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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

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

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

6

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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

The Job Training Mechanical Technician Course is the second phase of the development programme. It is intended specifically for Mechanical Maintenance Developees.

Objectives

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

Methodology

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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

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

Job Training—Mechanical Technician

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 orings. 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

A flammable material catches fire easily. It is a fire hazard.

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.

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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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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. Seal fluid connections Stationary seal

Stationary seal-half

Rotating sleeve

Rotating seal-half

Stepped section (a) Seal Running on Plain Sleeve

(b) Interlocking Seal

Figure 2.2: Two-piece Labyrinth Seals

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.

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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.

(a) Full Inner Half

(b) Split Outer Halves

Figure 2.3: Interlocking Labyrinth Seal Halves

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 2.4: Rotating Labyrinth in Plain Casing

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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.

Impeller

Impeller eye labyrinth seal Balance drum labyrinth seal

Balance drum

Shaft Shaft sleeve

Shaft labyrinth seal

Figure 2.5: Labyrinth Seal Locations in a Centrifugal Compressor

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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

A barrier stops forward movement.

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.

Escaping discharge gas returned to suction (recovery)

Seal gas leaving High pressure seal gas entering

Impeller

Bearing

Oil seal

Figure 2.6: Labyrinths Seal with Discharge Recovery and Seal Gas Connections

Labyrinth seals are often used in series with other types of seal to give improved and back-up sealing.

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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. Sealing liquid IN

Floating ring Labyrinth seal

Shaft sleeve

Labyrinth seal

Sealing liquid OUT

Figure 3.1: Liquid (Oil) Film Seal

The example shown in the figure uses labyrinth seals to reduce leakage of the sealing liquid.

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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.

Garter spring Garter spring Radially cut ring

Ring sets

Tangentially cut ring

Lube oil

Fluid pressure

Fluid pressure

Coolant

Casing sections Garter springs Figure 4.1: Carbon Ring Seal

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.

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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.

Labyrinth seals

Carbon ring seals

Oil rings for splash lubrication Figure 4.2: Shaft Sealing 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.

Now try Exercise 1 Dynamic Seals/Rev. 0.0

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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.

Figure 5.1: 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).

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Metal case Garter spring

Primary sealing lip

Garter spring

Primary sealing lip

Oil

Oil film

Shaft

(a) Main Parts

Metal case

Lip contact

(b) Lip Lubrication

Figure 5.2: Parts and Lubrication of a 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.

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The simplest and cheapest type has a single metal pressing as shown in Figure 5.3.

Figure 5.3: Basic Case

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.

Figure 5.4: Rubber-covered Case

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.

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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.

Figure 5.5: Double Metal Case

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.

Dust lip

Primary lip

Figure 5.6: Lip Types

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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.

(a) For Viscous Fluid Applications

(b) For Wiping in Hydraulic Cylinder Applications

Figure 5.7: Garterless Lip Seals

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

•

Things that are compatible can exist together without harming each other

seal dimensions

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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. Housing Bore Depth

Seal ID

Housing ID Shaft Diameter

Seal OD

Seal Width

Figure 5.8: Lip Seal Dimensions

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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.

Figure 5.9: Seal Identification Information

Look at the manufacturer’s information to identify a seal from the case markings.

Now try Exercise 2 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. Dynamic Seals/Rev. 0.0

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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.

Fluid pressure

Build-up of dirt behind seal and around spring

Fluid pressure

(a) NOT Correct

(b) CORRECT

Figure 5.10: Effects of Correct and Incorrect Seal Orientation

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..

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Always fit lip seals with the garter spring on the higher pressure side Higher pressure (oil side)

Lower pressure (air side)

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.

Liquids pushed this way by rotating shaft

Bigger angle

Smaller angle

Figure 5.11: Pumping Direction of Rotating Shaft

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.

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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.

Seal case marking Shaft rotation Fluid flow

Ribs

Ribs

(a) Clockwise Shaft Rotation

(b) Anti-clockwise Shaft Rotation

Figure 5.12: Single-direction Lip Seals

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 anticlockwise 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 5.13: Two-direction Lip Seals

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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).

Front of seal Back of seal Smoothed edges

Direction of seal installation (a) Back-first Installation

Direction of seal installation (b) Front-first Installation

Figure 5.14: Shaft Preparation for Seal Installation

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.

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Personnel & Training Division

Job Training—Mechanical Technician Keyway

Shoulder Mounting tool

Protective cap

Seal housing

(a) Cap to Slide over Shoulder; Tool for Flush Mounting

(b) Cap to Slide over Keyway and Splines; Tool for Recessed Mounting

Figure 5.15: Shaft Cap and Seal Mounting Tool

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). Housing ID Tool diameter

(a) Mounting Tool too Small

(b) Out of Square

Figure 5.16: Seal Installation ERRORS

Now try Exercise 3 Dynamic Seals/Rev. 0.0

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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.

