamber face to the face of the mating ring.
4. If the stationary face pilots into the seal chamber, add the dimension "X" to the operating length
to determine the position of the back of the seal.
5. If the stationary face is recessed in the gland, subtract the dimension "X" from the operating
length to determine the position of the back of the seal.
Most seal manufacturers give you "D", the seal set dimension, in an installation drawing.
Check all components to the assembly drawing to assure that the part numbers and dimensions match.
7.1.4 General Rules for Secondary Seals
!"The shaft or sleeve should be clean, free from set screw marks, sharp obstructions, rust and
sharp edges.
!"Threaded section of the shaft should be covered to prevent damage to the seal.
!"Keyways should be covered.
!"Seals should be pushed, not twisted or hammered, down the shaft.
!"If the seal has a dynamic elastomer, the shaft or sleeve should be highly polished with the
surface finish in the 16 to 32 microinch (RMS) range.
!"O-rings should never be removed from grooves using a sharp metal object. The scratch can set
up a leak path around the o-ring.
7.1.5 Pump Condition
Rapid back-and-forth motion of the seal of only a few thousandths of an inch will shorten seal life. This
section describes the alignments and parts of the pump that can cause these small but destructive
motions. The most critical alignment in installation is that of the mating ring assembly. Misalignment
leads to a back-and-forth motion of the seal head on each revolution of the shaft.
3600 RPM X 2 movements =
X 60 minutes =
X 24 hours =
7,200 per minute
432,000 per hour
10,368,000 per day
7.1.6 Shaft Straightness
The straightness of the shaft can be checked by dial indicating the shaft as you turn it. The general rule
is that there should be no more than 0.001" Total Indicator Reading (TIR) per inch of shaft diameter
radial runout. After 0.005" runout, most seals will start having problems.
7.1.7 Bearings
The end play should also be checked with a dial indicator. Before placing the indicator pin against the
shaft shoulder (impeller end), ensure that the shaft is all the way in, then push the end of the shaft near
the thrust bearing. This will verify the condition of the thrust bearing and its fit in the housing. Shaft lift
can be used to determine the condition of the radial bearing and its fit in the housing. This is checked by
placing a dial indicator at the end of the shaft on the impeller end and lifting up.
7.1.8 Couplings
A misaligned coupling can transmit vibration through the bearings to the seal. Rapid bearing wear will
result from this condition leading to bearing failure and often seal failure.
7.1.9 Unbalanced Impeller
Severe vibrations caused by an unbalanced impeller can result in face separation, shaft fretting, drive
lug wear, and premature seal failure.
Seal Basics
43
7.1.10 Pipe Strain
Pipe strain can cause misalignment between the stationary and rotary seal faces, resulting in seal
motion, face separation, leakage and failure.
7.1.11 Shaft Deflection
Shaft deflection due to unbalanced radial loads and the weight of the impeller will have the same effect
on the seal as with a bent shaft.
7.2 Centrifugal Pump Prep for Start Up
After the pump has been installed and coupling alignment completed, the following steps should be
implemented for successful start up.
!"Check the pump and driver for proper lubrication.
!"Check the driver for correct rotation.
!"Pump suction valve should be fully opened. Check for leaks.
!"Vent pump case and seal chamber (open vent at top of pump casing until all air is expelled from
casing.)
!"If the product is hot, use warm up lines and allow ample time for pump case to heat up. The pump
case and rotating assembly could distort from uneven heat transfer.
!"Before starting, rotate pump shaft by hand. It should be free with no rubbing.
!"Open flush lines, heating or cooling lines, and quench vent and drain lines.
!"Crack open discharge valve, but don't fully open. A partially closed discharge valve also will prevent
initial cavitation.
!"Start pump. Watch discharge pressure gauge. As soon as pump pressure stabilizes on gauge,
open discharge valve slowly watching discharge gauge. Discharge pressure may fall off for a few
turns of the valve. Once pressure is stabilized, fully open discharge valve.
IMPORTANT - Never allow pump to run too long with discharge valve closed. Because the energy
applied to the product through recirculation within the pump case is converted into heat, the
temperature of the liquid may reach its boiling point and vaporize. This could cause pump seizure
and mechanical seal failure.
!"Once the pump is running, check mechanical seal for leaks and proper circulation.
If you are satisfied with the results of start up and the pump has run for 30 minutes, it is good practice to
record the vibration level of the pump and driver for future reference.
7.3 Pump in Operation and Shut Down
!"During operation, a centrifugal pump requires occasional inspection.
!"Watch for fluctuations in suction and discharge pressure to verify the pump is not cavitating.
!"Check the pump and motor bearings to see if they are overheating. Always touch the motor with the
back of the hand so any shock will push the hand away.
!"The mechanical seal should be checked for leakage, particularly during the first hours of operation.
Minor leakage through the seal usually stops after a short time. If leakage continues, the pump
should be stopped and the seal fixed.
!"Trouble Checklist
No liquid or insufficient discharge from the pump may be caused by:
- Pump not primed
- Speed too low
- Verify the driver is operating correctly
- Suction lift too high - insufficient NPSH
- Impeller or piping plugged
- Wrong rotation
- Air leak in suction line
- Air pocket in suction line
- Discharge pressure higher than anticipated/rated
- Mechanical defects - impeller rings worn/impeller damaged
44
Seal Basics
Shutting down the pump - To prevent reverse flow, the discharge valve on a centrifugal pump should be
partially closed before the driver is stopped. Usually there is a check valve in the discharge line to prevent
such reverse flow. After the driver is stopped, completely close the discharge then the suction valves and
check them for tightness.
7.4 Trouble Shooting
The Rules
!"Collect the entire seal
!"Examine the seal faces
!"Check for chipping of seal faces (For more information, see the John Crane booklet titled Identifying
Causes of Seal Leakage)
!"Check the drive mechanisms
!"Check the spring or bellows
!"Check the elastomers and secondary seals
!"Check for rubbing
To gain a better understanding of seal failure analysis, each of these rules are discussed in detail below.
Collect the Entire Seal - Do not try to troubleshoot a seal by using only the parts that look important. You
must have both the rotating and stationary parts. If possible, you should also inspect the gaskets, O-rings,
shaft sleeve, gland, and the inside of the seal chamber. Exercise caution in handling seal parts that have
been exposed to toxic, corrosive, or chemical environments. The best way to ensure that all seal parts are
examined after disassembly is to tie the mating ring and the primary ring parts together and tag them with
any information that might be useful. If the two major components are separated after removal, it will be
virtually impossible to determine the actual cause of the seal failure. Store them until they are ready to be
rebuilt or discarded. In the mean time, they are available for troubleshooting and failure analysis.
Examine the Faces - Look for a wear track which is a circular pattern on the mating ring face produced by
the rotation of the primary seal face. A proper wear track is the same diameter and width as the primary ring
face diameter and width. This is an important sign because it tells you that the pump is in good alignment
and face leakage is probably not the cause of any seal problem you might have. A wide wear track indicates
that there is a serious misalignment of the pump assembly. This can be caused by bad bearings, shaft whip,
shaft deflection, a bent shaft or severe vibration from a cavitating pump. A widened wear track is usually
associated with seal hang up, shaft fretting, and seal leakage.If the seal is forced to move both radially and
axially on each revolution, there is a tendency for the seal faces to separate slightly on each revolution.
Options to correct it include: replace damaged pump components, alignment of items mentioned, reducing
pipe strain, operating the pump at the designed capacity, reducing the sliding friction on the shaft. If the seal
can follow the vibrations and motions with little drag, many of the problems caused by face separation can
be eliminated.
When the wear track is narrower than the thinnest face, this means that the seal has been over
pressurized and has bowed away from the pressure. This bowing caused the face to seal on its edge.
