mechanical seal principles manual - chapter 1

8/10/2019 Mechanical Seal Principles Manual - Chapter 1

1/28

CHAPTER 1MECHANICAL

SEALPurposeand Parts

4


2/28

CHAPTERMECHANIC


Introduction

Objective

TheMechanical

SealPurpose

Thepacked

stuffing box

5

Chapter 1Mechanical SealPurpose and PartsIntroductionMechanical seals continue to evolveusing today s technological advances.However, the purpose and the basicparts of a mechanical seal havenot changed since its inception.This chapter will explain the purposeof mechanical seals along with theirbasic parts and respective functions.

Mechanical seals are a very commonsealing device used extensivelythroughout industry today.This lesson will define a mechanicalseal and describe its purpose.

The packed stuffing box Before mechanical seals, theattempts to control leakage ofproduct around reciprocating orrotating shafts meant restrictingthe shaft and stuffing box wallclearance. This was accomplishedby packing a soft, resilient materialaround the shaft in what is typicallyreferred to as a stuffing box.Compression packings, referred toas mechanical packings, are stillused in many applications becauseof their low initial cost, availability,familiarity, and ease of installation.

However, there are issues withmechanical packings. They canbe expensive to maintain and insome cases result in excessiveproduct losses to the environment.This high potential expense isoften the result of improper packinginstallation or poor equipmentcondition. But the fact still remainswith few exceptions, All packingsmust leak to work properly .

They may leak flush water or theymay leak product, but they still leak.This fact is the number one reasonwhy mechanical packings are beingreplaced by mechanical seals.

Another problem with mechanicalpackings is that they will cause shaftor sleeve damage given enough time.Even the newer materials willeventually fret the shaft or sleeve.

ObjectiveUpon completion of this chapteryou will be able to describethe purpose of a mechanical seal,the various mechanical seal partsand their function.

The Mechanical Seal Purpose

Figure 1The packed stuffing box


3/28

Figure 2The purpose of a mechanical seal

The purposeof a mechanical sealMechanical seals were developedto address the disadvantages of andproblems with compression packings.The purpose of a mechanical sealis to reduce or, in most cases,eliminate leakage of product orother fluids to the environment.

A mechanical seal consists oftwo extremely flat surfaces, calledfaces, held together by productpressure and spring force to preventproduct from escaping to theenvironment. Visible leakage thatcomes from compression packingis usually eliminated. Non-visibleleakage (i.e., fugitive emissions ) is

often reduced by mechanical sealsin order to meet the environmentallaws of local, state, and federalregulatory agencies. Compressionpackings just cannot be used tocomply with these environmental laws.

Mechanical seals that are appliedcorrectly can reduce the operatingand maintenance costs of most plants.However, a higher level of trainingis required for engineering andmaintenance personnel in order toensure mechanical seal reliability.

It is important to note thatinitial installation costs for sealsmay be higher than compressionpackings. It is also important torealize increasing system reliabilitymeans that mechanical sealsmust be applied correctly andthe seal may require customdesigning for a certain application.

CHAPTER 1MECHANICAL


6

TheMechanical

SealPurpose

Thepurpose

of a

mechanicalseal

Mechanical Seal


4/28

CHAPTERMECHANIC


TheSections

of aMechanical

Seal

Mechanicalseal

construction

Theprimary

seal rings

7Figure 3The primary seal rings

All mechanical seals are constructedsimilarly. They all can be distilleddown to three basic sets of parts:the primary seal rings, the secondaryseals, and the metal parts.This lesson will define these partsand explain their function.

Mechanical seal constructionAlmost every mechanical seal isconstructed of the same three basicsets of parts. In no particular order,the parts are as follows:

Primary seal rings Secondary seals The metal hardware

Of course, each of the above iscomprised of many parts which arediscussed in the following sections.

The primary seal ringsThe primary seal rings are a setof two extremely flat surfacesheld together by process and springpressure to prevent product from

escaping. In a mechanical seal,one ring must rotate with the shaftwhile the other ring does not rotate.These rings are commonly referredto as the rotary seal ring and thestationary seal ring or seat, respectively.

The Sections of a Mechanical Seal

Primary Seal Rings


5/28

CHAPTER 1MECHANICAL


8

The secondary sealsAll mechanical seals willuse some type of secondarysealing device to eliminateleakage at all other areasoutside the primary seal rings.The two main places secondaryseals are used is betweenthe mechanical seal and theequipment shaft or sleeve;and between the seal glandand pump stuffing box face.These sealing devices cantake many forms. They canbe any one of the following:

O-rings Gaskets U-cups V-rings Teflon* wedges Molded rubber boots Chevrons Square packing

*Teflon is a registered trademark of DuPont Dow

The metal partsMechanical seals also haveplenty of metal hardware.Typical hardware may includethe following, just to name a few:

Shaft sleeves Gland rings

Collars Compression rings Pins Springs Bellows Drive lugs Snap rings Seal ring holders

TheSections

of aMechanical

Seal

Thesecondary

seals

Themetal parts

Figure 5The metal parts

Figure 4The secondary seals

Secondary Seals Metal Parts


6/28

CHAPTERMECHANIC


PrimarySeal Rings

Theprimary

seal rings

9Figure 9Worn soft face. Note: the shorter nose

Figure 6 The primary seal rings

Figure 7 Typical mechanical seal showing the narrow soft face and the hard wide face

Figure 8New soft face made of carbon.Note: the long nose

Understanding the function ofthe primary seal rings is the basisfor all mechanical seal discussions.This lesson will define seal faceflatness, describe how an opticalflat works and try to explain whattakes place between these rings.

