Cavitation Description Cavitation is a form of erosion-corrosion very much like particle erosion but caused by collapsing vapour bubbles that locally induce very high shock loads on the metal surface causing local deformation, micro crater formation and erosion of soft metal phases. This behaviour can occur in cases in which the liquid is static (as in a propeller spinning in a large body of water) or in motion (as in the case of liquid flow through pipes and fittings). Cavitation produces noise in a broad frequency range and is frequently described as a rattling sound similar to that which would be anticipated if gravel were in the fluid stream. Equipment damage, vibration and loss of performance often accompany cavitation attack. Cavitation is generally recognised to consist of four distinct events, that is, nucleation, growth, collapse, and rebound (Figure 1). All four events contribute to the overall extent of cavitation-related problems. However, the latter two events are the primary source of noise, vibration, and material damage. Figure 1 Cavitation corrosion Equipment affected by cavitation In general, cavitation damage can be anticipated wherever an unstable state of fluid flow exists or where substantial pressure changes are encountered. Susceptible locations include sharp discontinuities on metal surfaces, areas where flow direction is suddenly altered, and regions where the cross-sectional areas of the flow passages are changed. Hydraulic turbines, pump impellers, ship propellers, valves, pipelines, tube ends, tube sheets, and shell outlets in heat exchanger have been affected by cavitation. Also, cylinder liners in diesel engines, plain bearings, seals, orifices and other hydraulic structures in contact with high-velocity liquids subjected to pressure changes are affected by cavitation. Figure 2 provides examples of the appearance of cavitation damage. Figure 2(a) Internal surface of carbon steel pipe section damaged by cavitation Figure 2(b) Confined patches of small pits. (Magnification: 15x.) Cavitation was caused by high-frequency vibration of the tubes. Mechanism of the Cavitation Process Cavitation ensues when the local pressure in a fluidflow field falls below a critical value that is a function of temperature, surface tension, vapour pressure, and external pressure. Bernoulli’s principle states that an increase in fluid flow simultaneously causes a decrease in pressure. The pressure differentials and low pressures generated in highly turbulent flows, e.g. around a pump impeller or valve opening may cause the formation of areas where the local static pressure becomes lower than the vapour pressure of the liquid at the given temperature. When the local pressure in a liquid is reduced sufficiently, a gas filled bubble can nucleate and grow. If these bubbles subsequently pass into a region of higher pressure, they can implode in a few milliseconds and form microjets of liquid that attain velocities of from 100 to 500 m/sec and pressure up to 414 MPa (60 ksi). The shocks generated by the collapsing bubbles can deform the metal surface, form micro craters on the metal surface and cause removal of soft and weak phases in the metal microstructure (e.g. the ferrite from perlite in C-steel). For steels and alloys protected against corrosion by protective layers (e.g. stainless steels) cavitation can damage the protective layer and accelerate corrosion. Different Stages of Cavitation Erosion Evolution of cavitation erosion depends on many parameters, such as material, surface shape, liquid, and cavitation conditions. There is no universal law for erosion rate (mass loss per unit time) evolution with period of exposure to cavitation. In most cases, however, little mass loss is observed in the early stage of cavitation (incubation stage). This stage often is followed by a period of great increase of erosion rate (accumulation stage) or of a constant erosion rate (steady stage). After that, a decrease of erosion rate often is observed (attenuation stage) Three levels of cavitation have been defined by the American Water Works Association (AWWA). Incipient cavitation represents the beginning stage of cavitation where light popping noises are heard. Constant cavitation is a steady rumbling sound associated with start of possible valve damage. Choked cavitation is the point where the vaporisation of the fluid reaches sonic velocity in the valve port and limits the flow through the valve. The lower the value for the cavitation index, the more likely cavitation will occur. As a rule of thumb, manufacturers typically suggest that when the value for the cavitation index is less than 2.5, cavitation may occur. The erosion rate depends upon the cavitation intensity as defined by the fluid velocity and cavitation number. At constant velocity, with the cavitation number reducing from the value for inception, the erosion rate increases to a maximum and then decreases. At a constant cavitation number, however, the erosion rate varies with powers of velocities ranging from 3 to 10 although a narrower range of 6 to 8 is normally quoted. Harder or more brittle materials tend to have a higher power dependency than ductile materials The effects of velocity should not be considered independently of pressure, since the cavitation number, σ, which is a measure of the intensity of cavitation, is a function of both parameters σ = (p – pv) / 0.5 ρV2 where σ is cavitation number, p and pv are the absolute static pressure and the liquid vapour pressure, respectively, and V the free stream velocity. The cavitation number represents the ratio of pressure drop required to initiate cavitation to the velocity head available and therefore indicates the potential for cavitation. The number at which cavitation occurs depends on the boundary geometry and flow pattern. If the number for the onset of cavitation in a given situation is known through research or experience it can be used to test whether cavitation will occur over a range of velocities and pressures and (subject to small variations due to scale effect) in situations of geometrical similarity but different absolute dimensions, as in scale models. Morphology Figure 3(a) Heavily pitted condensate line. Attack resembles oxygen pitting, but was caused (at least in part) by cavitation Figure 3(b) Highly localised cavitation damage on steel feedwater-pump shaft. Adjacent regions are free of metal loss The appearance of cavitation is similar to pitting except that the surfaces in the cavitation pits are usually much rougher, Figure 3(a). Cavitation typically produces sharp, jagged, sponge like metal loss, even in ductile materials. The affected region is free of deposits and accumulated corrosion products if cavitation has been recent. Typically, affected surfaces are highly localised to specific