Severe wear in the hard face is common when particles embed in the carbon face causing a grinding
action. If the hard face is harder than the product particles, abrasive damage will be reduced or
eliminated.
A deep wear track in a hard face is often present on an outside mounted seal, on seals in misaligned
pumps, and on the inboard seal of a double seal arrangement. This condition is caused by face separation
that allows large particles between the faces. The particles imbed in the soft carbon face and grind the hard
face.
Hard face combinations will often stop this problem, provided they are harder than the abrasive particles.
Dual hard face combinations should only be used in products that are not too sensitive to heat and in seals
which are pressure balanced.
Seal Basics
45
No wear track normally indicates that neither face was rotating or the seal faces were never together
to begin with.
Shiny spots on the face with no complete circular wear track is caused by warping the face.Warping is
caused by too much pressure, improper bolting, clamping onto a bad seal chamber face, or faces that were
severely out of flat at installation. Shiny spots are important symptoms because they indicate that the seal
was probably leaking from start up. Cures for this problem include: checking the flatness of the hard face
before installing, facing off the seal chamber face to ensure a clean, smooth surface, and tightening gland
bolts evenly to prevent misalignment of the primary ring assembly.
Check for Chipping of Seal Faces - When examining the seal faces, look for chipping at the OD and ID.
Chipping at the OD of the rotating seal face is caused by a separation of the faces and consequent breaking
when they bang back into each other. This condition is most often associated with fluids that flash.To cure
this condition, the product has to be cooled by a method discussed earlier and or the pressure in the seal
chamber must be increased.
Severe cavitation of the pump coupled with a hung-up seal may cause the seal face ID to chip. These chips
are made when the seal face comes in contact with the rotating shaft. The cure is to recheck alignment and
start-up procedures to ensure proper seal environment. Flashing also may be the cause.
Ceramics are sometimes subject to thermal shock and can break.This occurs when the ceramic is heated
unevenly and then subjected to rapid temperature change. Pure ceramics are much less likely to break in
this situation.
The more corners and sharp edges (called stress risers) on a ceramic, the more likely it is to break due to
shock. As a general rule, the cure for broken ceramics is to change the material to either tungsten carbide or
silicon carbide. Ceramics are strong in compression, but when they are clamped against an uneven surface,
they sometimes shatter.
Carbon primary rings can be damaged when using recirculation lines from the pump discharge to the seal
chamber and the fluid contains abrasives that can impinge or erode the faces. If a recirculation flush is
necessary and the product is abrasive, the flush should be at a low velocity.
Check the Drive Mechanisms - Mechanical seal designs all use some way to transmit driving torque from
the shaft to the rotary face. This driving mechanism can be pins, drive lugs, a spring, or a drive collar. First
determine where the drive junction is located on the seal. Check for signs of wear at the pin, drive lug, dent,
spring, or carbon groove. Worn drive lugs or drive slots are usually caused by slip-stick. If the two faces
stick together, the drive pin will load with a high stress. This stress is transferred to the face, causing it to
accelerate and then stick again. Slip-stick is caused by high face friction and loss of face lubrication.
This lack of lubrication can be caused by:
a) installing the seal with too much spring compression
b) too much pressure on the faces
c) poor lubricating properties of the fluid
d) bad face combinations
e) pump cavitation
f) a gas bubble trapped in the seal chamber at the face.
Check the Spring or Bellows - Springs and metal bellows usually break from chemical attack at the same
time the device is being stressed. The possibility of chloride and sulfide stress corrosion should always be
checked. Stainless steel springs and bellows are subject to stress cracking in the presence of chlorine,
fluorine, bromine, and iodine. Nickel alloys, such as Hastelloy C, Inconel or Monel, are usually not subject to
breakage from stress corrosion and should be used in these applications.
46
Seal Basics
A metal bellows seal will clog or fail if the pumped product hardens, or particles become stuck at the inside
of the bellows. This occurs when there is excessive product leakage past the faces of the shaft seal. Pump
circulation, pump jackets or steam quenches could be considered for dealing with these conditions.
It is important to determine if the clogged seal is at its proper operating length. This can be done by
measuring the operating length of the frozen seal and verifying that is had been set properly. Also check for
shaft seal damage. This can be caused by installation problems and coking.
Check the Elastomers and Secondary Seals - A swollen, sticky or disintegrating elastomer is normally a
sign of chemical attack. It is solved by using a different material. An O-ring selection guide is helpful in
determining chemical compatibility. If the product is a mixed solvent or a chemical that is not compatible with
any elastomer, then PTFE or Grafoil sealing devices may be required.
Excessive face heat, caused by lack of lubrication, can cause hardening, charring, cracking, a burned
appearance, or shape changes on elastomers. The use of pump circulation, pump jackets, coolers or
heat exchangers effectively deal with heat problems.
Check for Rubbing - Look for worn spots or signs of discoloration on the seal from rubbing. When metals
are heated past 100°F / 38°C, they often form oxide coatings that discolor the metal and are very difficult to
remove. A color chart with relationship to temperature for stainless steel is below:
Straw Yellow
Brown
Blue
700°F – 800°F / 370°C – 425°C
900°F – 1000°F / 480°C – 540°C
1100°F / 600°C
Look for signs of rubbing on the seal chamber throat, on the shaft where it goes through the throat, and
especially on any gland pilot that sticks into the seal chamber.
Some other causes of rubbing are:
a. flush lines too far into the lantern connection
b. unpiloted glands touching the shaft
c. unpiloted gland gaskets slipping into the seal chamber
d. primary rings rubbing the shaft
e. scale build-up in the seal chamber
f. eccentric seal chamber
g. shaft whip caused by impeller imbalance
h. shaft deflection
i. set screws backed out of the seal
j. PTFE gaskets extruding into the seal chamber
k. pieces of mating ring breaking off
l. excessive pipe strain
m. worn bearings
n. excessive seal expansion due to elevated temperatures
Inconel is a product of Inco Alloys International, Inc.
Grafoil is a registered trademark of GRAFTECH, Inc., a member of UCAR International Inc.