The primary seal ringsThe primary seal rings consist oftwo extremely flat surfaces heldtogether by process and springpressure to prevent product fromescaping. In a mechanical seal,one ring must rotate with the shaft.This ring is commonly called therotary seal ring. The second ringdoes not rotate and is commonlycalled the stationary seal ring.

Dissimilar materials are commonlyused for the rotary and stationaryseal rings. One of the seal ringsis usually a softer ring relative tothe other. Because the ring is softer,it will wear as the mechanical sealrotates. The softer seal ring contactsurface or face is always morenarrow than the harder seal ringface material. As the narrow,softer face wears on the seal ring,it maintains contact with the harderface throughout the life of themechanical seal. The narrow softface can be on either the rotaryor stationary seal ring. Its locationdepends only on seal design and type.

When the sacrificial, narrow, softerface has worn down completely,the mechanical seal life has expired.One can see a similarity betweenmechanical seal face wear andtread wear on an automobile tire.When the seal face wears down,leakage is likely, and it s time for anew seal. Illustrations showing anew soft face made of carbon anda worn soft face are shown inFigures 8 and 9 .

Primary Seal Rings

Primary Seal Rings

Harder, Wider Seal Ring

New Carbon

Worn Carbon

Narrow, SofterSeal Ring


7/28

CHAPTER 1MECHANICAL


PrimarySeal Rings

Seal faceflatness

Flatnessdefined

10

Figure 10Primary seal rings

Seal face flatnessA mechanical seal consists of twoseal rings whose faces have to beextremely flat. There are numerousfactors that determine rate offluid flow between the seal faces,however, the distance betweenthe two face surfaces is the factorthat has the greatest influence.This means that it is vital thatthe distance between these twofaces be minimized.

To achieve optimum seal face flatness,the seal faces must be lapped andpolished. The first step is to lap orcreate a flat surface. This surface isthen polished to achieve a reflectivefinish. To ensure that a seal ring has

the proper face flatness, specializedequipment is necessary to measure it.

Flatness definedFlatness is a term that describesa level surface that has no elevationsor depressions. We use terms likewaviness, or concave and convexsurfaces to describe the conditionwhen we refer to the mechanicalseal faces. It is this flatness thatis of the most concern to us.Testing has shown that if thefaces are separated by a spaceof about two microns or more,the seal faces will show visibleleakage and, depending uponthe separation, let solids penetratethat might score or in some wayinjure these lapped faces.

We just said that the seal faces

should be separated by twomicrons or less to seal properly.Considering that the human eyecan, at best, see items that areforty microns or greater, it standsto reason that we cannotdetect the proper face flatnessby ourselves without help.Some understanding of theproper terminology is requiredto discuss how we measuredistances this small.


8/28

CHAPTERMECHANIC


PrimarySeal Rings

What isa Helium

Light Band?

11

What is a Helium Light Band?To understand how we measuresuch small distances we haveto know that it is a characteristicof light to travel in waves.These light waves can interferewith each other, causing thelight to disappear. This appearsas a black band on the surfaceof the measured surface.It results from the interferenceof the wavelength going andcoming from the reflective surfaceof the piece being measured.

When you discuss visible light, colorand wavelength mean the samething, so to make the measurementwe use a monochromatic or single

wavelength light source (mono meansone, and chromatic means color ).Any color (wavelength ) couldbe used, but most companies usea pink color that comes off ahelium gas light source. This colorhas a wavelength of just about0.6 microns (0.000023 inches ).

This monochromatic light operatesusing a very simple law of physics.This law is that if two lights with

identical wavelengths interfere witheach other the result is blackness,not light. Please review the twoillustrations Figures 11 and 12 .

Figure 12Helium light reflecting off a surfacecausing light waves to cancel andblack lightbands to form

Figure 11Helium wavelength

0.0000232 inches( 0.00059 mm )

Helium Light Source

Reflected Light

Cancel Points1/2 Helium Wavelength =

1 Helium Lightband =0.0000116 inches ( 0.000295 mm )

1 Heliumwavelength


9/28

CHAPTER 1MECHANICAL


PrimarySeal Rings

What isan

Optical Flat?

12

What is an Optical Flat?To measure seal face flatnessa precision ground and polishedclear glass of optical quality isrequired. This type glass is calledan optical flat. Optical flat glassis lapped flat on at least one sideto a certain accuracy standard.This working standard is amaximum of 0.000004 inches(4 mK) or 0.1 microns

The optical flat is placed onthe piece to be measured.The monochromatic light isaimed at the piece and this lightreflects off of the piece backthrough the optical flat causinginterference light bands.

