Cavitation Case Study

Cavitation Case Study

Cavitation occurs in fluid flow systems where the local static pressure is below vapor pressure

Cavitation is a common problem in pumps and control valves - causing serious wear, tear and damage. Under the wrong conditions cavitation reduces components life time dram Cavitation is a common problem in pumps and control valves - causing serious wear, tear and damage. Under the wrong conditions cavitation reduces components life time dramatically.

What is Cavitation?

Cavitation may occur when local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature. According the Bernoulli Equation this may happen when a fluid accelerates in a control valve or around a pump impeller.

The vaporization itself does not cause the damage - the damage happens when the vapor almost immediately after evaporation collapses when velocity decreases and pressure increases .atically. 

Wear due to impingement of particles and of fluids

Wear due to impingement of particles or fluids is called erosive wear or simply

erosion. three typical situations:

• erosive wear due to the action of turbulent two phase flow parallel to a wall;

• erosive wear due to particles or fluids impacting on a wall;

• cavitational wear due to implosion of cavitation bubbles.

Erosive wear occurs when solid particles, carried by a liquid or a gas, strike a surface. Erosive wear is not always damaging, an interesting example being sand blasting, which is a technique for cleaning metal surfaces from oxide scales by projecting sand particles on them to bring about controlled wear. The impact of water droplets or of a water jet against a wall can also cause erosive wear, sometimes 

referred to as impingement wear. The simultaneous action of erosive wear and corrosion is called erosion corrosion. Erosion corrosion is often encountered in pumps and pipes exposed to turbulent flow in the presence of suspended particles. It also occurs in other situations, for example 

in incinerators due to ash particles that are entrained by the exhaust gas. The damage caused by the combined effect of the impingement of a liquid and of corrosion is sometimes referred to as impingement corrosion or impingement attack. Low-pressure zones in a fluid system under turbulent flow conditions are at the origin of cavitational wear. When the hydrodynamicpressure at some location temporarily drops below the vapor pressure of the fluid a fraction of the liquid is transformed into gas bubbles. The recurrent formation and implosion of these bubbles leads to material damage by local fatigue. If simultaneously corrosion takes place we speak of cavitation corrosion.

Cavitation corrosion 

Cavitation corrosion refers to the progressive deterioration of a surface under the combined action of corrosion and the implosion of cavitation bubbles. These bubbles can form in fluids undergoing turbulent flow whenever a local low-pressure zone is created; this can occur at a position downstream from an orifice in a pipe, on certain parts of a boat propeller, or in hydraulic turbines.

Origins of degradation by cavitation 

In the turbulent flow regime, the local pressure of a fluid fluctuates. If, at certain positions, the pressure drops below the vapor pressure of the fluid, a small amount of liquid can evaporate, and we observe the formation of a vapor bubble, called a cavitation bubble (Figure 1). With diameters that can vary between several micrometer and a millimeter, these bubbles grow from particular sites, related to surface topography or the presence of inclusions

(FIG 1 Formation and implosion of cavitation bubbles (schematic

Because of the pressure fluctuations, the cavitation bubbles implode when the local pressure increases again. This phenomenon produces a shock wave. During a short period of several microseconds, the pressure at the metal surface can thus reach very high values, up to 1000 MPa. 

The repetition of this mechanism induces fatigue at the metal surface, and thus causes irreversible damage. Attack patterns of characteristic shape appear at the deteriorated zones

The implosion phenomenon is presented in more detail in Figure 2, which gives the result of a theoretical calculation on the evolution of the shape of a cavitation bubble as it implodes: it recedes at its center, and then a liquid jet forms which perpendicularly strikes the metal surface. High-speed photographs of imploding cavitation bubbles confirmed this behavior

FIG 2 Theoretical evolution of the form of an imploding cavitation bubble 

Experimental study of cavitation corrosion
Figure 3 shows three experimental configurations frequently used to study
cavitation corrosion:
• rotating disk in the turbulent-flow regime;
• vibrating device;
• Venturi system

The turbulent-flow rotating disk allows the operator to simultaneously study a number of samples. The rotation rate of the disk is easily varied and can attain very high linear velocities at the sample surface. On the other hand, the set up is not well suited for electrochemical studies. The placement of obstacles on the disk upstream from the samples facilitates the creation of cavitation bubbles.

FIG 3 Experimental study of cavitation corrosion: (a) rotating disk in the turbulentflow

regime; (b) vibrating device; and (c) Venturi system.

