An ounce ofprevention isworth a poundof cure.

Rotating labyrinth seals (commonlycalled “bearing protectors” and “bearingisolators”) can be found in many OEMand retrofit applications. Bearing isola-tors are an improvement over the olderstationary labyrinths. They come in differ-ent versions, however, and some may notperform as expected.

Pressure-relief poppet valves aremarketed for the bearing housings ofcentrifugal pumps and other machinery.It can be shown that they are rarely, if ever,of any merit.

Finally, a new generation of on-linemoisture-monitoring devices and desic-cant-containing absorber cans are offeredby competent manufacturers. They areintended for mounting at the top of thevapor space of pump bearing housingsbut are not always cost-justified.

Rotating-Labyrinth Bearing-Housing Seals

It has long been recognized that as manyas 91% of all the rolling element bearingsinstalled in the world’s machinery fall shortof reaching the manufacturer’s calculatedL-10 life. L-10 is defined as the numberof operating hours at which 10% of anidentical bearing population will eitherhave failed, or will exhibit visible ormeasurable damage. Simple subtractionreveals that 90% of bearings should beexpected to survive to L-10 life. Researchand follow-up analysis have establishedlubricant contamination as the predomi-nant cause of failure.

Much airborne contamination finds itsway into bearing housings throughopenings where shafts protrude through

bearing housings, or at vents and breatherson the lubricated assembly. Bearinghousings undergo temperature shiftsbetween day and night, and in operat-ing/idle cycles. With increasing temper-atures, the vapors above the liquid oil willexpand and, with decreasing tempera-tures, they will contract. In a closedvolume, increasing temperatures causepressures to go up, while decreasingtemperatures cause pressures to decrease.

In an effort to reduce the pressure-related, in-and-out flow of contaminatedair, conventional labyrinths are oftenreplaced by rotating-labyrinth seals (whichwe will describe as protector/isolators, forshort. A typical version is shown in Fig. 3). These devices are designed withinherent clearances, so an air gap separatesthe rotating and stationary elements.Except when the clearance is bridged byan oil film, this gap is large enough to allow the exchange of air with its ever-present contaminants, water vapor andairborne dust.

It has been pointed out that bearingprotector/isolators work best when thehousing vent is plugged. To quote oneprominent manufacturer (Ref.1):

“If the housing vent is left open, theslight vacuum created by the contam-inant expulsion elements will inducethe flow of airborne dust, dirt, vaporsand everything available in theimmediate environment through thebearing enclosure not unlike an oil-bath vacuum cleaner. This action isconstant and the amount of induceddebris build-up can be significant.”

As Published in PUMPS & SYSTEMS- October, 2001

Bearing Protection Devices and Equipment Reliability:Part II – What is Really Justified?

By Heinz P. Bloch, P.E, Consulting Engineer,Process Machinery Consulting

Bearing isolators fitted with dynamic O-rings (Fig. 4) try to close the gap throughwhich airborne contaminants can enter thebearing housing. The expectation is for theO-ring to effectively seal off the gap at stand-still (Ref. 2). The designer/manufacturerhopes that centrifugal force, acting on therotating O-ring during operation, will causethe ring to lift off sufficiently to avoid thescraping and galling wear modes noted oncircumferentially contacting dynamic O-rings (such modes are why O-ring manufac-turers do not recommend high-cycle,dynamic, circumferential sealing applica-tions). Bearing protector/isolators equippedwith dynamic “vapor blocking” O-rings arelikely to outperform isolators that do notincorporate a dynamic O-ring, in that theydo prevent air intrusion on shutdown.Current production of these componentsexceeds 175,000 per year (Ref.2).

Many practicing engineers, however,have voiced concerns with mistaken claimsthat these devices provide “hermetic”sealing. Reliability professionals correctlyreason that if the O-ring does lift off, therestill will be a gap through which contami-nated air moves. Conversely, if there is nogap, there will be wear. Thus, contrary towritten claims dating back about a decade,even bearing protector/isolators designedwith dynamic O-rings will not achievehermetic sealing during operation.

