Page 88 - Root Cause Failure Analysis

In-Line In-line configurations have all impellers facing in the same direction. As a
result, the total differential pressure between the discharge and inlet is axially applied
to the rotating element toward the outboard bearing. Because of this configuration. in-
line pumps are highly susceptible to changes in the operating envelope.

Because of the tremendous axial pressures created by the in-line design, these pumps
must have a positive means of limiting end play, or axial movement, of the rotating ele-
ment. Normally, one of two methods is used to fix or limit axial movement: (1) a large
thrust bearing is installed at the outboard end of the pump to restrict movement or (2)
discharge pressure is vented to a piston mounted on the outboard end of the shaft.

Method 1 relies on the holding strength of the thrust bearing to absorb energy gener-
ated by the pump’s differential pressure. If the process is reasonably stable, this
design approach is valid and should provide a relatively trouble-free service life.
However, this design cannot tolerate any radical or repeated variation in its operating
envelope. Any change in the differential pressure or transient burst of energy gener-
ated by flow change will overload the thrust bearing, which may result in instanta-
neous failure.

Method 2 uses a bypass stream of pumped fluid at full discharge pressure to compen-
sate for the axial load on the rotating element. While this design is more tolerant of
process variations, it cannot compensate for repeated, instantaneous changes in
demand, volume, or pressure.

Opposed Multistage pumps that use opposed impellers are much more stable and
can tolerate a broader range of process variables than those with an in-line configura-
tion. In the opposed-impeller design, sets of impellers are mounted back to back on
the shaft. As a result, the thrust or axial force generated by one of the pairs is canceled
by the other. This design approach virtually eliminates axial forces. As a result, the
pump requires no massive thrust bearing or balancing piston to fix the axial position
of the shaft and rotating element.

Since the axial forces are balanced, this type of pump is much more tolerant of
changes in flow and differential pressure than the in-line design. However, it is not
immune to process instability or the transient forces caused by frequent radical
changes in the operating envelope.

performance
This section provides the basic knowledge needed to evaluate a centrifugal-pump
application to determine its operating dynamics and identify any forcing function that
may contribute to chronic reliability problems, premature failure, or loss of process
performance.

Centrifugal pump performance is controlled primarily by two variables: suction condi-
tions and total system pressure or head requirements. Total system pressure comprises