Seal Basics
47
7.5 Troubleshooting by Symptoms
Broken Metal Bellows
!"Cause – running too close to vapor point, chemical attack, low/high viscosity product,
over pressurized
!"Corrective Action – identify specific cause and correct
Broken Ceramic
!"Cause – ceramics exposed to different temperatures experience thermal shock and break
!"Corrective Action – reduce different temperatures or change materials
Broken Metal Bellows Primary Ring Front Adapter
!"Cause – liquids with high pH, chlorides or sulfides
!"Corrective Action – do not use these liquids, check temperature limits
Broken Springs
!"Cause – stress corrosion or by over torquing, high speeds
!"Corrective Action -- change materials and check installation procedures
Chipped Faces on the OD
!"Cause – face separation because of pressure distortion
!"Corrective Action – control the pressure
Chipped Faces on the ID
!"Cause – face striking the shaft or sleeves on the ID, temperature distortion, running dry, poor flush
!"Corrective Action – total pump alignment, check operating temperatures
Clogged Springs
!"Cause – by particles collecting in small crevices
!"Corrective Action -- use clean external flush or change seal designs
Coated or Discolored Metal
!"Cause – excessive heat or sealed fluid deposits
!"Corrective Action – eliminate or reduce heat, change metal, or use a clean external flush
Coking
!"Cause – fluid decomposition
!"Corrective Action – reduce temperature, use steam quench (Plan 62), use rotating seal design, use
dual arrangement
Cut Elastomer
!"Cause – damage during installation
!"Corrective Action – chamfering the shaft shoulder or the end of the sleeve; use of a cone device at
the end of the shaft; use assembly lubricant
Elastomer Out of Shape, Hardened or Burned
!"Cause – excessive heat or chemical attack
!"Corrective Action – change the elastomer or reduce heat
Eroded Carbon
!"Cause – flush impingement, chemical attack, incorrect pipe connections
!"Corrective Action – redirect or reduce the flush or remove abrasives
48
Seal Basics
Excessive Face Wear
!"Cause - heavy load or poor lubrication
!"Corrective Action - correct the problems or change to a balanced seal, check flush, check springs
Excessive Outboard Seal Temperature
!"Cause - inadequate buffer/barrier flow
!"Corrective Action - check for piping restrictions; check for correct piping plan; check for pressure
losses form heat exchanger or valving; check for gland drill thru holes, resize if not to specification;
check pump rotation vs tangential gland porting; check for gas entrainment in the barrier fluid
Frozen Seal (Locked up Solid)
!"Cause - clogged small springs, wrong seal selection
!"Corrective Action - use flush injection, select a different seal
Hard Face Wear
!"Cause - abrasives trapped between the faces and embedded in the soft face; poor lubrication, or
lack of lubrication between the faces that greatly accelerates wear
!"Corrective Action - use flush or change face materials
High Primary Ring Wear
!"Cause - seal chamber too close to product vapor pressure; incorrect seal design for conditions;
product contaminants; gas entrainment in barrier fluid
!"Corrective Action - decrease seal chamber temperature; increase seal chamber pressure using
close clearance throat bushing; select appropriate seal design; use hard faces if applicable; use
Flush Plan 31 or 41; use Flush Plan 32 if applicable; if drained barrier fluid shows bubbling then add
an accumulator to pressurize system; change barrier fluid type
Heat Checking
!"Cause - wrong materials and/or excessive heat
!"Corrective Action - change materials, reduce heat generation, increase heat removal
Pitted Carbon
!"Cause -- media attack on the carbon
!"Corrective Action -- change the grade of carbon
Pitting and Corrosion of Metal Parts
!"Cause -- wrong choice of materials, scratched surfaces on passive stainless steel, galvanic
corrosion or stress corrosion
!"Corrective Action - change materials
Seal Leaks
!"Cause - nothing appears to be wrong
!"Corrective Action - check for squareness of seal chamber to shaft; align shaft, impeller, bearing,
etc. to prevent shaft vibration and/or distortion of gland plate and/or mating ring, refer to list under
"steady dripping"
Seal Locked on Shaft
!"Cause - fretting
!"Corrective Action - change to a non-pusher seal
Shaft Damage Under the Seal
!"Cause - fretting due to relative motion between the secondary seal and the shaft
!"Corrective Action - check equipment alignment or change to a non-pusher seal
Seal Basics
49
Steady Dripping
!"Cause - faces not flat; blistered carbon graphite seal faces; thermal distortion of seal faces
!"Corrective Action - check for correct installation dimensions; check for proper materials; check gland
plate distortion due to over torquing of gland bolts; clean out foreign particles between seal faces;
check for cracks and chips at seal faces during installation - replace if necessary
!"Cause - secondary seals nicked or scratched during installation; over-aged o-rings; compression
set to secondary seals (hard and brittle); chemical attack (soft and sticky)
!"Corrective Action - replace secondary seals, check for proper lead in chamfers, burrs, etc.
!"Cause - spring failure, erosion damage of hardware; corrosion of drive mechanisms
!"Corrective Action - replace parts; check for more appropriate materials
Sticky or Swollen Elastomer
!"Cause - chemical incompatibility between the elastomer and the pumped product
!"Corrective Action - change to a compatible elastomer
Worn Drive Lugs
!"Cause - slip-stick from poor lubrication, vibration, imbalanced impeller, wrong face load, flush
pulses, pipe strain, or coupling misalignment
!"Corrective Action - correct the identified problem
Worn or Shiny Spot on the Seal
!"Cause - contact with an obstruction in the seal chamber or by flush impingement
!"Corrective Action - remove the obstruction, check gland plate gaskets and tighten or
redirect the flush
Troubleshooting at the Pump
!"Determine the leak path.
!"A distorted mating ring can be out of flat due to uneven pressure from the gland bolts. It is
sometimes possible to regain flatness and a seal by adjusting the gland bolts.
!"A nicked O-ring will distort. It will seal while pump is running, but will relax and leak when the pump
is shut down.
!"An incorrectly set seal operating length will cause problems. Too low a spring force will result in
leakage at start up and shutdown and will not be able to seal a reverse pressure or vacuum. Too
high a spring force will result in excessive heat generation.
50
Seal Basics
8.0 Fluid Services
Mechanical seal applications can be grouped in many ways.
!"Crystallizing fluids
!"Abrasive fluids - slurries
!"Fluids that set up or harden
!"Fluids that flash - Light specific gravity
!"Dangerous fluids
!"Hot fluids (over 400ºF / 200ºC)
!"Cryogenic fluids
!"Non-lubricating fluids
!"Corrosive fluids
!"High speed (over 500 fpm)
!"High vacuum service
!"Water (above 180ºF / 80ºC)
!"High pressure
Crystallizing Fluids (also Hydrocarbon Coking) - Fluids that crystallize or convert to a solid after passing
across the faces encompass a wide group of products, including salts, caustics and hydrocarbons.
Circulation of the product from pump discharge keeps fresh fluid in the seal chamber at all times. Using a
quench fluid at the atmosphere side of the seal keeps the area under the seal faces clean. Quench fluids
should be selected for compatibility with the pumped product and availability at the pump site.
Selection of seal materials, face materials, and secondary elastomeric seals depends upon chemical
compatibility and temperature capabilities.
Abrasive Fluids - Abrasive fluids exist in basically all industries, from slurries in power plants and mining
complexes to chemical and petrochemical processing plants. The key to successfully sealing abrasive fluids
is to keep the abrasive solids in suspension and select seal face materials that will withstand the abrasive
attack. Fluid characteristics, such as hazardous or corrosiveness, need to be considered when selecting
seal materials and determining the use of circulation and quenching arrangements for seal protection
systems.
Fluids that Solidify or Harden - Thermosensitive and viscous fluids can harden or solidify in the pump seal
chamber if heated or cooled beyond certain limits. Examples of these fluids include resins, asphalt, heavy
crude oils, molasses and grease. Pumps equipped with jacketed seal chambers and/or jacketed glands may
be used to handle these products. Quench and drain gland connections should be used as needed to
provide a steam quench to heat or cool the seal areas and provide cleaning. Hard face seal combinations
are normally recommended with a tungsten carbide primary ring and a silicon carbide mating ring. Selection
of the seal materials and secondary seals is dependent on chemical compatibility and temperature
limitations.
Fluids that Flash/Light Specific Gravity - Flashing results when the operating temperature/pressure of the
fluid causes it to change into a vapor in the seal chamber area. Flashing generally occurs before the fluids
pass all the way across the faces. As the liquid flashes to a vapor, the seal faces may pop open, allowing
excess product between them. Any abrasive particles subsequently trapped between the faces may cause
severe wear to the seal faces, resulting in premature seal failure. The key to these applications is to keep
the product in a liquid state as far across the seal faces as possible. A general rule for fluids that flash is to
maintain the seal chamber pressure at 50 psi / 3.4 bar above the vapor pressure threshold. This can be
accomplished by raising the pressure or reducing the temperature in the seal chamber. However, each
application should be examined based on its own characteristics for proper seal recommendations.
Temperature in the seal chamber area can be regulated by using the API piping plans discussed earlier.
Pressure can be increased by the use of API Plan 11 in conjunction with a close clearance throat bushing in
the bottom of the seal chamber.
Seal Basics
51
The refining industry normally classifies liquids with specific gravity below .6 as "light ends". These product
range progressively from hydrocarbons such as methane, through ethylene, ethane, propylene, propane,
isobutane, butane, butadiene, to pentane and isopentane. All liquids require a certain amount of pressure to
keep from vaporizing at any given temperature. This is known as vapor pressure. The lower the specific
gravity, the higher the pressure required to keep it a liquid.
