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Constant level lubricators and other bearing protection devices: how smart retrofits will improve equipment reliability
Constant level lubricators and other bearing protection devices: how smart retrofits will improve equipment reliability To this day, some i m p o r t a n t lubricant application and bearing protection issues are still being overlooked or misunderstood in the typical m o d e r n process plant e n v i r o n m e n t . Satisfactory explanations are not f o r t h c o m i n g , leading the industry to loose faith in e q u i p m e n t reliability. :~ ~ ~ ~: ~ ', ~v!,. sees if he can find a solution. or many decades, constant level lubricators have been the standard lubricant supply system for huge numbers of bearing housings. They have been supplied for a wide spectrum of machines, including millions of centrifugal pumps, gear units, and electric motors equipped with oil-lubricated bearings. However, certain types of constant level lubricating devices can be misapplied and require an understanding of how they function. Obviously, when constant level lubricators are misapplied, bearings could be deprived of lubrication and might fail.
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A more recently developed machinery component, rotating labyrinth seals (commonly called 'bearing protectors' and 'bearing isolators') can now be found in many OEM and retrofit applications. Bearing isolators are an improvement over the old stationary labyrinth. Yet, they come in different versions and some of these may not perform as expected.
Then there are pressure relief poppet valves being marketed for the bearing housings of centrifugal pumps and other machinery. It can be shown that they are rarely, if ever, of any merit. And, finally, a new generation of online moisture monitoring devices and desiccant-containing absorber cans are now being offered by competent manufacturers. They are intended for mounting at the top of the vapor space of pump bearing housings but are not always cost-justified. This article will discuss where they do have merit and will comment on general application criteria this article will examine: • W h e n and why some constant level lubricators are often involved in equipment failures • Why some bearing housing seals, or bearing protectors/isolators may not fulfill their intended function
Figure 1. Unbalanced constant level lubricator (Source: Trico Mfg. Corp., Oewaukee, Wl; www.tricomfgco.com).
38
WORLD
PUMPS
A u g u s t 2001
0262 1762/01/$ - see front matter
• W h e n bearing housing pressure relief poppets serve no useful purpose • Where it will make economic sense to apply on-line moisture monitoring and/or moisture reduction devices.
Understanding different constant level lubricators To begin with, the user community needs to be aware of two distinctly different types of constant level oil lubricators available and in use on the majority of small machines. The most widely used 'open-to-atmosphere' (OTA) type is shown in Figure 1. It does not incorporate a balance line connection to the bearing housing and, if used on bearing housings with restricted venting provisions, may not adequately protect against catastrophic bearing failure. O n the OTA-type constant level lubricator shown here, the desired oil level is determined by the height setting of a wing nut threaded into a centrally located rod. The oil level at 'x' is open to the atmosphere and higher-than-atmospheric pressures in the bearing housing will cause level 'x' to rise, often to the point of overflowing, although the risk of creating an overflow condition can be minimized by using open-to-atmosphere constant level lubricators with extended-height surge chambers.
© 2001 Elsevier Science Ltd. All rights reserved
In any event, the level rise phenomenon is easily explained by Bemoulli's Law. With even a fraction of one psi pressure increase in the bearing housing, the fluid in the lubricator body near 'x' could rise in excess of one inch (25.4mm). Unless contained in a properly sized surge chamber, the rising fluid could easily run over the rim of the chamber, or reach an air vent passage occasionally found in certain models at an intermediate location somewhere along this chamber. Figure 2 shows a different constant level lubricator. A n internal O-ring or similar sealing means prevents the ambient atmosphere from reaching the oil level at 'x'. This closed-loop, pressure-equalized lubricator (PEL) incorporates a balance tube, or pressureequalizing line and gives full protection against airborne contaminants. Moreover, in the real world of machinery, pressure differentials can and will exist between the ambient environment and the housing-interior where lubricated components are mounted. There is evidence that installing bearing isolators and other narrow-gap housing closures increases the likelihood of potential problems with lubricators that had served perfectly well as long as not-so-close fitting labyrinth seals were being used. To overcome these potential problems, only PE"s and not OTA-type lubricators should be used in reliabilityminded plants. By providing a piping connection between the lubricator and bearing housing, lubricator port 'x' and bearing housing operate at the same pressure and the risk of oil leakage or unintended lowering of oil levels due to pressure buildup in bearing housings is eliminated.
W h a t makes housing pressures rise Note that the location of the wing nut in Figure 1 or slanted tube in Figure 2, determines the oil level in the bearing housing. Either the bottle height in Figure. 1, or the bottle-and-tube height in Figure 2 are usually being set with the machine not operating. At that time, temperature equilibrium
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X II
exists, i.e. the machine components, lubricating oil and surrounding environment are probably all at or near the same temperature. Most bearing housing vents, and especially small vents, offer a restriction that might allow a small amount of pressure build-up in bearing housings. Furthermore, it is also reasonable to expect that some oil will get flung into the closeclearance region 'B' typically fitted with lip seals or labyrinth seals (see insert, Figure 2). This oil now tends to bridge the gap between the rotating shaft and the surrounding stationary components; in other words, the oil has film strength, which makes it cling to surfaces. In that event, the trapped air above the oil level will constitute a closed volume. As this volume of air is being warmed by sun exposure or frictional heat generated in the bearings, its pressure will increase in accordance with the perfect gas law: (P2) = (P1)(T2/T1). Using Rankine temperatures and absolute pressures, the reader will readily see how relatively minor temperature increases may cause pressures to rise by amounts that cannot be ignored. Granted, as pressures go up beyond the rupture strength of oil films, the sealing oil film will be temporarily interrupted. The bearing housing seal will 'burp', or open up for a fraction of a second, but one of two undesirable events will take place:
lower surge chamber supporting the lubricator bottle (Bernoulli's law). The bottom of the bearing will be deprived of oil, or slinger ring immersion will no longer be sufficient for satisfactory lubrication.
