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......................................................................... Special Applications

Hard irons are used to resist erosion and, to a lesser extent, impact
wear.
Elastomeric materials are used when pumping concentrations of fine
material but total head is usually restricted to about 100 feet per stage.
Castable ceramic materials have excellent resistance to cutting erosion
but impeller tip velocities are usually restricted to 100 ft./second.
The last 2 sections are reproduced with permission of the Hydraulic
Institute.

8.2 Paper stock

Paper Stock consists of cellulose fibers, up to about 1/4 inch long,
suspended in water. Once the stock has been washed or screened to
remove unwanted chemicals or impurities, it is either beaten or refined
to enhance the sheet properties. Various additives, such as starch, alum,
size or clay fillers, are then introduced to create the required
characteristics of the paper product being produced.
When pumping Paper Stock, a number of difficulties are experienced,
all of which are attributable to the nature of the basic product. If these
difficulties were encountered individually, they could be handled quite
easily. Collectively, they present a bigger challenge.
The simple presence of the fibers necessitates a pump capable of
handling small particles that, in high densities, may have a tendency to
clog the impeller. While lighter duty services will use the conventional
chemical process pump, higher flows and stock consistencies are
handled by a Stock Pump (Figure 8.7). The latter is usually a much
more robust design than an equivalent ANSI model.

While the physical size of the individual fibers is not a problem, they do
have a tendency to float in water. This requires constant agitation of the
stock to minimize the possibility of stratification. Unfortunately, such
agitation can introduce air which, in addition to being detrimental to
the stock, can cause considerable pumping difficulties.
Paper stock acquires an affinity for water as it is beaten or refined. The
retention of water by the stock increases the friction factor which is not
usually significant when the piping velocities are maintained at their
normal rates. However, at higher velocities, a heavily beaten stock with
a low freeness value becomes slippery and difficult to pump.
The consistency of a pulp and water suspension is the percent by weight
of pulp in the mixture. Bone Dry (B.D.) consistency is the amount of
pulp left in a sample after drying in an oven at 212~ or above. Air Dry

131 m