In a similar manner, stress corrosion may begin in grooves
or crevices or where impellers are mounted on the shaft. As
corrosion proceeds, the useable area of the shaft is reduced and
failure eventually results.
Excess power may also be a cause. When pumping liquids
with high specific gravity, such as sulphuric acid, the normal
power required by the pump is increased proportional to the
specific gravity. The shaft may be designed to pump water but
high specific gravity can cause an overload.
Finally, the shaft material may not meet the original
specification. Replacement shafts may be procured from non-OEM shops which may not know when special materials are
needed.
Q. What is an inducer and how does it work?
A. Inducers are single stage axial flow helixes installed in the
suction eye of centrifugal pump impellers to lower the NPSHR
of the pump (see Figure 1. 79). This allows use of increased
rotating speed for a given NPSHA or a lower NPSHR for a
given speed. Shallow blade inlet angles are used to draw liquid
into the inducer channels, which are shaped to impart enough
energy to provide sufficient NPSH for the main impellers to
avoid detrimental cavitation.
Whereas centrifugal pump
impellers often have a suction
specific speed, S, of about
8000 to 12,000, use of inducers
can increase S to a range of
15,000 to 35,000.
Whereas centrifugal pump impellers often have a suction
specific speed, S, of about 8000 to 12,000, use of inducers
can increase S to a range of 15,000 to 35,000. This typically
allows lower flow pumps to operate at 11,000-rpm with the
same NPSH, which would be required at 3600-rpm without
inducer, or reduce NPSHR of all pumps to less than half at
the same speed.
Cavitation damage of inducer blades may occur above
certain experience-established inducer tip speeds. At lower
tip speeds, cavitation will not produce damage regardless of S
value. Allowable tip speed is also a function of inducer materials; e.g., titanium alloy is 20 percent higher than for stainless
steel. Tip speeds may also be increased for lower specific gravity liquids.
Figure 1. 79 – Inducer
Inducer flow range must be designed and selected to provide sufficient S over the intended operating range to avoid
cavitation in the main impeller. Suction specific speed typically is reduced at low and high ends of inducer performance
range.
P&S
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OEMs as a means of ensuring a healthy dialogue on subjects
of common technical concern.
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designed to help eliminate misunderstandings between the
manufacturer, the purchaser and/or the user and to assist the
purchaser in selecting and obtaining the proper product for
a particular need.
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