Figure 2. Balanced constant level lubricator (Source: Trico Mfg. Corp., Pewaukee, Wl; www. tricomfgco, com)
2. Outward expulsion of oil will deplete the volume of oil available in the bearing housing and pressures in the lubricator bottle will adjust to the new equilibrium pressures. As the cycle repeats itself, more and more oil will be lost. Fortunately, the pressure-balanced constant level lubricator in Figure 2 does not introduce the same risk. As mentioned earlier, the oil levels 'X' at Figure 3. Rotating labyrinth seal locations inside the bearing housing or bearing and inside the lubricator are always protector/Isolator. exposed to identical pressures. The (Source: InprolSeal Milan, Illinois; problem is solved, and another step Company, ww~.inpro-seal.com).
1. The oil level will go down in the bearing housing and will rise in the
WORLD P U M P S
August 2001
39
FEAXURE *
contaminants, water vapor and airborne dust. r
It has been pointed out that bearing protector/isolators work best when the housing vent is plugged. To quote one manufacturer (Reference. 1):
:king Ring ng urll
Rot,
Figure 4.Bearing protector/isolator with dynamic O-ring. (Source: Inpro/Seal Company, Milan, Illinois,"
www.inpro-seal.com).
~ing Housing car magnetic mah
ating Ring
63 V2/ Pressure
sureP2 l
3#
A O-Ring
Figure 5. Magnetic seal Used in aerospace applications.
(Source: Magnetic Seal Corp., Warren, RI; info@rnagseal,corn).
Seat Case SeatRing
towards increased equipment reliability has been implemented. Disadvantages ? The pressure-balanced constant level lubricator probably costs a few pennies more, but the resulting failure risk reduction far outweighs any disadvantages, both perceived and real.
Rotating labyrinth bearing housing seals It has long been recognized that as many as 91% of all the rolling element bearings installed in the world's machinery fall short of reaching the manufacturer's calculated L-10 life. L-10 life is the number of operating hours where 10% of an identical bearing population will either have failed, or will exhibit visible or measurable damage. Research and followup analysis by bearing producers and users has
40
WORLD PUMPS
established lubricant contamination as the predominant failure cause. Much of this contamination finds its way into bearing housings through openings where equipment shafts protrude through bearing housings, or at vents and breathers provided somewhere on the lubricated assembly. Bearing housings undergo temperature shifts between day and night, or when operating vs. non-operating.
August 2001
With increasing temperatures, the vapors floating above the liquid oil level will expand, with decreasing temperatures they will contract. In a closed volume, increasing temperatures cause pressures to go up, while decreasing temperatures cause pressures to decrease. As a consequence of these findings and in an effort to reduce the pressurerelated alternating in-and-out flow of contaminated ambient air, conventional labyrinths (see insert, Figure 2) are often replaced by rotating labyrinth seals, which we will describe as protector/isolators, for short. A typical version is shown in Figure 3. It should be noted that these devices are designed with inherent clearances. In essence, an air gap separates the rotating and stationary elements. Except for brief time periods when the gap may be bridged by an oil film, this air gap is large enough to allow the constant exchange or inflow/outflow of atmospheric air with its ever-present
"If the housing vent is left open, the slight vacuum created by the contaminant expulsion elements will induce the flow of airborne dust, dirt, vapors and everything available in the immediate environment through the bearing enclosure not unlike an oilbath vacuum cleaner. This action is constant and the amount of induced debris build-up can be significant." Bearing isolators fitted with dynamic O-rings (Figure 4) try to close the gap through which airborne contaminants can enter into the bearing housing. The expectation is for the O-ring to effectively seal off the air gap at standstill (Ref. 2). The designer/ manufacturer is hoping that centrifugal force acting on the rotating Oring will cause this O-ring to lift off. In other words, the design objective is for the lifting distance to be sufficient to avoid the scraping and galling wear modes noted on circumferentially contacting dynamic O-rings. It is for these rather obvious reasons that O-ring manufacturers do not recommend high-cycle dynamic circumferential sealing applications. Nevertheless, bearing protector/isolators equipped with dynamic 'vapor blocking' O-rings are likely to outperform isolators that do not incorporate a dynamic O-ring. Current production of these components exceeds 175,000 per year (Reference .2). It should be pointed out that many practicing engineers have voiced concerns with mistaken c[aims that these devices provide hermetic sealing. Reliability professionals are correctly reasoning that if the O-ring does lift off, there will still be the gap through which contaminated air moves either in or out depending on temperature and pressure profiles. Conversely, if there is no gap, there will be wear. It can thus be shown that, contrary to written claims dating
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Figure 6. Magnetic seal applied in process equipment (Source: Isomag Corporation, Baton Rouge, LA; [email protected])
back about a decade, even bearing protector/isolators designed with dynamic O-rings will not perform 'hermetic' sealing duties.
