www.che.com

December2004

Numerous technological devel-opments have occurred overthe past few years to give me-chanical seals greater relia-bility in reducing the escape

of liquid and gaseous process fluidsfrom rotating equipment, such as mix-ers, centrifugal pumps and compres-sors. Mechanical seals were originallydeveloped as a leak-free alternative topump packing. Over the years, me-chanical seals have undergone contin-uous improvement.

Today, numerous classifications anddesign features are available to endusers. Each mechanical seal design of-fers specific strengths that make itsuse advantageous for certain situa-tions, and tradeoffs that may make itsuse impractical or ill-advised for partic-ular applications. As the technologicalsophistication associated with mechan-ical seal design has increased, usersoften have a difficult time decidingwhich seals to use, and where to applythem most appropriately.

Some mechanical seal standardshave been written to assist in thismatter, but such standards often lagbehind the latest technological devel-opments and, in some cases, stifle in-novation and add unnecessary cost.Mechanical seal standards take yearsto develop and cannot keep up with

technology. Also, many standards arewritten around specific design typesand older technologies, resulting inthe slow adoption of more reliable,lower-cost technology. Ultimately, afull understanding of the options —including the advantages and disad-vantages of each — is required tomake informed choices.

MECHANICAL SEALCLASSIFICATIONS ANDCOMMON USESMechanical seals can be classified bydesign, or by application. Each of thedifferent types listed below is dis-cussed in this article.Classification by design:1. Component or cartridge seals

2. Spring-type seals3. Stationary or rotary seals4. Balanced or unbalanced seals5. Pusher or bellows seals6. Split vs. non-split seals

Classification by application1. Pump or mixer seals2. Metallic versus nonmetallic seals3. High-temperature seals4. Single versus dual seals (such as

tandem, back-back or face-face de-signs)

5. Wet lubricated seals or gas seals

Classification by design1. Component or cartridge seals. Acomponent mechanical seal is one thatdoes not come preassembled, but re-

Cover Story

Mechanical Seals Evaluating What s Right for YouBefore choosing the bestdevice to prevent shaft

leakage, you shouldthoroughly review the

pros and cons of today’s designs

Scott BoysonA.W. Chesterton Co.

FIGURE 1 (top left). Component seals consist of multiple assemblies that need

to be carefully mounted onto the equipment

FIGURE 2 (left). Cartridge seals are preassmbled and are typically installed

onto the equipment in three steps or less

FIGURE 3 (top right). Finger springs are non-clogging and are thus ideal for sealing

slurry pumps and large equipment

Finger springs

quires assembly of the rotary and sta-tionary parts on the equipment shaftor sleeve. The seal faces must remainclean and intact during handling andinstallation, and precise measure-ment is required for proper installa-tion (Figure 1). Too often, componentseals fail prematurely as a result ofimproper installation techniques.

By comparison, a cartridge mechan-ical seal (Figure 2) is a completely self-contained assembly that is pre-assem-bled on a seal sleeve and enclosed in agland. The seal faces remain in con-tact during handling and installation,thereby limiting potential damageand contamination. And, precise mea-surements by mechanics during in-stallation are not required. Thanks tothis simplified design, the use of car-tridge seals can help users to virtuallyeliminate improper installation as afailure mode.

Most cartridge seals require only athree-step installation procedure:• Tighten the gland to the stuffing box

to seal the stuffing box gasket• Tighten the set screws to the shaft• Remove the clips that center the sta-

tionary components to the shaft andset the spring compression

However, newer designs incorpo-rate the centering mechanism as anintegral part of the seal design. Thisreduces the installation procedure tojust the first two steps.

In general, cartridge seals continueto increase in popularity, thanks totheir ongoing technological improve-ments and ease of installation, whilecomponent seal use has been decliningworldwide, due to not only the highlikelihood of improper installation,but also to the fact that technologicalimprovements associated with compo-nent seal designs have been virtuallynon-existent over the past decade.Component seals are still used insmall equipment, where space is lim-ited, and in light-duty applications,such as some positive-displacementpumps and small water pumps.