Dual seals are sometimes required for methane, ethylene and ethane applications, while single seals can be
used on propylene, propane butane and hexane. However, environmental and safety considerations may
dictate dual seals for all these products. For hydrocarbons with specific gravities of .4 to .55, pressures
usually range around 300 psi / 20 bar. To keep these fluids in a liquid form, the seal chamber pressure must
be 50 psi / 3.4 bar above the vapor pressure of the fluid in the seal area. This may require a Plan 11, a
close clearance bushing in the bottom of the seal chamber, and cooling.
Dangerous Fluids - This group generally includes toxic, carcinogenic, and other fluids determined to be
hazardous to human life. To eliminate any leakage or contamination into the atmosphere, these applications
are generally handled with dual seal arrangements with pressurized barrier systems. The barrier fluid must
be compatible with the pumped product fluid. The barrier fluid pressure should be 10% higher that the seal
chamber pressure with 25 psi / 1.7 bar minimum differential pressure to ensure no leakage of the pumped
product into the atmosphere.
Hot Fluids (over 400ºF / 200ºC) - These fluids are generally present in the petrochemical industry,
particularly in oil refineries. They are also present in many other industries. Selection of seal materials and
secondary seals is critical in these applications. High temperatures can lead to accelerated corrosion rates,
coking, flashing, or hardening of the product in the seal chamber. It is therefore frequently necessary to
control the sealed fluid temperature through the use of flushes, gland jackets, pump jackets, and steam
quenches. NOTE: When pump jackets are used, the fluid in the seal chamber should not be circulated
unless API Plan 23 is used. The seal chamber is normally dead ended (API Plan 02).
Many major oil companies classify applications 350ºF to 400ºF / 175ºC to 200ºC as high temperature, with
an upper limit of 800ºF / 425ºC. The selection of mechanical seals specifically designed to function in these
high temperatures is paramount for peak performance. Desirable design features that metal bellows seals
have for high temperature service include:
!"Ability to retain its spring tension through temperature cycles from ambient to 800ºF / 425ºC. This
is accomplished with the use of metals such as heat treated AM350 stainless steel and Inconel 750
and Inconel 718 in special applications.
!"Ability to retain the seal face insert in the adapter shell at high temperatures. Many metal bellows
seals utilize Alloy 42 with its inherent low thermal expansion to maintain face retention at elevated
temperatures. However, this material is not chemically compatible with all high temperature
applications. Our patented shell design, made of a corrosion resistant material, solves the problem
of face retention and maintains face stability through a wide range of pressures and temperatures.
!"Suitable elastomers. With the development of Grafoil and expanded graphite fiber tape, the high
temperature, elastomer problem has been eliminated.
Cryogenics - These applications involve liquids at extremely low temperatures (<-100ºF / -73ºC). These low
temperatures are typically required to maintain the fluid in a liquid form. Most cryogenic fluids have poor
lubricating properties. Dual seals with a barrier fluid between them may be required for cryogenics.
Compatibility with the cryogenic fluid, lubricity, and the availability at the pump site should be considered
when selecting barrier fluids. Material selection is critical in these applications. If using a single seal, a dry
inert (N2) quench is required to prevent atmospheric icing on the inner diameter.
Non-Lubricating Fluids - For pumped products that have little or no lubricating characteristics, a method to
replace that fluid in the seal chamber must be determined. This can be accomplished by introducing a flush
fluid, with lubricating capability into the seal chamber area, and installing a close clearance throat bushing in
52
Seal Basics
the throat of the seal chamber to restrict the flow of the pumped product. In practice, the flush fluid is sealed
rather than the pumped fluid. Dual seal arrangements with pressurized barrier systems have proven to be
effective in these applications. The barrier fluid provides the face lubrication for both sets of faces.
Corrosive Fluids - Corrosive fluids are present in almost all industries. As temperature increases, corrosion
rates are accelerated. Corrosion in excess of .002 inches per year is detrimental to most mechanical seal
designs. The solution to these applications lies in the selection of the correct materials. Select metal
components (except bellows or springs) for the seal is as follows:
!"If the pump is made of iron, steel, or stainless steel, use stainless steal for the seal body.
!"If the pump is made of another material, use the same material for the seal body if it is available.
!"If the pump is non-metallic, the best choice might be an outside-mounted PTFE bellows seal.
High Speed (over 5000 sfpm) - High speed sealing with minimum lubrication lead to the development of
stationary seal head assemblies and rotating mating ring assemblies. These are used today on applications
where peripheral speeds exceed 5000 surface feet/minute (sfpm). The formula for peripheral speed in ft/min
is:
Mean Face Diameter x rpm x 3.14
12
High Vacuum Service - Hydraulically balanced seals can perform well in vacuum service. Sometimes dual
seal arrangements are used. When a single seal is used, the seal chamber pressure must be increased by
injecting a flush, either from discharge or an external source, and installing a close clearance throat bushing.
Water - There are two categories for sealing water: applications below 180ºF / 80ºC and applications above
180ºF / 80ºC. Applications below 180ºF / 80ºC are relatively simple and can be easily sealed using the
proper materials. Applications above are typically cooled to prevent flashing of the liquid between the faces,
leaving behind any undissolved solids as abrasive crystals. The following means are recommended to cool
the seal chamber if necessary:
!"Attach a pumping ring to the shaft behind the rotating seal to circulate water out of the seal chamber
through a cooling device, and back though a circulation connection on the gland (API Plan 23 /
ANSI Plan 7323)
!"For applications at less that 300ºF / 150ºC a circulation line from the pump discharge through an
exchanger to a gland connection (API Plan 21 / ANSI Plan 7321) at the seal faces is adequate.
!"Use a water-cooled seal chamber associated with a pump jacket. Water at low pressures and
temperatures up to 300ºF / 150ºC can be sealed satisfactorily using an unbalanced, elastomeric
bellows seal with carbon vs silicon carbide or tungsten carbide.
High Pressure - The definition of high pressure in the seal industry begins with 300 psi / 21 bar seal
chamber pressure. However, applications in the 3000psi / 206 bar range appear in the nuclear, oil field
injection, boiler circulation pumps, and special mixing equipment industries. All high pressure applications
should be evaluated on an individual basis.
Seal Basics
53
9.0 Non-Contacting Gas Lubricated Seals
9.1 Background
Typically, the sealing of rotating equipment has been accomplished by various forms of contacting seals.
Seal performance is closely related to the development of a liquid lubricating film and cooling supplied to the
contacting faces. Heat developed at the seal faces must always be removed to avoid damage. Contacting
seals have proven to be effective in controlling VOC emissions from pumps and other equipment. However,
the new laws regarding the Clean Air Act of 1990 call out the use of dual mechanical seals on some
services. Up until the 1980’s, there were two sealing solutions preferred to meet or exceed government
regulations: pressurized and non-pressurized dual mechanical seals and sealless pumps.
Pressurized dual mechanical seals require the careful selection of a barrier fluid and a costly support system
to ensure proper barrier pressures and levels. Conventional pressurized dual seals operate in a "contacting"
mode resulting in high energy dissipation and face wear.
Non-pressurized dual mechanical seals are used when the product cannot be released to the atmosphere
and the buffer fluid cannot mix with the process. They require the non-pressurized outboard or secondary
containment seal to handle full process pressure in the event of an inboard seal failure and be in line with a
vapor recovery system or a closed loop system to contain the contaminated buffer fluid.
Sealless pumps are generally limited to fluids of medium viscosity and low solids content. They may require
expensive monitoring equipment to guard against upset conditions.
Both solutions are still used with success, but with the introduction of dual non-contacting gas lubricated
seals for process pumps, a third option is now available. Gas seals provide an alternative solution to
conventional liquid contacting seals for controlling emissions while extending the mean time between
failures. Gas seals provide the safety of a pressurized dual arrangement while reducing the life cost of
running the equipment.
