YOUR PARTNER IN CONDITION MONITORING
VIBRATION AND NOISE IN PUMPS
5/26/2005
Introduction: Although certain amount of noise is to be expected
from centrifugal pumps. Unusually high noise levels in excesses of
100db particularly high frequencies are the indicator of potential
failures. The occurrence of significant noise levels indicates that
sufficient energy exists to be potential cause of vibrations.
Noise in pumping system can be generated by ---
• The mechanical motions of pump components.
• The liquid motion in the pump and piping system.
The component motion noise and liquid motion noise can be
transmitted to environment.
1
YOUR PARTNER IN CONDITION MONITORING
SOURCES OF PUMP NOISE.
1.Mechanical noise sources.
Common mechanical sources that may produce noise and vibrations.
Because of pressure variations that are generated in liquid or air .
• Impeller rubbing
• Seal rubbing
• Defective or damaged bearing
• Vibrating pipe walls
• Unbalanced rotor
Improper installation of couplings often causes mechanical noise at
two times pump speed harmonic---misalignment
If pump speed is near or passes through the critical speed noise can
be generated.
1. By high vibrations resulting from unbalance –
• Rubbing of bearings
• Rubbing of seals
• Rubbing of impeller.
Symptom of rubbing : it may be characterized by high pitched
sequel.
2. windage noise may be generated by
• Motor fans
• Shaft keys
• Coupling bolts
3. damaged bearing noise : high frequency.
2
YOUR PARTNER IN CONDITION MONITORING
LIQUID NOISE SOURCES
These are the pressure fluctuations produced directly by liquid
motion.
1. High velocity flow
2. Pulsations
3. Cavitations
4. Flashing
5. Water hammer
6. Flow separation
7. Impeller interaction with the pump cut water.
Pressure pulsations and flow modulations produce either a discrete
or broadband frequency component.
If generated frequencies excite any part of the structure or piping or
pump =} then noise may be radiated into environment.
Four types of pulsations
1. Discrete frequency components generated by the pump
impeller such as vane pass frequencies plus its multiples.
2. Flow induced pulsation caused by turbulence such as flow past
restriction and side branches in the piping system.
3. Broadband turbulent energy resulting from high flow velocities.
4. Intermittent bursts on broadband energy caused by cavitations,
flashing water and water hammer.
3
YOUR PARTNER IN CONDITION MONITORING
Causes of vibrations
1. Installation/maintenance
• Unbalance
• Shaft-shaft misalignment
• Seal rubs
• Case distortion caused by piping load
• Piping dynamic response to supports and restraints.
• Support structural response to foundation/anchor
bolts/grout.
• Improper assembly.
2. APPLICATION
• Operating off of design point
• Improper speed/flow
• In adequate NPSH
• Entrained air
3. HYDRAULIC
• Interaction of pump (head-flow curve) with piping
resonance
• Hydraulic instabilities.
• Acoustic resonance (pressure pulsations)
• Water hammer
• Recalculation
• Cavitations
• Flow induced excitation (turbulences)
• High flow velocities.
4
YOUR PARTNER IN CONDITION MONITORING
4.Design/manufacturing
• Lateral critical speed
• Tensional critical speed
• Improper bearings
• Improper seals
• Rotor instability
• Shaft misalignment in journals
• Impeller resonance
• Bearing/pedestal resonance
•
Unbalance:
Unbalance of rotating shaft can cause large transverse vibration at
certain speeds known as critical speed that coincide with that natural
or lateral frequency of shaft.
Damage due to unbalance response may range from seal or bearing
wipes to catastrophic failure of rotor.
Excessive rotor unbalance can result from rotor bow.
Unbalance of couplings, thermal distortion or loose parts
After period of operation the pump rotor may become unbalanced by
erosion, corrosion or wear.
Unbalance could also be caused by non-uniform plating of the
pumped product on to the pump impeller. In this instance cleaning the
impeller could restore the balance.
Erosion of the impeller by cavitations pr chemical reaction with the
product may cause permanent unbalance requiring repair or
replacement of the impeller. Wear of impeller or shaft caused by rubs
will require repair or replacement.
Another cause of unbalance can be occur if lubricated couplings have
an uneven build up of grease or sludge.
