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Excerpted, by permission of the author, from "Practical Lubrication for Industrial Facilities" and "Oil Mist Lubrication: Practical Applications."
The use of oil mist lubrication in the refining and petrochemical industry dates back to the 1960s when companies such as Exxon and Chevron began to apply oil mist to pump bearings.(1) By the early 1970s, oil mist lubrication was applied to rolling element bearings of electric motors in the refining industry.(2) Today, as indicated in a biannual survey conducted by Lubrication Systems Company, 77 percent of the major, multilocation U.S. refining companies have at least one large-scale oil mist system in at least one of their refineries.(3) The use of oil mist in the hydrocarbon processing and other industries, such as pulp and paper, worldwide is growing because of oil mist lubrication’s proven performance in delivering improved machine reliability, reducing maintenance costs and providing a pay back on the investment in the oil mist lubrication system.
Typical Design Features
The oil mist produced in the central oil mist generator is transported throughout the process unit through the header pipe. Drop piping and a manifold complete the piping system (Figure 1).
Figure 1. Header and Drop Piping
Today’s designs call for the header to be sloped back to the central generator; no part of the header should be sloped away from the generator.(4) This design promotes oil usage efficiency as the oil mist that coalesces in the header is redirected back to the mist generator for reuse. Older technology allowed sloping the header away from the mist generator toward drain legs.(5)
In older mist systems, the manifold was typically a rectangular metal block with ports drilled for mist flow. Most of these blocks were equipped with a snap-acting valve for draining the collected coalesced oil. New mist manifolds contain a high-temperature glass-viewing chamber, which allows for visual monitoring of the level of coalesced oil collected (Figure 2).(6)
Figure 2. Mist Manifold Assembly
The viewing chamber allows operators to see when oil needs to be drained. Draining is accomplished through an internally ported push valve that is channeled to a vent port on the side of the manifold. This port is tubed via a return manifold and into a collection container. When the manifold is drained, the flow of mist can be seen in the viewing chamber. Thus, mist flow is inspected without venting to atmosphere.
Dry Sump (Pure Mist) Applications
There are two different mist application methods, dry sump and wet sump, available for pumps or any machine with rotating shafts requiring dependable lubrication. In the dry sump method the bearing housing is drained of oil and all lubrication is accomplished by mist (Figure 3). A sight glass or other bottle-type device is installed at the bottom of the bearing housing to capture condensed oil. This catch pot is generally made of a transparent material to permit easy observation of water or other contaminants (Figure 4). Although it is not necessary to ensure through-flow of oil mist for lightly loaded single-row radial bearings, it is nevertheless good practice to do so. On thrust-loaded rolling element bearings, the oil mist should always be routed through the bearing rolling elements in order to ensure that complete lubrication and perhaps a small amount of additional cooling can occur. Recent editions of Standard API 610 mandates the through-flow application shown in Figure 3.
Venting the carrier air from the closed bearing housings is required to permit the continuous flow of oil mist into the housings. Relative placements of mist entries and vents are used to promote the movement of oil mist through certain assemblies. Proper venting of single-row bearings (for instance, pump radial bearings) equipped with dry mist lube is relatively simple. Vent tubing can be piped to a low-pressure collection header (Figure 4).
Figure 4. Sight Glasses are Used to Collect Condensed Oil Mist in Dry Sump Systems
Purge Mist Application
Purge mist is used on sleeve (journal) bearings or other installations where the technically superior dry sump method would not be as effective.
To provide controlled mist system venting with purge mist/wet sump applications, a unique purge mist vent-fill assembly has been developed (Figure 5). It is easily affixed to bearing housings and eliminates uncontrolled venting and oil accumulation on and around equipment receiving purge oil mist. It also avoids problems associated with refilling reservoirs.(7)
Figure 5. Purge Mist Vent-Fill Assembly
The device is incorporated in the bearing housing and provides clog-free mist flow into and from the cavity receiving purge mist (Figure 6). It incorporates a screw on/off cap with internal porting into the mist penetration tube. This porting is designed so that when the cap is removed, mist is not flowing into the housing and creating back pressure, which causes problems when adding oil. The 38-mm (1 ½-inch) wide funnel mouth allows for an easy, spill-free addition of oil.
The internal porting permits controlled venting of escaping purge mist through tubing into the companion oil level sight and constant level oiler assemblies. The oil level sight assembly protects against overfilling from both coalesced oil mist and liquid oil addition, and offers visual inspection of the oil level in the bearing housing. The constant level oiler assembly in (Figure 7) also protects against overfilling and will add oil that is lost, for example, through bearing housing seals.(7)
With these newer devices, excess oil mist and overflow oil are directed to the collection container. Thus, even with purge mist, venting at the equipment and oil drainage to base plates and foundations are virtually eliminated.
Oil Collection Container
This recently commercialized container is mounted to the equipment foundation (Figure 8). Oil mist, plus coalesced oil flow into the container. The internal overflow tube with liquid seal prevents the container from filling with oil if it is not emptied. If the container were to overfill, oil would rise into the bearing housing and block the flow of mist. The excess mist from the bearing housing that does not coalesce in the container travels through the container and tubing to the overhead return header. The container is also equipped with a manually operated pump that pushes collected liquid oil through piping into the overhead return header.(8)
Figure 8. Collection Container
The 3.8-liter (1-gallon) capacity of the container means it needs to be emptied only once per month. Because the frequency for evacuating the container is infrequent, incorporating automated, air-operated level controlled pumps has not been considered cost-effective. In fact, the monthly interval is significantly longer than that required for emptying the traditional sight bottle located under dry sump mist applications.
The oil collection container is connected via stainless steel tubing to the return manifold assembly internal porting with a check-valve. This accommodates the continuous flow of mist to the return header and the intermittent pumping of liquid oil to the same return header (Figure 9).
The vertical return line is not simply a length of pipe but rather a tube within a pipe. This arrangement allows for efficient field installation and provides for a more compact, less cumbersome piping arrangement. This design also allows for the simultaneous pumping of oil and the continuous flow of mist without creating blockage of mist flow or unwanted system back pressure.(9)
Improved components such as mist manifolds and newly developed devices such as vent/fill assemblies and automated chain legs make today’s systems cleaner and more efficient than prior technology and installation practices. The invention and commercialization of closed-loop, circulating oil mist systems and related demisting equipment have positioned oil mist for greater use by process industries in an environmentally conscious world. Oil mist now meets the requirements for clean, emission-free operation while still delivering the improved reliability results expected of oil mist.