Air Bubble and
Cavitation Vibration Signatures of a Centrifugal Pump
Lin Liu,
Zhuang Li,
Suri
Ganeriwala
SpectraQuest Inc., 8201 Hermitage Road, Richmond, VA 23228
Published: May, 2006
Abstract
In this work, a centrifugal pump was tested for
its vibration signatures under different operational conditions. The
two abnormal operating conditions studied are air bubble and
cavitation. A transparent plastic cover was used in the experiments
to observe the cavitation. It was found that the pump has higher
vibration amplitude in the axial direction than in the radial
direction. From the experiments, it was also determined that
significant amount of air bubbles will increase vibration component
associated with impeller vane pass frequency significantly.
Cavitation might excite high frequency structural resonance. It may
also reduce the impeller vane pass frequency vibration. Although
cavitation is less likely to happen on a slow speed pump, it will
develop very fast if it happens
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Introduction
A pump is a
mechanical device used to move liquids. Mechanical energy is
transformed into hydraulic energy at the pump. Pumps can be
classified into two categories: displacement pumps and centrifugal
pumps. In this work, a centrifugal pump was tested and studied.
In a centrifugal pump, a
rotating element called an impeller is enclosed in a case. The fluid
to be pumped enters into the case through the suction piping into
the impeller and is forced out the discharge side of the pump by
means of centrifugal force pushing the fluid out radially through
the impeller. The fluid is discharged at a higher pressure and a
higher velocity. The major portion of the velocity energy is then
converted into pressure energy by means of a volute or by a set of
stationary diffusion vanes surrounding the impeller periphery. In a
pump, vibration is caused by the interaction between the moving pump
impeller and the stationary parts of the pump such as the volute and
the diffuser vanes. Also, vibration is caused by the interaction
between the impeller blades and the fluid being pumped.
One of the important
phenomenon in pump is cavitation. Cavitation occurs when the
pressure of the fluid drops below the vapor pressure for the
temperature of the fluid. When this pressure drop occurs, whether it
is a system pressure drop or a localized pressure drop, voids or
cavities (bubbles) will form in the liquid. These bubbles implode or
collapse when the fluid moves through impeller to the high pressure
side of the pump, causing the impeller to erode. These implosions
tear out tiny pieces of the metallic surface near which they
implode. This can be very damaging and eventually the impeller will
fail.
Fig 1: Schematic
representation of the cavitation process.
Fig 2: Pump Vibration with
Appearance of Cavitation (3616 RPM)-Axial Vibration (1 KHz)
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