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BULLETIN BOARD

QUESTIONS AND ANSWERS



Dry versus wet critical speed and WR^2

Q
- Occasionally I would come across the term “dry critical speed” and “wet critical
speed”, or “dry WR^2” or “wet WR^2”. What do the words “dry” and “wet” refer to?

A - In the technical terms mentioned, “dry” refers to a dry rotor, meaning the rotor
is not submerged in the pumped liquid and the liquid’s effect is excluded in the
data, or analysis. “Wet” refers to a submerged rotor, meaning the liquid’s effect is
taken into consideration. The “wet” data are the data that matter most. The “dry”
data are typically used for comparison only, and are less important.

In some cases, a number of “dry” and “wet” analyses are done on simple rotor
systems to come up with empirical ratio between “wet” and “dry”. Then, in a
complex rotor system where a “wet” analysis maybe impractical” to do, a “dry”
analysis is done instead and the empirical factor is applied to estimate the “wet”
value.

This is particularly handy in estimating “wet WR^2”. It is relatively easy to calculate
“dry WR^2” and by applying an empirical factor the “wet WR^2” is easily estimated
without going through some tedious calculations.


Bearing lubrication tests

Q
- There are many ways of lubricating non-friction (ball) bearings – by oil ring,
mist oil, or purge oil method. Are there available data from actual tests that show
which method is most effective in keeping the lubricating oil temperature low?

A - We have several bearing temperature tests that show the relative oil and
bearing housing temperature rise, over a period of time, based on oil ring, mist
oil, and purge oil lubricating systems. The comparative test results are available
only by request.



How to make head rise to shut-off steeper

Q
- We sold two multistage pumps for boiler feed application where the contract
specifies their head rise to shut-off to be a minimum of 15% from the rated flow.
Based on historical test records the pumps will barely make the 15% head rise.
Now we are concerned that with small variance in performance the pumps may
fall short of the 15% rise during the witness test. Is there anything we can do to the
impellers to increase their head rise to shut-off to ensure our compliance with the
contract?

A - There is an effective way of machining the impellers to increase their head rise
and make the performance curve steeper. This method is repeatedly proven
effective in several tests in both single and multistage pumps. For a description of
the impeller machining method, contact CPO.



Pump efficiency

Q
- Is the shape of the pump volute throat area opening affects the pump
efficiency?

A - Yes, studies have shown the shape of the volute throat area opening and, by
extension, of the volute crossover has an effect in pump efficiency. This explains
why some shapes are more commonly used than others by pump designers.
This is one weakness of many performance prediction softwares - they do not
factor in the effect of the volute shape and its diffusion angle or area ratio.

Volute throat area opening have been designed with circular, square, rectangular
(tall), rectangular (wide), and trapezoidal shapes. Which shapes are
recommended? Contact CPO.


Impeller trimming

Q
- In trimming the diameter of an impeller, should both the vanes and shrouds
be cut to the same diameter, or should it be the vanes only?

A - In almost all cases, both the impeller vanes and shrouds should be cut the
same diameter. Cutting the vanes only, and leaving the shrouds at their original
diameter, will result in slightly lower head and efficiency due to the additional
friction loss (or disk friction) on the uncut portion of the impeller shrouds.

But there are few exceptional cases where it will be preferable to keep the
shrouds uncut such to mitigate a severe discharge flow recirculation problem.


Rotor vibration and balancing

Q
- Should a pump rotor be balanced regularly?

A - A rotor can become out of balance after a period of operation due to loss of
material resulting from normal wear, cavitation, corrosion, erosion, and the like. If
a pump starts to show an increasing level of vibration over a period of time, when
there has been no change in its operating environment, it is likely that the rotor
has become out of balance, and re-balancing the rotor can bring back the pump to
run smoothly.

Out-of-balance vibration is manifested by high vibration level at a frequency
corresponding to the pump rotational speed.


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