We often get questions relating to oil mist and how it affects electric motor windings. For a quick answer, contact us at Gulf Publishing via e-mail. Well steer you in the right direction by explaining that oil mist will not harm modern motors. But if you must minimize oil mist intrusion into electric motors, we recommend you insist on having the vendor prove IP66 compliance. IP is the universally recognized Ingress Protection Code; its discussed in Reference 1. At least one fully compliant sealing product is available; it was introduced in 2003 and has since been an unqualified success.
There are other historical facts relating to oil-mist lubricated motors. In the mid-1970s, oil mist had demonstrated its outstanding suitability for lubricating and preserving electric motor bearings. By that time, petrochemical plants on the US Gulf Coast, Caribbean and South America had converted in excess of one thousand electric motors to dry-sump oil-mist lubrication. In 1986, there were more than 4,000 electric motors on oil mist lube in the US Gulf Coast area alone. As of this writing (in 2011), there are an estimated 26,000 electric motors that run on pure oil mist with outstanding success. Many of these are in the Middle East.
However, universal acceptance did not come overnight. Conversely, it seemed logical to extend oil-mist feeder lines from centrifugal pump bearing housings to the adjacent electric motor bearings. On the other hand, concern was voiced that lube oil would enter the motor and cause damage to winding insulation, or cause overheating until winding failure occurred. Initial efforts were, therefore, directed toward developing lip seals or other barriers confining the oil mist to only the bearing areas. Those efforts date back to about 1975.
When, in the late 1970s, failures of old-style Vee-ring seals (Fig. 1) were experienced in operating motors, oil mist did enter, and it coated the windings with coalesced oil. The potential explosion hazard was again investigated on this occasion and confirmation obtained that the oil/air mixture of a plant-wide oil mist system remains substantially below the sustainable burning point. Experiments had shown the concentration of oil mist in the main supply manifolds ranging from .005 to as little as .001 of the concentrations generally considered flammable. The fire or explosion hazard of oil-mist-lubricated motors is thus no different from that of NEMA-II motors. No signs of overheating were found, and winding-resistance readings conformed fully to the initial, as-installed values.
Fig. 1. A worn Vee-ring
removed from an electric
With the introduction of epoxy motor-winding materials several decades ago, it was shown that these winding coatings will not deteriorate in an oil-mist atmosphere. This has been conclusively proven in tests by users and motor manufacturers. Among them were Reliance Electric (Cleveland), Continental Electric (Newark), and an oil refinery in the Caribbean where windings coated with epoxy varnish were placed in beakers filled with various types of mineral oils and synthetic lubricants. Next, these windings were oven-aged at 170°C (338°F) for several weeks, and then cooled and inspectedno problems.
Decades ago, experimentation with motor winding and cable terminations in conduit boxes showed that a Teflon-based wrap should be used in the conduit box for best results. Other materials, including silicone tape, seemed to exhibit a tendency to swell or become gummy when exposed to oil mist. It was then decided to provide sealant between the motor frame and conduit box to reduce (an open system) mist emissions at the conduit enclosure. Mist supply and condensed oil-drain ports were made accessible without the need for covers and guards. A simple pipe nipple or similar extension was considered just fine in the 1980s. Today, environmentally friendly closed circuit oil-mist systems would be used in industry, but oil mist is still the best way to lubricate. HP
Fig. 2. A successful oil-mist-
lubricated motor bearing
dating to the mid 1970s.
1 Bloch, H. P. and A. Budris, Pump Users Handbook: Life Extension, 3rd Edition, The Fairmont Press, Inc., Lilburn, Georgia, 2010, pp. 477-478
|The author |
Heinz P. Bloch is Hydrocarbon Processings Reliability/Equipment Editor. The author of 18 textbooks and over 490 papers or articles, he advises process plants worldwide on reliability improvement and maintenance cost-reduction opportunities. For more details, see his Practical Lubrication for Industrial Facilities, ISBN 0-88173-579-5.