9.2 Non-Contacting, Gas-Lubricated Seals for Pumps
Satisfactory life for any mechanical seal depends on the ability of the design and the materials of
construction to minimize the effects of contact friction. Without the proper design and material consideration,
a seal will fail due to the thermomechanical effects of contact friction. For gas seals, the non-contacting
design eliminates the contact friction allowing the seals to be used where energy levels are too high to run
dry running contacting seals.
There are several different designs to achieve the non-contacting feature of a mechanical seal, such as
spiral grooves, T-slots, U-grooves, stepped grooves, and wavy faces.
Hydrodynamic gas seals ride on a gas film generated by the spiral grooves while the shaft is rotating, as
shown in Figure 51. Spiral grooves are recessed into the harder face materials as it is easier to control the
manufacturing process. The sealing dam is the area from the inner diameter of the spiral groove to the
inside diameter of the face of the opposing face.
Spiral groove seals operate by using the principles of fluid mechanics. As the seal rotates, gas flows into the
spiral groove by a viscous shearing action and is compressed. At the sealing dam, gas is expanded. The
combined film pressure results in a opening force greater than the closing force that separates the faces
approximately 100 µin. At shutdown, hydrostatic forces along with the spring load act to close the faces.
Seal balance and the design of the grooves prevent damage to the faces at start up and shutdown prior to
separation.
The sealing dam plays an important role in the performance of a gas seal. It restricts the barrier gas from
passing across the sealing faces and creates the pressure drop of the barrier gas.
54
Seal Basics
Figure 51 - Spiral Groove Mating Ring
CLOCKWISE SHAFT ROTATION
SPIRAL
GROOVE
INNER
GROOVE
SEALING
DAM
OPPOSING
FACE INNER
DIAMETER
OUTER
DIAMETER
Rotating Mating Ring
The pressure profile of a gas seal is illustrated in Figure 52. At P3 maximum pressure is achieved, but a
linear pressure drop across the sealing dam occurs reducing the gap and pressure differential, thus,
reducing leakage. In reverse pressure situations the sealing dam restricts the product from entering the
barrier chamber.
Figure 52 - Gas Film Pressure Distribution
FC
CLOSING FORCE
S
FC=FO
FO
OPENING FORCE
P
Grooves
P3 EXPANSION
S = SPRING LOAD
P = HYDRAULIC PRESSURE
Sealing
Dam
GAS FILM
PRESSURE
DISTRIBUTION
The sealing faces incorporating the spiral groove pattern can be rotating or stationary. The grooves must
also correspond to the shaft rotation and pressure location. The grooves can be designed to handle bidirectional rotation, but efficiency is lost at slower speeds, such as 1450 rpm.
Eliminating contact friction on mechanical seals has many advantages, such as reduced face operating
temperature, reduced horsepower consumption, and increased seal life. Face temperature rise on a
contacting seal may exceed 150°F / 83°C while gas seal faces generate less than 20°F / 12°C.
Energy consumption of a gas seal is a fraction of a dual pressurized liquid seal due to the elimination of
contact friction. For any liquid seal, there are frictional losses that can be calculated based on friction and
face velocity. There are also horsepower losses due to the viscous drag of the sealing fluid being sheared
between two closed areas. For a pump this can be one to four horsepower (hp). This is called parasitic
Seal Basics
55
horsepower, since it is a direct drain on the main driver. For pumps that have seal support systems where
the reservoir is at atmospheric pressure, there is another horsepower loss. There is the loss involved with
bringing the barrier fluids to the proper sealing pressure.
A simplified barrier support system (Figure 53) is another advantage to using a gas seal over a conventional
liquid seal. Inert gas is connected to the panel and then filtered through a coalescing element to remove
particles and moisture. The gas is then regulated and measured. A low pressure alarm is incorporated to
warn of the loss of barrier pressure. Most plant nitrogen or air supplies are at or below 120 psig. If higher
barrier pressures are required, an amplifier can be added to the panel that is capable of a 4:1 pressure
increase.
Figure 53 - Gas Seal Control Panel
UE
CAUTION
80
60
10
100
4
8
6
D
3
120
40
F
4
2
2
1
20
140
160
0
ATTENTION
5
H
PSI
2
1
.5
.4
.3
E
.2
.1
C
A. Ball Valve
B. Coalescing Filter
C. Pressure Regulator
D. Pressure Gauge
E. Low Range Flow Meter
F. High Range Flow Meter
G. High Flow Alarm Switch
H. Low Pressure Alarm Switch
I. Check Valve
G
B
I
A
INLET: 250 PSI
The non-contacting, gas lubricated seal is a solution to many problems identified by pump users. An
installation to a pump is illustrated in Figure 54. This type of pump seal technology does not require the
circulation of liquid for cooling. Instead, a static head of an inert gas is used to pressurize the space
between the seals. Nitrogen gas is normally used to create the barrier between the process liquid and the
environment. The gas barrier pressure is normally 20 to 30 psi / 1.4 to 2 bar above seal chamber pressure.
This technology presents a solution for those problems that effect cooling and lubrication at the seal faces.
These have been identified as loss of seal flush, dry run, start up without venting, low NPSH, and cavitation.
Cavitation will also result in the vibration of equipment. The vibration limit for this sealing concept is 0.4 in./s
(10 mm/s).
Non-contacting, gas lubricated seals can be used on a wide range of process liquids. If the process fluid
contains more than 6% solids by volume, consult John Crane Engineering with specific details about the
liquid being sealed. Designs are available for liquids with a concentration of solids to 20% by volume.
56
Seal Basics
Figure 54 - Non-Contacting, Gas Lubricated Seal
Inert Gas In
20 - 30 PSI Above
Process Pressure
12
11
13
2
1
Shaft Rotation is
Counter-clockwise
Direction of View
3 4
5
9
6
1. Mating Ring - Stationary
2. O-Ring
3. Primary Ring
4. O-Ring
5. Retainer
6. Spring
7. Disc
7
8 13 10
8. Snap Ring
9. Retaining Sleeve
10. Mating Ring - Stationary
11. Liner Assembly
12. Gland Plate Assembly
13. Collar
Shaft Rotation is
Clockwise
Direction of View
The operating envelope for a non-contacting, gas lubricated seal for liquid pumping services is shown in
Graph 2. Since rubbing contact at the seal face has been eliminated, the seal can be operated at the vapor
pressure of the liquid being sealed. In addition, there is no limiting factor due to the pressure-velocity
relationship and wear at the seal faces. The limiting factor in the application of the seal is pressure that has
an effect on seal face deflection.
Graph 2 - Gas Seal Operating Envelope
Pressure Limit of Seal
OPERATING
ENVELOPE
Boiling Point
Curve
Pressure
Temperature
Seal Basics
57
Dual pressurized non-contacting, gas lubricated seals have been designed to fit oversized and small bore
seal chambers. Oversized bore seal chambers allow for a larger cross section which can handle greater
pressures. This original gas seal for pumps is shown in Figure 54. Many existing pumps in the field have
small bore seal chambers and do not require the same pressure capability as the Type 2800 seal.
Figure 55 - 2800E
Inert Gas In
20 - 30 PSI Above
Process
Pressure
Inboard View
Mating Ring
Face
Outboard View
Mating Ring
Face
10
7
6
2
1
1. Mating Ring - Rotating
2. O-Ring
3. Primary Ring
4. O-Ring
5. Spring
3
4
5
8
9
6. Sleeve Assembly
7. Inner Gland Plate
8. Outer Gland Plate
9. Collar
10. Spacer
To meet this requirement, the Type 2800E shown in Figure 55 was designed. To meet the need for still
higher pressures on pumping applications with the ability to run dry, the Type 2800HP was designed. This
seal fits the same space envelope as the Type 2800 seal and is being used in petroleum refining and other
industries. The operating limits for these seals are shown in Table 4.
