38681211 dynamic seals
TRANSCRIPT
Abu Dhabi Gas Liquefaction Company Ltd
Job Training
Mechanical Technician Course
Module 10
Dynamic Seals
ADGAS Personnel & Training Division
Personnel & Training Division Job Training—Mechanical Technician
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Contents
Page No. Abbreviations and Terminology................................................. 5 1 Introduction ………………………………………………………….. 6 2 Labyrinth Seals............................................................................ 8 3 Liquid Film Seals......................................................................... 13 4 Carbon Ring Seals....................................................................... 14
5 Lip Seals....................................................................................... 16 5.1 Types of Lip Seal.............................................................. 17 5.2 Seal Identification............................................................. 20 5.3 Removing and Fitting Lip Seals...................................... 22 6 Mechanical Seals......................................................................... 28 6.1 Main Parts of a Mechanical Seal...................................... 29 6.2 Types of Mechanical Seal................................................ 31
6.2.1 Rotating and Stationary Seals.............................. 31 6.2.2 Balanced and Unbalanced Seals.......................... 32 6.2.3 Pusher and Non-pusher (Bellows) Seals............. 33 6.2.4 Internal and External Seals................................... 34 6.2.5 Conventional and Cartridge Seals........................ 36
6.3 Dual Seals.......................................................................... 37 6.4 Seal Fluids......................................................................... 39
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Contents
Page No. 7 Summary...................................................................................... 45 8 Glossary....................................................................................... 46 Appendix A................................................................................... 47 Appendix B................................................................................... 48 Exercises 1-5................................................................................ 49
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Pre-Requisite Completion of A.T.I. Maintenance Programme, ADGAS Induction Course and Basic Maintenance Technician Course.
Course Objectives
The Job Training Mechanical Technician Course is the second phase of the development programme. It is intended specifically for Mechanical Maintenance Developees.
On completion of the Course the developee will have acquired an awareness of some of the equipment, terminology, and procedures related to mechanical maintenance of ADGAS LNG plant. Appropriate safety procedures will continue to be stressed at all times.
Module Objectives
On completion of this module, the developee will be able to correctly :
• identify types of dynamic seals and describe their applications
• identify parts of a lip seal and describe their functions
• identify parts of a mechanical seal and describe their functions
• describe the function of seal fluids
• remove and replace carbon ring seals
• remove and replace a lip seal
• remove, dismantle, re-assemble and replace a dynamic seal
Methodology The above will be achieved through the following:
• pre-test
• classroom instruction
• audio visual support
• tasks & exercises
• post-test
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Abbreviations and Terminology
API American Petroleum Institute
PTFE Polytetrafluoroethylene—a low-friction polymer also known by its trade name: Teflon
Barrier Something that blocks a path.
Bed in A small amount of initial wear between two surfaces that allows them to match.
Buffer Something that exists between two extremes and reduces the effect of one on the other.
Ceramic A very hard, heat-resistant material made of clay that has been permanently hardened by heating.
Contaminants Materials that make a substance impure; unwanted additions to a substance.
Elastomer A natural or synthetic rubber.
Emery cloth A flexible material with an abrasive coating for finishing.
Flush To clean by passing a large quantity of water, etc., through or over.
Garter spring A helical spring with its ends joined to form a circle. Goes around something and applies a radially inward force.
Honing A very fine finishing process using an oilstone or whetstone to remove small amounts of material from a surface.
Inert Something that does not chemically react.
Quench To rapidly cool something.
Tandem Describing two things that work together, usually in series.
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1 Introduction
Seals prevent, or reduce to a minimum acceptable level, leaks of gas or liquid from
between component surfaces. They also prevent dirt from entering through those
surfaces.
There are two main types of seals:
• static seals
• dynamic seals
Static seals stop leaks between components that do not move relative to each other. A
typical use is to seal flange joints. The most common static seals are gaskets and o-
rings. These are described in the Gaskets module of this course.
Dynamic seals control leaks of gas or liquid where there is movement between
components. They are used on the rotating and reciprocating parts of valves, pumps,
compressors, gearboxes and prime movers. They are often used to keep dirt from
entering bearings and to keep lubricant from leaking out.
Dynamic seals either make contact with the moving part or leave a very small gap. In
both cases there will be some leakage. If there is clearance, fluid can leak through the
gap.
Non-contact seals include:
• labyrinth seals
• liquid (oil) film seals
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If there is contact, there must be lubrication to stop excessive wear of the seal. Any
fluid used to lubricate the seal will leak out from between the sealed surfaces.
Contact seals include:
• packing glands
• carbon ring seals
• lip seals
• mechanical seals
Modern seals are designed to reduce leakage to a very small amount.
