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8.2.2 Excessive discharge throttling
While it is realized that excess capacity is normally required over the
paper machine output in tons per day, the 'over-selection' of pumps on
the basis of capacity and head usually results in the necessity of
throttling the pump at the vane in the discharge line. Since the vane is
frequently located adjacent to the pump, the restriction of the vane and
the high velocity within the valve will result in some dehydration and
cause vibration due to slugs of stock. Vibration at the valve due to
throttling is transmitted to the pump and may reduce the normal life of
the pump rotating element.

Centrifugal pumps operating at greatly reduced capacity have more
sever loading internally due to hydraulic radial thrust as discussed in
Chapter 2.5. Hence pumps selected too greatly oversize in both
capacity and head have the combination of the vibration due to
throttling plus the greater internal radial load acting to reduce the life
of the rotating element. As a general rule, stock pumps should not be
operated for extended periods at less than one quarter of their capacity
at maximum efficiency. When excessive throttling is required, one of
the two methods below should be employed.
Revise capacity requirements and check the static and friction head
required for the capacity desired. Reduce the impeller diameter to
meet the maximum operating conditions. This will also result in
considerable power savings.
Install a by-pass line upstream from the discharge vane back to the
suction chest below the minimum chest level, if possible, and at a
point opposite the chest opening to the pump suction. This by-pass
line should include a vane for flow reduction. This method is
suggested where mill production includes great variation in weight
of sheet.

.2.3 Filters and additives
The presence of fillers and chemical additives such as clay, size and
caustics can materially increase the ability of paper stock to remain in
suspension. However, overdosing with additives such as alum, may
cause gas formation on the stock fibers resulting in interruption of
pumping.
The last 3 sections are reproduced with permission of Goulds Pumps, ITT
Industries

8.2.4 Paper stock pumps
Standard centrifugal pumps have difficulty handling stock over 6% and
consequently special designs are necessary, and are currently referred to
as Medium Density and High Density pumps.

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