Table 4 - Operating Limits for 2800 Series Seals
Seal Type
2800
2800E
2800HP
58
Pressure
Vacuum to
300 psig / 21 bar g
Vacuum to
230 psig / 16 bar g
Vacuum to
500 psig / 34.5 bar g
Temperature
-40ºF to 500ºF
-40ºC to 260ºC
-40ºF to 500ºF
-40ºC to 260ºC
-40ºF to 500ºF
-40ºC to 260ºC
Speed
to 5000 fpm
25 m/s
to 5000 fpm
25 m/s
to 5000 fpm
25 m/s
Seal Basics
9.3 Non-Contacting, Gas Lubricated Seals for Compressors
Since the late 1970's, an ever increasing number of centrifugal compressors are being sealed with modern
non-contacting dry gas seals. This sealing technology, based on the design concept of spiral grooves, has
become the industry standard for pipelines, offshore applications, refineries, petrochemical and gas
processing plants.
Commercial interest in dry running, gas lubricated seals began with the desire to eliminate a costly seal oil
lubrication system which was not reliable and a high maintenance cost item in the operation of the
compressor. Prior to the development of spiral groove seal technology, most compressor sealing systems
relied on various forms of contacting seals and bushings to prevent the process gas from reaching
atmosphere. The ability to run for extended periods of time was not possible with early seal designs. Oil
consumption, normally three 55-gallon barrels per day, could increase to as high as ten barrels per day
before the unit would be shut down to replace the seals. With the development of dry running, gas lubricated
seals, the use of seal lubricating oil is completely eliminated. Some refineries plan for a plant shutdown to
replace seals every five years, even though the seals have successfully run for over ten years.
Seal arrangement is an important part of installation for a compressor. Operating conditions such as the type
of gas to be sealed, pressure, temperature and speed, as well as abrasive contaminants are considered in
the selection of the seal arrangement. When the process fluid is inert or non-toxic, a single seal will be
selected. Process gas at the outside diameter of the seal face will flow to the atmospheric side of the seal.
Single seals are limited to service pressures of 400 psig / 27 bar g.
When the gas being sealed is hazardous or toxic, a tandem seal (Figure 56) is used. In this case, the space
between the seals is vented to a vapor disposal system or flare system. In a tandem arrangement, the
inboard seal handles the full differential pressure across the inboard seal. The outboard seal prevents any
leakage from reaching atmosphere and also acts as a safety seal in the event of any inboard seal failure.
This type of seal arrangement is limited to 1200 psig / 82 bar g, 400ºF / 204ºC, and 590 fps / 180 mps.
Figure 56 - Tandem Gas Compressor Seal
8
7
1. Mating Ring
2. O-Ring
3. Primary Ring
4. Retainer Assembly
Seal Basics
2
1
3
6
4
5
5. Spring
6. Disc
7. Sleeve Assembly
8. Labyrinth Assembly
59
Leakage is dependent on speed, pressure and temperature. The effect of seal size and speed is shown in
Graph 2. As seal size increases, leakage will also increase. The effect of pressure and temperature is
shown in Graph 3. Again, there is a slight increase in leakage with increasing pressure and temperature.
Gas seal power consumption is very small. The power consumption is shown in Graph 4. The power loss is
due to shearing gas at the seal faces. This can be a significant value for large seals at high pressure
operating at significantly high shaft speeds.
Graph 2 - Leakage as a Function of Size and Speed
Graph 3 - Leakage as a Function of Size, Temperature, and Pressure
Graph 4 - Power Consumption as a Function of Size, Speed, and Pressure
60
Seal Basics
In certain applications, there is a recognized need for the seal to work in the reverse rotation for an
extended period. There are two specific circumstances that can cause a compressor to run in the reverse
direction after shut down. These are:
!"A leaky or "stuck open" discharge valve or
!"A large volume of gas between the compressor and its discharge valve.
This has resulted in an optimized bi-directional groove profile. (Figure 57) Even though this design has
optimized bi-directional seal performance, a single uni-directional spiral groove design still provides a
superior level of performance across the operating envelope.
Figure 57 - Bi-Directional Groove Profile
This technology has resulted in significant increases in MTBM (Mean Time Between Maintenance).
Compressors are now operating as intended from maintenance period to maintenance period. Triple tandem
seals have become the standard for refinery services for hydrogen recycle compressors. One refinery
reports an annual savings of $1.7 million for just the hydrogen recycle compressor.
9.4 Non-Contacting, Gas Lubricated Seals for Mixers and Agitators
Prior to the development of the Type 2800SS, non-contacting, gas lubricated seals were limited to a
minimum face speed of 6.5 feet / 2 meters per second. Below this limit contact would occur, resulting in seal
face wear. This represented a design challenge to engineers. Today, non-contacting seal technology has
been extended into the area of slow speed services. These new products are specifically designed for
vertical shaft, top entering mixers, agitators, and reactors.
Seal Basics
61
10.0 GLOSSARY OF TERMS
A
ANSI - American National Standards Institute.
Anti-Extrusion Ring - A ring which is installed on the low pressure side of an O-ring to prevent extrusion of
the secondary seal.
Anti-Rotational Device - A device such as a key or pin, used to prevent rotation of one component relative to
an adjacent component in a seal assembly.
API - American Petroleum Institute. Develops standards and specifications for centrifugal pumps and
mechanical seals for general refinery service.
Auxiliary Equipment - Additional equipment used with a seal, such as environmental controls, reservoirs,
lubricators, etc.
Axial Movement - Movement along the axis or parallel to the center line of a shaft.
B
Balance Ratio - The ratio of hydraulic closing area to the hydraulic face area, typically expressed in a
percentage. Unbalanced seal > 100%, Balanced seal < 100%.
Balance Holes - Openings near the hub of an impeller on a centrifugal pump that reduce the seal chamber
pressure by equalizing the pressure behind the impeller with the suction pressure of the pump.
Balanced Seal - A mechanical seal arrangement whereby the effect of the hydraulic pressure in the seal
chamber on the seal face closing forces has been modified through seal design. Usually needed for higher
pressure applications and identified by a step in the primary ring and shaft/sleeve.
Barrier Fluid - A fluid which is introduced between dual mechanical seals to completely isolate the pump
process liquid from the environment. Pressure of the barrier fluid is always higher than the process pressure
being sealed.
Bearing - Part of a pump that keeps the shaft aligned. There are usually two types of bearings, radial and
thrust.
Bellows Convolution - In a welded bellows, an assembly of two single-ply or multiple-ply formed plates
welded at the ID.
Bellows Seal/Elastomeric - A pusher type of mechanical seal which uses an elastomeric bellows for
secondary sealing.
Bellows Seal/Metal - A type of mechanical seal which uses a flexible metal bellows to provide secondary
sealing and spring loading.
Bellows Seal/PTFE - A type of mechanical seal which uses PTFE bellows with multiple convolutions in
conjunction with a coil spring for secondary seal and spring loading.
Bellows Pitch - The distance between convolutions.
62
Seal Basics
Buffer Fluid - A fluid used as a lubricant or buffer between dual mechanical seals. The fluid is always at a
pressure lower than the pump process pressure being sealed.
Bushing - A square or rectangular cross section device used to restrict fluid flow.
Bypass flush - An environmental control where a line is taken from the pump discharge and is piped into the
seal chamber area to provide fluid flow to the seal chamber product (API Plan 11/ANSI Plan 7311). A
bypass flush can also be attached from the seal chamber area to the pump suction (API Plan 13/ANSI Plan
7313).
