Page 175 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Works well through heat exchangers with refrigerated water for
additional cooling in large high-speed bearings.
There are some disadvantages to using oil, including:
Difficult to seal and retain inside the bearing chamber requiring
frequent refilling.
Oil levels must be checked with more frequency as the temperature
rises.
Lower viscosity and more leaks as the temperature rises.

Grease
If you combine petroleum based or synthetic based oil with a suitable
thickening agent, you get grease. The thickening agent can represent
between 3% and 30% of the total volume of grease.
The amount of thickening agent and the oil’s viscosity will determine
the consistency or stiffness of the grease. For general industrial service,
popular thickening agents used to make grease are soaps of calcium,
sodium, aluminum, and lithium, and combinations of these. All greases
have certain characteristics, but they should not be categorized for
usage according to the thickening agent. For example, just because
grease is thickened with sodium soap doesn’t mean it is necessarily best
in a particular service. No particular grease can satis@ all requirements,
although considerable effort has been invested in developing a multi-
purpose lubricant.

Grease is selected for use in a pump bearing for its advantages,
including its resistance to dripping and running, and it’s easy to seal
and retain in the bearings. Grease also has some disadvantages. It is
subject to separation and oxygen decomposition. It is difficult to clean
and remove old grease from the bearing assembly.

Bearing lubrication in pumps
The majority of industrial pumps are designed with a horizontal shaft
assembly. Therefore, more pump bearings are lubricated with oil
contained in the bearing chamber. Grease is preferred for bearings on
vertical pump shafts. Lip seals, labyrinth seals or mechanical seals
are used to seal the bearing housing where the spinning shaft
passes through, keeping any contamination out of the bearings and
holding the lubricant into the housing. Seals are discussed later in this
chapter.

Correct oil level is critical. Consult the owner’s manual. As a general
guideline, the oil level should be half way up the lowest ball or roller in
the bearing assembly. Too much, or too little oil will lead to premature