If the distance betweenthe optical flat and the piecewe are measuring is one halfthe wavelength of helium,or an even multiple of thenumber, a dark band is formed.This is referred to as a heliumlight band and because it isone half the wavelength ofhelium it measures 0.3 microns,or 0.0000116 inches.

To understand this measurementwe might mention that thesmallest object that can be seenwith the human eye is 40 micronsor 0.0015 inches. Another way tounderstand this measurement isto know that the average coffeefilter is in the range of 10 to 15microns or 0.0004 - 0.0006 inches.Experienced seal people knowthat this means that solidscannot penetrate between theseal faces unless they open.

We check flatness of our seal faceby comparing the pattern we seeto a chart that is supplied by themeasuring equipment manufacturer.A sample of this chart is shown inFigure 15 .

Figure 13Monochromatic light and optical flat

Figure 14Optical flat under a monochromatic light showinghelium light bands on a mechanical seal ring

Figure 15 Typical flatness interpretation chart showing light wave band pattern guide


10/28

Reading Light BandsWhen using an optical flatthere are two methods that canbe used to determine face flatness.The two methods are thewedge and contact methods.

Wedge Method

This method is usually used whenthe surfaces of the work and theflat are nearly parallel.The flat willcontact the work at one point.Use a tissue at this point betweenthe flat and the work.Read thebands in two directions by changingthe pressure point by 90 degrees.The amount of the curve indicatesflatness. If the band curves acrosstwo adjacent bands then thepiece is flat within 23.2 mK.

Contact Method This is the best method for RingShaped Work. The optical flatrests on the highest points of thework. Establish an imaginary lineparallel to the bands in the centerof the piece. Count the bandsbetween one side of the line,then on the other side of the line.Divide the largest number bythe smallest number. The resultindicates the flatness in lightbands. Localized distortions aremeasured by taking an imaginaryline across the light bands.

Count the number of light bandsthe line crosses. This indicatesthe flatness in light bands.

Silicon carbide, tungsten carbideand ceramic seal faces are lesslikely to be out of flat than carbon.Flatness is a good indicator ofwear on the wide face of the seal.

CHAPTERMECHANIC


PrimarySeal Rings

ReadingLightBands

13

Figure 17 Flatness interpretation chart

showing the contact method for various rings

Figure 16 Flatness interpretation chart

showing the wedge method for various symmetric pieces of work

Contact Method 1 Light Band

0.0000116 inches0.3 microns

Wedge Method

Contact Point Contact Point Contact Point Contact Point

Wedge Method Wedge Method Wedge Method

O p t i c a l F l a t

Work Work Work Work

O p t i c a l F l a t O p t i c a l F l a t

O p t i c a l F l a t

Contact Method 2 Light Band

0.000023 inches0.6 microns


11/28

CHAPTER 1MECHANICAL


PrimarySeal Rings

FlatnessReadings

Rulesof Thumb

Seal facelubrication

Theasperitytheory

14

Flatness Readings Rules of ThumbHard seal faces should read less thanthree light bands for seal faces witha mean diameter up to four inches.

There should be no visible leakage.Leakage is always subject to definition,but three light bands of flatnesswill allow a mechanical seal to seala vacuum down to a measurement ofone torr (one millimeter of mercury ).

Carbon graphite faces relax afterlapping. Although lapped to lessthan one light band by the sealmanufacturer, you will see readingsas high as three light bands if youcheck the faces. These faces shouldreturn to flat once they are placed

against a hard face that is flat.Most large seal manufacturers usefinite element analysis techniquesto design these faces. Some repairand smaller seal facilities supply,replace or repair these faces withno provision for keeping them flatduring temperature and pressuretransients.

Carbon is a flexible material.

It can go out of flat easily.It should go back flat again whenit presses against the hard face.

Some seal companies lap facesconcave or convex on purpose.That is why three helium lightbands is often the specification.

Tests done with two hard faces(they do not relap easily ) show thatvisible leakage starts to occur atabout five helium light bands.

It is not a good idea to relap carbongraphite faces. Imbedded solidsare pushed even further in, causingscoring and wearing of the hard face.Remember carbon cannot weara hard face, only foreign materialstuck in the carbon can do that,and relapping cannot remove it.

Seal face lubricationMechanical seals typically requirea layer of gas or liquid lubricationbetween the rotary and stationaryseal faces. Seal face lubricationis crucial in maintaining seal lifeand reducing energy consumption.Even though mechanical seals havebeen in operation since the early1900 s in one fashion or another,it is still not known what actuallyhappens between seal faces.There are at least five commontheories of what may be happeningbetween the faces. These areexplained in more detail as follows.

The asperity theoryThis theory was proposed bythe Battell Memorial Institute backin1963. They were commissionedby the U.S. Air Force to findout once and for all what washappening between seal faces.