Hermetic sealing is the preferred solution Hermetically sealing the bearing housing implies that nothing enters and nothing escapes. Only face-toface sealing devices meet this definition. However, in view of the generally limited axial space available between bearing housings and fluid casings of centrifugal pumps and other machinery, relatively narrow-width, magnetically closing face seals have been developed in recent decades.
Whenever there exists a thin fihn of clean oil between either springactivated or magnet-activated seal faces, long seal life and hermetic containment (Source: InprolSeal of the lubricating fluids results. Tens of Company, Milan, lUinois; thousands of the magnetic face seals wv~ac.inpro-seal.com). Figure 7. Inpro 'RMS 700' (Three-Piece) repulsion magnetic seal.
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42
WORLD P U M P S
~i
August2001
shown in Figure 5 have been used in aircraft task pumps, as aircraft generator seals, and on vertical stabilizer units (Reference 3). They can tolerate rubbing velocities as high as 86m/s (17,000fpm) and temperatures as high as 200 ° C (392 ° F) and have frequently served to everyone's satisfaction for over 50,000 operating hours. These seals have performed equally well in such industrial applications as gun drills, gearboxes and pump housings. They use a single A1-Ni-Co magnet annulus to attract the opposing seal face. A rather similar seal, illustrated in Figure 6, incorporates a series of strong rare-earth magnet rods that attract the opposing face. Superior face material combinations achieve coefficients of friction that, even without lubrication present, rival those of PTFE. The stationary seal face of the product in Figure 6 (also shown in insert, Figure 8) has a diamond-like hardness, RC90 (Ref.4). Many thousands of fluid machines in different parts of the industrial world have been fitted with this particular type of magnetic seal. Current yearly production exceeds 60,000, with primary applications in centrifugal pumps of all size ranges in the hydrocarbon processing and related industries. Figure 7 shows a magnetic seal whose contacting faces are pushed together by the repelling action of rod magnets of like polarity. Although repulsion magnetic seals embody certain advantages over the pulling configurations in seals per Figures 5 and 6, they are more expensive to make and take up more axial space than comparable 'pulling-faces-together' type magnetic seals. Therefore, very few of these seals have been sold since their introduction in 1992. Only a few dozen of these seals were produced in calendar year 2000. It should be pointed out that magnetic seals obtain lubrication and cooling from the ever-present oil fog that surrounds oil-lubricated bearings. Properly designed, using appropriate face materials and applying suitable selection criteria, they represent the ideal choice of a bearing housing seal that prevents
both egress of lube oil and ingress of atmospheric contaminants. Should the temperature-dependent pressure inside a non-vented bearing housing rise and overcome the magnetic closing force, the magnetic face seal would instantly release this pressure by opening and immediately re-closing. Magnetic seals of the type shown in Figures 5 and 6 will perform flawlessly, either in conjunction with pressurebalanced constant level lubricators or non-vented bearing housings filled with a finite amount of liquid lubricant. Moreover, they are the ideal bearing housing closure for closed, environmentally acceptable oil mist lubrication systems. In a closed oil mist application, the oil mist is introduced in the space between magnetic seal and bearing (Reference 5). Excess liquid or vaporized oil is led off or collected at the bottom center location of the bearing housing, Figure 8 (Reference 6).
Pressure relief poppet valves are rarely needed Pressure relief poppets similar to the one shown in Figure9, have been offered as retrofit items for bearing housings of centrifugal pumps and other machinery. As can be visualized from the illustration, the weight of the ball exerts a downward force on the seat. Whenever the product of pressure inside the bearing housing multiplied by the exposed area (=upward force) exceeds the downward force on the seat, the ball will unseat itself and will allow excess internal pressure to escape to atmosphere. A steel ball of 3/4in (9mm) diameter weighs 0.00641b (2.9gm). Assuming an exposed area of 0.1 lsq. in, it would take a pressure increase of 0.059psi, [or 1.6in (40mm) of H 2 0 column over atmospheric] to unseat the steel ball, although it has been pointed out that PVC balls would relieve at lower pressures. If equipment is fitted with labyrinth seals or bearing protector/ isolators either with or without O-rings, slightly negative pressures may actually exist inside the bearing
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MANIFO[~ aSSEMBLY
PUR~ O L Ul API STANDAR MIST APPLICI REAR,NG ANE NO STRAY M TO ATMOSPH
Figure 9. Pressure relief poppet valve. Figure 8. Magnetic seal hermetically sealing-in oil mist applied to modern centrifugal pump bearing.
(Source: Isomag Corporation, Baton Rouge, LA; [email protected]).
housing due to minor pumping action brought on by the contaminant expulsion elements mentioned in Referecne 1. Bearing protector/ isolators with designs that avoid pumping action have a liberal separation gap between rotating and stationary elements. There will thus always be pressure equalization between housing interior and the surrounding external atmosphere. Therefore, consistent pressure build-up can occur only in truly hermetically sealed bearing housings, i.e. ones equipped with magnetic seals. However, lightly-loaded seals with face orientations per Figure 8 would 'burp' before the poppet would release. Only heavily-loaded magnetic face seals would benefit from poppet relief valves or the expansion chambers described below (Figurel0). Experience shows that the seal of Figuress. 6 and 7 will not need either device.