The failure of a seal can result in anything from a small water or oil leak to the escape
of flammable or toxic fluids. The planned replacement of seals to
prevent failure is part of the routine maintenance of rotating
equipment.
The most common type of dynamic seal uses packing in a stuffing box. This type has
been described in the module on Gland Packing. Packing of this kind is used mainly
on valve stems and smaller pumps.
In many applications, gland packing has been replaced by other types of dynamic
seals that are more reliable and easier to replace.
You have met dynamic seals in this course in the modules on Pumps and
Compressors. They are described here in more detail.
A flammable material catches fire easily. It is a fire hazard.
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2 Labyrinth Seals
A labyrinth is a long and complicated path or network of paths; the kind of place that
you can easily get lost in. The word is used to describe seals that provide a long
leakage path that makes any leaking fluid squeeze through a series of very small gaps.
Labyrinth seals do not reduce leaks by rubbing on the shaft. They do not make
contact with the shaft but leave very small clearances between shaft and seal or
between stationary and rotating parts of the seal.
They have grooves machined on the surface, leaving many sharp, knife-edged rings,
as shown in Figure 2.1.
Figure 2.1: Simple Labyrinth Seals
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Rotating sleeve
Stationary seal
Stationary seal-half
Rotating seal-half
Stepped section
Seal fluid connections
(a) Seal Running on Plain Sleeve (b) Interlocking Seal
As fluid passes through the labyrinth it does not follow a straight path. It is constantly
changing direction to squeeze through the gaps. This creates a lot of fluid friction that
results in pressure loss in the fluid. By the time the fluid reaches the end of the seal
its pressure has dropped so much that it is no higher than the outside pressure and it
can not flow out.
The advantage of a non-contacting seal is that there is no contact wear between
surfaces as long as clearance is maintained. Wear only results when worn bearings
allow the shaft to move so that clearances are lost.
Labyrinth seals can operate directly on the shaft, as shown in Figure 2.1 but it is more
usual for them to operate on a shaft sleeve or rotating seal-half that is fixed to the
shaft as shown in Figure 2.2.
The rotating seal-half may also have grooves and knife-edges that fit between those
on the stationary half as shown in Figure 2.2(b). This gives an even longer leakage
path with a greater pressure drop. This drawing shows two other features you may
find on labyrinth seals. The stepped section helps to stop leakage from right to left in
the figure. Much of the leaking fluid rotates with the shaft and centrifugal action
stops it from flowing inwards towards the shaft centre. Seal fluid connections allow
fluids to be injected and removed from the seal at points along its
length. This is described later in this section.
Figure 2.2: Two-piece Labyrinth Seals
To inject something is to feed it under pressure into a space or into another substance.
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The outer half of an interlocking seal is split to allow assembly. Figure 2.3(a) and (b)
shows full inner and split outer sections of interlocking labyrinth seals.
Sometimes the grooved seal rotates with the shaft and seals against a plane section of
casing. This is the case for the small turbine seal shown in Figure 2.4.
Figure 2.3: Interlocking Labyrinth Seal Halves
Figure 2.4: Rotating Labyrinth in Plain Casing
(a) Full Inner Half (b) Split Outer Halves
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Labyrinth seals are used where pressure differences are not very great. They are often
used between stages of centrifugal compressors and turbines. Figure 2.5 shows
typical labyrinth seal locations in a centrifugal compressor.
Figure 2.5: Labyrinth Seal Locations in a Centrifugal Compressor
Shaft labyrinth seal
Impeller eye labyrinth seal
Balance drum labyrinth seal
Shaft Shaft sleeve
Impeller
Balance drum
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If the pressure of the contained fluid is below atmospheric, injecting a fluid into the
seal at a higher pressure stops air entering the system. The principle is the same as
that for lantern-ring gland packing systems described in the Gland Packing module in
this course. Fluids flow from high to low pressure. No air can enter a seal that
contains fluid at a higher pressure.
If the contained fluid is hazardous and no leakage is acceptable a harmless fluid is
injected at a higher pressure. This fluid forms a barrier past which
the contained fluid can not escape. Small quantities of this seal fluid
may escape without danger of pollution or hazard to health and safety. Figure 2.6
shows an example of a labyrinth seal on the discharge end of a centrifugal compressor
shaft. A second labyrinth seal contains oil in the bearing housing.
Labyrinth seals are often used in series with other types of seal to give improved and
back-up sealing.
A barrier stops forward movement.
Figure 2.6: Labyrinths Seal with Discharge Recovery and Seal Gas Connections
Escaping discharge gas returned to suction (recovery)
High pressure seal gas entering
Seal gas leaving
Oil seal
Bearing
Impeller
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3 Liquid Film Seals
Liquid film seals are another type of non-contact seal. The seal housing contains a
floating ring that is free to rotate in the housing and has clearance on the shaft.