Hermetic Sealing is the Preferred Solution

Hermetically sealing the bearing housingimplies that nothing enters and nothingescapes. Only face-to-face sealing devicesmeet this definition. In view of the gener-ally limited axial space between bearinghousings and fluid casings of centrifugalpumps and other machinery, narrow-width,magnetically closing face seals have beendeveloped in recent decades.

Whenever there exists a thin film of cleanoil between either spring-activated ormagnet-activated seal faces, long seal lifeand hermetic containment of the lubricat-

ing fluids result. Tens of thousandsof the magnetic face seals shownin Fig. 5 have been used in aircrafttask pumps, as aircraft generatorseals, and on vertical stabilizerunits (Ref.3). They can toleraterubbing velocities as high as 86 m/s(17,000 fpm) and temperatures to200 °C (392°F), and havefrequently served to everyone’ssatisfaction for over 50,000 operat-ing hours. These seals haveperformed equally well in suchindustrial applications as gundrills, gearboxes and pumphousings. They use a single Al-Ni-Co magnet annulus to attract theopposing seal face.

A rather similar seal (Fig. 6)incorporates a series of strong rare-earth magnetic rods to attract the opposing face.Superior face-material combina-tions achieve coefficients of friction that, even without lubri-cation present, rival those of PTFE.The stationary seal face of theproduct in Fig. 6 (also shown ininsert, Fig. 8) has a diamond-likehardness (RC90, Ref.4).Thousands of machines in

“To this day, some important lubricant-application and bearing-protection issues are overlookedor misunderstood in the typical process-plant environment.” In Part I of this series concerningthose issues, the author compared the types of available constant-level lubricators, and found thepressure-equalized (PEL) designs superior to those that were “open-to-atmosphere” (OTA). In thisconclusion, the noted lubrication expert reviews various contamination-prevention and pressure-relief alternatives, with focus on cost and reliability-based justifications.

Figure.4: Bearing protector/isolator with dynamic O-ring (Source: Inpro/Seal Company, Milan, Illinois; www.inpro-seal.com)

PUMPS & SYSTEMS www.pump-zone.com OCTOBER 2001

Figure.3: Rotating labyrinth seal (bearing protector/isolator)(Source: Inpro/Seal Company, Milan, Illinois; www.inpro-seal.com)

different industries have been fitted with thisparticular type of seal, and yearly produc-tion exceeds 60,000 (with primary appli-cations in centrifugal pumps in hydrocarbonprocessing and related industries).

Figure 7 shows a magnetic seal whosecontacting faces are pushed together by therepelling action of rod magnets of like polar-ity. Although repulsion magnetic sealsembody certain advantages over the pullingconfigurations they are more expensive tomake and take up more axial space. Veryfew of these seals have been sold since theirintroduction in 1992, and only a few dozenwere produced in 2000.

Magnetic seals obtain lubrication andcooling from the ever-present oil fog thatsurrounds oil-lubricated bearings. Properlydesigned, using appropriate face materialsand applying suitable selection criteria, theyrepresent the ideal choice of bearing-housingseal to prevent both egress of lube oil andingress of atmospheric contaminants. Shouldthe temperature-dependent pressure insidea non-vented bearing housing rise above themagnetic closing force, the seal would releasepressure by opening and immediately re-closing.

Magnetic seals of the typeshown in Figs. 5 and 6 willperform flawlessly, either withpressure-balanced constant- levellubricators or non-vented bearinghousings, and they are ideal forclosed, environmentally accept-able oil-mist lubrication systems.In a closed, oil-mist application,the oil mist is introduced in thespace between magnetic seal andbearing (Ref.5). Excess liquid orvaporized oil is led off or collectedat the bottom-center location ofthe bearing housing (Fig. 8, andRef.6).

Pressure-Relief PoppetValves are Rarely Needed

Pressure-relief poppets, similarto the one shown in Fig.9, havebeen offered as retrofit items forbearing housings of centrifugalpumps and other machinery. Theweight of the ball exerts adownward force on the seat.Whenever the product of pressureinside the bearing housing multi-plied by the exposed area (= upward force) exceeds the

downward force on the seat, the ball willunseat itself and will allow excess internalpressure to escape to atmosphere.

