There is a better way to lubricate your pump

Oil mist lubrication systems avoid the need for some 
maintenance activities and reduce operating temperatures

I had the pleasure of working with a small Texas Gulf Coast asphalt plant with the retrofit installation of a oil mist system on the majority of pumps in the plant. The throughput of this plant is less than 30,000 barrels per day. However, their pump applications--process and temperature--is typical of other
asphalt plants.

Pump modifications such as removing the slinger rings, plugging the oil drain back ports, and having two reclassifiers on each pump have always been an issue of concern when justifying pump retrofits using oil mist lubrication. Many engineers feel that removing the slinger rings along with plugging the drain back ports is necessary before retrofitting to pure oil mist. Some also require that two oil mist inlet ports must be available to direct oil mist through both the thrust and radial bearings.

Other large refineries retrofitted oil mist to their pumps without any such modifications and noted the results in technical papers. I personally worked with several refineries in which no such modifications were made and no failures were experienced when converting the pumps to pure oil mist lubrication. This does not mean that failures cannot occur. I have also seen oil sumps drained as oil mist was applied followed by bearing failure shortly there after.

Oil mist suppliers are learning that horsepower, rotating speed, and temperature are factors that must be considered when converting from oil sump to pure oil mist lubrication.

During the late 70s and the early 80s many plants had their first experiences with oil mist lubrication and not all experiences were favorable. Some plants, however, had personnel that were intrigued by this unique method of lubrication and did more than install a system and leave it to perform on it's own.

Several of the major refiners studied this technology to learn more about where oil mist could be used and how to apply it. These companies experienced a decrease in pump and bearing failures. There, oil mist lubrication became a standard while other companies are still struggling with justification.

During the past ten years several companies adopted oil mist as their only means of lubrication and retrofitted systems throughout their domestic facilities, and recommended oil mist to be installed in foreign locations.

What is an oil mist lubrication system?
An oil mist system is a centralized lubrication system that uses the energy of a instrument air supply to atomize lubricating oil. The oil is atomized into particle sizes from 1 to 3 microns--that is, a dry mist. Air flow conveys these small oil particles through a low-pressure distribution system. Oil mist can be transported easily in a 600 foot radius from the centralized generator. Oil mist resembles cigarette smoke when seen drifting through the air. However, the resemblance stops there. Oil mist is an aerosol--not a vapor--and is nonhazardous and nonflammable.

At the point of lubrication, the oil mist passes through a restricting orifice-- a reclassifier fitting--that performs two functions. The reclassifier controls the amount of lubricant applied to each bearing and converts the dry mist into wet mist. As the dry oil mist passes through the reclassifier it causes turbulence and the small particles impinge on each other and grow in size. The rapid motion of the bearing's rolling elements also create turbulence and a wetting action of the oil mist. These droplets plate out and provide a oil film on the rolling elements of antifriction bearings.

Benefits of oil mist lubrication
Oil mist lubrication of antifriction bearings is preferred in the dry sump method. That means that no oil level is maintained in the bearing housing and oil mist is the only means of lubrication.

This was not always the preferred method of lubrication by oil mist users since some systems were unreliable. This has now changed as reliability of the oil mist generators has increased, system monitoring has improved, and system installation practices have changed to make the system more efficient. During the past 20 years pure mist became the preferred method of lubrication for rolling element bearings.

Facts supporting the use of pure oil mist show bearing temperature normally drops 20 to 30 degrees F after removing the oil sump. This is important since temperature reduction is a major factor for increased bearing life. For every 10 degree rise in bearing temperature the fatigue life is shortened by approximately 11 percent.

Further, by removing the oil sump, wear particles that are a factor in reducing bearing life are not recirculated through the bearings causing additional damage. Only oil is applied to the bearings.

Oil mist creates a slight positive pressure in the bearing cavity that prevents the intrusion of airborne contaminates from thermal cycling. Thermal cycling is a major contributor to oil sump contamination and is caused by changes in ambient temperature and the start and stop operation of equipment. This slight positive pressure in wet sump applications is effective for preventing contamination of oil in gear boxes and steam turbine bearing housings.

