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Suction Conditions

the-shelf' pump styles that are used by the vast majority of industry.
These are special pumps only. All pump manufacturers design their
standard pump range to operate with an NPSH value that is tested at a
3% head drop.
Consequently, every major pump manufacturer can identify the NPSH
required by their pump when operating at a particular Head-Capacity
condition. However it is important to recognize that, if no more than
that amount is supplied, the pump will be cavitating, but at such a low
level of energy that the resulting symptoms (i.e. noise, vibration and
impeller damage) will be difficult to detect, and the long term
detriment to the operation of the pump will be minimal.

There is a tendency in many areas to try and combat Cavitation by
reducing the NPSH Required by the pump. It is worthwhile to realize
that, to accomplish this, there are only a limited number of possibilities.

4.4.1.1 Increase the eye area of the impeller
As this option can cause more trouble that it solves by introducing
recirculation difficulties, it is not recommended. It should only be
considered as a last resort, and only with the full design involvement of
the pump manufacturer.

4.4.1.2 Install a suction inducer

As very few pump manufacturers have suction inducers available, the
practical application of this option will be severely limited. Even the few
that are available must be approached with caution as they are likely to
affect the pump performance at lower flows.

4.4.1.3 Use a double suction impeller
As the liquid flows into the impeller through two opposing eyes, a
double suction impeller uses approximately 67% of the NPSH that is
required by a single suction impeller in an equivalent size. This
modification would necessitate a change of pump.

4.4.1.4 Use a slower speed pump
A slower speed requires less NPSH and will also necessitate a change to
a much larger pump with a bigger impeller in order to accommodate
the same performance conditions.

4.4.1.5 Use lower capacity pumps
A smaller, lower capacity pump also requires less NPSH, but will
necessitate a change to multiple pumps in order to accommodate the
same performance conditions.

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