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Close verses Open Loop Oil-Mist Systems

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Close verses Open Loop Oil-Mist Systems

Post by ioncube on Sun Mar 25, 2012 9:46 am

For decades, plant-wide oil-mist systems have been used to lubricate rolling-element bearings in high-speed applications throughout the petrochemicals, textiles, pharmaceuticals, mining and other chemical process industries (CPI). Applications range from small motors to large rotating cylinders and a variety of fluid movers. Plant-wide oil-mist systems provide a centralized way to supply lubricating oil to all such components, and often eliminate the need for frequent maintenance and corrective action.

Closed-loop oil-mist systems (Figure 1) have been widely used in these industries for decades. In fact, the CPI have been leaders in the use of plantwide oil-mist systems since the 1960s.
Due to their lower initial cost, openloop oil-mist systems have traditionally been more popular in the U.S. While they may be less costly up front, open-loop systems consume oil in a once-through fashion (Figure 2) and they can discharge spent or “stray” oil mist to the atmosphere, which creates environmental issues.

FIGURE 1. Shown here is a closed-loop oil-mist system that provides lubrication for bearings in process pumps and their drivers. If desired, the small, blue collection containers that are shown in this image can be furnished with automated or manual means of pumping the coalesced oil into the spent mist-return header

By comparison, closed-loop systems (as shown in Figures 1 and 3) allow the oil mist that has passed through the equipment bearings to be collected, filtered and reused. As a result, the lifecycle cost of a well-designed closed-loop system is often less than that of a comparably sized, plant-wide open-loop system. As noted above, one particular downside of open-loop systems is that they allow a considerable amount of stray mist to escape from the pump bearing housings (Figure 2). In the face of strict, plant-specific environmental requirements, many facilities now prefer to implement closed-loop technology, whereby the mist is routed in compliance with API 610. API 610 is a widely used and strongly recommended — although not compulsory — industry specification, which calls for oil-mist introduction between a modern face-type or rotating labyrinth-style bearing housing protector seal and the adjacent rolling-element bearing. This is shown in Figure 3. The introduction of an oil mist into this open space prevents process vapors or contaminated atmospheric air from accumulating there. Similarly, the existence of oil mist in those spaces at slightly over atmospheric pressures precludes the ingress of moist or dirt-laden atmospheric air. It is worth noting that lip seals — which are widely used in inexpensive pumps — are not considered to be acceptable, per the API standard. It is generally assumed that elastomeric lip seals start leaking after about 2,000 operating hours.

FIGURE 2. Old-style, open-loop oilmist systems can result in considerable mist escaping to the environment. Bearing protector seals are purposely left off here so as to promote throughflow. As one knows from basic laws of physics, a pressure difference is needed for gas to flow from one location to another

FIGURE 3. This pump bearing housing has face seals that allow for appropriate oil mist flow, per the API-610 standard. The task of manually emptying a 2-quart container once or twice per year is considered to be more cost-effective by many than automating the means of returning oil from the bottom of a bearing housing to a more-remote collection point. Modern magnet-closed bearing protector seals are shown; these will prevent atmospheric air from being drawn into the bearing housing. During operation, oil mist or carrier air arriving at the bearing housing drain will have traveled through one of the bearings

In contrast, some API-recommended face and rotating labyrinth seals run well in excess of 50,000 hours. In the modern face-type bearing-protector seal (as shown in Figure 3), a closing force is applied by a series of magnets. These magnets are equally spaced around the circumference of the stationary element of the seal. In API 610-compliant systems, the oil mist must flow through the bearing (or bearings) on its way to the central outlet port. Central outlet ports — usually located at the 6 o’clock position of the bearing housing, as shown in Figure 3 — can be connected to a common return header, which operates at a pressure below that of the oil-mist supply line. This application of a slight vacuum will increase the flowrate through the system.
The spent mist in the drain header can then be sent through a final coalescer element, which will capture residual oil droplets that are still entrained in the carrier gas. Instrument air is generally used as the carrier gas. While other carrier gases are entirely feasible, instrument air is fully satisfactory in virtually all plant-wide oil mist installations.
A closed-loop system that applies oil mist to the bearing housings creates virtually oil-free carrier air, which can ultimately be discharged to atmosphere, or be recompressed for use as motive air elsewhere in the facility. However, the volume of this spent air is typically only about 1% the typical suction-air requirement of a facility’s instrument air compressor. Thus, the economics typically do not favor either recompression, or the piping that would be necessary to return the spent carrier air to the facility’s instrumentair inlet system.
The choice of the most appropriate coalesced-oil collector (from either the drain or the outlet port) is left to the user, and many different collector bottle configurations are available. In general, the average pump-bearing housing produces less than two quarts (about two liters) of coalesced lube oil per year. Only closed-loop systems are suitable for locations where the use of best available lubricant application technology is preferred from a failure risk-reduction point of view. In some cases, open oil-mist systems are simply disallowed because they discharge excess oil into the surroundings and because protecting the environment takes precedence.
Moreover, open systems consume significantly more oil than properly managed, closed-loop systems. Regarding the ability to reuse the collected oil, any potential concern that this oil may have been overheated — and thus might have lost some of its required properties — is not supported by field experience. From a practical perspective, the temperature rise that is encountered in the rolling-element bearing of many process pumps causes no measurable degradation of the premium- grade synthetic lubrication oils that are widely used by today’s reliability- focused plants in their closedloop oil-mist systems. In general, the initial as-installed cost of closed-loop, oil-mist systems will be higher than that of open systems. However, the ability to both eliminate the unwanted discharge of stray oil mist (and thus protect the environment) and to recover lube oil for reuse helps to lower the overall lifecycle costs of the system, compared to open-loop designs. In fact, for many facilities, the benefits of closed oil-mist systems clearly outweigh the disadvantages of less costly, but messy and inefficient open systems. Close adherence to the configuration shown in Figure 3 has been shown to result in increased profitability and significant reductions in the numbers of pump failures in many facilities worldwide.

Author: Heinz P. Bloch, P.E., is a consulting engineer residing in Westminster, Colo., (hpbloch@ He has held machinery-oriented staff and line positions with Exxon affiliates in the U.S., Italy, Spain, England, The Netherlands and Japan, during a career that spanned several decades prior to his retirement as Exxon Chemical’s regional machinery specialist for the U.S. Bloch is the author of 18 comprehensive texts and over 500 other publications on machinery-reliability improvement. He advises process plants worldwide on strategies

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