Noise Reduction by Cavitations Control

8/3/2019 Noise Reduction by Cavitations Control

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NOISE REDUCTION BY

CAVITATIONS CONTROL

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INTRODUCTION:

CAVITATION:

When we heard the word cavitations then the first thing whichstrikes in our mind is something related to formation of cavity or

we can say formation of void

The literal meaning of cavitations is related to formation of

bubbles in liquid when we are dealing with hydraulic machinery.

Although cavitations can be defined in number of ways but

basically it is defined as a formation and instantaneous collapse of

innumerable tiny voids or cavities with in a liquid subjected rapidand intense pressure changes, or we can say it is the phenomenon

of production of voids in a liquid system due to extreme reduction

of internal pressure .Collapse of these voids produce very large

impulsive pressure which can cause considerable damage to near

by surfaces,

This process is very frequent in high speed impellor blades.

Cavitations is referred as a condition where a low pressure isformed on the surface of the impellor , this low pressure will cause

water to vaporize into steam and when these steam bubbles are

passing through the area having high pressure these bubbles will

implode and cause severe damage to the impellor in co-operate

with noise and vibration.

It occurs in hydraulic system as a result of low fluid level that draw

air into the system producing tiny bubbles that expand explosively

at the pump outlet causing metal erosion and eventual pump

destruction.


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OCCURENCE OF CAVITATIONS:-

Cavitations occurs when the velocity of liquid increases to the

point where the consequent pressure drop reaches the pressure of

vaporization of liquid, when this happens, vapor pockets or

bubbles from the liquid and later collapse when subjected to high

pressure at some other point in the flow.

The collapse of the vapor bubble can takes place with considerable

force. This effort is coupled with the rather corrosive action of

vapor bubbles moving at high speed can severely pit and corrode

impellor surface sometime even pump casing , in some cases

cavitations there is structural failure of impellor blades.Whenever cavitations occurs it is frequently signed by a clearly

audible noise and vibration.

Cavitations can also be caused by improper operation of the pump,

for instance cavitations can occur when sudden and large demand

for liquid are made upon the pump . As the liquid discharged from

the pump is rapidly distributed and used downstream, a suction

effect is created on the discharge side of the pump, thus pressurehead on discharge decreases. Velocity of the liquid flowing across

the impellor increases up to a point where cavitations occurs.


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DAMAGES CAUSED BY CAVITATIONS :-

1.Abnormal sounds and vibrations:-As we know during cavitations , there is movement of bubbles

from low pressure area to high pressure area and subsequent

collapse that creates shock waves producing abnormal sounds and

vibrations .It has been estimated that during collapse of bubbles ,

the pressure of the order of 10


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particularly impellors, the bubble is trying to collapse from all the

sides but if the bubble is lying against apiece of metal it cannot

collapse from that side , so the fluid comes from opposite side at

this high velocity and bangs against the metal and creating a long

term impression called PIT, which is defined as plasticdeformation of very small dimension. This damage is called

cavitations erosion or pitting .

The most sensitive area where cavitations erosion occurs is low

pressure side of the impellor where the bubble actually originates.

There are some instances , cavitations has been severe enough to

wear holes in the impellor to such an extent ,the impellor becomes

totally ineffective.


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Cavitations Corrosion:-

Frequently cavitations is combined with corrosion. The implosions

of bubble destroys existing protective layer making the metal

surface permanently activated for the chemical attack, thus in this

case , even a small cavitations leads to severe damage

METHODS TO REDUCE NOISE CAUSED BY

CAVITATIONS:-

Although there is no full proof methods to control or reduce noiseproduced during cavitations but following two methods are

implemented to reduce certain level of noise .

1.Noise reduction by Polymer Addition.

2. Noise reduction by Ultrasonic.

Noise reduction by polymer addition:-

In order to control noise, designers must first determine where the

noise source or the source how much each is contributing to the

overall noise.

Once the source have been identified ,they can be ranked by how

much contributes to the overall noise level. This is most important

because if lower level noise contributors are silent first, this will

not reduce the overall noise level.

When trying to control noise in any type of machinery, designers

generally use basically four types of element called- absorption,

barrier, dampers and gasketing materialsAbsorption material:-

A good material will soak up air borne sound energy waves by

changing the wave energy into heat as it passes through the

absorption medium. Absorption material are generally either


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fibourous or cellular. Common fibre materials are fibre glass,

mineral wool and ceramics. These are applied as blankets or semi

rigid sheets which can be cut to shape. Generally they are film

faced to prevent the fibres from being dislodged and causing

problem in air-handling system.Newly developed melamine and polyamide cellular materials offer

significant advantages over traditional urethanes in many respects.

The melamine required only 40% of weight of comparable

urethanes and has no smoke or non-toxic by products of

combustion.

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BARRIERS:-

These are two types of barriers- those that already exist ( wall,

cabinets, enclosures etc) and supplement barriers are those which

are add if existing wall is not thick enough, in this case mass is the

key to control noise.

Mass law for homogeneous material gives rough approximation of

the amount of noise that can be controlled .

However the same noise reduction cannot be achieved at all the

frequencies. There is less noise reduction in lower frequencies than

the higher frequencies for given mass.


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But this concept is related to concept of perfect walls when there is

no opening, when openings are created for pipe wires, air and

products to enter and exit, noise is let out.