Battell made one of the faces outof glass and photographed theresult. The test was run on a carbongraphite face running against thisglass face. The sealing medium wasMIL7808 oil, a high grade turbine oil.

Battell observed that the faceswere separated by vapors comingfrom the asperities in the seal faces.Figure 18-A describes the sealface lapped flat. Being a mixtureof carbon and graphite, the graphitetransfers Figure 18-C to thehard face, leaving asperities(roughness of surface).Unlikeother materials that tend to wearsmooth, these asperities continueto appear as the faces wear(graphite is a natural lubricant ).

Battell observed vapors comingfrom the asperities Figure 18-B .The British picked up on this ideaand came out with vapor phaseseals in which the seal faceswere heated to vaporize the fluid.


12/28

CHAPTERMECHANIC


PrimarySeal Rings

Thepressure

droptheory

Thepressurewedgetheory

15

Figure 18 Asperity theory

The pressure drop theoryThis theory has some similaritiesto the asperity theory. It assumes,that as asperities develop, the fluidgoes through a series of pressuredrops across the face until allpressure is lost at the atmosphericside of the seal faces. In addition,a meniscus of fluid forms on theinside diameter of the face andis held there by centrifugal force.

The pressure wedge theoryThis theory is the one we use whendiscussing mechanical seal balance.It claims that the faces are runningon a film of liquid that produceshydrodynamic forces, keepingthe faces apart. The liquid is forcedbetween the faces by a combinationof pressure and capillary action.

The pressure drop across this wedge is assumed to be linearfor most applications. This meansthat as the fluid travels fromthe process pressure side (highpressure ) to the atmospheric side(low pressure ) the pressure dropsby the same amount for every0.0001inches or 0.01mm we move

across the seal face. However, thedrop may not be linear across thewedge and seal manufacturersneed to be aware of this toproperly design mechanical seals.

Figure 20Pressure drop across the pressure wedgefrom the process pressure sideto the atmospheric pressure side

Figure 19Pressure drop theory

The one problem with this theoryis it does not explain how we are ableto run ceramic against ceramic ortungsten carbide against tungstencarbide. These materials do not haveasperities on the seal face.

CarbonGraphite

RotatingFace

Pressure

Miniscus held by centrifugal force

Stationary Face

Non-Linear Pressure Drop

L i n e a

r P r e s

s u r e

D r o p

S e a

l F

a c e

Process Pressure Side

Atmosphere Pressure Side

A C B

CarbonGraphite


13/28

CHAPTER 1MECHANICAL


PrimarySeal Rings

Thedry running

theory

Thethree band

theory

16

The dry running theoryThis theory proposes that nolubricant is necessary because theseal is running on a combinationof carbon versus graphite.The hard face is there to providea surface for the graphite to stick to.While there is evidence that sealsrun well in solvents and hotwater that provide no lubrication,this theory does not explainhow hard faces can be run inthese non-lubricating applications.

The three band theoryThis theory uses the observationof three distinct areas, or bands,between some seal faces as its basis.A band of liquid forms on the outsidearea, a vapor band in the middle anda dry band on the inside diameter.

All of the conditions noted previouslyhave been observed when usingmechanical seals. The observationof a certain phenomenon or theorymay be a function of the facematerial and the fluid being sealed.Tribological researchers really do notknow. The only thing we know forsure is that something is happening.Whenever we install seals we try tokeep the faces immersed in liquid.

Seals immersed in lubricating fluidseem to last longer, as the facesdo not wear as quickly. One day wewill be able to explain what happensbetween the faces and design sealswith extremely long lives.

Figure 21Three band theory

Dry Vapor Liquid


14/28

CHAPTERMECHANIC


SecondarySeals

Thegland seal

Theshaft seal

17

Figure 22The gland seal

Figure 23Typical dynamic shaft seal

Figure 24Dynamic shaft seal being moved by the springsto the right as the soft seal face wears

Figure 25Static shaft seal plus another secondary sealthat is dynamic and moves to the rightas the soft seal face wears

Secondary SealsIn a mechanical seal there arenumerous secondary seals thatfunction to keep the liquid fromleaking to the atmosphere. Thislesson will describe the types andfunctions of various secondary seals.It is very important to understandthe limitations of each type of sealso that it will be used properly.

The gland sealThe gland seal is a static seal.A static seal is a seal betweentwo surfaces that have no relativemotion to each other. It functions toprovide sealing between the glandand the face of the stuffing box.The gland seal is usually a gasketor an o-ring and can be made ofmany different materials. As with allsecondary seals, this seal needs to becompatible with the fluid being sealed.

The shaft sealThe shaft seal is the part betweenthe mechanical seal and the shaft(or sleeve ) that prevents fluid fromleaking along the shaft out to theatmosphere. The shaft seal can comein various types and configurations.Some common ones are o-rings,V-rings (chevrons ), U-cups, wedges,and boot-type seals.