Converting to pure oil mist also allows for the removal of cooling water to bearing housing that have a oil sump cooling line. Without the oil sump there is no need to have cooling water in the bearing housing. Removal eliminates maintenance on the external water system, eliminates water leaks in the bearing housing, and reduces cooling water cost.

Oil mist is the application of only pure oil to the bearings; the oil particles of 1 to 3 microns in size are physically too small to carry solid particulates or other contaminates to the bearing. The weight of the contaminant causes the oil mist particle to fall back into the reservoir before leaving the oil mist generator. Thus, only pure oil particles travel throughout the distribution system providing superior lubrication for the bearings.

Computerized tracking of equipment failures before and after oil mist lubrication is applied revealed another benefit along with increased bearing life--mechanical seal failures are reduced 25 to 30 percent. Oil mist is not applied to the mechanical seal but premature bearing failure is a contributing factor to seal failures. Therefore, if bearing life is improved then some number of seal failures will be avoided.

Documents are now available from a Texas Gulf Coast Refinery that shows the final results up to ten years after the installation of oil mist. Many other charts are available that show one year before and one year after--along with others that show repair cost before and after--the installation of oil mist.

Computerized tracking of equipment failures before and after oil mist lubrication is applied has revealed another benefit along with increased bearing life--mechanical seal failures are reduced 25 to 30 percent.

Along with the items previously noted, oil mist lubricated bearings run with about 25 percent less friction than oil sump lubricated bearings. Under identical endurance tests of load, speed, and exterior conditions the L₁₀ life of the oil mist lubricated bearing is about 6 times the L₁₀ life of a oil sump lubricated bearing.

Research also shows that certain oils build a carbonaceous layer on the hot operating surfaces of bearings. This layer was absent on oil sump lubricated bearings. Maintenance costs and downtime caused by bearing failures are significantly reduced.

Because oil mist systems are simple and almost trouble free, the tendency is to start them and walk away. Yet, periodic inspection and maintenance are required to keep the systems in tune and maintain the highest degree of performance and reliability. The key to a successful oil mist system is careful attention to design and construction. The systems must be matched to the operating equipment. A properly applied and installed oil mist system can reduce maintenance costs significantly by reducing bearing failures as much as 90 percent.

Types of installations
Oil mist system installations are defined as either a grass roots installation or as a retrofit installation. Oil mist systems are justified with typical savings of $40,000 to $50,000 per year from reduced failures and maintenance cost from a system serving 30 to 40 pumps. A 20 year life gives the system a good rate of return on investment.

Grass roots installations are normally included in the capital budget due to past experience with oil mist systems and company policy to reduce the total life cycle cost of the rotating equipment. Equipment such as process pumps and motors are specified for oil mist lubrication and are available from the manufacturer for that service.

The 7th and 8th edition API Pumps makes installation easy by omitting the slinger rings, the plugging drain back ports, and providing two lubrication points on overhung pump bearing housings. Some ANSI pumps also have these arrangements.

As an extra benefit, oil mist is often used for the preservation of rotating equipment during the construction phase of a project. That has a dramatic effect on reducing premature bearing failures at start up.

The lack of justification for oil mist on grass roots projects comes from the inflated price tag that engineering and construction companies put on the oil mist system with little or no added value to the final product. Recent experience with a grass roots installation in a domestic ethylene plant showed the quoted installation cost from the oil mist supplier had been doubled by the E&C.

A foreign steam cracker project with startup in 1999 to 2000 had oil mist omitted due to the E&C company inflating the oil mist system price tag 4 to 6 times. These experiences caused several large oil mist users to go directly to the supplier for their grass roots oil mist installations. This eliminates the large handling fee by the E&C company and provides a better return on investment.

Retrofit installations can be direct from the supplier to the end user and installed on existing pumps, turbines, gear boxes, and sometimes motors. Normally, older pumps do not have two lubrication points and, with the traditional sump lubrication, they are equipped with slinger rings or flingers, and the drain back ports are open. Here opinions on the best approach differ. One point of view suggests the removing the slingers, plugging drain back ports, and installing a second lubrication point. Another suggests installing a single reclassifier, leaving the slingers, and ignoring the drain back ports.