Sealing liquid, usually oil, enters the seal, filling the spaces between the floating ring,
shaft and housing. This liquid is at a higher pressure than the contained fluid. As
fluids can only flow from high to low pressure, no contained fluid can flow into the
seal. Figure 3.1 shows an example of a liquid film seal.
The example shown in the figure uses labyrinth seals to reduce leakage of the sealing
liquid.
Figure 3.1: Liquid (Oil) Film Seal
Sealing liquid IN
Sealing liquid OUT
Floating ring
Labyrinth seal
Labyrinth seal
Shaft sleeve
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4 Carbon Ring Seals
Carbon ring seals make contact with the shaft and their casing and so they will wear.
Although they leave no gap for leakage there must be lubrication between the rubbing
surfaces. This may be provided by the contained fluid, in which case some will leak
out. If some other lubricant is fed to the seal, some of that will leak out. If there is no
leakage at all from a seal it must be running dry and will soon wear and fail.
A number of rings fit inside a casing as shown in Figure 4.1.
The casing is made up of a series of sections. It may be an integral part of the
equipment casing or a separate unit that can fit into a standard stuffing box, as shown
in the figure. Each section contains a set of rings, normally made up of one
tangentially cut ring and one radially cut ring. A garter spring around the outside of
each ring holds the parts of the ring together and keeps them in light contact with the
shaft. In operation, fluid pressure acts on the rings in each set to push them axially:
together and against one side of the housing. It also pushes the rings radially onto the
shaft surface. These forces from the fluid help the rings to seal.
Figure 4.1: Carbon Ring Seal
Tangentially cut ring
Radially cut ring
Garter spring
Garter springs
Fluid pressure
Casing sections
Ring sets
Coolant
Garter spring
Fluid pressure
Lube oil
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The rings are traditionally made of carbon but may now be made of other low-friction
materials such as PTFE (polytetrafluoroethylene). Seven packing sets are most
common although up to twenty sets are used for special applications.
Cases used for high-pressure, or in some high temperature applications, may have
lubricant and/or cooling fluid supplied.
Figure 4.2 shows carbon ring and labyrinth seals on a small steam turbine.
The carbon rings seal the steam in the main turbine casing. These rings fit directly
into the turbine casing. Labyrinth seals are fitted each side of the bearing to prevent
lubrication oil leakage. Bearing lubrication is by a simple splash lubrication system
using oil rings that are turned by the shaft and dip into oil in the oil reservoir.
Figure 4.2: Shaft Sealing on a Small Steam Turbine
Carbon ring seals
Labyrinth seals
Oil rings for splash lubrication
Now try Exercise 1
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5 Lip Seals
Lip seals, also called radial shaft seals, are another type of contact seal. They are
used mainly to reduce leakage of lubricant from bearings and gearboxes, etc., to a
minimum and to keep dirt or other contaminants out. They are only used for small
pressure differences, up to 1 or 2bar. Figure 5.1 shows a typical lip seal.
The main parts of a lip seal are:
• casing
• lip
• garter spring
The lip is usually made of a rubber material (elastomer) that is bonded onto a metal
casing. The garter spring holds the lip against the shaft. In operation, any pressure
difference between the contained fluid and the outside should help to hold the lip
against the shaft.
The main parts of a lip seal are shown in Figure 5.2(a).
Figure 5.1: Lip Seal
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As this is a contact seal, lubrication is necessary to avoid excessive wear of the lip.
Some of the oil being contained forms a film between the lip and the shaft as shown in
Figure 5.2(b).
5.1 Types of Lip Seal
The type of lip seal depends on:
• case design
• lip design
• whether or not a garter spring is fitted
There are three main types of case:
• single metal pressing
• a single metal pressing covered with the rubber lip material
• a double metal pressing
Most lip seal cases are made of steel.
(a) Main Parts (b) Lip Lubrication
Figure 5.2: Parts and Lubrication of a Lip Seal
Garter spring
Metal case
Primary sealing lip
Oil film Oil
Lip contact
Primary sealing lip
Metal case
Garter spring
Shaft
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The simplest and cheapest type has a single metal pressing as shown in Figure 5.3.
This basic type is designed to fit into a housing that is machined accurately and which
has a smooth surface finish.
To give more flexibility in the fit between seal and housing the case can be covered
with the rubber lip material as shown in Figure 5.4.
A rubber-covered case gives a better seal between case and housing and allows for a
rougher finish. It also allows for thermal expansion of the housing.