Expansion chambers: use w h e r e n==ded Expansion chambers (Figure 10) are designed to absorb the expansion of gases or fluids in closed (hermetically sealed) systems. A rolling diaphragm provides a variable volume that, when correctly dimensioned, will reduce pressure buildup. On some seal models, maintaining pressures below certain limits will extend seal life or prevent 'burping'.
Figure 10. Bearing housing expansion chamber.
(Source: Trico Mfg. Corp.; Pewaukee, WI; www.tricomfgco.com).
M o i s t u r e monitors and desiccant cans There is ample evidence that free water in lubricating oils is seriously curtailing bearing life. Moist air that has intruded
into bearing housings will have its water vapor condense once saturation limits are exceeded. As of the late 1990s, this fact prompted competent manufacturers to develop and actively market on-line monitoring devices (Figure 11) that are capable of annunciating maximum safe levels of relative humidity (Reference 7). Also, add-on air dryers are now available and one such device is depicted in Figurel2. Desiccant technology works; it has been around for hundreds of years and packets of a chemically suitable desiccating substance are found inthe packing containers of cameras, sunglasses, kitchen appliances, etc. At issue is the technical and cost justification of installing an air dryer requiring replacement upon color change and thus future maintenance labor. Smart user companies design-out maintenance, not add to it. On the overwhelming majority of pumps and similar process plant machinery it makes far more sense to invest in failure avoidance, i.e. preventing moisture intrusion, than investing in either moisture removal or moisture annunciation. Again, hermetic sealing is the right failure avoidance strategy. Where no moisture intrudes, investing in measures to remove moisture is simply not necessary. There are, however, applications where moisture monitoring and/or desiccant-based moisture removal makes technical and economic sense. Among these are machine components that both rotate and undergo axial movement. There are also gearbox installations in steel mills and other industries where hermetic sealing is not practical. This is where moisture monitoring and removal are often easily justified.
Conclusions are supported by basic physics A reliability-minded process plant will give serious consideration to upgrading from the 'traditional' nonbalanced constant level lubricator. Pressure-balanced
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WORLD P U M P S
August 2001
www.worldpumps.com
FEATURE •
configurations are clearly becoming the norm among Best-Of-Class performers: • Bearing protector/isolators perform better than lip seals and/or labyrinth seals.
ir D r y e r
• Bearing protector/isolators with dynamic O-rings perform better than similar products without the dynamic O-ring. • Bearing protector/isolators with expulsion vanes have been known to create small pressure differences that promote the outward leakage of oil. • Magnetic bearing housing seals are a cost-effective means of precluding the alternating in-and-out movement of airborne contaminants. • Magnetic seals are the only practical hermetic bearing housing closure. Hermetic sealing optimally extends the life of lubricants and bearings. Precluding lubricant contamination makes the use of more expensive, superior synthesized hydrocarbon lubricants economically attractive. • Magnetic seals render constant level lubricators obsolete. Constant level lubricators are no longer needed in hermetically sealed bearing housings. • Poppet relief valves are very rarely needed. Their usefulness is a function of seal closing forces and must be determined on a case-by-case basis. • Moisture monitoring and removal means are of economic value in equipment whose geometry and component features are unable to preclude the migration of water into the bearing housing. • Expansion chambers are low-cost, readily justified addons where undue pressure rises may either jeopardize the life of hermetic sealing devices, or allow outward leakage of lubricant. There are compelling reasons, then, to keep up with an old dictum that probably predates even the industrial revolution: "An ounce of prevention is worth a pound of cure." We have the means and the knowledge to reduce the risk of bearing failure. Superior constant-level lubricators will do that. Hermetically sealing the hearing housings of industrial equipment is feasible and will prevent the intrusion of water and airborne dust. Expansion chambers will protect certain hermetic seals against pressure-induced decreases in anticipated life. Desiccant-based means of moisture removal and costly moisture monitoring devices will not be needed once the well-known water entry passageways have been closed in a manner that duplicates the sealing action of literally hundreds of millions of mechanical face seals. However, moisture monitoring and removal are important reliability improvement measures in equipment where moisture intrusion cannot be prevented by economic means. •
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--Viewport
References:
Figure.l l: On Line monitoring device
1. Orlowski,D.C., G/b/tGamb/ts,Volume 1, No. 71 ,June 13, •989 2. Inpro/Seal Corporation, P.O. Box 260, Milan, IL 61264 3. Magnetic Seal Corporation, 365 Market Street, Warren, RI 02885
capableof testing for saturated relative humidity.
(Source: TricoMfg.
Corp., Pewaukee, Wl; www.tricomfgco.com).
4.Isomag Corporation, 11871 Dunlay Ave, Baton Rouge, LA 70809
5. Bloch, Heinz P. and Abdus Shamim; 'Oil Mist Lubrication: PracticalApplications,' The Fairmont Press, Lilbum, GA, 1998 6. Bloch, Heinz P. 'PracticalLubrication for Industrial Facilities,' The Fairmont Press, Lilbum, GA, 2000 7. Rake, Brad. 'Water Contam/nat/on of Equipment-Lubricating Oil,' Pumps and Systems, January 2001, pp.28-35 CONTACT Heinz P. Bloch, P.E, Process Machinery Consulting, 128 Dawn's Edge, Montgomery, Texas 77356 Tel. 936-588-4611, e-mail: [email protected]
Figure. 12: Desiccantbased air dryer. (Source: Trico Mfg. Corp.; Pewaukee, WI; www. tricom fgco.com).
g~pansion c h ~ b e r at normal tempemtu~.