2. Spring-type seals. The most com-mon spring type in new seal designsinvolves the use of multiple small coilsprings. Compared to large singlesprings — which are still seen in someolder designs — these newer springsoffer more-even spring loading. Ingeneral, it is beneficial that the springmechanism is not in contact with theprocess fluid.

The use of larger single springs canmake a mechanical seal susceptible touneven spring loading on the sealface, which can cause face distortion.Finger springs, which apply springforce through a cantilever effect, areoften seen in specialty designs, suchas split seal and slurry seals (Figure3). These springs are non-clogging,have relatively short axial space re-quirements, and can offer increases inmotion capability to compensate forlarge axial shaft movement commonin large equipment.

Bellows springs are available invarious elastomers and metals. Theelastomeric bellows typically offer alower-cost option while metal bellowscan offer performance advantagesand greater reliability in aggressive-chemical and high-temperature appli-cations.

3. Stationary and rotary seals. Sta-tionary seals — seals that incorporatethe spring mechanism in the station-ary component of the seal — are in-creasing in popularity. These seals au-tomatically compensate for any lack ofsquareness of the stuffing box face tothe shaft centerline. Thus, when thestuffing box face is not perfectly at a

Spring mechanism does not rotate Spring mechanism rotates

Rotary components

Stationary components

Springs

Springs

Process fluid pressure

Soft seal face wears

Shaft seal is “pushed” by springs

FIGURE 4. Springs mounted in the sta-tionary part of this stationary spring sealassembly automatically compensate forany lack of squareness between thestuffing box face and the shaft

FIGURE 5. Rotary seals do not easilycompensate for a stuffing box face thatis not perfectly square to the shaft cen-terline

FIGURE 6. Newer pusher seals pushthe elastomer along a micropolished,non-oxidizing surface rather than thanagainst the shaft, as shown here

FIGURE 8a and 8b. Split seals have all of their components split into two identicalhalves, simplifying assembly and disassembly. Use of split seals is increasing inpoularity for a large variety of equipment, such as mixers and large pumps

FIGURE 7. Bellows seals compensatefor movement by compressing or ex-tending a bellows, which eliminates theneed for a dynamic elastomeric element

right angle to the shaft, stationarysprings seals easily compensate forthis without compromising seal relia-bility (Figure 4).

Rotary seals have their springs lo-cated in the rotary component of theseal and do not have the ability tocompensate very well for any lack ofstuffing box squareness. The springson a rotary seal will try to compressand extend with every shaft revolu-tion. Pump shafts typically rotate at25 to 60 times per second. At thesespeeds, rotary seals can’t always reactfast enough, so reliability is reduced.

Higher speeds, larger equipmentwith greater tolerances and higher-temperature pumps that can distortdue to temperatures all create moreproblems for rotary seal designs, mak-ing it hard for these designs to delivergreater reliability than stationary de-signs (Figure 5).

4. Balanced or unbalanced seals.Unbalanced mechanical seals are sealarrangements in which the hydraulicpressure of the seal chamber acts onthe entire seal face area without anyof the force being reduced through theseal design. Unbalanced seals usuallyhave a lower pressure limitation thanbalanced seals.

Among the common problems associ-ated with unbalanced seals are these:

• If the pressure acting on the face ishigh enough, seal face lubricationmay be compromised

• These seals tend to have higher heatgeneration than balanced seals be-cause there is excessive closing forceapplied in an attempt to keep theseal faces together

• More rapid face wear occurs becauseof the higher closing forces

• Higher power consumption occursbecause of the extra drag caused bythe higher closing forces

A balanced mechanical seal arrange-ment reduces the hydraulic forces act-ing on the seal faces through mechan-ical seal design. As the seal faces rubtogether, the amount of heat gener-ated is determined by the amount ofpressure applied, the lubricating filmbetween the faces, the rotationalspeed, and the seal ring materials.

Balanced seals reduce the seal ringarea on which the stuffing box pres-sure acts. With the reduction in area,the overall closing force is diminished.This allows for better lubrication, re-sulting in reduced heat generation,face wear and power consumption,compared to unbalanced seals.