5
YOUR PARTNER IN CONDITION MONITORING
MISALIGNMENT:
Angular misalignment between two shafts connected with a flexible
coupling introduces a additional driving force that can produce
tensional or lateral vibrations.
The force in a typical industrial coupling is similar to universal joint.
When a small angular misalignment occurs the velocity ratio across
the coupling is not constant. If one shaft speed assumed constant the
other shaft speed has faster rotational rate for part of the revolution
and slower rotational rate for part of the revolution.
This variation of rotating speed results in a second harmonic vibration
component.
PIPING AND STRUCTURE
The pump relatively isolated from the piping. The weight and thermal
loading on suction and discharge connections should be minimized.
Static forces from piping may misalign the pump from its driver or for
excessive loading the pump case may distorted and case or seal and
bearing damage.
Vibrations of piping or the support structure can be mechanically
transferred to the pump.
The piping and the structure should not have their resonant
frequencies or multiples.
The vibration transferred to the piping to the structure can be
minimized by using a visco-elastic material.(belting material between
the pipe and piping clamp.
6
YOUR PARTNER IN CONDITION MONITORING
APPLICAT ION
Improper application or changing conditions can result in variety of
problems. Operation at high flow, low head conditions can cause
vibration of rotor case, inadequate NPSH can result in cavitations that
will cause noise and vibration
BEARINGS
General purpose and small pumps in processes plants generally
have rolling element bearings. Noise and vibrations are commonly a
result of bearing wear. As the rolling element or races wear, the worn
surfaces or defects initially produce a noise and as wear increase
vibration become noticeable and several high frequencies may occur
that depend on the geometry of the bearing component and their
relative rotational speeds. The frequency generally above the
operating speed. Many ball bearing failure are due to contamination
in the lubricant that have found their way into bearing
SEALS
The fluid dynamics of flow through seals have a dramatic effect on
rotor dynamics. Hydrodynamic forces involved may contribute the
stabilization of rotating machinery or make it unstable. Seals with
large axial flow in the turbulent range such as in the water pumps
tend to produce large stiffness and damping co-efficient that are
beneficial to the rotor vibration and stability. Wear of the seals will
increase the clearance and cause greater leakage and possibility to
change the rotor dynamics characteristics of the seal resulting in
increased vibrations. After the machine has been placed in operation.
Common contaminants moister, dirt and other miscellaneous
particles which when trapped inside the bearing may cause wear or
permanently indent the balls and raceway under tremendous stresses
generated by the operating load.
7
YOUR PARTNER IN CONDITION MONITORING
The hydrodynamic bearing is superior to rolling element bearings for
high speed application. The hydrodynamic bearing supports the rotor
on a film of oil as it rotates. The geometry of hydrodynamic bearing
and oil properties play a important role in controlling the lateral critical
speeds and consequently the vibration characteristics of the pump
HYDRAULIC EFFECTS
Hydraulic effects and pulsations can result almost any frequency of
vibration of the pump or piping from one per revolution to vane pass
frequency and its harmonics. frequencies below running speed can
be caused by acoustical resonance. Generally these effects are due
to the impeller passing and discharge diffuser or some other
discontinuity in the case any non-symmetry of these internals of the
pump may produce an uneven pressure distribution that can result in
forces applied to the rotor.
TRANSIENTS
Starting and stopping pumps with the attendant opening and closing
of the valves are major causes of sever transients in piping system.
The resulting pressure surge referred to as water hammer, can apply
sudden impact force to the pump and its internals and the piping.
Sever water hammer has caused cracks in concrete structures to
which the pipe was anchored.
Rapid closer of conventional valves used in feed water lines can
cause severe water hammer. Increasing the closer time of the valve
can reduce the severity of surge pressure.
CAVITATIONS AND FLASHING.
For many liquid-pumping systems it is common to have some degree
of flashing and cavitations associated with the pump or with pressure
control valves in piping system. A high flow rate produces more
severe cavitations because of greater flow losses through restriction.
8
YOUR PARTNER IN CONDITION MONITORING
Cavitations produces high local pressure that may be transmitted
directly to the pump or piping and may also be transmitted through
fluid to other area of piping. Cavitations are one of the most
commonly occurring and damaging problem in liquid pump systems.