Gland Seal

Dynamic Shaft Seal

Shaft Seal is Moved by Springs

Static Shaft Seal

DynamicSeal

Soft Seal Face Wears


15/28

CHAPTER 1MECHANICAL


SecondarySeals

O-ringseals

V-ring(Chevron)

seals

18

O-ring sealsAn o-ring is a sealing ring witha circular shaped cross section.O-rings come in many differentsizes and cross sections dependingon the application. They are verycommon in mechanical sealsand have two distinct advantagesover most other secondary seals.

It is impossible to installan o-ring the wrong way.Think about it.You can t do it.

An o-ring can seal bothpositive pressure and vacuum.This is important if the pressurein the stuffing box can fluctuatebetween these two extremes.

V-ring (Chevron) sealsThe V-ring, or Chevron, is a sealingdevice that requires constantloading in order to seal properly.The V-ring must be oriented so the V opens toward the fluid pressure.If the V-ring is installed backwards,the pressure in the stuffing boxcould force the fluid underneaththe ring and leak to atmosphere.

Unlike o-rings, Chevrons can onlyseal in one direction. In other words,they can seal either positive pressure(when installed as illustrated below )or vacuum (when installed backwards ),but not both. Most V-rings are loadedby the springs and the process pressureand are usually dynamic shaft seals.

Figure 26O- ring seal

Figure 27V-ring ( Chevron ) seal with V opento process pressure

O-ring Seal V-ring Seal


16/28

CHAPTERMECHANIC


SecondarySeals

U-cupseals

Wedgeseals

19

U-cup sealsThe U-cup seal is another sealingdevice that requires constantloading in order to seal properly.The cup must be oriented so the U opens toward the fluid pressure.If the U-cup is installed backwards,the pressure in the stuffing boxcould force the fluid underneaththe ring and leak to atmosphere.

Like Chevrons, U-cups can only sealin one direction. That is, they can sealeither positive pressure or vacuum,but not both. Most U-cups are loadedby the springs and the process pressureand are usually dynamic shaft seals.

Wedge sealsThis type of secondary sealis a wedge, usually made of Teflon,that is spring-loaded and matesbehind the rotating primary seal ring.The spring and process pressureskeep the wedge in contact with theshaft. The wedge must be oriented sothat it provides a leak-free seal whenexposed to this process pressure.

Like Chevrons and U-cups, wedgescan only seal in one direction.Again, this means they can sealeither positive pressure or vacuum,but not both. Because of thetendency for Teflon to cold-flow ,almost all wedges need to beloaded by one or more springs

along with the process pressure.And in almost all mechanical sealsthat use the wedge, they areusually dynamic shaft seals.

Figure 28U-cup seal

Figure 29Wedge seal

U-cup Seal

Wedge Seal


17/28

CHAPTER 1MECHANICAL


SecondarySeals

Boot-typeseals

Additionalsecondary

seals

20

Boot-type sealsThis type of secondary seal usuallyconsists of a rubber boot anda large single-coil spring that loadsthe boot against the back of therotating primary seal ring.The boot is made of any numberof rubber materials and providestwo things for the mechanical seal.

The boot is the shaft sealand prevents leakagealong the shaft or sleeve.

The boot, along with the spring,provides the drive mechanismthat attaches to the shaft androtates one of the primary sealrings. Drive mechanisms will bediscussed later in this chapter.

Because boot-type seals are loadedby the spring and do not movein relation to the shaft, they areconsidered to be static shaft seals.Also, because the boot has to adhereto the shaft to work, almost allmechanical seals that use this typeof secondary seal and need to bereplaced require a completely new seal,as they cannot be rebuilt very easily.

Additional secondary sealsAdditional o-rings or secondaryseals may be located throughouta mechanical seal design.Typically, these seals will allowfor radial or axial movementof the primary seal rings while stilleliminating leakage. These sealscan be dynamic or static in nature.As we said earlier, all secondaryseals must be compatiblewith the product and operatingtemperature so it will notdeteriorate and produce leakage.

Figure 30Boot-type seal

Figure 31Mechanical seal with numerous secondary seals,including the shaft and gland seals

Boot-type Seal


18/28

CHAPTERMECHANIC


MechanicalSeal

Hardware

Thegland

21Figure 32The gland

Mechanical Seal HardwareAs mentioned earlier,mechanical seals also containa multitude of other parts andhardware, mostly made of metal.Understanding the type andfunction of this hardwareis important to ensure propermechanical seal operation.This lesson will describe thevarious pieces of hardwarefound in most mechanical sealsand define the function of each.

The glandThe gland holds the non-rotatingparts of the mechanical seal,including the stationary seal ring.The gland is also called the

stationary holder, gland plate,end plate, or flange. The glandis mounted to the seal chamber,by means of various types of bolts,to prevent the stationary partsfrom moving. It also providesan opposing surface to mountthe static seal to the seal chamberas previously discussed.

Gland


19/28

CHAPTER 1MECHANICAL


MechanicalSeal

Hardware

Thespring

mechanism

Largesingle coil

springs

Multiplesmall

springs

22

The spring mechanismThe spring mechanism is a machinedcomponent that stores energy and,when required, releases that energy.The spring provides the force to holdthe rotary and stationary rings togetherwhen the seal is not pressurized.The fluid pressure in the seal chamberprovides the majority of the closingforce holding the seal rings together.