C
Cartridge Seal - A completely self-contained unit (including seal, mating ring, gland plate, sleeve, , and the
like), which is pre-assembled and preset before installation.
Cavitation - A condition in which vapor or gas bubbles form locally in liquids as a result of an abrupt
decrease in pressure. The subsequent collapse of these bubbles causes high local impact pressure which
can contribute to equipment wear and reduced seal life.
Centrifugal Pump - A classification of pump in which the fluid pressure is increased by means of an impeller
and a volute (or diffuser).
Closing Force - The sum of the seal chamber fluid pressure and the mechanical seal spring force acting
together to maintain seal face contact in opposition to the opening force.
Coking - The severe oxidation of a material into a hard, black carbon deposit. Coking is usually encountered
on the atmospheric side of high temperature seals and can hang up seal components.
Cyclone Separator - A cone shaped device that separates heavier solid particles form the lighter density
fluid by centrifugal force.
D
Dead-ended - A term used to describe a seal chamber that has no circulation (API Plan 02/ANSI Plan
7302).
Differential Pressure - The difference in pressure between two points in a system, e.g. the difference
between the discharge pressure and suction pressure in the pump.
DIN - Deutsches Institut fur Normung e.V. German industry standard.
Drive Collar - The part of the cartridge seal that mechanically connects the sleeve to the shaft to transmit
rotation and prevent axial movement of the sleeve relative to the shaft.
Drive Mechanism - A device such as a pin, key or structural part of the seal designed to transmit torque
from the shaft to the rotating seat.
Dry Running - A term describing a mechanical seal that is in operation without liquid lubrication between its
faces.
Dual Mechanical Seal - A seal arrangement using more than one seal in the chamber in any orientation
which can utilize either a pressurized barrier fluid or a non-pressurized buffer fluid (previously referred to as
a double or tandem seal).
Seal Basics
63
E
Elastomer - The term elastomer refers to rubber-like materials which have resilience. When they are
stretched they return to their original shape.
Eye - The inlet center of a centrifugal pump's rotating impeller.
Externally Mounted Seal - A seal arrangement that is mounted outside the seal chamber of the pump or
vessel being sealed.
Externally Pressurized Seal - A seal that has pressure acting on the seal parts from an external independent
source of supply.
F
Face - Either of the two lapped surfaces (primary ring and mating ring) that rotate against one another to
form the primary seal. Face area is the nominal contact area between the primary ring and the mating ring.
Face Load - The sum of the pneumatic or hydraulic force and the spring force divided by the contacting area
of the sealing faces.
Film Thickness - The distance separating the two surfaces which form the primary seal of a mechanical
seal.
Flashing - A rapid change in fluid state, from liquid to gaseous. This frequently occurs when frictional energy
is added to a liquid as it passes between the primary seal faces or when the fluid pressure is reduced below
the fluid's vapor pressure because of a pressure drop across the sealing faces.
Flatness - In sealing technology, the degree to which the surface of a seal face deviates from a perfect
plane. Expressed in helium light bands where one helium light band is equal to 11.6 millionths of an inch.
Flush - A small amount of fluid which is introduced into the seal chamber on the process fluid side in close
proximity to the sealing faces and usually used for cooling and lubricating the seal faces.
Flush Connection - A tap in the seal chamber or gland plate to allow circulation of the product being sealed,
the injection of a clean external flush, or the recirculation of sealing fluid.
Fretting - A combination of corrosion and wear. In a mechanical seal, a common example of fretting occurs
when the rubbing motion of a secondary seal continually wipes the oxide coating from a shaft or sleeve.
G
Gasket - A material used between two static surfaces to prevent leakage.
Gas Lubricated - A type of mechanical seal that uses a gas to separate the seal faces in the primary sealing
area.
Gland Plate - An end plate which connects the stationary assembly of a mechanical seal to the seal
chamber.
H
Harmonic resonance - A rhythmic harmonic motion having a specific vibration frequency.
64
Seal Basics
Head - The height of a column or body of fluid above a given point, expressed in linear units.
Heat Checking - A condition of minute radial cracks on and beneath the surface of a seal face caused by
highly localized thermal stresses.
Hook Sleeve - A cylindrical shaft sleeve with a step or hoop at the product end, placed over the shaft to
protect it from wear and corrosion. This step is usually abutted against the impeller to hold in place a gasket
between the shaft and the step (hook).
Hydropads - Grooves, scallops, slots, etc. cut into the seal faces to enhance face lubrication and reduce
head build up due to friction, by creating a hydrodynamic opening force between the seal faces.
I
Impeller - A rotating element of a centrifugal pump driven by a motor or turbine. The impeller has vanes or
grooves to impart rotary velocity to the product.
Insert - The seal face that is located in the shell of a metal bellows seal assembly.
Inboard Seal - The seal next to the product being sealed in a dual pressurized or non-pressured seal
arrangement.
Inside Mounted Seal - A mechanical seal located inside the seal chamber. No parts of the seal's flexible
element or stationary faces are external to the gland.
J
Jacket - A closed passageway around the pump chamber, the seal chamber or both to aid in fluid
temperature control.
L
Lantern Ring - A device used to separate rings of compression packing within a pump seal chamber.
It permits the injection, under pressure, of a fluid to provide cooling and lubrication.
Lapping - A finishing operation used to produce an extremely smooth and flat surface on mechanical seal
faces.
Leakage Concentration (VOC {Volatile Organic Compound} or other regulated emissions) - A measure of the
concentration of VOC in the environment immediately surrounding the seal, not a VOC leakage rate.
Usually measured as PPM (Parts Per Million) with a calibrated analyzer.
Leakage Rate - The volume of a fluid (compressible or incompressible) passing through a seal in a given
length of time. For compressible fluids, it is normally expressed in standard cubic feet per minute (SCFM),
and for incompressible fluids, in terms of cubic centimeters or drops per minute.
Load - A term that can denote force or pressure. The load which is applied to a seal to ensure contact
between the mating surfaces, without regard to fluid pressure, usually accomplished by a spring or bellows.
M
Mating Ring - A disc or ring-shaped member mounted on the shaft or in a gland. It provides the primary seal
when in proximity to the face of an axially adjustable face seal assembly.
Seal Basics
65
Maximum Allowable Working Pressure (MAWP) - The greatest discharge pressure at the specified pumping
temperature for which the pump casing is designed.
Maximum Dynamic Sealing Pressure (MDSP) - The highest pressure expected at the seal (or seals) during
any specified operating conditions and during start up and shutdown. In determining this pressure,
consideration should be given to the maximum suction pressure, the flush pressure, and the effect of
clearance changes with the pump.
Maximum Static Sealing Pressure (MSSP) - The highest pressure, excluding pressures encountered
during hydrostatic testing, to which the seal (or seals) can be subjected while the pump is shut down.
Metal Fatigue - A condition usually caused by bending and flexing of a metal part, characterized by a
decrease of physical properties such as tensile strength. Metal failure due to repeated or fluctuating stresses
which are frequently below the yield strength of the material.
Multi-stage Pump - A type of centrifugal pump having two or more impellers acting in series in one casing.
N
Non-Flashing - A fluid state that does not change to a vapor phase at any normal operating condition or
temperature.
Non-Pusher Type Seal - A mechanical seal in which the secondary seal forms a static seal between the tail
section of the bellows and the shaft. The bellows convolution compensates for any wear or misalignment.
NPSH (Net Positive Suction Head) - Energy in the liquid at the suction connection to the pump, over and
above the energy in the liquid due to its vapor pressure.
O
Opening Force - The portion of the seal chamber fluid pressure found between the seal faces.
O-Ring - An elastomeric sealing ring with an O-shaped (circular) cross section. It may be used as a
secondary seal or as a gasket.
O-Ring Groove - The space into which an O-ring is inserted and retained.
Outboard Seal - The outermost seal in a dual seal arrangement designed to retain a barrier or buffer fluid.
