dynamic seals

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

Dynamic Seals/Rev. 0.0 of 49

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



Contents

Page No. 7 Summary...................................................................................... 45 8 Glossary....................................................................................... 46 Appendix A................................................................................... 47 Appendix B................................................................................... 48 Exercises 1-5................................................................................ 49



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



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.



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



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.



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



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.



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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)



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



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



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



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



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



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




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




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.



Figure 6.13: Dual Seals Mounted Back-to-back

Figure 6.14: Dual Seals Mounted Face-to-face



Contained fluid



Contained fluid



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.



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



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



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



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



Figure 6.19: Compressor Seal Oil Forced Circulation System P&ID

Now try Exercises 4 and 5



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.



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.



Appendix A Typical Lip Seal Codes



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.



Exercises

dynamic seals

Documents

mechanical seals

stationary seals

unbalanced seals

external seals

cartridge seals

labyrinth seals

dual seals

types of dynamic seals