Balanced seals typically have higherpressure limitations than unbalancedseals. Some standard balanced sealsrun extremely cool and have pressurelimits of 450 psig (30 bar g).

5. Pusher and bellows seals. Pusherseals are those seals that move anelastomeric element such as an O-ringalong a surface to compensate for wear(Figure 6). These seals perform reli-ably in the vast majority of process flu-ids. While the availability of elas-

tomeric elements made from materialssuch as ethylene propylene, fluorocar-bon and perfluorocarbon compoundsoffers the user a wide range of chemi-cal compatibility options, seal reliabil-ity may still be compromised by thechemical compatibility issues, so closeattention to this is required duringspecification.

Bellows seals compensate for move-ment by compressing or extending abellows. Be providing both sealing andmovement, the bellows eliminates theneed for a dynamic elastomeric ele-ment (Figure 7).

Sealing elements are required inother areas of the mechanical seal.However, with bellows seals, thesesealing areas are all static and allowfor the use of non-elastomeric seals,such as graphite gaskets; thus, theyare also less sensitive to elastomericchemical compatibility issues.

6. Split/Non-split seals. Non-splitseals, such as those shown in Figures1, 2 and 3, require equipment disas-sembly to install, remove and reinstallthe seal. On some equipment, such asmixers and large horizontally splitcase pumps, this can be quite difficult.

By comparison, split mechanicalseals, such as those shown in Figures8a and 8b, have all of their compo-nents split into two equal halves. Splitseal designs are now available inmany configurations for sealing mix-ers, high-pressure pumps and dryers.

Classification by application1. Pump or mixer seals. Centrifugalpumps typically incorporate an im-peller that is either overhung but still

Process fluid

Metal components contact process fluid

Process fluid

Metal components do not contact

process fluid

FIGURE 10. Metallic seals require the metal to be compatible to the process fluid

FIGURE 11. Nonmetallic seals are oftenused on pumps handling such harsh flu-ids as hydrochloric acid, as an inexpen-sive alternative to exotic metallurgy seals

FIGURE 9. Mixers can be difficult toseal effectively, due to large vibration-in-duced shaft movements they typicallyexperience

Cartridge mixer seal eases assembly

Long cantilever shaft

Radial shaft movementfrom mixer blades

relatively close to two sets of bearings,or is mounted between the bearings.In both cases, the mechanical seal ismounted relatively close to the twosets of bearings. Shaft movement inthe radial direction is relatively lim-ited, requiring less than 0.016 in. (0.5mm) of runout capability in the seal tocompensate for the shaft movement.However, in large mixers, the seal ismounted much further away from thebearings. Large mixer blades can in-duce vibration resulting in largeamounts of shaft movement, requiringthe seal to compensate for as much as0.125 in (3 mm) of shaft runout ormore (Figure 9). Also, large mixersalso typically require the mechanicalseal to compensate for shaft axialmovement or growth due to thermalexpansion. Designs are available thateasily allow for increased axial and ra-dial shaft motion without affectingspring force.

Mixer shafts typically enter into thetop of the vessel; therefore the me-chanical seals used in these applica-tions are not lubricated by the processfluid, but run dry. A dual seal is com-monly used to supply external lubrica-tion although dry-gas and dry-run-ning seals are also being used. If shaftrunout is large enough to threatenseal reliability, then an integral bush-ing or bearing can be incorporatedinto the seal design. Split mixer sealsare gaining in popularity as they en-able users to eliminate mixer disas-sembly to change out the seal.

2. Metallic versus nonmetallic seals.A metallic seal in described as suchnot because it is made of metal. It is

described in this way because it hassome metal parts that are in contactwith the process fluid. If any metalparts are in contact with the processfluid, then the device is called a metal-lic seal (Figure 10).

Similarly, a nonmetallic seal is de-scribed as such not because is has nometallic parts. Rather, nonmetallicseals have no metal parts in contactwith the process fluid (Figure 11) Onlythe nonmetallic seal faces, O-rings andgaskets contact the fluid process fluid.