The spring mechanism may belocated on the rotary, stationary,or both depending on mechanicalseal design. There are basicallyfive types of spring mechanisms.All, except one, are metal parts.The one that is made of anelastomer must be compatible

with the product and operatingtemperature so it will notdeteriorate and produce leakage.

Large single coil springsLarge single coil springs wereone of the first spring mechanismsused in early mechanical sealdesigns and are still used in awide range of applications today.They are found in older seal designs,but they work very well whenused properly. However, there areseveral limitations to this spring.

They have a tendency todistort at high surface speeds.This means that large sealsat high rotational speed canbe affected by this problem.

There is a large axialand radial space required.Because there is just onespring, it has to be of

sufficient mass to providethe proper spring load. There is a need to stock

a different size springfor each seal size.

The large coil springs,by design, cannot provideeven closing pressure for theentire seal ring. This couldcause uneven seal face wearand premature failure.

Multiple small springsMany of the newer mechanicalseal designs incorporate multiplesmall springs as the springmechanism.These small springsoperate better in high speedmechanical seal applicationsas well as low speed applications.Because of their quantity,these small springs are not proneto distortion. Consequently,they exert an even closing forceon the seal ring at all times.Unlike the large single coil spring,the multiple small springs maybe used with a wide range of shaftsizes. Also, because of their size,they do not require as much axialand radial space as large coil springs.

Figure 33Large single coil spring

Figure 34Multiple small springs

Large Single Coil Springs

Multiple Small Springs


20/28

CHAPTERMECHANIC


MechanicalSeal

Hardware

Metalbellows

23

Metal bellowsMetal bellows are anotherform of spring mechanism usedin mechanical seals. The weldedmetal bellows is formed by weldingseparate thin (~0.005 inches or0.13 mm ) plates of metal togetherto form the bellows assembly.This one-piece unit provides thespring loading required to maintainface contact. Because metal bellowsmechanical seals are often designedwithout elastomers (i.e., no o-rings ),they are typically used in hightemperature applications. Figure 35

Typical welded metal bellows mechanical seal with vibration damper

Figure 36Metal bellows. Note: this seal uses no elastomer

Vibration Damper

Metal Bellows

Thin ( 0.005 inches or 0.13 mm )Welded Metal Plates

that Form the Bellows


21/28

CHAPTER 1MECHANICAL


MechanicalSeal

Hardware

Somecommonproblems

with

weldedmetal

bellowsseals

Fingersprings

24

Some common problems withwelded metal bellows sealsWelded metal bellows seals workwell when applied properly. However,there are some inherent problemswith them that are listed below.

Welded metal bellows are very

sensitive to vibration (eitherharmonic or slip-stick ). Vibrationproblems can be recognized bya cracking of the bellows nearthe end-fittings. Vibration dampingis a serious problem at extremetemperatures because the shaftand the vibration damper (notcheson the inside diameter of thebellows ) are growing at differentrates. The shaft is usually growingfaster. If the vibration dampercauses the seal to fail by stickingto the shaft, rub marks will bepresent on the shaft.

Some bellows materials arenot very corrosion resistant. Thiscan cause problems if the seal iscleaned with an acid or a solventbecause the bellows is very thin.

Recirculation lines can actas a sand blaster and rupturethe thin bellows.

If the product that is being sealedhas a tendency to harden orset-up between the seal faces,the bellows can twist and rupturebecause the faces have stucktogether. Remember the bellowsis only 0.005 inches (0.013mm ) thick.

When using welded metal bellowsin high temperature petroleumproducts (or other organics ),the proper environmental controlsmust be used or coking will causethe seal to fail. Coking is thebuild-up of a hard black organicresidue caused by over-heating.This coke builds up on theinside diameter of the seal andcan fill the spaces between themetal bellows plates to stop thebellows from acting like a spring.

* Hastelloy C is a registered trademark ofHaynes International Incorporated

** Inconel is a registered trademark ofInternational Nickel Company

Stainless steel should notbe used as a metal bellowsmaterial because of thepossibility of chloride stresscorrosion (to be discussed later ).Better materials like AM350,Hastelloy*C or Inconel**718should be used instead.

Welded metal bellows seals havelimited use in slurry applications.The seal can fail from bellowsrupture due to wear or corrosion.It can also fail when the slurryclogs the bellows.

Because the most common failurefor these seals is bellows breakage,metal bellows seals are very costlyto repair. If this occurs, the wholebellows requires replacement.

Finger springsFinger springs (sometimes calledleaf springs ) are spring mechanismsthat are typically located on theoutside of the mechanical seal gland.These springs are a new designthat provide the force requiredto hold the stationary and rotaryseal rings together. Because thesesprings are outside the mechanicalseal, they are much less prone

to clogging from the product.

