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8.2.4.1 Medium density stock pumps
Medium density applications can be handled successfully by a
centrifugal pump. However, this capability is greatly dependent on the
fluidity of the stock and the ability of the system to deliver it freely to
the impeller eye.
Special stock pump designs have been produced and are reputed to be
able to deliver the same head and flow for stock as for water at
consistencies of up to 5-7%. As the power consumption and efficiency
remain almost unchanged, this would result in dramatic reductions in
horsepower requirements.

Other centrifugal pumps such as that shown in Figure 8.8, are being
used on increasingly higher consistencies using a special rotor device
extending into the suction line. This device fluidizes the stock and
allows it to be handled by the centrifugal impeller.
The piping arrangement on the inlet side of the pump is especially
important. Stock must be delivered freely to the pump, and with good
cross-sectional distribution, to prevent slugs of stock from entering the
impeller eye. The pipe should be a minimum of one to two pipe sizes
larger than the pump inlet and should be straight with no high spots
where air pockets can collect.
Unlike conventional pumps however, the stock pump often requires a
short suction line and a short, stepped suction reducer at the pump

Figure 8.8. Medium density pump (Reproduced with permission of Sulzer Pumps)

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