Nonmetallic seals can offer advan-tages over metallic seals in fluidswhere common metallurgy may be at-tacked by such common fluids as sea-water, brine and strong acids. Non-metallic seals may also offer a low-costalternative to specialty metallurgies,such as those based on titanium. How-ever, most nonmetallic seals havelower pressure and temperature lim-its when compared to metallic seals,thus limiting their use.

3. High-temperature seals. Temper-ature limits on mechanical seals arebased on any elastomers that areused. High-temperature perfluorelas-tomers are typically suggested for reli-able use at process temperature limitsbelow 500°F (260°C). At low tempera-tures (beyond –45°F or -45°C), elas-tomer flexibility is not adequate to en-sure dynamic sealing. Applicationsencountering these temperature ex-tremes require seals that have noelastomers in contact with the fluid.For such cases, metallic bellows sealsthat use non-elastomeric sealing ele-ments such as graphite work reliablywell (Figure 12).

4. Single versus dual seals. A singleseal has one set of seal faces. One sealface rotates while the other remainsstationary. Pressed against each otherby a spring mechanism and hydraulicforce, the seal faces contact each otherand are lubricated by the process fluid.The downside is that in any singleseal, there is always a process fluid-to-atmosphere interface at the seal faces.At this interface, fluid vaporization,crystallization and oxidation mayoccur, compromising seal reliability.

By comparison, a dual seal has twosets of seal faces and incorporates ei-ther a barrier fluid or a buffer fluid(Figure 13). This fluid acts as a bar-rier between the process fluid and theatmosphere. By elimination of this in-terface, seal reliability can be in-creased. In addition, seal leakage willbe to the buffer fluid rather than tothe atmosphere, ensuring greatersafety for applications involving haz-ardous fluids.

Dual seals are used where they offergreater reliability than single sealsand greater safety. Dual seals can alsoprovide a backup or spare seal capa-bility to minimize emergency shut-down and maintenance activity, byacting as an installed spare seal.

Meanwhile, a dual seal can easily actas two independent seals mounted onone shaft. This can be useful in batchprocesses where one seal can seal thefluid while the other idles on the bufferfluid. The inboard set of faces does thehard work and when it begins to leak,the outboard set of seal faces takesover, allowing for extended runtimes.This approach not only allows for im-

FIGURE 12. With no elastomers, this metallicbellows seal can handle a wide range of tempera-ture extremes, sealing effectively to temperaturesabove 500°F (260°C) and below 45°F(45°C)

FIGURE 13. Dualseals have two

sets of seal facesfor added reliabil-

ity and safety

Outboard seal

Pressurizedbarrier fluid

tank

Inboard seal

Graphite seals eliminate elastomers

proved process operation, but it also al-lows the maintenance engineers toshift from reactive to planned mainte-nance to manage seal changeouts.

Dual seal classifications:a. Tandem seals. A tandem design is

sometimes referred to as an inline de-sign. It uses two sets of seal faces thatare mounted as in series on a shaft(Figure 14). Many of today’s high-per-formance pump seals use thisarrangement as it offers the best com-promise of pressure capability andparticulate handling. However, tan-dem seals are typically larger in sizeand are more difficult to fit in verysmall equipment.

b. Back-to-back seals. Back-to-backseals use seal faces that are opposedto each other (Figure 15). The springmechanism is located in the center ofthe seal, and pushes one face inboardand the other outboard. The barrierfluid is typically located on the outerdiameter of both sets of seal faces.

A downside of this design is thatsince the inboard is pushed towardthe process fluid, it can get stuck orhang up and lose sealability. In addi-tion, the inboard seal faces are insidethe design, which can be problematicwhen sealing fluids that contain par-ticulates. The back-to-back design isstill widely used in top- mounted mix-ers and reactors, as it is an ideal con-figuration for sealing high pressure,clean fluids and vapors.

c. Face-to-face seals. This design in-volves seal faces that are pushed to-ward each other. These seals typicallydo not have a pumping ring to circu-late the barrier fluid and must rely onconvective flow (Figure 16).

All double seal types should incor-

porate a double balance feature for theinboard seal. This allows the seal tooperate reliability regardless ofwhether the barrier/buffer fluid is athigher or lower pressures compared tothe process pressure. This is critical tomaintain reliability and safe opera-tion during operating pressure tran-sient conditions.

5. Wet lubricated seals or gas seals.Wet lubricated seals operate in themixed lubrication regime. The processfluid between the faces carries some ofthe load on the seal faces. At the sametime, the seal faces are also in contactand thus generate heat and wear. Wetlubricated seals require lubricationfrom the process liquid, or a pressur-ized barrier fluid or an unpressurizedbuffer fluid. A wet dual seal with apressurized barrier lubrication sys-tem requires a barrier liquid that iscompatible with the process fluid.Small amounts of leakage and heatgeneration at the contacting sealinginterfaces must be tolerated.

A dual seal with a buffer fluid sys-tem is typically used on vaporizingfluids. The seal buffer tank can thenbe connected to a central vapor-recov-ery system.

A typical gas seal design uses hydro-dynamic lift-off to separate the rotaryand stationary seal faces. Spiralgrooves in the rotary seal face collectthe gas. As the seal rotates, gas is com-pressed toward the end of the groove,creating an opening pressure. Thispressure exerts an opening force that isgreater than the closing force separat-ing the seal faces. This slight separa-tion allows the gas, typically inexpen-sive and inert nitrogen, to flow acrossthe seal faces. Thus, the seal faces ride

on a pressurized, gaseous fluid film. Unlike the case with wet lubricated

seals, gas seal use is not limited by thetribological characteristics of thesealed fluid, because of the inert nitro-gen gas that separates the seal faces.Heat generation is non-existent, sothat potential mode of failure is notrelevant here. Vaporizing and non-lu-bricating fluids are easily sealed.Process upsets are tolerated due to thestiff gas sealing film and reliable con-ventional sealing capabilities offeredby these designs. Expensive vapor re-covery systems used with buffer fluidsystems are not required.

Advances in gas seal design havefocused on enhancing the reliabilityand practicality of using such designs(Figure 17). Because the gas is di-rected right into the sealing inter-face, numerous opportunities are cre-ated for the seal design engineer. Gasseals using newer technology are ableto combine both hydrostatic and hy-drodynamic opening forces. This al-lows for a stiffer, self-regulating gasfilm (Figure 18). In addition, closingforces can be hydrostatically con-trolled during operation allowing forbetter sealing control during equip-ment operation.

Older style gas seals typically useremotely mounted gas control panels,which are mounted on a platform nearthe pump to adjust gas pressure as re-quired. As pump and mixer applica-tions have variable operating pres-sures, the gas pressure must often beset at a pressure greater than maxi-mum operating pressure, resulting ingreater nitrogen flows at normalequipment pressures.

By comparison, newer designs elimi-

FIGURE 14. Tandem seals offer an ideal configuration, as they act like two seals mounted on one shaft

FIGURE 15. Back-to back seals are not ideal inparticulate or slurry service, since solids can accu-mulate at the inside diameter of the inboard seal

Process fluid

Inboard seal Outboard seal

Barrier fluidl

Identical seal rings setsmounted in-series

Process fluid Inbound

sealOutbound seal

O-ring slides inboard toward process fluid

Middle seal rings mounted back-to-back

Barrier fluid Dynamic o-rings

nate the remote gas panel arrangementby using an in-gland control systemthat regulates the gas supply pressurewith respect to process pressure.

Sealing mixers, agitators, reactorsand other slowly rotating shafts pre-sent particularly challenging designissues for gas seals. Typically, suchequipment has long, unsupportedshafts, that result in large amounts ofmovement. And they often operate atcircumferential speeds that are toolow for hydrodynamic lift-off. How-ever, as the sealing device location forsuch equipment is typically at the topof the vessel, they can benefit greatlyfrom gas sealing as a gas seal will nat-urally operate in a dry environment.

As emission regulations tighten onagitator vessels, dry running singleseals and braided packing are nolonger a viable option for many fluids.The other sealing option for thisequipment has been the use of conven-tional dual wet lubricated seals anddual split seals. While these sealingarrangements can provide longtermoperating reliability, they both rely ona pressurized barrier fluid arrange-ment when mounted at the top of avessel. Barrier fluid leakage will even-tually migrate and mix with theprocess fluid. This may not be an ac-ceptable or desirable alternative inmany high-purity processes.

Meanwhile, mixer shafts with largeradial runout can create unstable op-eration in gas seal designs that relysolely on hydrodynamic lift-offgrooves. By combination of a hydro-static pressure with the hydrody-namic lift-off, a dual compensationsystem is created, minimizing unsta-ble operation.

Mixer shafts operate at speeds belowthose typically required for hydrody-namic lift-off and separation of the sealfaces. Special lift-off grooves developedfor the specific application are typicallyrequired. These special designs requirea minimum circumferential speed to bereached for lift-off to occur. At speedsbelow this, contact occurs, causingwear and possible process contamina-tion from wear debris. In addition, gasconsumption can be high on some de-signs due to high gas differential pres-sure requirements.

Some seals use a combination of hy-drostatic and hydrodynamic pressuresto achieve lift-off. Stable lift-off caneasily be achieved from zero rpm tospeeds as high as 5,000 rpm. Speciallift-off grooves are not required.

The split dry gas seal is ideal for seal-ing critical equipment that is difficult todisassemble. As mentioned above, non-split seals require equipment disassem-bly. As many mixers and reactors arelarge, their disassembly can be timeconsuming and expensive. This, cou-pled with the fact that they do not haveback-up spares, make them critical to aplant’s operating efficiency.

The split dry gas seal ideally willhave a lift-off speed of zero rpm due toits use of both hydrostatic and hydro-dynamic lift-off. This minimizes wearand allows for both very-low-speedand very-high-speed operation. Theseal faces should be resilientlymounted (discussed below) for maxi-mum reliability.

Seal featuresOnce a set classifications has beenidentified for the equipment orprocess fluid, it is important to iden-

tify specific features that are desirablein a mechanical seal. Some features toconsider are discussed below.

Resilient mounting. Older seal de-signs mount seal faces directly intometal housings. However, even withmodest increases in temperature, themetal housing expands at muchgreater rates then the seal material,causing the face to lose its flatness.Research and experience has shownthat this distortion has a negative im-pact on reliability and leakage control.Single piece, monolithic seal faces areused to prevent the thermal distortionthat results from differential rates ofthermal expansion.

Today, most seals are resilientlymounted to minimize the impact oftransient axial and radial loads. Re-silient (or cushioned) mounting re-duces the negative impact of equip-ment vibration. Such designs not onlyreduce transmission of vibration tothe seal face but also reduce the im-pact of high torque that can occur dur-ing startup, especially with viscousfluids. Seal face distortion from axialsealing pressures are also minimizedwith cushioning.

Double-balanced capabilities. Dualseals use a barrier fluid that is set at ahigher pressure when compared tostuffing box pressure. They can alsouse a buffer fluid that is set a lowerpressure. But often during the life ofthe seal, pressures fluctuate, requir-ing the seal to operate in a “reverse”pressure mode. Double-balanced dualseals perform reliably during pressurereversals, unlike single-balanced dualseals that may fail. Double balancecan be created by O-ring shift, whichrequires wider seal faces, or by using

Common stationary

Barrier fluid

Process fluid

Inboard seal Outboard seal

Limited barrier fluid circulation

Compact seal gland

Cartridgeseal sleeve

Cartridgelock ring

Nitrogen gas port

FIGURE 16. Face-to-face seals are often limited due to their lack of barrier pumping capability

FIGURE 17. Gas seal usage is increasing as newerdesigns enhance reliability and user friendliness

geometric double balance, which iscreated by establishing two bal-ance areas with two O-rings. Dou-ble balance created by shifting O-rings use extra wide seal facesthat generate high amounts offrictional heat.