In a dual non-pressurized (tandem) seal, the outboard seal is generally designed to perform back up sealing
in the event of inboard seal failure.
Outside Mounted Seal - A mechanical seal mounted outside the seal chamber that holds the fluid to be
sealed.
P
Packing - Any variety of materials such as carbon, cotton, hemp, or synthetic materials fit into a seal
chamber. The packing material is used to make sealing contact with the shaft by adjustment of a gland
which compresses the material against the shaft sleeve and bore of the seal chamber.
Positive Drive - The use of pins, lugs, bellows and other devices to transmit motion from the shaft to the
rotary unit of a mechanical seal.
Pressure Casing - The composite of all the stationary pressure containing parts of the seal, including seal
66
Seal Basics
chamber, seal gland, and barrier/buffer fluid chamber and other attached parts, but excluding the stationary
and rotating members of the mechanical seal.
Pressure (absolute ) PSIA - The sum of gauge pressure and atmospheric pressure.
Pressure (atmospheric) - The force exerted on a unit by the weight of the atmosphere. The pressure at seal
level is approximately 14.7 psi.
Pressure (gauge) PSIG - Corrected pressure, the difference between a given pressure and that of the
atmosphere.
Pressure, Discharge - The actual pressure at the pump discharge connection as measured by a gauge. It is
equal to suction pressure plus total head developed by the pump.
Pressure, Seal Chamber - The pressure acting on the product side of a seal.
Pressure, Suction - The actual pressure, positive or negative at the pump suction connection as measured
by a gauge.
Primary Ring - A flexibily mounted,ring shaped component (usually carbon), that forms the primary sealing
face with the mating ring.
Primary Seal - The seal faces, primary ring and mating ring. The closure at the mating faces.
Product - The liquid, gas or slurry being moved, pumped, agitated, mixed, or processed by a pump, mixer or
some similar type of equipment.
Pumping Ring - A device attached to a rotating shaft within a seal chamber that circulates fluid through a
closed loop for cooling purposes.
Pump Shaft Rotation - Either clockwise or counter-clockwise as viewed from the driver end.
Pusher Seal - A mechanical seal in which a secondary seal is mechanically pushed along the shaft or
sleeve to compensate for face wear and equipment misalignment.
PV Factor - The product of face pressure and relative sliding velocity. The units customarily used are
pounds per square inch-feet per minute. PV is normally considered to provide some measure of severity of
service, and thus relates to a seal's life.
Q
Quench - A neutral fluid, often water, nitrogen, or steam, introduced on the atmospheric side of the seal to
retard formation of solids that may interfere with seal movement.
S
Seal Chamber - A component, either integral with or separate from the pump case (housing), that forms the
region between the shaft and casing into which the shaft seal is installed.
Seal Face - Either of the two lapped surfaces in a mechanical seal assembly forming the primary seal.
Seal Face Width - The radial dimension of the sealing face measured from the inside edge to the outside
edge.
Seal Basics
67
Secondary Seal - A device, such as an O-ring or bellows, that allows axial movement of the seal face
without undue leakage. The term is also sometimes applied to the other O-rings or gaskets which prevent
leakage around seal components.
Service Condition - The maximum/minimum temperature and pressure under static or dynamic condition.
Shaft Sleeve - A cylindrical sleeve placed over the shaft to protect it from wear and corrosion. It may also be
used to provide spacing for the impeller, or as a device to provide a step in the shaft to achieve balance, or
as a device upon which a cartridge seal can be mounted.
Spring - A machined element which is capable of storing energy and releasing it as required. In face seals,
its principal use is to keep the faces of the primary sealing elements together.
Starting Torque - The torque required to initiate rotary motion.
Static Seal - A seal between two surfaces which have no relative motion.
Stationary Seal - A mechanical seal in which the flexible members do not rotate with the shaft.
Spiral Groove Seal - A seal that uses a gas to lift the sealing faces apart. Also referred to as a noncontacting seal.
Split Seal - A mechanical seal which has its primary sealing elements and other components split. It can be
mounted on a pump or other equipment with out equipment disassembly.
Stick Slip - A function phenomenon which can be described as a jerky motion which sometimes results
when one surface is dragged against another. Normally associated with a non-lubricated or boundary
lubricated condition.
Surface Finish - The smoothness of a shaft, sleeve, housing, gland, etc., expressed in micro-inches / µin or
micro-meters / µm
T
Throat Bushing - A device that forms a restrictive close clearance around the sleeve (or shaft) between the
seal and the impeller.
Throttle Bushing - A device that forms a restrictive close clearance around the sleeve (or shaft) at the
outboard end of a mechanical seal gland.
Torque - As applied to sealing, the force required to rotate a shaft against the resistance caused by the
frictional drag at the seal faces. It is normally expressed in foot/pound or in/pound units.
Total Indicated Runout (TIR) - Also known as total indicator reading, is the runout of a diameter or face
determined by measurement with a dial indicator. The indicator reading implies an out-of-squareness or an
eccentricity equal to half the reading. TIR is measured by securing a dial indicator to either the stationary or
rotating component, setting the dial indicator to zero, and then rotating either component.
U
Unbalanced Seal - A mechanical seal in which the balance ratio is equal or greater than 1.
68
Seal Basics
V
Vacuum - Refers to pressures below atmospheric.
Vaporization - The process by which a liquid becomes a gas.
Vapor Pressure - A pressure at a given temperature below which a liquid will convert to a gas. It is
measured in pounds per square inch absolute (psia) and is a function of the temperature of the liquid.
Vent - The act of eliminating gas or vapor from the seal chamber. This is normally accomplished through a
gland connection, such as the flush connection.
Vibration Dampener - A device added to prevent a seal from going into harmonic vibration.
Viscosity - Internal resistance to flow exhibited by a fluid. Viscosity of fluids is usually affected by
temperature. A temperature increase will reduce viscosity and a temperature decrease will increase
viscosity.
Volatile Hazardous Air Pollutants (VHAP) - Any compound as defined by Title 1, Part A, Section 112 of the
National Emission Standards for Hazardous Air Pollutants (Clean Air Act Amended).
Volatile Organic Compound (VOC) - Term used by various environmental agencies to designate regulated
compounds. Emissions are measured as PPM (Parts Per Million) with a calibrated analyzer.
Volute - That portion of a centrifugal pump casing which houses the impeller.
W
Wave Spring - A disc washer-type of spring that has been deformed to have a multiple wave pattern in a
plane perpendicular to its axis.
Wear Track - The mark or path worn on the mating ring seal face during use.
Wedge-Type Seal - A type of secondary seal with a wedge-shape cross section used in mechanical seals.
Welded Metal Bellows - A seal assembly bellows fabricated by welding together a series of thin metal plates
to form an accordion-type structure. When assembled to other components in the seal assembly, it acts as
both the secondary seal and the spring loading mechanism.
Seal Basics
69
Notes:
Europe
Slough, UK
Latin America
São Paulo, Brazil
Middle East, Africa, Asia
Dubai, United Arab Emirates
North America
Morton Grove, Illinois USA
Tel: 44-1753-224000
Fax: 44-1753-224224
Tel: 55-11-3371-2500
Fax: 55-11-3371-2599
Tel: 971-4-3438940
Fax: 971-4-3438970
1-800-SEALING
Tel: 1-847-967-2400
Fax: 1-847-967-3915
For your nearest John Crane facility, please contact one of the locations above.
If the products featured will be used in a potentially dangerous and/or hazardous process, your John Crane representative should be consulted prior to their selection and use.
In the interest of continuous development, John Crane Companies reserve the right to alter designs and specifications without prior notice. It is dangerous to smoke while handling
products made from PTFE. Old and new PTFE products must not be incinerated.
©2001 John Crane Print 10/02/02
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