Processing Magazine

Are lip seals obsolete?

September 26, 2006

Thirty-eight million rubber lip seals are produced for industrial use in the United States each year. At least half of them are probably misapplied and should not be installed in industrial rotating equipment. According to their manufacturers, even the best lip seals have a median life to failure of only 1,844 hours or 77 days of operation. Half last longer than that and half last less than the mean time hours to leakage. More than 90 percent of them will quit sealing within the first 3,000 hours of operation.

After that, they either groove the shaft or burn to a crisp at the point of contact with the rotating shaft. Why, then, are they applied to protect rotating equipment that is designed to run uninterrupted for five years and bearings that are rated for much longer than that?

On average, each lip seal consumes 147 watts of power. A plant with 600 pumps, assuming two lip seals per pump, consumes 176.4 KW per hour of operation. Assuming 7.56 cents per KWh, that amounts to $117,041 per year spent just to overcome lip seal drag. The low initial purchase price of lip seals represents only a tiny percentage of their total operating cost. Fortunately, or unfortunately, the run-time will be considerably less than a full year and the energy costs are not as much as they could be unless the lip seals are changed out every time that they quit sealing. The odds of that happening are not great.

When the lip seal quits sealing and loses contact with the shaft, very undesirable things are liable to happen. Lubricant is free to exit the bearing enclosure and as the equipment cycles, moisture in the environment is drawn into the enclosure where it condenses and contaminates the lubricant. Even a minute amount of water in the oil is very detrimental to the bearing fatigue life projection.

It was not until the late 1970s that alternative, permanent, sealing devices for industrial/process rotating equipment were made available with the invention of the Bearing Isolator by what is now the Inpro/Seal Company. Even the term “Bearing Isolator” was invented at Inpro/Seal Company. Bearing isolators are made to order for the various sizes and shapes of industrial rotating equipment.

In order to service the customers as well as the lip seal manufacturers with 25,000 part numbers in their catalogs, bearing isolators must be able to be shipped the same day that they are ordered. Rotating equipment repair, as stated before, usually has a very short-term deadline for completion.

Other contact seals, such as spring or magnetically loaded face seals, have been used for bearing sealing, but with only limited success. Even the highly touted double face magnetic seal is projected to last no more than 18,000 hours and is somehow expected to protect 200,000 hour bearings. All contact seals have a finite life and a 100 percent failure rate over time.