The term of cavitations refers the formation and subsequent collapse
of vapor bubbles in a liquid caused by dynamic pressure variation
near the vapor pressure. Cavitations can produce noise, vibrations,
loss of head and capacity as well as severe erosion of impeller and
casing surfaces.
Before the pressure of the liquid flowing through a pump is increased,
the liquid may experience a pressure drop inside the pump case. This
is due to part of acceleration of liquid into eye of the impeller and flow
separation from impeller inlet vanes. If flow is an excess of design or
the incident vane angle is incorrect, high velocity, low pressure
eddies may form. If liquid pressure is reduced to vaporization
pressure the liquid will flash. Later in flow path the pressure will
increase. The implosion, which flow causes, what is usually referred
to as cavitations noise. The collapse of the vapor pockets, usually on
the non-pressure side of the impeller vanes, causes severe damage.
(Vane erosion) in addition to noise.
When a centrifugal pump is operated at flows away from the point of
best efficiency, the noise is often heard around the pump casing. The
magnitude and noise may vary from pump to pump and are
independent on the magnitude of the pump head generated, the ratio
NPSH required to NPSH available, and amount by low deviates from
ideal flow. Noise is often generated when the vane angles of the inlet
guides, impeller and diffuser are incorrect for the actual flow rate.
Observing the complex wave or dynamic pressure variation using an
oscilloscope and a pressure transducer can best recognize
cavitations. The pressure waveform will be non-sinusoidal with sharp
maximum peaks (spikes) and rounded minimum peaks occurring at
vapor pressure. As the pressure drops it cannot reduce a vacuum
less than vapor pressure.
Cavitations like noise can also be heard at flows less than design,
even then available inlet NPSH is an excess of pump required NPSH,
This has been puzzling problem.
9
YOUR PARTNER IN CONDITION MONITORING
Noise of a very low, random frequency but very high intensity results
from backflow at the impeller eye or at the impeller discharge or both.
Every centrifugal pump has this recirculation under certain condition
of flow reduction operation in recirculation condition can be damaging
to the pressure side of the inlet and or discharge impeller vanes(also
to casing vanes). Recirculation is evidenced by an increase in
loudness of a banging type, random noise, and an increase in suction
and or discharge pressure pulsation as flow is decreased sound level
measured at the casing of an 8000hp pump and near the suction
piping during cavitations produced a wide band shock that excited
may be high frequency., however in this case the vane passing
frequency (number of impeller vanes times revolution/sec and
multiplies are predominant). Cavitations noise of this type usually
produces very high frequency best described as “ crackling “
Flashing is particularly common in hot water systems (feed water
systems) when the hot, pressurized water experiences a decrease in
pressure through restrictions (flow control valve). This restriction of
pressure allows the liquid to suddenly vaporize or flash which results
in noise similar to cavitations.
To avoid flashing after restriction sufficient backpressure should be
provided. Alternatively, the restriction could be located at the end of
line so that flashing energy can dissipate in to layer volume.
FLOW TURBULENCE
Pump generated dynamic pressure sources include turbulence
(vortices or wakes) produced in the clearance space between
impeller tips and stationary diffuser or volute lips. Dynamic pressure
fluctuations or pulsations produced in this manner can cause impeller
vibrations or can result in shaft vibrations as the pressure peals
impinge on impeller.
Flow past an restriction or restriction in the piping may produce
turbulence or flow induced pulsations. These pulsations may produce
both noise and vibration over a wide frequency band. The
frequencies are related to the flow velocity and geometry of the
obstruction. These pulsations may cause resonant interaction with
other parts of the acoustic piping system. Most of these unstable flow
10
YOUR PARTNER IN CONDITION MONITORING
patterns are produced by shearing at the boundary between a high
velocity and low velocity region in a fluid field.
Typical example of this type of turbulence includes flow around
obstruction or past dead water regions (closed water bypass line) or
bi-directional flow. The shearing action produce vortices or eddies
that are converted to pressure per turbulences at the pipe wall that
may result in localized vibration excitation of the piping or pump
components. The acoustic natural response modes of piping systems
and the location of the turbulence have strong influence on the
frequency and amplifications. This vortex shedding experimental
measurements have shown that vortex flow is more severe when a
system is acoustic resonance coincides with the generation
frequency of the source. The vortices produce broadband turbulent
energy centered around frequency.
11
YOUR PARTNER IN CONDITION MONITORING
12