The vibrating device functions on the principle of magnetostriction or piezoelectricity. A transducer supplies vibrations at a frequency on the order of 1 kHz, with an amplitude of 20 to 100 𝛍m typically. During each oscillation, cavitation bubbles appear as the sample is pulled upwards, and these then implode as the sample is pushed back downwards. This experimental arrangement stands out by its simplicity; it does not require any heavy equipment and it requires only small volumes of solution.
In addition, it is well suited for electrochemical experiments. On the other hand, because the entire surface is exposed to intense cavitation, the application of the results to other flow systems may pose problems. Venturi systems exist in a variety of forms, but their principle of operation is always the same: a fluid in the turbulent flow regime flows in a pipe and passes through a contraction that generates cavitation bubbles. These are entrained by the fluid and implode downstream at the point where the sample is located. Implosion of cavitation bubbles at this point is induced by an enlargement of the pipe diameter, which causes a deceleration of the fluid velocity and therefore an increase in the local pressure. Among the three systems shown in Figure 3, the Venturi system
provides experimental conditions that are closest to those found in practice. In addition, it can easily be adapted to carry out electrochemical measurements or to optical observation of the cavitation bubbles. However, Venturi systems require a powerful pumping system and a large volume of solution, which makes them rather cumbersome. Furthermore, to obtain representative results, experiments must often be run for prolonged periods of time.

Interpretation of the results

Because of the large number of parameters that come into play in cavitation corrosion, the interpretation of the results obtained in the laboratory and their application to the design of technical installations poses many problems. The cavitation number N🇰 is often used to compare cavitation conditions among experimental configurations of comparable geometry. The cavitation number expresses the ratio between the pressure difference 𝛥P = P – Pvap, and the kinetic energy

 Ekin = (1/2)𝜌v²  of the fluid.

N🇰(𝞼)=( P – Pvap)/((1/2)𝜌v²)

Here, 

  P        represents the pressure of the fluid 

 Pvap    is its vapor pressure at the temperature of the experiment;

  v       designates the flow velocity 

 𝜌     is the density of the fluid. 

The degree of deterioration due to cavitation is proportional to the difference:

ΔN🇰 = N🇰,crit – N🇰

N🇰,crit  The critical cavitation number

The Cavitation Number or Cavitation Parameter is a "special edition" of the dimensionless Euler Number. The Euler Number - an introduction to and a definition of the Euler Number

The Cavitation Number is useful when analyzing fluid flow dynamics problems where cavitation may occur.

units

N🇰(𝞼)= Cavitation number 

P = reference pressure (Pa) 

Pvap = vapor pressure of the fluid (Pa) 

𝜌= density of the fluid (kg/m3) 

v = velocity of fluid (m/s)

To prevent cavitation

• avoid low pressure - pressurize supply tanks if necessary 
• reduce fluid temperature 
• use larger suction pipe diameters - reduce minor losses 
• use cavitation resistant materials or coatings 
• small amounts of air supplied to the suction system may reduce the amount of cavitation damage 
• keep available NPSH well above required NPSH.

The critical cavitation number N🇰,crit , which appears in expression (4), is an empirical quantity, whose value varies as a function of the geometry of the experimental apparatus. Cavitation attack can only take place if   N🇰 is smaller than the critical cavitation number, in other words if ΔN🇰 > 1.

FIG 4 Evolution of rate of cavitation corrosion as a function of the test duration

The formation of cavitation bubbles depends on the rugosity, which often changes during the test. The rate of cavitational erosion therefore does not remain constant.One generally observes an onset period, followed by a transition period during which the rate of cavitation corrosion passes through a maximum before it reaches a steady value (Figure 4). The duration of these periods depends on the material. To compare the results obtained for different materials the transition period must be short 

relative to the time scale of the experiment.

Mechanisms of cavitation corrosion 

Metal degradation by cavitation is above all a mechanical phenomenon, but corrosion also contributes. Figure 5 illustrates this statement. It shows the results of cavitation corrosion tests carried out on steel using the vibrating device described above, in different environments and at different temperatures. The mass loss as a function of temperature passes through a maximum. This behavior can be explained by postulating that two factors, acting with opposite effect, contribute to the deterioration. On one hand, higher temperatures increase the vapor pressure and thus favor cavitation. On the other hand, the solubility of the corrosive agent, in this case oxygen, decreases with temperature, thereby decreasing the corrosion rate. Further evidence for the influence of corrosion is the increased mass loss observed in solutions of greater conductivity.

Cavitation corrosion is still poorly understood. Two mechanisms have been proposed: 

         • the removal of material by depassivation-repassivation events; 

          • embrittlement of the metal by hydrogen.

According to the depassivation-repassivation mechanism the metal loss is due to an oxidation reaction. The implosion of the cavitation bubbles on a passive surface

FIG 5 Effect of electrolyte on cavitation corrosion. Weight loss of a carbon steel after

15 minutes as a function of temperature during cavitation tests using a vibrating device: (a) in

distilled water; (b) in water buffered to pH 8; and (c) in a 3% NaCl solution

creates shock waves that damage the passive film, exposing the underlying metal.Between shocks the surface repassivates and each repassivation event transforms a small amount of metal into oxide. By repeating themselves, these reactions can lead to significant material loss. Figure 6 shows the anodic polarization curve of a steel specimen measured in presence and absence of cavitation using a vibrating device In the presence of vibrations, the passive current density is significantly higher

FIG 6 Effect of electrode vibration on the anodic polarization curve of steel in aphthalate solution 

