What Makes Bearing Housing Pressures Rise?- A Reliability Issue

Most bearing-housing breather vents, and especially small vents, offer a restriction that might allow a small amount of pressure build-up in bearing housings. It is also reasonable to expect that some oil will get flung into the close clearance region, typically fitted with lip seals or labyrinth seals, where the shaft penetrates the bearing housing. The oil has film strength, which makes it cling to surfaces. This oil now tends to bridge the gap between the rotating shaft and the surrounding stationary components.

In that event, the trapped air above the oil level will constitute a closed volume. As this volume of air is warmed by sun exposure or by frictional heat generated in the bearings, its pressure will increase in accordance with the perfect gas law:

(P2) = (P1)(T2/T1)

Using Rankine (absolute) temperatures and absolute pressures, it is easy to see how relatively minor temperature increases may cause pressures to rise by amounts that cannot be ignored. As pressures go up beyond the rupture strength of oil films, the sealing oil film will be temporarily interrupted. The bearing housing seal will open up for a fraction of a second.However, rising bearing housing pressure may lead to one of two undesirable events:

• During a pressure rise, the oil level will go down in the bearing housing and will rise in the lower surge chamber supporting the lubricator bottle (Bernoulli’s law).If the housing level drops sufficiently, the bottom of the bearing may be deprived of oil, or slinger ring immersion may no longer be sufficient for satisfactory lubrication.

• If oil overflows the surge chamber during a pressure rise, it is lost from the system, while the pressure in the lubricator adjusts to the new housing pressure. Oil from the bottle refills the housing when the system re- equilibrates with each“burp.” As the cycle repeats itself, more and more oil will be lost. These problems are caused by system features that allow for pressure differentials between the housing interior and the environment. There is evidence that installing bearing isolators and other narrow-gap housing closures increases the likelihood of potential problems with lubricators that had served perfectly well as long as not-so-close fitting labyrinth seals were being used. Fortunately, the pressure-balanced constant-level lubricator does not introduce the same risk. As mentioned earlier, the oil levels “x” at locations inside the bearing housing and inside the lubricator are always exposed to identical pressures. The problem is solved, and another step towards increased equipment reliability has been implemented. Only Pressure equilibrated level and not OTA-type lubricators should be used in reliability-minded plants. By providing a piping connection between the lubricator and bearing housing, lubricator port “x” and the bearing housing operate at the same pressure and the risk of oil leakage or unintended lowering of oil levels due to pressure buildup in bearing housings is eliminated. This approach provides protection from airborne contaminants, as well. Disadvantages? The pressure-balanced constant-level lubricator probably costs more, but the resulting reduction in the risk of failure far outweighs any disadvantages, both perceived and real.