Page 183 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Rolling element bearings lubricated with grease can operate safely in
the 200” F range. In fact, the upper temperature limit of the grease is
the real operating limit of the bearing. It’s not the bearing metallurgy.
The temperature at which the grease carbonizes is the bearing’s
operating limit. Bearings are perfectly safe at 160” F. This is actually
good for a bearing because of the expected lubricant flow at this
temperature.

It’s obvious that all bearings will operate at some temperature above
the surrounding environment, without additional cooling. The
resulting temperature is composed of three factors. First, frictional heat
is generated inside the bearings from contact between the rolling and
stationary elements. Second, conductive heat is added to frictional heat.
This is heat from the shaft, bringing the temperature of the pumped
liquid, and also the radiated heat of surrounding equipment in the area.
Third, the amount of heat to be dissipated away from the pump’s
bearings is a hnction of the conductivity of the lubricant, the surface
area of the bearing housing, and the temperature, and motion of the
surrounding air. These three factors work to bring about a stable
operating tcmpcraturc. This temperature should be less than the upper
limit of the oil or grease.
You need to investigate an unexplained rise in the bearing temperature.
This could indicate imminent failure. You can add one more shot of
grease, but if the temperature doesn’t reduce immediately, don’t
continue adding more grease. First, rule out obvious reasons for the
increase. It could be that the temperature in the surrounding area has
changed. Has the weather changed? Has new heat generating
equipment been installed in the vicinity? Has there been a change in the
temperature of the pumped liquid? Next, check the assembly for
unnecessary thrust and radial loading, coupling misalignment, or over-
tightened pump packing.
Remember again that the temperature can go up from improper
lubrication practices. Excessive grease causes the bearing to sling and
pack the grease against the internal housing wall. The grease becomes
an insulator and the bearing will run dry if the grease cannot return to
the sump. Excessive oil causes foaming and air bubble entrainment as
the rollers and balls crash into the fluid. Air is a good insulator and the
air doesn’t lubricate or dissipate heat. Insufficient oil or grease leads to
increased friction from metal to metal contact. Inadequate oil and
grease are also sources of excessive frictional heat.

Pumps that handle hot fluids have bearing chambers designed with
thermal jackets and heat exchangers installed at the factory. These
devices have connections for isolated water flow through, or around the
bearing housing. You should not use high temperature grease with
artificially cooled bearings. The grease won’t flow properly. The result