gap between the lining and the shaft.
The new O-ring seal design was implemented and has
been successful in eliminating that failure mode for this pump
design in the plant. All was quiet again with parts issues from
the OEM until another strange failure mode appeared in
the form of large divot marks (i.e. rubbing wear damage) on
the back of the PFA lining on one pump impeller. Figures 8
and 9 show the details of the divots found on the back of the
impeller.
More meetings have been held and discussions on corrective actions are ongoing with the OEM. To date, no significant
concrete changes have been proposed by the OEM to address
Figure 9. Impeller divot close-up.
any of the suspected root causes for this newly observed defect.
Evidence thus far suggests this defect is the result of thermal
deformation and “lifting” of the PFA lining, which spans the
five “fingers” of the metal core. In this regard, the design flaw
may be a combination of inadequate stiffness or inadequate
restraint of the PFA lining on the back of the impeller, combined with overly tight impeller-to-housing clearances.
In this context, the result is a product that does not have
a rugged or tolerant operating envelope. Thus it may work
acceptably in many services, but fail repeatedly in others. The
pertinent point here is that this type of failure may occur solely
as the result of design “frailness” and not from meaningful