Pump bearing and seal expenses were documented for 2 quarters--6 months--before the installation of oil mist and the immediate 3 quarters--9 months--after installation--the time during which this article was written. Pump reliability was significantly improved without additional expense for modifications. The winner is the Asphalt Plant where the system was installed and is now performing beyond expectations.

The situation
Before oil mist lubrication, quarterly bearing repair cost was approximately $14,000 and quarterly mechanical seal repair cost was in the $10,000 range. Bearings and seals were a major expense to this small plant with 36 to 40 pumps total.

The oil mist system was laid out to include 28 pumps including 4 between bearing pumps and 24 overhung pumps. Eighteen overhung pumps rotate at 3,600 RPM and the remainder of the pumps rotate at 1,800 RPM. After oil mist was installed bearing failures were reduced by 95 percent and seal failures went down on average by 50 percent.

Bearing and seal failures were ongoing, I surveyed this plant 2 to 3 years earlier for a oil mist system but installation was delayed until April 1997. The bearing failures were caused by reasons that included steam, salt air humidity, and dust intrusion into the bearing housing. Because of the size of the facility and the small quantity of pumps; operations cycles the pumps weekly to help maintain reliability. This frequent starting and stopping of the pumps may have caused more failures than normal from thermal cycling.

The hottest pair of pumps are ANSI overhung pumps having lip seals, a product temperature of 680 degrees F and driven by 100-hp motors rotating at 3,600 rpm. A single reclassifier was installed and the oil sump was drained. Slinger rings were not removed and no changes have been made to the pump design.

Other hot applications included 4 pumps with a product temperature of 575 degrees F that have 150-hp, 3,600 rpm drivers and 3 pumps with a product temperature of 450 degrees F that also have 150-hp, 3,600 rpm drivers. These particular pumps are late model API overhung pumps that have the double taps as supplied from the manufacturer and two reclassifiers were installed. Still no slinger rings were removed and no changes were made to the pump design.

Eight additional pumps are larger than the hot pumps. Two of these eight are rotating at 3,600 rpm with 500- and 300-hp drivers. The remaining six pumps are rotate at 1,800 rpm with 200- and 300-hp drivers. These eight pumps are also late model API overhung with the double tap arrangement and two reclassifiers were installed. Again, no modifications of any type were made on the pumps. The remaining 11 general process pumps are less than 100 -hp with ambient product temperatures; a single reclassifier was installed and no modifications were made.

The resolution
As the oil mist system went in each pump was set up as wet sump until the system was balanced and before training was available to personnel. The operating pumps--half of the population--was converted to pure mist while documenting bearing temperatures before and after conversion to pure oil mist. The ANSI pump having the hottest product temperature had a drop of 20 degrees F on the thrust bearing and 10 degrees F on the radial bearing. The following week the second half of all pumps were converted to pure mist with similar results. The additional eight hot pumps averaged a 10 degrees F drop on the thrust bearings and the radial bearings averaged a seven degrees F drop.

The conclusion
This installation did not require additional pump modifications and there have been no lubrication related bearing failures since startup. In applying oil mist to overhung pumps, special attention must be given when the driver is rotating at 3,600 rpm and the horse power is 200 or greater. The overhung pumps involved in the case study that were 200-hp or above were already set up for two lubrication points so no modification was made or requested.

Further, some overhung pumps may require directional reclassifiers to direct the oil mist into the rolling elements of the bearings depending on the load. Suppliers should always install directional reclassifiers on between
bearing pumps.

Some types of 7th & 8th edition API overhung pumps require plugging the drain back ports due to the quantity of and the port ID size; older model overhung pumps seldom require this modification. Slinger ring removal is not often required. As we gain more experience on retrofit systems, the failure rate associated with it--about one percent--is reduced consistently.

These failures are not always a result of inadequate lubrication by the oil mist. Often, this failure is from bearings on the verge of imminent failure even with oil sump lubrication. These few failures are often dramatized to discredit oil mist, but little merit is given to oil mist for the hundreds or thousands of pumps, within the same unit or plant, that operate flawlessly on pure oil mist lubrication.