Page 229 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

The O-ring seal gripping the shaft moves
1,800 rpm x 2 movements/revolution = 3,600 movements/
minute.

3,600 movements/min x 60 minutes/hour = 216,000
movements/hour
216,000 movements/hour x 24 hours/day = 5,184,000
movements per day
At 5,184,000 movements (rubs) per day on a shaft spinning at 1,800
rpm the constant friction will eat a fret mark (groove) into the shaft or
sleeve in just a few days (Figure 14-12). The next O-ring seal, installed
onto this pump, will ride in the groove (cut by the previous seal) and
never give good service. Again, this is an alignment problem and not a
seal problem. Many seal companies have addressed fretting corrosion
with a product called self-aligning faces. Both the rotary and stationary
faces are spring loaded as the faces push against each other. The
opposing springs tend to cancel themselves and the union between the
seal faces will always be perpendicular to the shaft axis. This prevents
the flexibly mounted shaft seal from dancing and rubbing on the shaft if
the pump parts should be out of alignment. Another way to resolve
fretting corrosion is to align the pump parts upon rebuilding the pump.

SHAFT
[ FRElTlNG

I I -- I

t

Figure 14-12 __

Incorrect installation dimension

Seal faces loaded with over-compressed spring tension, can generate too
much heat. This will damage the O-ring and even fiacture the seal faces
from thermal expansion (Figure 14-13). If the spring tension is
inadequate, the faces can leak after a short while as the softer face wears
against the harder face, and the spring tension relaxes completely. The
installation dimension is critical to long seal life.
Most seal companies provide a set of instructions and engineering
schematic with each mechanical seal to help the mechanic determine
the correct installation dimension. These instructions assume: