Cavitation In Depth
Part 5 - Diagnosing Cavitation
Diagnose Cavitation by Sound
Low level cavitation in pumps may be inaudible, but higher levels generate distinctive sounds that we hear and call cavitation. This sound can be a diagnostic clue to the experienced practitioner. Cavitation makes different sounds depending on the equipment and conditions, and according to the type of cavitation. Following are some of the descriptions of cavitation sounds:
Pumps (water or similar weight liquids)
Crackling or sizzling
Small steel shot rapidly striking against metal.
Hissing, rushing, swishing, or a static like sound similar to radio or television static.
Suction Recirculation Cavitation can produce loud knocking, hammering, or crackling sounds, that are distinctive from other cavitation types.
Valves (Valve disc riding close to seat)
High pitch squeal
High pitch singing
Valves (High Flow)
The sound of high flow rate through a full open valve is not proof of cavitation. The high flow rate sound can be similar to the sound of cavitation in a pump, but there may or may not be cavitation occurring in the valve. This sound is described as a swishing, rushing, or hissing sound.
The key diagnostic observation to prove or disprove the presence of cavitation in a valve, is to very slowly increase and decrease flow through the valve. If a well defined point exists, where a very small change in flow rate causes the noise to appear or disappear, cavitation is the likely cause. If the noise appears or disappears slowly in response to large changes in flow rate, then cavitation is not a likely cause.
General Sound Levels
If there is no pattern or distinctive sound, or if the person listening cannot distinguish a specific type of sound, the general sound level can be diagnostic as follows:
If the sound lessens or disappears as the flow rate is reduced, suction cavitation is probably occurring in the pump.
If the sound lessens or disappears as the flow rate is increased, then recirculation cavitation may be the cause.
If the sound disappears as suction pressure is increased, suction cavitation may be occurring.
One pump with suction cavitation due to clogged intake screens, produced 86 decibels in the key of C. After the intake screens were cleared, the sound level dropped to 66 decibels in the key of C.
NOTE: The key of C is commonly used to measure general noise.
Watch a video and listen to sound of Cavitation
We are grateful to Dr. Roger E. A. Arndt, Professor at St. Anthony Falls Laboratory for this video. We have followed Professor Arndt's research for years and are grateful for his permission to share this video and sound track with you.
Select the Picture Below to play the video. Play the video repeatedly to learn as much as possible.
Clear liquid is moving at high speed from right to left.
Fluid velocity changes throughout the video.
As liquid moves across the tip of the round blunt profile fluid pressure falls below vapor pressure on the downstream side of the profile, causing the gas phase of the liquid to occur at that point.
The cavities are then swept along with the fluid producing a visible thread of cavities.
See if you can detect the following: Sound changes from a crackling noise to a steady high pitch "singing" noise, and then returns again to the crackling noise.
As you notice the sound change, watch the whitish thread of cavities, and also notice the change in the size and shape of the thread in relation to the sounds you hear.
Click on the photograph below to download and play an mpeg file (2.26 Mb)
Visit Dr. Arndt's Page at St. Anthony Falls Research Laboratory
Experienced persons may be able to diagnose cavitation by its unique sound qualities. Cavity collapse has specific sound qualities that distinguish cavitation from sounds made by entrained gas bubbles, and also from the sound of failed bearings and other machinery noises.
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The trained ear may be able to distinguish just by the sound if cavitation is the source or not.
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Cavitation sounds can start and stop quickly in response to changes in flow rate.
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Cavitation sounds exhibit precise repeatability, the noise is always the same under identical conditions.
Entrained gas bubbles moving through a pump or valve, make a softer and lower frequency sound than cavitation because of the immense difference in energy levels. Sound from entrained gasses changes slowly to variations in flow rate, whereas, cavitation sounds appear and disappear quickly in response to small changes in flow rate, and sounds caused by cavitation have precise repeatability.
Diagnose Cavitation by Visual Examination of Damage
Visual examination of supposed cavitation damage to pump components is often the best way to determine exact cause. The key observation is usually the location of the damage. The photographs below are a guide to diagnosing cavitation damage by location.
Discharge Recirculation Cavitation
Explanation of the two photographs above of the same impeller.
Severe discharge recirculation cavitation has damaged the discharge side of the impeller, specifically the vane tips and the outer edges of the front and rear vane shrouds.
Moreover, discharge recirculation creates cyclic axial thrust loads that can fatigue the shaft, and in this case, the impeller attaching bolt, causing fatigue failure of the bolt. The impeller bolt can be the weakest axial component in overhung end suction pumps such as the one in the photographs, so the bolt failed instead of the shaft.
Discharge Recirculation Cavitation
This close up photograph of an enclosed impeller, shows discharge recirculation cavitation damage to the vane tips and the outer edges of the front and back vane shrouds.
Suction Cavitation
Two photographs above of the same impeller.
Suction cavitation has damaged the leading edge and suction side of the vane, and also damage is observed on "corner" surfaces leading into the vane. The suction side of the vane is the side facing the viewer.
Suction cavitation on this pump was severe enough that cavities formed in the fluid before the fluid reached the impeller. When the fluid reached the leading edge of the vanes and surrounding areas, the cavities collapsed onto the vane and surrounding areas eroding the impeller material.
If the pressure side of the vanes were damaged, (back side of the vane that can only be seen with a mirror), then suction recirculation cavitation would have been the cause.
Suction Cavitation
Close-up photograph of the leading edge of a vane damaged by suction cavitation.
Again, the suction cavitation was severe enough that cavities occurred in the pumpage before the pumpage reached the pump. When pressure increased in the area just ahead of the vane leading edge the cavities collapsed onto the vane causing the observed damage.
End of Cavitation - Part 5