Sealing faces Spring loading

Sealing faces

Spring loading

Figure 6.1: Mechanical Seal

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6.1

Job Training—Mechanical Technician

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.

Spring (or springs)

Shaft collar (or sleeve)

Primary seal (between faces) Stationary ring

Primary seal Spring (or springs)

Secondary seal

Rotating ring

Shaft collar (or sleeve) Secondary seal Rotating ring

Stationary ring

Housing

Shaft Shaft Housing Secondary seals

(a) Basic Mechanical Seal

(b) Drawing of Basic Mechanical Seal

Figure 6.2: Basic Parts of a Mechanical Seal

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.

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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. Rotating face

Drive lug

Shell

Spring

Collar

Figure 6.3: Drive for Rotating Face

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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.

Housing

Spring-loaded face held in fixed housing

Seat located on shaft sleeve Shaft

Figure 6.4: Stationary-type Mechanical Seal

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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). Axial part of contained fluid pressure

(a) Unbalanced Axial parts of contained fluid pressure

(b) Balanced

Figure 6.5: Balanced and Unbalanced Seals

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.

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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. Sliding secondary seal

Figure 6.6: Pusher Seal

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. Non-sliding secondary seal

Bellows

Figure 6.7: Non-pusher or Bellows Seal

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.

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Figure 6.8 shows elastomer and metal bellows seals.

Bellows

Spring

Bellows

(a) Elastomer Bellows

(b) Metal Bellows

Figure 6.8: 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.

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Job Training—Mechanical Technician Face

Gland throat Seat

Fluid pressure

Atmospheric pressure

Figure 6.9: Internal Seal

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.

Seat

Face

Gland throat

Atmospheric pressure

Fluid pressure

Figure 6.10: External Seal

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. Dynamic Seals/Rev. 0.0

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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.

Face

Seat

Fluid pressure Sleeve

Atmospheric pressure

Figure 6.11: Cartridge Seal

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6.3

Job Training—Mechanical Technician

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.

Inboard primary seal

Outboard primary seal

Contained fluid

Figure 6.12: Dual Seals Mounted in Tandem

*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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Inboard primary seal

Contained fluid

Outboard primary seal

Figure 6.13: Dual Seals Mounted Back-to-back

Contained fluid

Inboard primary seal

Outboard primary seal

Figure 6.14: Dual Seals Mounted Face-to-face

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6.4

Job Training—Mechanical Technician

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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Job Training—Mechanical Technician Flush liquid out Quench liquid out

Jacket liquid out Throat bushing

Outboard seal or throttle bushing

Quench liquid in

Jacket liquid in Flush liquid in

Figure 6.15: Seal Fluids used for Single and Dual Seals

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. Dynamic Seals/Rev. 0.0

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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.

Buffer fluid out

Atmospheric pressure P3

Contained pressure P1

Buffer fluid in at pressure P2

Buffer pressure P2 P1>P2>P3

Figure 6.16: 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.

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Barrier fluid in at pressure P2

Barrier fluid out

Atmospheric pressure P3

Contained pressure P1

Barrier pressure P2 P2>P1>P3

Figure 6.17: Barrier Fluid

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.

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Reservoir

Hot return

Seal Cold feed Figure 6.18: Seal Fluid Supply by Thermosyphon

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.

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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

Job Training—Mechanical Technician

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

Job Training—Mechanical Technician

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.

First Used on Page:

Part of Speech

Barrier

12

noun

Compatible

20

Flammable

Inject

Word

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Meaning

Example of Use

Something that blocks the path

A barrier across the entrance is lifted when you show your security pass.

adjective

Able to exist or be used together without problems

It is difficult to work with someone with whom you are not compatible.

7

adjective

Easily set on fire

Never leave flammable liquids standing in direct sunlight.

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

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Typical Lip Seal Codes

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Appendix B

Job Training—Mechanical Technician

Typical Synthetic Lip Seal Material Codes and Applications

LIP MATERIAL

NITRILE

POLYACRYLATE

SILICONE

FLUOROELASTOMER

N

P

S

V

-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

2BG715B14B34 E014 EO34EF11EF21

SDH710A26B16 B36EO16EO36

2GE8O7A19B3 7 EO16EO36G11

2HK710A110B38

Material Code Temperature Range *

ASTM D2000 Spec.

* maximum temperature limits depend on other operating conditions.

Key: E G F X

Excellent Good for most applications. Fair, can be used if no other materials available. Not recommended.

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Exercises

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