A steel ball of 3/8 inch (9.5 mm) diame-ter weighs 0.00646 lb (2.9 gm). Assumingan exposed area of 0.11 sq. in., it would takea pressure increase of 0.059 psi [or 1.6 in. (40 mm) of H2O column] over atmos-pheric to unseat the steel ball (although PVCballs would relieve at lower pressures).

If equipment is fitted with labyrinth sealsor bearing protector/isolators (either withor without O-rings), slightly negativepressuresmay actually exist inside the bearing housingdue to pumping action brought on by thecontaminant expulsion elements (Ref. 1).Bearing protector/isolators with designs thatavoid such pumping action have a liberalseparation between rotating and stationaryelements, and will provide pressure equal-ization between housing interior and thesurrounding, external atmosphere. Consis-tent pressure build-up can occur only in trulyhermetically sealed bearing housings,(i.e.,with magnetic seals). However, lightly loadedseals with face orientations per Fig. 8 would

“burp” before the poppet would release. Only heavily loaded magnetic face sealswould benefit from poppet relief valves orexpansion chambers (described below).Experience shows that the seal of Figs. 6 and 7 will not need either device.

Expansion Chambers:Use where Needed

Expansion chambers (Fig. 10) aredesigned to absorb the expansion of gasesor fluids in closed (hermetically sealed)systems. A rolling diaphragm provides avariable volume that, when correctly dimensioned, will reduce pressure buildup.On some seal models, this will extend seal life or prevent “burping.”

Moisture Monitors and Desiccant Cans

There is ample evidence that free waterin lubricating oils seriously curtails bearinglife. Water vapor, from in-leaking moist air,will condense once saturation limits areexceeded. Since the 1990’s, competentmanufacturers have offered on-line monitor-

ing devices, capable of annun-ciating maximum safe levels ofrelative humidity (Ref. 7). Also, add-on air dryers are now available.

Desiccant technology works.It has been around for hundredsof years and packets of a chemi-cally suitable desiccatingsubstance are found in thepacking containers of cameras,sunglasses, kitchen appliances,etc. At issue is the technical andcost justification of an air dryerrequiring replacement upon colorchange and, thus, future mainte-nance. Smart companies designaway maintenance, not add to it.On the overwhelming majorityof pumps and similar processplant machinery, it makes farmore sense to invest in failureavoidance (i.e. preventing themoisture intrusion), rather thaninvesting in either moistureremoval or moisture annuncia-tion. Hermetic sealing is usually the right strategy, and renders measures to remove moisture unnecessary.

Figure.5: Magnetic seal used in aerospace applications (Source:Magnetic Seal Corp., Warren, RI; info@magseal.com)

Figure.6: Magnetic seal applied in process equipment (Source: IsomagCorporation, Baton Rouge, LA; isomagcorp@aol.com)

PUMPS & SYSTEMS www.pump-zone.com OCTOBER 2001

There are, however, applications wheremoisture monitoring and/or desiccant-basedmoisture removal makes technical andeconomic sense. Machine components thatboth rotate and undergo axial movement,or gearbox installations where hermetic sealingis not practical, are where moisture monitor-ing and removal often are easily justified.

Conclusions are Supported by Basic Physics • A reliability-minded process plant will

give serious consideration to upgradingfrom the “traditional” non-balancedconstant-level lubricator. Pressure-balanced configurations are clearlybecoming the norm among Best-Of-Class performers.

• Bearing protector/isolators performbetter than lip seals and/or labyrinthseals.

• Bearing protector/isolators withdynamic O-rings perform better thanthose without the O-ring.

• Bearing protector/isolators with expul-sion vanes have been known to createsmall pressure differences that promotethe outward leakage of oil.

• Magnetic bearing-housing seals are acost-effective means of precluding thealternating, in-and-out movement ofairborne contaminants.

• Magnetic seals are the only practicalhermetic bearing-housing closure.Hermetic sealing optimally extends thelife of lubricants and bearings. Preclud-ing lubricant contamination also makesthe use of more expensive, but superior,synthesized hydrocarbon lubricants

economically attractive. • Magnetic seals render constant-level

lubricators obsolete. Constant-levellubricators are no longer needed inhermetically sealed bearing housings.