The amount of noise reduction expected from a perfect wall

diminishes drastically in the real world. As the opening sizeincreases as a percentage of total enclosures area, the actual noise

reduction decreases.

The above concept can be understood with the help of an example

with transmission potential of 20 decibel or greater and 10%

opening, designers can never get more than 10 decibel noise

reduction, However noise reduction can be achieved drastically by

sealing around wires, pipes and air ducts in addition to this noise

reduction is increased by providing absorption lined tunnels.

DAMPING:-

All materials have natural frequency when the are excited by some

external source at this normal frequency, they will vibrate, this

causes the air surrounding the material to vibrate and produce

noise sometimes referred as oil can phenomenon.

Properly applied damping material will only work, if the metal towhich they are applied are vibrating at a near of their resonant

frequency.

Damping material are temperature sensitive so they must be

selected both for their temperature range and the operating

temperature of the material.

Damping material work to reduce the vibration in the material to

which they are applied by dissipating the vibration energy as heat.

Damping materials are termed vico-elastic because of their viscous

and elastic properties.Homogeneous or free layer damping material are generally vinyls

which have platelet type fillers in them.

As the material to which they are applied vibrates, the platelets

slide against each other and the friction between them vibration

energy into heat.


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The other type of damping material is called constrained layer,

here the vico-elastic polymer is homogeneous and sandwitched

between two plates. These are bonded together usually with a

structural epoxy adhesives.

Polymer thickness is determined by the frequencies to beattenuated, generally thicker the polymer, lower the frequency and

conversely the thinner the polymer, higher the frequency.

Each formulation of polymer has a finite temperature range, over

which it will be effective. With the help of damping material about

14 decibel of sound reduction can be achieved.

GASKETING:-

Generally this concept is given less importance ..

Gasketing material are generally soft, urethanes, neoprenes,

pliable formed vinyls.

The important characteristic they exhibit for noise control is

sealiblity or conformability to the irregular surfaces between which

they are placed. Closed cell materials make better gaskets than

open cells but their design is more complicated because they areharder to compress due to entrapped gas in each cell.

A good gasket material properly designed for the application will

easily reduce noise by 6 decibel or more.

NOISE REDUCTION BY ULTRASOUNDS

Ultra sounds can produce temperature as high as those on thesurface of the sun and pressure as that of bottom of the ocean.


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Applications of ultrasonic to reduce the abnormal sound produced

during cavitations is related to chemistry of ultra sound called sono

chemistry..

Ultrasounds are high intensity sound waves which can be produced

with the help of electric energy, the electric energy is used to causethe motion of solid surface such as piezoelectric crystal,

piezoelectric materials expand and contract when electric field is

applied to them, for production of ultrasounds alternating current is

allowed to pass through a piezoelectric crystal.

When ultrasounds are pass through a liquid, the expansion cycle

can create cavities in the liquid , this will happen when negative

pressure exceeds the local tensile strengththis negative pressureis created by expansion cycle.

The local tensile strength of liquid depends upon the type and

purity of a liquid, since most of the liquids are contaminated by

sufficient amount so cavitations process is initiated at small

negative pressure.

Ultrasounds absorb the sound waves produced by air bubbles and

increasing the bubble size, thus by applying ultrasounds we can

reduce the noise but bubble size is increase..


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When we consider the concept of ultrasound in solid liquid

interface, which is quiet common in cavitations process, then we

found that there are basically two proposed mechanisms for the

effect of cavitations near the surfaces , micro jet impact and shockwave damage . The symmetry of the environment near the

interface induces a deformation of the cavity , this deformation is

self reinforcing and it sends a fast moving streams of liquid at the

surface having a velocity of 1000m/sec.


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The second mechanism invokes shockwaves created by cavity

collapse in the liquid .The existence of both the mechanisms have

been established, and both these mechanisms are responsible for

the creation of localized erosion responsible for ultrasonic

cleaningunder these conditions the ultrasound may absorb the

sound waves but it leads to creation of big air bubbles.

Thus ultrasound may absorb the audible noise produced during

cavitations but indirectly enhance the cavitations process byincreasing the bubble size.


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Our ideas:1. By propagating reverse waves in the region where we think

there is possibility of maximum cavitations formation. In theinitial stage as the liquid starts flowing through the region of

maximum cavitations formation, as the liquid starts flowing and

there is change of pressure in the region, bubble formation and

collapse of these bubbles takes place, due to this phenomena

noise & vibration would be produced. These activities are high

enough to be easily detected in case of large machinery. These

vibrations are detected by the pneumatic load cells. The

vibrations put stress on the membrane of the pneumatic loadcells, and these will send the activating signal to the electronic

wave pulse generator .and due to potential difference created

there will be a wave propagated. Only two things we have to

look out for that is

(i) propagation of the wave should be synchronized.

(ii) We have to see whether the waves propagated are in

phase or out of phase.

2. Flushing out the region with the prepared slurries which woulddispel the liquid from the walls of the region by adhering to the

walls of the region. Although it is at the cost of efficiency yet it

seems to be quite achievable.

The feasibility of these two methods are still not sure but if they

are implemented under proper conditions and guidance they

lead to effective result.


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CONCLUSIONFinally we would like to conclude that Polymer addition and

Ultrasounds are the most coveted methods of acoustic control.

They are pioneer steps towards maximum noise reduction and are

still behemoth subjects of research going on at leading institutionsof research.

Noise Reduction by Cavitations Control

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