Page 94 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 2 .13 Pump Best Practices
the field, the largest loss of revenue is usually due to lost Regardless of the proposed action, be sure to show lost
product revenue resulting from unplanned critical equipment product revenue against capital investment for the problem
downtime. solution.

Best
Best
Best Practice 2.13Practice 2.13Practice 2.13
Obtain references from pump vendors for pump bearings Lessons Learned
with DN numbers above 350,000 when ring oil lubrication is Pump bearings with a DN number above 350,000 are prone
being offered to determine if oil circulation in the bearing to low bearing MTBF, and should be supplied with a small
housing is required. dedicated pressurized lubrication system.
DN is a measure of the surface speed of the shaft. It is the product Many pump bearing low MTBFs (less than 12 months) have been
of the shaft speed in revolutions per minute and the bore of the bearing resolved by installing small, dedicated, lube oil circulation systems
in millimeters. whenever pump vendor experience dictates that the pump will operate
The higher the DN number, the greater the frictional heat that will be at a DN above 350,000.
generated by the bearing.
DN values above 350,000 produce a significant amount of heat, and Benchmarks
require oil circulation through a cooler to maintain the proper oil vis- This best practice has been used since the mid-1990s to increase
cosity in the bearing. pump high DN bearing MTBFs from less than 12 to greater than 100
Small oil consoles will solve the problem and result in a pump with months.
a high MTBF. They can be easily retrofitted in the field if not originally
supplied.

B.P. 2.13. Supporting Material The L-10 rating life for anti-friction bearings is defined as the
number of hours at rated bearing load that 90% of a group of
identical bearings will complete or exceed (25,000 hours of
Bearings operation) before the evidence of failure. Failure evidence is
generally defined as a 100% increase in measured vibration.
Functions of bearings API Standard 610 recommends applying pressurized hydro-
Bearings provide support for the rotating element. They are dynamic bearings when the product of the pump rated horse-
required to carry radial and axial loads. The basic relationship for power and the rated speed, in revolutions per minute, exceeds
any bearing design is: 2.7 million. Pressure lubrication systems may be integral or
F
P ¼
Where : A
Increased process pipe forces and moments
Foundation forces (‘soft’ foot, differential settlement)
P ¼ Pressure on the supporting oil film
Fouling or plugging of impeller
ðhydrodynamicÞ or element ðanti-frictionÞ Misalignment
in P:S:I: Unbalance
F ¼ The total of all static and dynamic forces acting Rubs
on the bearing in lb force Improper assembly clearances
2 Thermal expansion of components (loss of cooling medium,
A ¼ The bearing area in in excessive operating temperature)
Radial forces (single volute – off design operation)
In many cases, a pump bearing may perform for years and
Poor piping layouts (causing unequal flow distribution to the
suddenly fail. This abrupt change in performance characteristics
pump)
is usually due to a change in the forces acting on it. Factors that
can increase bearing load are shown in Figure 2.13.1.
In this section we will discuss the functions and various types of Fig 2.13.1 Sources of forces
bearings used in pumps. There are three (3) basic types of bearings
that are used in centrifugal pumps (refer to Figure 2.13.2).
Anti-friction
Application guidelines Hydrodynamic ring oil lubricated
Hydrodynamic pressure lubricated
Centrifugal pumps in the process industry are normally fitted
with bearings appropriate to the application and pump design
(refer to Figure 2.13.3). Fig 2.13.2 Basic types of bearings

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