Expansion ¢ h ~ h e r at higher temperature
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WORLD PUMPS
~1
August 2001 4 5
F
A more recently developed machinery component, rotating labyrinth seals (commonly called 'bearing protectors' and 'bearing isolators') can now be found in many OEM and retrofit applications. Bearing isolators are an improvement over the old stationary labyrinth. Yet, they come in different versions and some of these may not perform as expected.
Then there are pressure relief poppet valves being marketed for the bearing housings of centrifugal pumps and other machinery. It can be shown that they are rarely, if ever, of any merit. And, finally, a new generation of online moisture monitoring devices and desiccant-containing absorber cans are now being offered by competent manufacturers. They are intended for mounting at the top of the vapor space of pump bearing housings but are not always cost-justified. This article will discuss where they do have merit and will comment on general application criteria this article will examine: • W h e n and why some constant level lubricators are often involved in equipment failures • Why some bearing housing seals, or bearing protectors/isolators may not fulfill their intended function
Figure 1. Unbalanced constant level lubricator (Source: Trico Mfg. Corp., Oewaukee, Wl; www.tricomfgco.com).
38
WORLD
PUMPS
A u g u s t 2001
0262 1762/01/$ - see front matter
• W h e n bearing housing pressure relief poppets serve no useful purpose • Where it will make economic sense to apply on-line moisture monitoring and/or moisture reduction devices.
Understanding different constant level lubricators To begin with, the user community needs to be aware of two distinctly different types of constant level oil lubricators available and in use on the majority of small machines. The most widely used 'open-to-atmosphere' (OTA) type is shown in Figure 1. It does not incorporate a balance line connection to the bearing housing and, if used on bearing housings with restricted venting provisions, may not adequately protect against catastrophic bearing failure. O n the OTA-type constant level lubricator shown here, the desired oil level is determined by the height setting of a wing nut threaded into a centrally located rod. The oil level at 'x' is open to the atmosphere and higher-than-atmospheric pressures in the bearing housing will cause level 'x' to rise, often to the point of overflowing, although the risk of creating an overflow condition can be minimized by using open-to-atmosphere constant level lubricators with extended-height surge chambers.
© 2001 Elsevier Science Ltd. All rights reserved
In any event, the level rise phenomenon is easily explained by Bemoulli's Law. With even a fraction of one psi pressure increase in the bearing housing, the fluid in the lubricator body near 'x' could rise in excess of one inch (25.4mm). Unless contained in a properly sized surge chamber, the rising fluid could easily run over the rim of the chamber, or reach an air vent passage occasionally found in certain models at an intermediate location somewhere along this chamber. Figure 2 shows a different constant level lubricator. A n internal O-ring or similar sealing means prevents the ambient atmosphere from reaching the oil level at 'x'. This closed-loop, pressure-equalized lubricator (PEL) incorporates a balance tube, or pressureequalizing line and gives full protection against airborne contaminants. Moreover, in the real world of machinery, pressure differentials can and will exist between the ambient environment and the housing-interior where lubricated components are mounted. There is evidence that installing bearing isolators and other narrow-gap housing closures increases the likelihood of potential problems with lubricators that had served perfectly well as long as not-so-close fitting labyrinth seals were being used. To overcome these potential problems, only PE"s and not OTA-type lubricators should be used in reliabilityminded plants. By providing a piping connection between the lubricator and bearing housing, lubricator port 'x' and bearing housing operate at the same pressure and the risk of oil leakage or unintended lowering of oil levels due to pressure buildup in bearing housings is eliminated.
W h a t makes housing pressures rise Note that the location of the wing nut in Figure 1 or slanted tube in Figure 2, determines the oil level in the bearing housing. Either the bottle height in Figure. 1, or the bottle-and-tube height in Figure 2 are usually being set with the machine not operating. At that time, temperature equilibrium
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X II
exists, i.e. the machine components, lubricating oil and surrounding environment are probably all at or near the same temperature. Most bearing housing vents, and especially small vents, offer a restriction that might allow a small amount of pressure build-up in bearing housings. Furthermore, it is also reasonable to expect that some oil will get flung into the closeclearance region 'B' typically fitted with lip seals or labyrinth seals (see insert, Figure 2). This oil now tends to bridge the gap between the rotating shaft and the surrounding stationary components; in other words, the oil has film strength, which makes it cling to surfaces. In that event, the trapped air above the oil level will constitute a closed volume. As this volume of air is being warmed by sun exposure or frictional heat generated in the bearings, its pressure will increase in accordance with the perfect gas law: (P2) = (P1)(T2/T1). Using Rankine temperatures and absolute pressures, the reader will readily see how relatively minor temperature increases may cause pressures to rise by amounts that cannot be ignored. Granted, as pressures go up beyond the rupture strength of oil films, the sealing oil film will be temporarily interrupted. The bearing housing seal will 'burp', or open up for a fraction of a second, but one of two undesirable events will take place:
lower surge chamber supporting the lubricator bottle (Bernoulli's law). The bottom of the bearing will be deprived of oil, or slinger ring immersion will no longer be sufficient for satisfactory lubrication.