This proves that in these experiments cavitation indeed damaged the passive film of the metal

Cavitation corrosion damage due to hydrogen embrittlement can be explained as follows. When the surface of an active metal is exposed to an electrolyte, it can react with protons to form hydrogen. Under certain conditions, the hydrogen may diffuse into the metal and increase its brittleness . This then lowers the fracture energy and facilitates the formation of metal particles due to fatigue cracking under the effect of repeated shock waves from imploding cavitation bubbles. Somewhat speculative, the proposed mechanisms of cavitation corrosion need further experimental proof. Anodic oxidation, for example, does not explain the specific shape of the attack patterns created by cavitation, which normally exhibit a morphology that is very different from that typically found after anodic dissolution or pitting corrosion. Rather, the morphology of cavitation pits resembles that expected for particle detachment by brittle fracture due to fatigue. Mechanical damage of the metal, rather than of the passive film would then be the cause of cavitation corrosion. A hydrogen embrittlement mechanism could be responsible for such behavior, but it is in contradiction with certain experimental observations. In particular, a cathodic polarization of the metal in the region of hydrogen generation is sometimes applied to suppress cavitation corrosion

To reduce cavitation corrosion damage in cooling circuits, corrosion inhibitors are sometimes added. This underscores the electrochemical nature of cavitation corrosion

Control valves and cavitation, application ratio and multi stage control valves 

If the speed through the valve is increases enough, the pressure in the fluid drops to a level where the fluid may start to boil, bubble or flash. And when the pressure recovers sufficiently the bubbles will collapse upon themselves. This collapse causes cavitation

Cavitation may be noisy but is usually of low intensity and low frequency. This situation is extremely destructive and may wear out the trim and body parts of a valve in short time

Application Ratio
A common way to characterize potential cavitation condition is the "applications ratio" (or "the incipient cavitation index") which can be expressed as

AR = pi - po / (pi - pv)

where
AR = Application Ratio
pi = inlet pressure, absolute
po = outlet pressure, absolute
pv(Pvap) = vapor pressure of the fluid, absolute

For application ratios above 1 - the fluid flashes. This is not the same as cavitation, but the closer the ratio is to 1, the higher is the potential for cavitation.

Example - Flashing Water 

If we know the boiling point and the absolute pressure of a fluid (Steam Table with saturated steam properties) the minimum outlet pressure from a valve to avoid flashing can be calculated. 

For an application ratio like one (AR = 1), equation  can modified to 

AR = 1 

       = pi - po / (pi - pv) 

or transformed to 

        po = pv

Using the "Steam Table" with saturated steam properties we can conclude that
-  for a water temperature of 17.51 oC and absolute inlet pressure of 1 bar - the minimum outlet                    pressure is 0.02 bar to avoid flashing
-   for a water temperature of 81.35 oC and absolute inlet pressure of 1 bar - the minimum outlet                    pressure is 0.5 bar to avoid flashing
-    For a water temperature of 99.63 oC and absolute inlet pressure of 1 bar - the minimum outlet                  pressure is 1 bar to avoid flashing 

Avoiding Cavitation 

Cavitation can in general be avoided by 

--increasing the distance (pressure difference) between the actual local static pressure in the fluid        and the vapor pressure of the fluid at the actual temperature 

This can be done by: 

-- re-engineering components initiating high speed velocities and low static pressures 

--increasing the total or local static pressure in the system 

--reducing the temperature of the fluid

Re-engineering of Components Initiating High Speed Velocity and Low Static Pressure 

Cavitation and damage can be avoided by using special components designed for the actual rough conditions. 

              - conditions with huge pressure drops can - with limitations - be handled by Multi Stage                         Control Valves

              --challenging pumping conditions with fluid temperatures close to the vaporization                                 temperature can be handled with special pumps - working after other principles than                             centrifugal pumps

Increasing the Total or Local Pressure in the System 

By increasing the total or local pressure in the system the distance between the static pressure and the vaporization pressure is increased and vaporization and cavitation can be avoided. 

The ratio between static pressure and the vaporization pressure - an indication of the possibility of vaporization, is often expressed by the Cavitation Number. 

Unfortunately it is not always possible to increase total static pressure due to systems classifications or other limitations. Local static pressure in components may be increased by lowering (elevation) the component in the system. Control valves and pumps should in general be positioned in the lowest part of a system to maximize the static head. 

This is a common solution for boiler feeding pumps receiving hot condensate (water close to 100 oC) from condensate receivers in steam plants.

Reducing the Fluid Temperature 

Vaporization pressure depends of fluid temperature. The vapor pressures for water - our most common fluid - are indicated below:

Temperature (oC)	Vapor Pressure (kPa, kN/m2)
0	0.6
5	0.9
10	1.2
15	1.7
20	2.3
25	3.2
30	4.3
35	5.6
40	7.7
45	9.6
50	12.5
55	15.7
60	20
65	25
70	32.1
75	38.6
80	47.5
85	57.8
90	70
95	84.5
100	101.33