All mechanical machines vibrate and pumps are no exception. Even the best
designed and high quality built pumps produce vibration of magnitudes that are
acceptable under most conditions. It is when the magnitudes of vibration exceed
established standards, or some mutually agreed upon limits between the pump
users and vendors, that the vibration can be troublesome and possibly lead to
pump damage and failure.

The principles of rotordynamics in centrifugal pumps, and the underlying
concepts involving rotor vibration issues, their root causes and solutions, are
complex enough that many articles and handbooks are published on this topic
alone. Vibration specialists are given various levels of accreditation depending
on their increasing training, qualifications, and competence to handle this
complex issue. This article is meant only to provide basic and simplified
information on the subject of centrifugal pump vibration.


Understanding pump vibration

Vibration is the continuous back and forth motion of an object from its neutral
position in reaction to an applied external force. A simple example of vibration is
when a mass is suspended on a metal spring; if downward force is applied to the
mass to pull it down and then released, the mass will move up and down
repeatedly from its neutral position.

Now imagine the same mass but, instead of straight down, it is pulled at an angle
from its vertical position - the mass will move not only up and down, but also
sideways, to and pro. In other words, the mass will vibrate both vertically and
horizontally. On the other hand, if the mass is pulled down and the spring is
twisted before it is released, the mass will also move in angular motion.

If the magnitude of the applied force is varied the vibration magnitude will
change in response to the force variation. And if force is re-applied repeatedly
the mass will continue to vibrate indefinitely until the spring breaks due to fatigue
failure.

Pump vibration is a complex extension of the simple mass-and-spring example. A
pump has spring-like properties and its rotor is akin to a suspended mass. Its  
vibration results from the displacement of its shaft  from its neutral position due
to some external forces, both mechanical and hydraulics, generated when the
shaft  rotates. Due to the complexity of these forces, the shaft displacement
occurs in horizontal, vertical, axial, and angular directions. If the vibration occurs
indefinitely the pump will fail prematurely depending on the severity of the
vibration.

When dealing with vibration, it is important to know its severity (or magnitude)
and its frequency. Severity is a measure of how smooth or rough a vibration is,
and frequency is an indicator of what is, or are, causing the vibration.

There are many ways of measuring severity, depending on the instrument used
- it can be measured in terms of amplitude (displacement), velocity, or
acceleration.

Amplitude or displacement is measured in  mils peak-to-peak.
Velocity is measured in inch per second (ips)
Acceleration is measured in gs (gravity)

Vibration readings can be either filtered (discreet), or unfiltered (overall).
Filtered vibration refers to the vibration at a specific frequency, whereas
unfiltered is the overall vibration resulting from the combination of vibrations at
different frequencies.

Vibrations in pumps with ball bearings are measured on the bearing housing,
perpendicular to the shaft, in inch per second (ips) unit, or in acceleration unit
(g).

Vibrations in pumps with sleeve bearings are measured on the shaft, also in
perpendicular direction to the shaft.


Common causes of pump vibration:

  • unbalance of rotating parts
  • misalignment of shaft, coupling, or bearing housing
  • rubbing, or looseness of parts
  • interference or eccentricity
  • bent shaft
  • defective bearings
  • oil whip, or whirl
  • acoustic, mechanical, or structural resonance
  • hydraulic forces
  • torque variations
  • aerodynamic forces
  • dirt caught between mating parts

In order to identify the cause, or causes, of vibration it is important to know its
magnitude, direction, and frequency.


The common causes of pump vibration can be grouped into:

Mechanical
- such as rotor unbalance, parts misalignment, looseness of parts,
soft foot, imbedded foreign materials, etc.

Hydraulics - cavitation, low flow recirculation, release of dissolved or entrained
gas or vapor, tight volute B-gap, excessive flow velocities, etc.

Resonance - mechanical resonance with bearings, foundation, valves, and
piping; resonance with impeller vane pass frequency; acoustic resonance; rotor
critical speed, etc.

In order to identify accurately the root cause of high vibration, and find the
effective solution to the problem, it is essential to identify:

  • The vibration amplitude or magnitude
  • The discreet frequency of the vibration, or vibration signature
  • Direction of dominant amplitude - axial, horizontal, or vertical
  • Symptoms or damage to the pump such as bearing or seal damage, high
    bearing temperature, material erosion, etc.

High vibration amplitudes exhibited by pumps driven by variable speed drivers,
or with VFDs, at certain speed but not in others are likely caused by acoustic
resonance or critical speeds.

The second part of this article discusses about potential solutions.


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Vibration
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