Modularity. Most recent seal de-signs claim a high degree of modu-larity to allow for common part use.However, such modularity often ben-efits the manufacturer, not the enduser. In some cases, the design of theseal is sacrificed to increase the useof common components. In fact, oneof most common areas for such sacri-fice is one of the most critical aspectsof any seal — the seal face width,which has the greatest impact on theamount of frictional heat generatedat the seal face. Such heat generationshould be kept to a minimum, as itincreases the likelihood of processfluid vaporization, greater leakage,inadequate lubrication, polymeriza-tion, oxidation, crystallization andelastomer failure.

In many seal designs, the inboarddual seal face has to be extra wide toallow for double balance design as dis-cussed earlier. When this is done andthat same seal face is also used on thesingle seal and outboard of the doubleseal, the entire seal line performancehas been sacrificed to aid in commonparts usage. This wider seal face gen-erates more frictional heat and is nowused throughout throughout the sealsolely for modularity purposes. Thesemodular seal designs can be easilyidentified as they will have their max-imum pressure capabilities reduced to300 psi (20 bar) maximum due to theuse of a wide seal face and its result-ing higher heat generation.

Some models do utilize modularityin such a way to benefit the end user.For instance, some designs can easilybe swapped over from a single seal toa dual seal using cassette technology(Figure 19). The cassettes easily slideinto a common seal gland, to enablethe user to easily move from one sealtype to another.

Responsiveness. First-generationseals in many instances wore groovesinto equipment shafts, causing dam-age and calling for the use of equip-ment sleeves. These fretting grooves

also cause seal failure due to lack ofresponsiveness.

Second-generation seals, whichoften incorporated metal sleeves intothe their design, allowed for the use ofstronger, solid equipment shafts.However, grooves resulting in sealfailure were still created.

Third-generation seals eliminatefretting grooves by the use of hard,non-oxidizing, seal face materialssuch as silicon and tungsten carbideinstead of softer oxidizing metals suchas stainless steel. In addition, thesesurfaces are now micropolished so sealresponsiveness and reliability aremaximized (Figure 20).

Seal environment. Changing the en-vironment around the seal can en-hance performance. Most seals aremounted inside stuffing boxes de-signed for packing. Seal chambers areincreasing in popularity and are avail-able in numerous design configura-tions from taper bores to cylindrical oreven C-shaped chambers. Another op-tion is to use special devices mountedin the bottom of the stuffing box orseal chamber, which modify the fluid

flow around the seal. These devicestypically use the centrifugal and cen-tripetal forces that result from fluidrotation inside the seal chamber toclean the seal chamber of particulatesduring operation (Figure 21).

There are many factors to considerwhen selecting a mechanical seal.Choosing the best seal for a given ap-plication requires a complete under-standing of the advantages and disad-vantages of each seal design. �

Edited by Suzanne Shelley

AuthorScott Boyson is the businessdevelopment manager for theA.W. Chesterton Co. (860Salem St. Groveland, MA01834; Phone 781-738-1905;Fax: 781-481-7060; Email:boysons@chesterton.com). Heholds a B.S. degree in me-chanical engineering fromNortheastern University anda Certificate in GraduateStudies in Business Adminis-

tration from Harvard University. Since gradua-tion, he has worked as a project engineer forChesterton's mechanical seal and centrifugalpump divisions. As a training coordinator withChesterton, Boyson has developed and deliveredmechanical seal courses to thousands of endusers worldwide. As a business developmentmanager, he works closely with major chemicalcompanies to increase efficiency and reducecosts by supplying innovative improvements.

FIGURE 20. Micro-polished sur-faces enhance seal responsiveness,especially under transient conditions

FIGURE 21. By seamlessly changing theenvironment around the seal in the stuff-ing box, seal reliability can be dramaticallyincreased

FIGURE 18. (above) Gas seals operate on a fluid film of inert nitro-gen or clean gas rather than liquid

FIGURE 19. (right) Cassette sealinggives the user the full benefits of mod-ularity

Non-split seal

chamber devices

Split seal

chamber device

Reprinted on behalf of seepex, Inc. Chemical Engineering, December 2004. '2004 Access Intelligence LLC