Figure 2. Balanced constant level lubricator (Source: Trico Mfg. Corp., Pewaukee, Wl; www. tricomfgco, com)
2. Outward expulsion of oil will deplete the volume of oil available in the bearing housing and pressures in the lubricator bottle will adjust to the new equilibrium pressures. As the cycle repeats itself, more and more oil will be lost. Fortunately, the pressure-balanced constant level lubricator in Figure 2 does not introduce the same risk. As mentioned earlier, the oil levels 'X' at Figure 3. Rotating labyrinth seal locations inside the bearing housing or bearing and inside the lubricator are always protector/Isolator. exposed to identical pressures. The (Source: InprolSeal Milan, Illinois; problem is solved, and another step Company, ww~.inpro-seal.com).
1. The oil level will go down in the bearing housing and will rise in the
WORLD P U M P S
August 2001
39
FEAXURE *
contaminants, water vapor and airborne dust. r
It has been pointed out that bearing protector/isolators work best when the housing vent is plugged. To quote one manufacturer (Reference. 1):
:king Ring ng urll
Rot,
Figure 4.Bearing protector/isolator with dynamic O-ring. (Source: Inpro/Seal Company, Milan, Illinois,"
www.inpro-seal.com).
~ing Housing car magnetic mah
ating Ring
63 V2/ Pressure
sureP2 l
3#
A O-Ring
Figure 5. Magnetic seal Used in aerospace applications.
(Source: Magnetic Seal Corp., Warren, RI; info@rnagseal,corn).
Seat Case SeatRing
towards increased equipment reliability has been implemented. Disadvantages ? The pressure-balanced constant level lubricator probably costs a few pennies more, but the resulting failure risk reduction far outweighs any disadvantages, both perceived and real.
Rotating labyrinth bearing housing seals It has long been recognized that as many as 91% of all the rolling element bearings installed in the world's machinery fall short of reaching the manufacturer's calculated L-10 life. L-10 life is the number of operating hours where 10% of an identical bearing population will either have failed, or will exhibit visible or measurable damage. Research and followup analysis by bearing producers and users has
40
WORLD PUMPS
established lubricant contamination as the predominant failure cause. Much of this contamination finds its way into bearing housings through openings where equipment shafts protrude through bearing housings, or at vents and breathers provided somewhere on the lubricated assembly. Bearing housings undergo temperature shifts between day and night, or when operating vs. non-operating.
August 2001
With increasing temperatures, the vapors floating above the liquid oil level will expand, with decreasing temperatures they will contract. In a closed volume, increasing temperatures cause pressures to go up, while decreasing temperatures cause pressures to decrease. As a consequence of these findings and in an effort to reduce the pressurerelated alternating in-and-out flow of contaminated ambient air, conventional labyrinths (see insert, Figure 2) are often replaced by rotating labyrinth seals, which we will describe as protector/isolators, for short. A typical version is shown in Figure 3. It should be noted that these devices are designed with inherent clearances. In essence, an air gap separates the rotating and stationary elements. Except for brief time periods when the gap may be bridged by an oil film, this air gap is large enough to allow the constant exchange or inflow/outflow of atmospheric air with its ever-present
"If the housing vent is left open, the slight vacuum created by the contaminant expulsion elements will induce the flow of airborne dust, dirt, vapors and everything available in the immediate environment through the bearing enclosure not unlike an oilbath vacuum cleaner. This action is constant and the amount of induced debris build-up can be significant." Bearing isolators fitted with dynamic O-rings (Figure 4) try to close the gap through which airborne contaminants can enter into the bearing housing. The expectation is for the O-ring to effectively seal off the air gap at standstill (Ref. 2). The designer/ manufacturer is hoping that centrifugal force acting on the rotating Oring will cause this O-ring to lift off. In other words, the design objective is for the lifting distance to be sufficient to avoid the scraping and galling wear modes noted on circumferentially contacting dynamic O-rings. It is for these rather obvious reasons that O-ring manufacturers do not recommend high-cycle dynamic circumferential sealing applications. Nevertheless, bearing protector/isolators equipped with dynamic 'vapor blocking' O-rings are likely to outperform isolators that do not incorporate a dynamic O-ring. Current production of these components exceeds 175,000 per year (Reference .2). It should be pointed out that many practicing engineers have voiced concerns with mistaken c[aims that these devices provide hermetic sealing. Reliability professionals are correctly reasoning that if the O-ring does lift off, there will still be the gap through which contaminated air moves either in or out depending on temperature and pressure profiles. Conversely, if there is no gap, there will be wear. It can thus be shown that, contrary to written claims dating
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Figure 6. Magnetic seal applied in process equipment (Source: Isomag Corporation, Baton Rouge, LA; [email protected])
back about a decade, even bearing protector/isolators designed with dynamic O-rings will not perform 'hermetic' sealing duties.
Hermetic sealing is the preferred solution Hermetically sealing the bearing housing implies that nothing enters and nothing escapes. Only face-toface sealing devices meet this definition. However, in view of the generally limited axial space available between bearing housings and fluid casings of centrifugal pumps and other machinery, relatively narrow-width, magnetically closing face seals have been developed in recent decades.
