VIBTOOL 02.2009
39
Cavitation measurement
What is cavitation?
If the liquid pressure at the inlet of a pump housing
becomes less than the vapor pressure, a portion of the
liquid turns into saturated vapor and forms bubbles. As the
pressure increases, the vapor condenses and the bubbles
implode. These implosion forces - known as cavitation - can
damage or even destroy the impeller. Fortunately, these
cavitation forces exhibit certain characteristics which allow
them to be measured quite accurately. Damage can there-
fore be anticipated well in advance of actual machine
failure.
VIBTOOL
allows measurement of cavitation in pumps,
turbines and other rotating equipment and valves. Cavita-
tion is generally considered to cause a 2-3% loss in
efficiency. Before now, some pump manufacturers and
pump users have had the wrong opinion of how to prevent
cavitation. It has been assumed that cavitation could
always be heard and therefore somehow prevented. This is
not the case and never will be. Depending on the pump
construction and on the running condition of the pump,
cavitation cannot normally be heard.
If cavitation is detected in its early stages, damage to
impellers and pump housings can be prevented, the condi-
tion can be remedied and the lifetime of the pump can be
extended. This is why PRÜFTECHNIK Condition Monitor-
ing has developed VIBTOOL with the cavitation measure-
ment function. This function allows the measurement of
the high frequency pulses in the pump housing. The source
of the pulses can be localized by simply placing the
VIBTOOL sensor on the inlet of the pump housing while the
pump is running. What is more important is that it is
possible to change the pump velocity or the suction height
while measuring. The point at which cavitation ceases in
the pump can then be clearly seen on VIBTOOL display.
Cavitation measurement
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