Section 6.1.1 - Centrifugal Pump Bearings: Operation

Operation   Heat is generated during bearing operation for various reasons. The lubricant
is subject to shear forces as it circulates in the bearing. Excess oil in the bearings increases
churning and shearing resulting in elevated operating temperatures. Energy from
deflection of the bearing metal occurs on both the bearing races and the rolling element
itself, so a highly loaded bearing will run hotter than a lightly loaded bearing. Friction
occurs from the rolling element as it revolves in the retainer pocket. Friction occurs from
any relative motion or sliding of the rolling element to the bearing race. Bearings whose
rolling elements are skidding run very hot. Highly preloaded bearings also tend to run
hotter than lightly preloaded or non-preloaded bearings, provided no skidding is occurring.

The operating speed, the type of rolling element, angular operation of a rolling element,
the type of bearing retainer, the number of rolling elements, and ambient temperature can
also influence operating temperature. Higher speeds result in increased lubricant shear
and retainer friction. Roller-type bearings increase lubricant shear, retainer friction, and
sliding friction compared to ball-type bearings. Bearings whose rolling elements operate at
an angle such as angular contact ball bearings and spherical roller bearings are more
prone to sliding and skidding which increases friction. These types of bearings also utilize
a heavy-duty retainer that operates in close proximity to the rolling element, resulting in
more lubricant shear and friction. Maximum capacity bearings (when allowed) have a
higher quantity of rolling elements than standard-capacity bearings, which increases
lubricant shear and friction. Bearing operating temperature is approximately proportional
to ambient temperature.

As can be surmised from the above discussion, bearing operating temperatures vary
widely. Bearing manufacturers generally claim a maximum allowable bearing metal operating
temperature of 250^oF (121^oC), but this is rarely utilized in practice. The practical
limit of operation is considered to be 200^oF (93^oC), though there are many successful applications
operating at higher temperatures. 180^oF (82^oC) is the upper temperature limit of
mineral-type oils to achieve a reasonable lubricant life. Mineral oils operating above this
temperature oxidize very rapidly, resulting in what has become known in the industry as
black oil. Synthetic oils are utilized successfully at higher bearing temperatures. The following
operating temperature limits have been established for API Standard 610 (ISO
13709) pumps, based on an ambient temperature of 110^oF (43^oC):

• For ring-oiled or splash systems, the oil sump temperature should not exceed 180^oF
  (82^oC). Bearing outer ring temperatures should not exceed 200^oF (93^oC).
• For pressurized lubrication systems, the outlet oil temperature should not exceed 160^oF
  (70^oC) and bearing metal temperatures should not exceed 200^oF (93^oC).
• During a shop test with a pressurized system, under the most severe operating
  conditions specified, the bearing oil temperature rise should not exceed 50^oR (28 K).

References

1. American Petroleum Institute Standard 610. “Centrifugal Pumps for Petroleum, Petrochemical
   and Natural Gas Industries.” 10th Edition, October 2004, www.api.org
2. Specifications for Centrifugal Pumps for Chemical Process: ANSI/ASME B73.1M–
   1991, Horizontal End Suction; American Society of Mechanical Engineers, New York,
   NY www.asme.org
3. ISO 281, “Rolling bearings—Dynamic load ratings and rating life.”
4. ISO 5753, “Rolling bearings—Radial internal clearance.”
5. ISO 13709:2003, “Centrifugal pumps for petroleum, petrochemical and natural gas
   industries.”

Operation   Heat is generated during bearing operation for various reasons. The lubricant
is subject to shear forces as it circulates in the bearing. Excess oil in the bearings increases
churning and shearing resulting in elevated operating temperatures. Energy from
deflection of the bearing metal occurs on both the bearing races and the rolling element
itself, so a highly loaded bearing will run hotter than a lightly loaded bearing. Friction
occurs from the rolling element as it revolves in the retainer pocket. Friction occurs from
any relative motion or sliding of the rolling element to the bearing race. Bearings whose
rolling elements are skidding run very hot. Highly preloaded bearings also tend to run
hotter than lightly preloaded or non-preloaded bearings, provided no skidding is occurring.

The operating speed, the type of rolling element, angular operation of a rolling element,
the type of bearing retainer, the number of rolling elements, and ambient temperature can
also influence operating temperature. Higher speeds result in increased lubricant shear
and retainer friction. Roller-type bearings increase lubricant shear, retainer friction,...