Page 227 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

The pump shaft must be free to slide through the O-ring. However,
crystallizing liquids, accumulated solids and pastes, and products that
harden can cause the O-ring to hang-up or stick to the shaft losing its
freedom. Oversized shafts and sleeves can also be blamed. If the seal is
misaligned onto the shaft the O-ring can fret the shaft and hang into its
groove. Heat and chemical attack can also make the O-ring vulcanize to
the shaft. Any of these can occur. As the shaft moves within the
tolerance of the bearings, it can drag the seal faces open or crush them
together if the O-ring sticks to the shaft.

The springs clog and jam

Suspended solids, crystals and sediment in the pumped liquid can lodge
into the seal springs and restrict their movement. Jammed springs
cannot flex to maintain the seal faces united while the shaft moves
within the bearing axial tolerance. It’s best to use seals designed for
slurries and solid particles. Many of these designs have the springs
placed outside the pumped fluid. The seal chamber design and piping
also has an influence on the seal’s ability to survive while handling solid
particles. Seal chambers designed with tapered, spiraled, and open bores
facilitate the handling of suspended solids (Figure 14-9).
The discharge re-circulation line is a dinosaur of design, held over from
the days when pumps had packing. With packing, the discharge bypass
line prevented the entrance of air through the packings on pump start-
up, and also provided some cooling to the packing rings. With a
mechanical seal in slurry service, this discharge bypass line blasts the
mechanical seal and chamber with the highest concentration of solids in
the pump. It will destroy the seal in short order. If some cooling and
flushing is desired, a suction bypass line, from the seal chamber to the
pump suction nozzle, is preferred over a discharge bypass line, from the
pump discharge to the seal chamber.

II 1
I I I

Fiaure 14-9

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