Figure 5.3: Basic Case
Figure 5.4: Rubber-covered Case
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The third main type of case has no rubber covering but a second metal pressing to
give the seal case more strength. This is shown in Figure 5.5.
Lip designs can also vary but there are two main types:
• single lip
• single lip and dust lip
The seals shown above are of the single-lip type. Where the outside of the seal is
open to the surroundings and there is danger of dirt entering, an extra (secondary)
rubber lip is added to keep it away from the main (primary) lip. Both types are shown
in Figure 5.6.
Figure 5.5: Double Metal Case
Figure 5.6: Lip Types
Primary lip
Dust lip
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Lip seals without garter rings are used for more viscous fluids like grease. They are
also used on hydraulic cylinders for wiping hydraulic fluid or dirt from reciprocating
components. Examples of these are shown in Figure 5.7.
The material of the lip depends on the fluid being sealed. Almost all are of some kind
of rubber but, as natural rubber is attacked by hydrocarbons, e.g. oil and grease, most
are made of one of the many synthetic rubbers. If the fluid being sealed attacks the
steel casing a rubber coated case is used. This may be fully coated, as shown in
Figure 5.4 or just coated on the fluid side, as shown in Figure 5.7(b).
5.2 Seal Identification
Lip seals are identified by their:
• casing type—as described in the last section, plus some additional designs
• lip type—as described in the last section, plus many more
• lip material—mostly synthetic rubbers which must be
compatible with the fluid they contact
• seal dimensions
Figure 5.7: Garterless Lip Seals
Things that are compatible can exist together without harming each other
(a) For Viscous Fluid Applications (b) For Wiping in Hydraulic Cylinder Applications
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Lip and casing types are identified by code letters and numbers. These may be
different for different seal manufacturers. Look at the manufacturer’s catalogue to
find the coding for the seal you need. An example of a typical seal type coding
system is shown in Appendix A of this module.
Lip materials depend on the fluid they contact and the operating temperature. A table
of applications for different rubbers is shown in Appendix B of this module. Lip
materials are identified by a code letter.
The basic dimensions for a lip seal are:
• shaft diameter
• housing diameter
• seal width
and sometimes
• seal OD
The main dimensions are shown in red in Figure 5.8. Other dimensions sometimes
needed are shown in black in the figure.
Figure 5.8: Lip Seal Dimensions
Seal Width
Shaf
t Dia
met
er
Seal
ID
Hou
sing
ID
Housing Bore Depth
Seal
OD
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Seal type, material and size information is usually marked on the metal case of the
seal as shown in Figure 5.9.
Look at the manufacturer’s information to identify a seal from the case markings.
5.3 Removing and Fitting Lip Seals
The main thing to remember when removing an old seal is not to damage the housing
bore. The seal can normally be levered out using a sharp tool behind the seal case.
After removing the old seal, clean the shaft and housing and inspect them for
scratches or burs, especially on shoulders, splines and keyways. Check the shaft for
excessive wear. Remove scratches and burs by honing and finishing with fine emery
cloth. Clean and dry all surfaces.
Before fitting a new seal, inspect it carefully for any damage. Make sure that the
garter spring is located correctly and the seal is clean and free of dust. Check that the
seal is the correct replacement for the one you have removed. Make very sure that the
lip material code is correct for the application.
Figure 5.9: Seal Identification Information
Now try Exercise 2
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You must be aware of two important facts before you fit a lip seal:
• the higher pressure should always push on the garter spring side to help the lip
to stay in contact with the shaft
• most lip seals are designed for a particular direction of shaft rotation:
clockwise or anti-clockwise
In Figure 5.10 you can see the right and the wrong way to fit a lip seal.
In Figure 5.10(a) the seal is fitted so that you can see the garter spring from outside.
This is not correct as the pressure of the contained fluid tries to lift the seal lip off the
shaft causing excessive leakage. Dirt also can collect in the seal and can effect the
operation of the garter spring.
In Figure 5.10(b) the seal is fitted with the garter spring on the inside. This is
correct and pressure of the contained fluid helps to keep the seal lip against the shaft.
Dirt can not build up so easily and can not affect the garter spring..
Figure 5.10: Effects of Correct and Incorrect Seal Orientation
(a) NOT Correct (b) CORRECT
Fluid pressure
Build-up of dirt behind seal and around spring
Fluid pressure
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Another reason for fitting seals in the way shown in Figure 5.10(b) is the difference
in lip angles. One side of the lip is at a greater angle than the other as shown in
Figure 5.11.
Tests have shown that in operation the shaft rotation pushes liquid from the side with
the small angle to the side with the big angle. If fitted correctly, this helps to keep the
liquid behind the seal. If fitted the wrong way around it pushes liquid out from the
seal.
Figure 5.11: Pumping Direction of Rotating Shaft
Always fit lip seals with the garter spring on the higher pressure side
Higher pressure Lower pressure (oil side) (air side)
Bigger angle
Smaller angle
Liquids pushed this way by rotating shaft
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Many lip seals are designed for a particular direction of shaft rotation. They have ribs
moulded into the outside face of the seal as shown in Figure 5.12.
These ribs help the pumping action of the rotating shaft. As the shaft rotates it drags
fluid around with it. The ribs are in a direction that carries any leaking fluid back
towards the sealing edge of the lip. The direction of rotation is clockwise or anti-
clockwise as you look at the end of the shaft from outside the seal. The rotation
direction may be marked on the seal with an arrow, as shown in the figure, or may be
part of the manufacturer’s seal code.
Lip seals designed for shaft rotation in both directions often have ribs in both
directions as shown in Figure 5.13.
Figure 5.12: Single-direction Lip Seals
Figure 5.13: Two-direction Lip Seals
(a) Clockwise Shaft Rotation (b) Anti-clockwise Shaft Rotation
Seal case marking
Shaft rotation
Fluid flow
Ribs Ribs
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When you are sure that you have the correct replacement seal and that the housing and
shaft are in good condition you can install the seal.
Lubricate the lip before sliding it onto the shaft. Use the same fluid that will be
contacting the seal during operation. Another lubricant may not be compatible with
the seal material.
If you are sliding the back (outside) face over the end of the shaft, the shaft should be
radiused as shown in Figure 5.14(a).
If you are sliding the front (inside) face over the end of the shaft, the shaft should be
chamfered as shown in Figure 5.14(b).
If the shaft is not machined as shown in the figure or if the seal must slide over a
shoulder, splines or a keyway, use a cap over the end of the shaft. Two examples are
shown in Figure 5.15: one to slide the seal over a shoulder and one to slide it over a
keyway or splines.
Figure 5.14: Shaft Preparation for Seal Installation
Direction of seal installation
Direction of seal installation
Back of seal Front of seal
Smoothed edges
(a) Back-first Installation (b) Front-first Installation
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The seal is an interference fit in the housing. It is very important to fit the seal with a
force that is spread evenly around the seal, very much like the way in which you
press-fit a bearing. Two examples of suitable mounting tools are shown in Figure
5.15 above. If one is not available you can use a correctly sized tube, with an OD
slightly smaller than the housing ID.
Apply the mounting force steadily, using a press wherever possible, and as close to
the outside as possible to avoid bending the casing as shown in Figure 5.16(a).
Take great care to fit the seal square in its housing, not as shown in Figure 5.16(b).
Figure 5.15: Shaft Cap and Seal Mounting Tool
Figure 5.16: Seal Installation ERRORS
(a) Cap to Slide over Shoulder; Tool for Flush Mounting
(b) Cap to Slide over Keyway and Splines; Tool for Recessed Mounting
Mounting tool
Protective cap
Seal housing
Shoulder
Keyway
(b) Out of Square
Housing ID
Tool diameter
(a) Mounting Tool too Small
Now try Exercise 3
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6 Mechanical Seals
Mechanical seals can protect against leakage across much higher pressure differences
than the other seals described. They reduce leakage to such a small amount that it can
not be seen. Any liquid leakage usually evaporates before it can be detected. This
does not mean that there is no leakage and, as with other contact seals, some fluid
must pass between the sealing surfaces to lubricate and help cool them. By using
more than one seal and by injecting harmless fluids between them we can stop any
hazardous fluids from leaking into the environment.
The seal is made between the very smooth, very flat faces of two rings. One is
attached to and rotates with the shaft. The other is attached to the housing and is
stationary.
The sealing faces are held together by a spring force. During operation this force is
usually increased by the pressure of the contained fluid.
Figure 6.1 shows the two sealing faces of a mechanical seal.
Figure 6.1: Mechanical Seal
Spring loading
Sealing faces
Sealing faces Spring loading
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6.1 Main Parts of a Mechanical Seal
There are many different designs of mechanical seals but they all contain the basic
parts shown in Figure 6.2.
The spring-loaded ring is often just called the face. The other ring is often called the
seat. There may be a single spring as shown in Figure 6.2(a) or a number of springs
as shown in Figures 6.1 and 6.2(b).
In most mechanical seals it is the face that rotates against the stationary seat as shown
in Figure 6.2. The dynamic seal between these surfaces is called the primary seal.