Figure 37Finger springs.Note: springs are external to the seal


22/28

CHAPTERMECHANIC


MechanicalSeal

Hardware

Rubberbellows

Somecommonproblems

withrubberbellows

seals

25

Some common problemswith rubber bellows sealsSome of the more common problemswith rubber bellows are listed below:

Many of the existing rubberbellows are made of Buna-Nrubber. This rubber has a finite

shelf-life and is easily attackedby sunlight and ozone. Mechanical seals using a

rubber bellows often fail dueto bellows breakage. This failureis usually catastrophic in nature.This is different from o-ring sealfailure because o-ring problemsusually begin slowly andgradually deteriorate, allowingfor time to schedule repairs.

The rubber bellows suffers fromrepair problems because theelastomer bellows bonds to theshaft in order to obtain a properseal and drive mechanism.Duringa repair the bellows usuallymust be scraped clean from theshaft after the mechanical sealhas been removed.

Installation of this seal type isoften difficult.The rubber bellowslocation is critical and there is noway to set it. The only recourseto this issue is to relocate thespring, however, this does notalways solve the problem.

The rubber bellows must belubricated to slip it over the shaft,however, once installed therubber bellows must bond tothe shaft. Most other mechanicalseals use silicon grease to lubricatethe elastomer. Silicon greaseshould never be used witha rubber bellows seal becauseit will not allow the bellows tobond to the shaft. The shaftwill spin through the bellows,thereby causing premature failure.

Rubber bellowsThe rubber bellows, althoughnot made of metal, also acts asa spring mechanism when usedwith a single coil spring. Because itis a bellows, it has speed limitations.Like all elastomer parts, it must becompatible with the product andoperating temperature so it will notdeteriorate and produce leakage.

The rubber bellows is required tobond to the shaft to work properly.Silicone grease should never beused to install this bellows.Rubberbellows seal designs were oneof the first mechanical seal designsavailable to industry over 50 yearsago. Due to its longevity, and low

cost, this is a popular seal choice fororiginal equipment manufacturer(O.E.M.) pump companies.

Figure 38Rubber bellows with single coil springbonded to a shaft to perform its shaft sealand spring mechanism duties

Spring Stationary Seal Ring

Rubber Bellows

RotatingSeal Ring

Shaft Surface Adhered to Bellows


23/28

CHAPTER 1MECHANICAL


MechanicalSeal

Hardware

Theshaftsleeve

26

The shaft sleeveThe shaft sleeve is a cylindrical-shaped piece of metal or compositematerial placed over the shaft,usually inside the stuffing box.Some common reasons for usinga shaft sleeve are listed below:

Probably the most commonis to provide protectionfrom wear to the shaftdue to mechanical packing.

The second most commonis to provide protectionfrom wear to the shaftdue to mechanical seals.

Shaft sleeves are oftenused to provide the properspacing for the impeller.

They also can be used toprovide a step in the shaftto achieve hydraulic balancefor the mechanical seal(to be discussed later ).

Some sleeves are installedbecause the fluid is extremelycorrosive and it would becostly to make a shaftfrom the sleeve material.

Lastly, cartridge mechanicalseal designs use a sleeve asan integral part of the seal.

Figure 39Common shaft sleeve used to protect the shaft and provide impeller spacing

Figure 40Shaft sleeve used to provide hydraulic balance in a mechanical seal and protect the shaft

Hook Shaft Sleeve

Shaft Sleeve


24/28

CHAPTERMECHANIC


MechanicalSeal

Hardware

Thedrive

mechanism

27

The drive mechanismThe drive mechanism is the partof the mechanical seal that providespositive contact to the rotating shaft.This mechanism, once secured tothe shaft, allows the rotating partsof the seal to rotate with the shaft.

Probably the most common drivemechanism is a group of set screws.However, other mechanismssuch as a clamp, rubber boot(vulcanized or bonded to the shaft )or o-ring drive are also used.

Figure 41Clamp drive mechanism

Figure 43Vulcanized rubber boot drive mechanism

Figure 44O-ring drive mechanism

Figure 42Set screw drive mechanism

Clamp

Set Screw

O-ring Drive

Vulcanized Rubber Boot


25/28

CHAPTER 1MECHANICAL


ReviewQuestions

1 through 10

28

Review Questions1. For the most part,

all packings must leak to work.a. True.b. False.c. It depends.

2. The purpose of a mechanical sealis to reduce or eliminate visibleleakage to the environment.

a. True.b. False.

3. Mechanical seals areconstructed of:

a. Primary seal rings.b. Secondary seals.c. Metal hardware.

d. All of the above.4. Which statement is correct?

a. In a mechanical seal,all the seal ringsrotate with the shaft.

b. The ring that rotatesis commonly referredto as the stationaryseal ring.

c. The seal ring that rotatesis commonly referred toas the rotary seal ring.

d. The seal ring that doesnot rotate with theshaft is called therotary seal ring.

5. Mechanical seal secondary seals

can be .a. O-rings.b. U-cups.

c. Rubber boots.d. All of the above.

6. Typical mechanical sealmetal hardware includesall of the following:Shaft sleeves, Gland rings,Teflon wedges, Pins,Springs, Bellows.

a. True.b. False.