regions, Figure 3(b), although if cavitation is severe and widespread, the area affected may be extensive. Although corrosion usually accelerates metal loss, attack can occur in its absence. Even glass can be attacked by cavitation. A striking feature of cavitation damage is the localised nature of attack. Because of the unique characteristics of this damage, microscopic observations often conclusively show that cavitation has occurred. Factors affecting cavitation The resistance of a metal to cavitation erosion is influenced by several factors, among which are: hardness, the strain energy to fracture, and the corrosion-fatigue properties. Circumstances that may induce cavitation include vibration, excessively high flow rates, insufficient supply of fluid to the suction side of pumps, and operation of valves in a partially open or closed position. Temperature, air content, pressure, and chemical composition of the fluid can affect the tendency of the fluid to cavitate. For example, the presence of minute air bubbles in the fluid can act as nucleation sites for cavitation bubbles, thereby increasing the tendency of the fluid to cavitate. Increasing pressure decreases susceptibility to cavitation; decreasing pressure increases susceptibility to cavitation. Cavitation is favoured by: Rough rather than smooth surfaces Sharp surface discontinuities Rapid change in direction Abrupt changes in cross section Most common materials of construction including copper and brass, cast iron, carbon steel, low alloy steels, 300 Series SS, 400 Series SS and nickel base alloys are affected by cavitation. Alloy composition also influences attack. Soft, ductile metals and brittle, low-strength alloys such as gray cast iron are easily attacked. Alloys such as chromium stainless steels are resistant to attack in many environments. Figure 4 Cavitation damage on the internal surface of the condenser tube. Note longitudinal crack. The surfaces are covered with orange, air-formed iron oxides that formed subsequently to the removal of the condenser tube. Turbulent flow and abrupt pressure changes promote attack. In many cases, cavitation is a threshold phenomenon. Cavitation does not simply gradually decrease in intensity, but ceases altogether below some critical turbulence level. Cavitation in valves When a liquid passes through a partially closed valve, the static pressure in the region of increasing velocity and in the wake of the closure member drops and may reach the vapour pressure of the liquid causing cavitation. The cavitation performance of a valve is typical for a particular valve type, and it is customarily defined by a cavitation index, which indicates the degree of cavitation or the tendency of the valve to cavitate. Cavitation Index for cavitation characteristics of butterfly, gate, globe, and ball valves is C= (Pd - Pv) / (Pu - Pd) Where C = cavitation index Pv = vapour pressure relative to atmospheric pressure (negative) Pd = pressure in pipe 12 pipe diameters downstream of the valve seat Pu = pressure in pipe 3 pipe diameters upstream of the valve seat Cavitation in pumps Cavitation may occur at the entry to pumps or at the exit from hydraulic turbines in the vicinity of the moving blades. The dynamic action of the blades causes the static pressure to reduce locally in a region which is already normally below atmospheric pressure and cavitation can commence. The phenomenon is accentuated by the presence of dissolved gases which are released with a reduction in pressure. Figure 5 Wear on pressure surface of centrifugal pump impeller by cavitation and solid particle erosion. Pump cavitation is often caused by too high a pressure differential between suction and discharge sides. Pump cavitation can result from insufficient available net positive suction head (NPSH), from a high suction piping velocity, and from a poor suction piping configuration. Throttling on the suction side of pumps promotes a large pressure differential. Gas entrainment due to leaking packing, decomposition of water chemicals, and gas effervescence can also promote bubble formation. Incorrectly sized impellers and other pump components can also cause difficulties. With moderate cavitation in a centrifugal pump, the pump will sound as though it is pumping gravel or a slurry of sand and gravel. Severe cavitation will cause the discharge pressure to fall and become highly erratic and produce both flow and pressure pulsation. Cavitation in piping systems Cavitation in pipe systems is possible wherever there are changes in section or flow direction such as expansions, bends and branches. However, serious erosion problems are normally only associated with components within which flow is severely constricted and consequently accelerated. In most systems, this situation applies to devices used to control the fluid flow, namely orifice plates and various types of valve. Also, local cavitation in the pipes may occur due to water hammer. Direct steam injection into liquid lines such as in a direct steam heating system can lead to cavitation damage from the rapid collapse of the condensing steam. Figure 6(a) Severe, localised wastage on internal surfaces due to cavitation caused a blowdown line to fail. A perforation occurred near the attachment of pipe to tee. Figure 6(b) Metal loss downstream of flange wall expansion due to cavitation Blowdown lines are especially susceptible to damage if flow is excessive and the discharge direction abruptly changes. Attack usually occurs during intermittent manual blowdown when flow direction is severely changed in pipe tees and elbows. Attack can be intense if blowdown rate is high and lines are undersized. Cavitation in elbows/curved pipe bends The cavitation in a curved pipe occurs due the forces on an element of fluid as it passes through the bend. As the surface curves away from the element, an inward force must act on it to keep it against the inner surface. The outer wall pressure cannot increase; hence, the pressure difference required to keep the particle against the inner surface must be supplied by a reduction in the inner wall pressure. The maximum pressure difference is attained when the inner surface pressure has fallen to the vapour pressure of the liquid. However, if the outward radial velocity component of the particle has not fallen to zero, the pressure difference is not great enough to cause the particle to follow the path of the bend. Therefore, it separates from the inner wall and cavitation occurs. Prevention/Mitigation Several approaches are available to alleviate or eliminate cavitation damage problems: Change of materials Use of coatings Alteration of environment Alteration of operating procedures Redesign of equipment Reducing the temperature of the fluid Increasing the total or local pressure of the fluid