Whenever there exists a thin fihn of clean oil between either springactivated or magnet-activated seal faces, long seal life and hermetic containment (Source: InprolSeal of the lubricating fluids results. Tens of Company, Milan, lUinois; thousands of the magnetic face seals wv~ac.inpro-seal.com). Figure 7. Inpro 'RMS 700' (Three-Piece) repulsion magnetic seal.
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WORLD P U M P S
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August2001
shown in Figure 5 have been used in aircraft task pumps, as aircraft generator seals, and on vertical stabilizer units (Reference 3). They can tolerate rubbing velocities as high as 86m/s (17,000fpm) and temperatures as high as 200 ° C (392 ° F) and have frequently served to everyone's satisfaction for over 50,000 operating hours. These seals have performed equally well in such industrial applications as gun drills, gearboxes and pump housings. They use a single A1-Ni-Co magnet annulus to attract the opposing seal face. A rather similar seal, illustrated in Figure 6, incorporates a series of strong rare-earth magnet rods that attract the opposing face. Superior face material combinations achieve coefficients of friction that, even without lubrication present, rival those of PTFE. The stationary seal face of the product in Figure 6 (also shown in insert, Figure 8) has a diamond-like hardness, RC90 (Ref.4). Many thousands of fluid machines in different parts of the industrial world have been fitted with this particular type of magnetic seal. Current yearly production exceeds 60,000, with primary applications in centrifugal pumps of all size ranges in the hydrocarbon processing and related industries. Figure 7 shows a magnetic seal whose contacting faces are pushed together by the repelling action of rod magnets of like polarity. Although repulsion magnetic seals embody certain advantages over the pulling configurations in seals per Figures 5 and 6, they are more expensive to make and take up more axial space than comparable 'pulling-faces-together' type magnetic seals. Therefore, very few of these seals have been sold since their introduction in 1992. Only a few dozen of these seals were produced in calendar year 2000. It should be pointed out that magnetic seals obtain lubrication and cooling from the ever-present oil fog that surrounds oil-lubricated bearings. Properly designed, using appropriate face materials and applying suitable selection criteria, they represent the ideal choice of a bearing housing seal that prevents
both egress of lube oil and ingress of atmospheric contaminants. Should the temperature-dependent pressure inside a non-vented bearing housing rise and overcome the magnetic closing force, the magnetic face seal would instantly release this pressure by opening and immediately re-closing. Magnetic seals of the type shown in Figures 5 and 6 will perform flawlessly, either in conjunction with pressurebalanced constant level lubricators or non-vented bearing housings filled with a finite amount of liquid lubricant. Moreover, they are the ideal bearing housing closure for closed, environmentally acceptable oil mist lubrication systems. In a closed oil mist application, the oil mist is introduced in the space between magnetic seal and bearing (Reference 5). Excess liquid or vaporized oil is led off or collected at the bottom center location of the bearing housing, Figure 8 (Reference 6).
Pressure relief poppet valves are rarely needed Pressure relief poppets similar to the one shown in Figure9, have been offered as retrofit items for bearing housings of centrifugal pumps and other machinery. As can be visualized from the illustration, the weight of the ball exerts a downward force on the seat. Whenever the product of pressure inside the bearing housing multiplied by the exposed area (=upward force) exceeds the downward force on the seat, the ball will unseat itself and will allow excess internal pressure to escape to atmosphere. A steel ball of 3/4in (9mm) diameter weighs 0.00641b (2.9gm). Assuming an exposed area of 0.1 lsq. in, it would take a pressure increase of 0.059psi, [or 1.6in (40mm) of H 2 0 column over atmospheric] to unseat the steel ball, although it has been pointed out that PVC balls would relieve at lower pressures. If equipment is fitted with labyrinth seals or bearing protector/ isolators either with or without O-rings, slightly negative pressures may actually exist inside the bearing
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MANIFO[~ aSSEMBLY
PUR~ O L Ul API STANDAR MIST APPLICI REAR,NG ANE NO STRAY M TO ATMOSPH
Figure 9. Pressure relief poppet valve. Figure 8. Magnetic seal hermetically sealing-in oil mist applied to modern centrifugal pump bearing.
(Source: Isomag Corporation, Baton Rouge, LA; [email protected]).
housing due to minor pumping action brought on by the contaminant expulsion elements mentioned in Referecne 1. Bearing protector/ isolators with designs that avoid pumping action have a liberal separation gap between rotating and stationary elements. There will thus always be pressure equalization between housing interior and the surrounding external atmosphere. Therefore, consistent pressure build-up can occur only in truly hermetically sealed bearing housings, i.e. ones equipped with magnetic seals. However, lightly-loaded seals with face orientations per Figure 8 would 'burp' before the poppet would release. Only heavily-loaded magnetic face seals would benefit from poppet relief valves or the expansion chambers described below (Figurel0). Experience shows that the seal of Figuress. 6 and 7 will not need either device.
Expansion chambers: use w h e r e n==ded Expansion chambers (Figure 10) are designed to absorb the expansion of gases or fluids in closed (hermetically sealed) systems. A rolling diaphragm provides a variable volume that, when correctly dimensioned, will reduce pressure buildup. On some seal models, maintaining pressures below certain limits will extend seal life or prevent 'burping'.