The primary seal surfaces are lapped to very high precision of flatness and surface
finish. Even the small amount of acid in your sweat can damage them so you should
never touch them with bare fingers. The face is usually made of a softer material than
the seat. This allows the face to bed in and prevents the harder seat from wearing.
The face is often made of carbon, a natural solid lubricant, which reduces wear during
start-up and shut-down, before a fluid film can form between the faces.
Figure 6.2: Basic Parts of a Mechanical Seal
(a) Basic Mechanical Seal
Spring (or springs)
Stationary ring
Rotating ring
Secondary seal
Secondary seal
Shaft collar (or sleeve)
Shaft
Housing
Primary seal (between faces)
(b) Drawing of Basic Mechanical Seal
Shaft
Housing
Rotating ring
Stationary ring
Secondary seals
Primary seal
Shaft collar (or sleeve)
Spring (or springs)
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The seat is made from a metal or ceramic material. Both surfaces must be compatible
with the fluid they contact.
Static secondary seals stop the contained fluid from leaking along the shaft, under the
collar and rotating ring. A secondary seal also stops leakage between the stationary
ring and its housing. There may be other static secondary seals at points where
leakage between stationary or axially sliding surfaces is possible.
Rubber o-rings are the most common type of secondary seal but other polymers
(PTFE for example) and sections (wedge, chevron and u-cups), as well as gaskets, are
also used.
A collar or sleeve is fixed to the shaft by a key or by set screws. This collar drives the
spring (or springs) and the rotating ring. The drive is usually through a positive drive
mechanism that allows the rotating ring to move axially on the shaft. This must
happen to form the seal and take up any wear on the faces. An outer shell and pins or
lugs often provide this drive as shown in Figure 6.3.
Figure 6.3: Drive for Rotating Face
Rotating face
Shell
Collar
Drive lug
Spring
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6.2 Types of Mechanical Seal
Mechanical seals can be grouped in a number of ways, depending on:
• primary seal design
o rotating and stationary
o balanced and unbalanced
• secondary seal design
o pusher and non-pusher (bellows)
• location and method of fitting
o internal and external
o conventional and cartridge
6.2.1 Rotating and Stationary Seals
In most mechanical seal designs it is the spring-loaded face that rotates with the shaft
and the seat that is fixed in a stationary housing. This is a rotating mechanical seal.
If the spring-loaded face is fixed in the housing and the seat rotates with the shaft, the
seal is of the stationary type. All the seals shown in figures so far have been of the
rotating type. Figure 6.4 shows a stationary-type seal.
Figure 6.4: Stationary-type Mechanical Seal
Spring-loaded face held in fixed housing
Seat located on shaft sleeve
Housing
Shaft
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6.2.2 Balanced and Unbalanced Seals
In most designs, the pressure of the fluid being contained helps to keep the primary
seal surfaces pressed together. The force pushing them together depends on the fluid
pressure and the area the pressure pushes on.
In an unbalanced seal, all the axial part of the force pushes the face onto the seat as
shown in Figure 6.5(a). This is good up to a certain pressure but for higher pressures
it can break down the lubricating film between surfaces.
By changing the shape of the spring-loaded face, some of the contained pressure can
be used to push back as shown in Figure 6.5(b).
In the balanced seal, only a part of the fluid pressure pushes the seal surfaces together
as some is balanced by a force in the opposite direction. Balanced seals can continue
to operate under higher pressures than unbalanced seals.
Figure 6.5: Balanced and Unbalanced Seals
Axial part of contained fluid pressure
(a) Unbalanced
(b) Balanced
Axial parts of contained fluid pressure
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6.2.3 Pusher and Non-pusher (Bellows) Seals
As the seal face wears, it is pushed closer to the seat by the spring or springs.
In a pusher seal, the secondary seal is located so that it slides along the shaft with the
seal face as shown in Figure 6.6.
This is the most common type. The disadvantage with this arrangement is that the
secondary seal can stick, or hang up, so that the primary face can not take up wear.
In a non–pusher seal, the secondary seal is located under the collar and does not slide
with the primary face. This type uses a metal or rubber (elastomer) bellows to keep
fluid away from the shaft downstream from the secondary seal. This arrangement is
shown in Figure 6.7.
Seals with elastomer bellows need a spring to push the primary face against the seat.
If the bellows is made of metal it can also act as a spring.
Figure 6.6: Pusher Seal
Figure 6.7: Non-pusher or Bellows Seal
Sliding secondary seal
Non-sliding secondary seal
Bellows
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Figure 6.8 shows elastomer and metal bellows seals.
6.2.4 Internal and External Seals
Most seals are mounted internally. The rotating seal face, collar, spring, etc., are
mounted inside the seal gland. This has the advantage that fluid pressure helps to
keep the face pushed against the seat.