• Poppet relief valves are very rarelyneeded. Their usefulness is a functionof seal closing forces and must be deter-mined on a case-by-case basis.

• Moisture monitoring and removal are

of economic value in equipment whosegeometry and component featurespreclude the hermetic exclusion of waterfrom the bearing housing.

• Expansion chambers are low-cost,readily justified add-ons where unduepressure rises may either jeopardize thelife of hermetic sealing devices, or allowoutward leakage of lubricant.

Figure.7: Inpro “RMS 700” (three-piece) repulsion magnetic seal (Source: Inpro/SealCompany, Milan, Illinois; www.inpro-seal.com)

PUMPS & SYSTEMS www.pump-zone.com OCTOBER 2001

Fig.8: Magnetic seal hermetically sealing-in oil mist applied to moderncentrifugal pump bearing (Source: Isomag Corporation, Baton Rouge, LA;isomagcorp@aol.com)

Fig.10: Bearing housing expansion chamber (Source: Trico Mfg. Corp.;Pewaukee, WI; www.tricomfg.com)

Fig.9: Pressure relief poppet valve

FIRST-CLASS MAILU.S. POSTAGE

PAIDPEWAUKEE, WIPERMIT NO. 720061235 Hickory Street

Pewaukee, WI 53072

There are compelling reasons, then, toadhere to the old dictum: “An ounce ofprevention is worth a pound of cure.” Wehave the means and the knowledge toreduce the risk of bearing failure. Superiorconstant-level lubricators prevent pressure-driven oil loss. Hermetically sealing thebearing housing is feasible, and preventsthe intrusion of water and airborne dust.Expansion chambers protect certainhermetic seals against pressure-induceddecreases in anticipated life. Desiccant-based means of moisture removal and costlymoisture monitoring devices will not beneeded once the well-known water entrypassageways have been closed (in a provenmanner, which duplicates the sealing action of literally hundreds of millions ofmechanical face seals). However, moisturemonitoring and removal are important reliability-improvement measures in equip-ment where moisture intrusion cannot beprevented by economic means. �

Heinz P. Bloch, P.E., is a consulting engineerand ASME Fellow with offices in Montgomery,Texas. He advises industry world-wide on relia-bility improvement and maintenance cost-reduction issues, and continues to teach in-plantcourses on all six continents. Before retiringfrom Exxon in 1986 after over two decadesof service, Mr. Bloch’s professional careerincluded an assignment as Exxon Chemical’sRegional Machinery Specialist for the UnitedStates, as well as machinery-oriented staff andline positions with Exxon affiliates in the USA,Italy, Spain, England, The Netherlands andJapan. He is the author or co-author of thirteentexts and over 200 technical papers andarticles. In his spare time, he functions as the

Reliability/Equipment editor of HydrocarbonProcessing Magazine and chairs the yearlyInternational Process Plant Reliability Confer-ence. Email him at hpbloch@juno.com.

(Editor’s Note: This excerpt is adaptedfrom “Constant Level Lubricators and OtherBearing Protection Devices: Smart Retrofitswill Improve Equipment Reliability,” byHeinz P. Bloch, P.E.)

References:Ref. 1: Orlowski, D.C., Gibit Gambits,

Volume 1, No. 71, June 13, 1989Ref. 2: Inpro/Seal Corporation, P.O. Box

260, Milan, IL 61264Ref. 3: Magnetic Seal Corporation, 365

Market Street, Warren, RI 02885

Ref. 4: Isomag Corporation, 11871 DunlayAve, Baton Rouge, LA 70809

Ref. 5: Bloch, Heinz P. and Abdus Shamim;“Oil Mist Lubrication: PracticalApplications,” The Fairmont Press,Lilburn, GA, 1998

Ref. 6: Bloch, Heinz P.; “Practical Lubri-cation for Industrial Facilities,” TheFairmont Press, Lilburn, GA, 2000

Ref. 7: Rake, Brad; “Water Contaminationof Equipment-Lubricating Oil,”Pumps and Systems, January2001, pp. 28-35

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