A recent Hydrocarbon Processing (HP) article conveyed an industry finding that dates back several decades: Oil mist successfully lubricates operating machinery, protects and preserves standby equipment, and provides superior lubrication to electric motors driving process pumps. In a follow-up communication, an HP reader had questions regarding these findings. He believed that closed-loop oil-mist systems would possibly: 1) require oil sampling and oil changes at times, 2) not protect or preserve standby equipment and 3) be of no use to drive motors. Under those conditions, he believed his facility would continue to depend on inferior grease lubrication. Some of this reader’s conclusions and beliefs are refuted by decades of highly satisfactory experience.

Oil sampling recommendations

In response to point No. 1, a prudent practitioner of predictive maintenance (PdM) would occasionally sample the oil that has passed through the equipment bearings. We believe the coalesced bulk oil in a coalescer-collector vessel, as shown in Fig. 1, should be sampled twice per year. Sampling the coalesced mist drawn from the bottom of an equipment-bearing housing is optional because the total bearing condition is best determined by widely available PdM devices. These devices monitor vibration amplitude and frequency spectra. Still, with superior synthetic lubricants made cost-effective due to very low oil consumption and cooler running bearings, a five-year oil service life can be attained without difficulty.

Fig. 1. A small oil-mist console manages 20   pumps. It is shown on the left, and the return   mist coalescer-collector vessel is on the right.   Source: Lubrication Systems Co., a Division   of Colfax Industries, Houston, Texas.

Rely on ‘pump wisdom’

In response to point No. 2, protective action on standby equipment is very important and has been fully proven since about 1965. In over 5,000 large plant-wide oil-mist systems—including hundreds of closed systems and many open-air equipment storage yards—the oil mist envelops the antifriction bearings in a protective fog. Existing at a slightly higher-than-ambient pressure in bearing housings, oil mist keeps out ambient air typically contaminated by water vapor and airborne dust. Properly applied oil mist travels through the bearings and to the bearing housing drain port. Thus, the oil mist protects non-running or standby equipment. Without this preservation method, standby equipment is exposed to the risk of the lubrication oil being “wiped off” due to vibration transmitted from the adjacent running equipment.

In addition, without this preservation method, corrosion of bearings in non-running equipment is a greater probability. The rate of corrosion is a function of ingress and egress of air (“breathing” action) of the affected bearing housings. This breathing action occurs only in unprotected bearing housings, and it is not possible in bearing housings filled with oil mist at about 1 psi over atmospheric pressure. Industry now has in excess of four decades of experience with this protection method on many thousands of pumps and motors. In 1978, a user company commissioned 132 electric motors driving vertical pumps. We were told that none of them had ever failed in 32 years.

Regarding No. 3, all motors with rolling-element bearings will benefit from dry-sump oil-mist applications. Thousands of motors have been lubricated in this manner for over 40 years. Many books and papers have been written about this subject. Inferior grease lubrication has been superseded by superior dry-sump oil-mist systems for decades. A recent book on pumps again shows how oil mist eliminates slinger rings, constant level lubricators, dessicant breathers and bulls-eye sight glasses.¹ Oil mist safeguards against over-greased motor bearings and greatly reduces bearing failures. Closed-oil-mist systems are proven technology; they excel by further reducing lubricant consumption and protecting the environment. HP

LITERATURE CITED
¹ Bloch, H. P., Pump Wisdom: Problem Solving for Operators and Specialists, John Wiley & Sons, Hoboken, New Jersey, April 2011.