The disadvantage is that inside the seal gland the seal is exposed to the contained
fluid. If the contained fluid is very corrosive, the seal parts must be made of
expensive, corrosion-resistant materials. Figure 6.9 shows a typical internal seal.
Figure 6.8: Bellows Seals
(a) Elastomer Bellows (b) Metal Bellows
Bellows
Bellows
Spring
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For very corrosive fluids an external seal may be cheaper. The seal is reversed and
the moving parts are mounted outside the gland. Only the seat and face are exposed
to the contained fluid, as shown in Figure 6.10.
These seals are easier to access for maintenance but, being outside, they are more
exposed to damage. Fluid pressure acts to open the seal so they are not suitable for
high pressures.
Figure 6.9: Internal Seal
Figure 6.10: External Seal
Seat Face
Gland throat
Fluid pressure Atmospheric pressure
Seat
Face
Gland throat
Fluid pressure Atmospheric pressure
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6.2.5 Conventional and Cartridge Seals
The seals described so far are conventional seals. The face and seat have to be
assembled on site and must be set and aligned carefully.
Cartridge seals are pre-assembled on a shaft sleeve and include a gland. They fit
directly onto a shaft of the correct size or a second shaft sleeve. This design does not
need setting and alignment on site and reduces maintenance time and cost. Figure
6.11 shows a typical cartridge seal.
Figure 6.11: Cartridge Seal
Seat
Face
Sleeve
Fluid pressure Atmospheric pressure
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6.3 Dual* Seals
Two mechanical seals may be mounted together to:
• provide a back-up to protect against failure of one seal
• allow higher pressures to be sealed or to reduce the pressure drop across the
inside (inboard) seal
• prevent leakage of hazardous or toxic fluids
• seal corrosive or abrasive fluids
There are three possible arrangements of dual* seals:
• tandem*— both seals facing the same direction
• double* seals mounted back-to-back
• double* seals mounted face-to-face
These seal arrangements are shown in Figures 6.12, 6.13 and 6.14.
Figure 6.12: Dual Seals Mounted in Tandem
Inboard primary seal
Outboard primary seal
Contained fluid
*Note: The words dual, tandem and double all have the same meaning. They
describe two things that work together. The API preferred term for all of these
seals is dual.
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Figure 6.13: Dual Seals Mounted Back-to-back
Figure 6.14: Dual Seals Mounted Face-to-face
Inboard primary seal
Outboard primary seal
Contained fluid
Inboard primary seal
Outboard primary seal
Contained fluid
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6.4 Seal Fluids
Fluid can be injected into the seal gland area for several reasons:
• flushing—to wash out any unwanted fluids or solids that might build up in the
seal or to keep abrasives away from primary seal surfaces
• quenching—to control temperature and remove solids, etc., that might build up
outboard of the seal
• jacketing—to cool the stuffing box area, including the seal
• buffer—to reduce the total pressure difference across the seals in two steps
• barrier—to stop any leakage of toxic or hazardous fluids
Seal fluids must be compatible with the seal materials. In some cases, some will leak
into the contained fluid and this must be acceptable to the final product.
All these fluids may help to control the temperature at the seals. Temperature control
at the primary seal surfaces is important to maintain the lubricating film between
them. Temperature affects the viscosity of a fluid. The higher the temperature the
lower the viscosity and the easier it is for the fluid film to break down. Also, if the
fluid pressure in the film is close to its vapour pressure the fluid may vapourise
causing cavitation between the surfaces. Cavitation is described in the Pumps module
in this course.
Flushing, quenching and jacketing fluids can be used with single and dual seals.
Figure 6.15 shows a single seal with connections for these.
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If the fluids used are liquids they should enter at the bottom and leave at the top. This
makes sure that no air is trapped inside during filling. If gas or vapour, e.g. steam, is
used the flow direction is reversed—in at the top and out at the bottom.
Flushing fluid is directed towards the primary seal surfaces at a pressure higher than
that of the contained fluid. It is a clean fluid that keeps the surfaces clear of solid
build-up and harmful liquids or vapours and helps to cool the seal surfaces. If an exit
connection is not provided the flushing fluid enters the contained fluid through the
throat bushing.
Quenching fluid, also called vent and drain fluid, is injected into the area outboard of
the seal. This does a similar job to the flush but cleans and cools the seal from the
outside. Quench liquid does not enter the contained fluid.
Jacketing fluid is used for some seal glands where temperature control of the whole
gland area is necessary. These glands have spaces around the seal to allow fluid to
circulate.