Figure 10. Bearing housing expansion chamber.
(Source: Trico Mfg. Corp.; Pewaukee, WI; www.tricomfgco.com).
M o i s t u r e monitors and desiccant cans There is ample evidence that free water in lubricating oils is seriously curtailing bearing life. Moist air that has intruded
into bearing housings will have its water vapor condense once saturation limits are exceeded. As of the late 1990s, this fact prompted competent manufacturers to develop and actively market on-line monitoring devices (Figure 11) that are capable of annunciating maximum safe levels of relative humidity (Reference 7). Also, add-on air dryers are now available and one such device is depicted in Figurel2. Desiccant technology works; it has been around for hundreds of years and packets of a chemically suitable desiccating substance are found inthe packing containers of cameras, sunglasses, kitchen appliances, etc. At issue is the technical and cost justification of installing an air dryer requiring replacement upon color change and thus future maintenance labor. Smart user companies design-out maintenance, not add to it. On the overwhelming majority of pumps and similar process plant machinery it makes far more sense to invest in failure avoidance, i.e. preventing moisture intrusion, than investing in either moisture removal or moisture annunciation. Again, hermetic sealing is the right failure avoidance strategy. Where no moisture intrudes, investing in measures to remove moisture is simply not necessary. There are, however, applications where moisture monitoring and/or desiccant-based moisture removal makes technical and economic sense. Among these are machine components that both rotate and undergo axial movement. There are also gearbox installations in steel mills and other industries where hermetic sealing is not practical. This is where moisture monitoring and removal are often easily justified.
Conclusions are supported by basic physics A reliability-minded process plant will give serious consideration to upgrading from the 'traditional' nonbalanced constant level lubricator. Pressure-balanced
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WORLD P U M P S
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FEATURE •
configurations are clearly becoming the norm among Best-Of-Class performers: • Bearing protector/isolators perform better than lip seals and/or labyrinth seals.
ir D r y e r
• Bearing protector/isolators with dynamic O-rings perform better than similar products without the dynamic O-ring. • Bearing protector/isolators with expulsion vanes have been known to create small pressure differences that promote the outward leakage of oil. • Magnetic bearing housing seals are a cost-effective means of precluding the alternating in-and-out movement of airborne contaminants. • Magnetic seals are the only practical hermetic bearing housing closure. Hermetic sealing optimally extends the life of lubricants and bearings. Precluding lubricant contamination makes the use of more expensive, superior synthesized hydrocarbon lubricants economically attractive. • Magnetic seals render constant level lubricators obsolete. Constant level lubricators are no longer needed in hermetically sealed bearing housings. • Poppet relief valves are very rarely needed. Their usefulness is a function of seal closing forces and must be determined on a case-by-case basis. • Moisture monitoring and removal means are of economic value in equipment whose geometry and component features are unable to preclude the migration of water into the bearing housing. • Expansion chambers are low-cost, readily justified addons where undue pressure rises may either jeopardize the life of hermetic sealing devices, or allow outward leakage of lubricant. There are compelling reasons, then, to keep up with an old dictum that probably predates even the industrial revolution: "An ounce of prevention is worth a pound of cure." We have the means and the knowledge to reduce the risk of bearing failure. Superior constant-level lubricators will do that. Hermetically sealing the hearing housings of industrial equipment is feasible and will prevent the intrusion of water and airborne dust. Expansion chambers will protect certain hermetic seals against pressure-induced decreases in anticipated life. Desiccant-based means of moisture removal and costly moisture monitoring devices will not be needed once the well-known water entry passageways have been closed in a manner that duplicates the sealing action of literally hundreds of millions of mechanical face seals. However, moisture monitoring and removal are important reliability improvement measures in equipment where moisture intrusion cannot be prevented by economic means. •
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--Viewport
References:
Figure.l l: On Line monitoring device
1. Orlowski,D.C., G/b/tGamb/ts,Volume 1, No. 71 ,June 13, •989 2. Inpro/Seal Corporation, P.O. Box 260, Milan, IL 61264 3. Magnetic Seal Corporation, 365 Market Street, Warren, RI 02885
capableof testing for saturated relative humidity.
(Source: TricoMfg.
Corp., Pewaukee, Wl; www.tricomfgco.com).
4.Isomag Corporation, 11871 Dunlay Ave, Baton Rouge, LA 70809
5. Bloch, Heinz P. and Abdus Shamim; 'Oil Mist Lubrication: PracticalApplications,' The Fairmont Press, Lilbum, GA, 1998 6. Bloch, Heinz P. 'PracticalLubrication for Industrial Facilities,' The Fairmont Press, Lilbum, GA, 2000 7. Rake, Brad. 'Water Contam/nat/on of Equipment-Lubricating Oil,' Pumps and Systems, January 2001, pp.28-35 CONTACT Heinz P. Bloch, P.E, Process Machinery Consulting, 128 Dawn's Edge, Montgomery, Texas 77356 Tel. 936-588-4611, e-mail: [email protected]
Figure. 12: Desiccantbased air dryer. (Source: Trico Mfg. Corp.; Pewaukee, WI; www. tricom fgco.com).
g~pansion c h ~ b e r at normal tempemtu~.
Expansion ¢ h ~ h e r at higher temperature
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