Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

536 Centrifugal Pumps: Design and Application
minute) did not exceed 300,000 [15]. Studies showed, on the other hand,
that close-fitting seals can cause high frictional heat and that loose fitting
seals cannot effectively exclude atmospheric air and moisture that will
cause grease deterioration. It should be assumed that these facts preclude
the use of lubed-for-life bearings in installations that expect "life" to last
more than three years in the typical petrochemical plant environment.
Moreover, we believe this to be the reason why some bearing manufacturers
advise against the use of sealed bearings at DN values in excess of
108,000. At least one bearing manufacturer considers the DN range between
80,000 and 108,000 the "gray'* area, where either sealed or open
(shielded) bearing would be acceptable. The DN range below 80,000 is
generally considered safe for sealed bearings. This would generally exclude
sealed bearings from all but the smallest centrifugal pumps.
Oil Viscosity Selection
The Oil Viscosity Selection Chart (Figure 20-2) may be used as a guide
for the selection of the proper viscosity oil. The chart may be used for
applications where loads are light to moderate with moderately high conditions
of speed and temperature.
Applications of Liquid Lubricants in Pumps
The amount of oil needed to maintain a satisfactory lubricant film in a
rolling element bearing is extremely small. The minimum quantity required
is a film averaging only a few micro-inches in thickness. Once this
small amount has been supplied, make-up is required only to replace the
losses due to vaporization, atomization, and seepage from the bearing
surfaces [12].
How small a quantity of oil is required can be realized when we consider
that 1/1000 of a drop of oil, having a viscosity of 300 SUS at 100°F
(38°C) can lubricate a 50 mm bore bearing running at 3,600 RPM for
one hour. Although this small amount of oil can adequately lubricate a
bearing, much more oil is needed to dissipate heat generated in high
speed, heavily loaded bearings.
Oil may be supplied to rolling element bearings in a number of ways.
These include bath oiling, oil mist from an external supply, wick feed,
drip feed, circulating system, oil jet, and splash or spray from a slinger
or nearby machine parts.
One of the simplest methods of oil lubrication is to provide a bath of oil
through which the rolling element will pass during a portion of each revolution.
It can be shown that only a few drops of oil per hour would
satisfy the lubrication requirements of a typical rolling element bearing in