Figure 6.15: Seal Fluids used for Single and Dual Seals
Jacket liquid in
Jacket liquid out
Flush liquid in
Flush liquid out
Quench liquid out
Quench liquid in
Throat bushing
Outboard seal or throttle bushing
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In addition to the seal fluids that can be used for single or dual seals, there are two that
are used only with dual seals.
Buffer fluids are injected between dual seals at a pressure between that of the
contained fluid and the outside atmosphere. This reduces the pressure drop across
each seal, allowing higher contained pressures to be sealed. Figure 6.16 shows
tandem seals with a buffer fluid.
Barrier fluids are injected between dual seals at a pressure higher than that of the
contained fluid. This makes sure that no contained fluid escapes into the space
between the seals and so none can escape to atmosphere. Barrier fluids are used to
stop any trace of leak of a hazardous or toxic fluid. Figure 6.17 shows back-to-back
seals with a barrier fluid.
Figure 6.16: Buffer Fluid
Contained pressure P1
Buffer pressure P2 P1>P2>P3
Atmospheric pressure P3
Buffer fluid in at pressure P2
Buffer fluid out
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The choice of seal fluid used depends on the application. Fluid can be taken directly
from pump or compressor suction or discharge if the pressure and cleanliness of the
fluid is suitable. Water is often used for pumps and air or an inert gas like nitrogen
for compressors.
Seal liquids taken from an outside source may be circulated by gravity and convection
or pumped under pressure. Figure 6.18 shows a natural convection supply system,
often called a thermosyphon system.
Figure 6.17: Barrier Fluid
Contained pressure P1
Barrier pressure P2 P2>P1>P3
Atmospheric pressure P3
Barrier fluid in at pressure P2
Barrier fluid out
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As the seal liquid temperature increases inside the seal it expands, becoming less
dense. The more dense cooler liquid falls to the lowest point in the system, displacing
the hotter less dense liquid and pushing it up into the reservoir. In this way the
convection currents set up circulate the liquid around the system.
For many applications a forced feed system is used in which seal fluid is pumped
from the reservoir, through coolers and filters and then to the seals. This system is
very similar to a forced lubrication supply to bearings. Figure 6.19 shows a P&ID of
a typical compressor seal oil system.
Figure 6.18: Seal Fluid Supply by Thermosyphon
Seal
Cold feed
Hot return
Reservoir
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Figure 6.19: Compressor Seal Oil Forced Circulation System P&ID
Now try Exercises 4 and 5
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7 Summary
In this module the types of dynamic seals not covered in the module on Gland
Packing are described. These seals are mentioned in the modules on Pumps and on
Compressors but they are described in much greater detail here.
You should now be able to identify most of the seals used on the plant, know their
applications and have had practice in fitting some of them. You should be able to
identify the different types and arrangements of mechanical seals and know the types
of seal fluids used and what they are for.
The procedure for removing, dismantling, re-assembling and fitting a mechanical seal
depends on the type and design of the seal. Exercise 5 gives you practice at following
a procedure for one type of mechanical seal.
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8 Glossary
Here are some words used in this module that might be new to you. You will find
these words in coloured italics in the notes. There is a short definition in a box near
the word in the notes.
Word First Used on
Page:
Part of Speech Meaning Example of Use
Barrier 12 noun Something that blocks the path
A barrier across the entrance is lifted when you show your security pass.
Compatible 20 adjective Able to exist or be used together without problems
It is difficult to work with someone with whom you are not compatible.
Flammable 7 adjective Easily set on fire Never leave flammable liquids standing in direct sunlight.
Inject 9 verb To feed something into another substance, usually under pressure
Sometimes a doctor injects a drug directly into your blood.
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Appendix A Typical Lip Seal Codes
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Appendix B Typical Synthetic Lip Seal Material Codes and Applications
LIP MATERIAL NITRILE POLYACRYLATE SILICONE FLUOROELASTOMER
Material Code N P S V
Temperature Range *
-40 F ~ 250 F (-35 C ~ 120 C)
-20 F ~ 300 F (-30 C ~ 150 C)
-80 F ~ 400 F(-60 C ~ 200 C)
-30 F ~ 400 F (-35 C ~ 200 C)
Oil Resistance E E G E
Acid Resistance G F F E
Alkali Resisitance G X X F
Water Resisitance G F G G
Heat Resistance G E E E
Cold Resistance G F E F
Wear Resistance E E G E
Ozone Resistance G E E E
ASTM D2000 Spec.
2BG715B14B34E014
EO34EF11EF21 SDH710A26B16 B36EO16EO36
2GE8O7A19B37
EO16EO36G112HK710A110B38
* maximum temperature limits depend on other operating conditions.
Key:
E Excellent G Good for most applications. F Fair, can be used if no other materials available. X Not recommended.
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Exercises