7. Which statement is not correct?a. Dissimilar materials

are commonly usedfor the rotary andstationary seal rings.

b. One of the seal ringsis usually a softer ringrelative to the other.

c. The softer seal ringis always widerthan the harderseal ring material.

d. Because the ring issofter, it will wear as themechanical seal rotates.

8. To achieve seal faceflatness, the seal faces mustbe lapped and polished.

a. True.

b. False.9. Surface finish is a term

that describes a levelsurface that has noelevations or depressions.

a. True.b. False.

10. What gas is usually usedin a monochromatic lightto measure seal face flatness?

a. Helium.b. Hydrogen.c. Argond. Nitrogen.


26/28

CHAPTERMECHANIC


ReviewQuestions

11 through 19

29

11.is used to measure mechanicalseal face flatness.

a. Only the human eye.b. An optical flat.c. A very accurate straight-edge.d. A micrometer.

12. The methods usedto measure light bandswith an optical flat are the

methods.a. contact and triangle.b. wedge and interference.c. interference and triangle.d. contact and wedge.

13. When does visible leakagestart between two mechanicalseal faces?

a. 2 lightbands.b. 5 lightbands.c. 10 lightbands.d. 20 lightbands.

14. Which statement is correct?a. Mechanical seals

typically require a layerof gas or liquid lubricationbetween the rotaryand stationary seal faces.

b. Seal face lubricationis crucial in maintainingseal life and reducingenergy consumption.

c. We really don t knowwhat happens betweenseal faces.

d. All of the above.

15. The asperity theory assumesthat small holes appearon the soft face as the facewears providing places forlubricant to reside.

a. True.

b. False.16. The pressure drop theory

assumes that the asperitiesformed on the seal faceact as an infinite numberof pressure reducers tobring the process pressureto atmospheric pressure.

a. True.b. False.

17. The pressure wedge theoryassumes a film of liquidexists between the seal facescausing them to be pushedapart.

a. True.b. False.

18. What theory proposesthat mechanical seal facesrun without the fluid

to act as a lubricantbetween them?a. The pressure wedge theory.b. The dry running theory.c. The pressure drop theory.d. The three band theory.

19.proposes the separationof the lubricated fluidinto a liquid, vapor and dry

are between the seal faces.a. The dry running theory.b. The pressure wedge theory.c. The pressure drop theory.d. The three band theory.


27/28

CHAPTER 1MECHANICAL


ReviewQuestions

20 through 30

30

20. Which of the followingis a static seal?

a. The primary seal rings.b. The wedge seal.c. The gland seal.d. None of the above.

21. Is the o-ring shownin the above illustration,static or dynamic?

a. Static.b. Dynamic.

22. O-rings can be installedbackwards and still work.

a. True.b. False.

23. Can a V-ring sealbe installed backwardsand still work properly.

a. Yes.b. No.

24. U-cup secondary sealscan seal vacuum and highpressure.

a. True.b. False.

25. A wedge seal is

a. a primary seal.b. a secondary seal.c. a tertiary seal.

d. None of the above.26. Boot-type secondary seals

are dynamic seals.a. True.b. False.

27. The gland holds the rotatingparts of a mechanical seal.

a. True.b. False.

28. Which of the followingstores energy and releasesit as required?

a. The gland.b. The drive mechanism.c. The spring mechanism.d. The o-ring.

29. What spring mechanismis limited in its abilityto provide even face load.

a. The bellows.b. The large coil spring.c. The set of multiple springs.d. The set of finger springs.

30. Multiple small springsare recommended for highsurface speed applicationsversus a single coil spring.

a. True.b. False.


28/28

CHAPTERMECHANIC


ReviewQuestions

31 through 37

31

31. What spring mechanismis constructed of thin metalplates welded together?

a. Single coil spring.b. Multiple coil springs.c. Metal bellows.

d. Rubber bellows.32. What is the most common

failure of bellows mechanicalseals?

a. O-ring failure.b. Bellows breakage.c. Bellows hang-up

due to coking.d. Bellows corrosion.

33. What spring mechanismis usually mounted outsidethe gland?

a. The large coil spring.b. The set of finger springs.c. The metal bellows.d. The set of multiple small

coil springs.

34. Rubber bellows sealsare required to bond tothe shaft in order to

perform their functions.a. True.b. False.

35. If a rubber bellowsis lubricated with siliconegrease for installation,what usually happens?

a. The shaft will spininside the bellows.

b. The bellows will not

seal properly.c. The seal will failprematurely.

d. All of the above.

36. Which of the followingis a cylindrical-shaped pieceof metal or composite materialplaced over the shaft toprovide protection to the shaft?

a. Metal bellows.

b. The spring mechanism.c. The gland.d. The sleeve.

37. Which of the followingis the part of the mechanicalseal that enables the rotaryseal ring to rotate?

a. The spring mechanism.b. The gland.c. The drive mechanism.d. The shaft seal.

mechanical seal principles manual - chapter 1

Documents