1. PREVENT CAVITATION
Cavitatio range.n of the pump should not occur throughout its operating capacity
2. MINIMIZE LOW FLOW OPERATION
Continuous operation of centrifugal pumps at low flows i.e. reduced capacities, leads to a number of unfavorable conditions. These include reduced motor efficiency, excessive radial thrusts, excessive temperature rise in the pumping fluid, internal re-circulation, etc. A certain minimum continuous flow (MCF) should be maintained during the pump operation.
Operating a pump under the condition of cavitation for even a short period of time can have damaging consequences for both the equipment and the process. Operating a pump at low flow conditions for an extended duration may also have damaging consequences for the equipment.
§ The condition of cavitation is essentially an indication of an abnormality in the pump suction system, whereas the condition of flow indicates an abnormality in the entire pumping system or process.
The two conditions are also interlinked such that a low flow situation can also induce cavitation.
The concept of cavitation is explored in detail under following topics:
1. Meaning of the term ‘cavitation’ in the context of centrifugal pumps.
2. Important definitions: Static pressure, Dynamic pressure, Total pressure, Static pressure head, Velocity Head, Vapor pressure.
3. Mechanism of cavitation.
4. General symptoms of cavitation and its effects on pump performance and pump parts.
5. Types of cavitation:
a. Vaporous cavitation
i. Classic cavitation
ii. Internal re-circulation cavitation
b. Gaseous cavitation
i. Air ingestion induced cavitation
7. Methods to prevent cavitation
The topics 1 to 4 are covered in detail in this part of the article. The topics 5 to 6 shall be explored in next part of the article.
Readers of Part I showed keen interest and appreciation about the approach in which the topic of Centrifugal Pumps has been discussed. The same enthusiasm, response and feedback are solicited from the readers.
Concept of Cavitation
Cavitation is a common occurrence but is the least understood of all pumping problems. Cavitation means different things to different people. Some say when a pump makes a rattling or knocking sound along with vibrations, it is cavitating. Some call it slippage as the pump discharge pressure slips and flow becomes erratic. When cavitating, the pump not only fails to serve its basic purpose of pumping the liquid but also may experience internal damage, leakage from the seal and casing, bearing failure, etc.
In summary, cavitation is an abnormal condition that can result in loss of production, equipment damage and worst of all, personnel injury.
The plant engineer’s job is to quickly detect the signs of cavitation, correctly identify the type and cause of the cavitation and eliminate it. A good understanding of the concept is the key to troubleshooting any cavitation related pumping problem.
Meaning of the Term "Cavitation" in the Context of the Centrifugal Pump |
The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.
In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of the term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion.
In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.
Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles.
1. Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as Vaporous Cavitation.
2. Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system). The cavitation condition induced by the formation and collapse of gas bubbles is commonly referred to as Gaseous Cavitation.
Both types of bubbles are formed at a point inside the pump where the local static pressure is less than the vapor pressure of the liquid (vaporous cavitation) or saturation pressure of the gas (gaseous cavitation).
Vaporous cavitation is the most common form of cavitation found in process plants. Generally it occurs due to insufficiency of the available NPSH or internal recirculation phenomenon. It generally manifests itself in the form of reduced pump performance, excessive noise and vibrations and wear of pump parts. The extent of the cavitation damage can range from a relatively minor amount of pitting after years of service to catastrophic failure in a relatively short period of time.
Gaseous cavitation occurs when any gas (most commonly air) enters a centrifugal pump along with liquid. A centrifugal pump can handle air in the range of ½ % by volume. If the amount of air is increased to 6%, the pump starts cavitating. The cavitation condition
is also referred to as Air binding. It seldom causes damage to the impeller or casing. The main effect of gaseous cavitation is loss of capacity.
is also referred to as Air binding. It seldom causes damage to the impeller or casing. The main effect of gaseous cavitation is loss of capacity.
The different types of cavitation, their specific symptoms and specific corrective actions shall be explored in the next part of the article. However, in order to clearly identify the type of cavitation, let us first understand the mechanism of cavitation, i.e. how cavitation occurs. Unless otherwise specified, the term cavitation shall refer to vaporous cavitation.
Important Definitions |
To enable a clear understanding of mechanism of cavitation, definitions of following important terms are explored.
· Static pressure,
· Dynamic pressure,
· Total pressure,
· Static pressure head,
· Velocity head, and
· Vapor pressure.
Static Pressure, ps
The static pressure in a fluid stream is the normal force per unit area on a solid boundary moving with the fluid. It describes the difference between the pressure inside and outside a system, disregarding any motion in the system. For instance, when referring to an air duct, static pressure is the difference between the pressure inside the duct and outside the duct, disregarding any airflow inside the duct. In energy terms, the static pressure is a measure of the potential energy of the fluid.
Dynamic pressure, pd
A moving fluid stream exerts a pressure higher than the static pressure due to the kinetic energy (½ mv2) of the fluid. This additional pressure is defined as the dynamic pressure. The dynamic pressure can be measured by converting the kinetic energy of the fluid stream into the potential energy. In other words, it is pressure that would exist in a fluid stream that has been decelerated from its velocity ‘v’ to ‘zero’ velocity.
Total pressure, pt
The sum of static pressure and dynamic pressure is defined as the total pressure. It is a measure of total energy of the moving fluid stream. i.e. both potential and kinetic energy.
Relation between ps, pd & pt
In an incompressible flow, the relation between static, dynamic and total pressures can be found out using a simple device called Pitot tube (named after Henri Pitot in 1732) shown in Figure 1.
Figure 1: A Simple Sketch of a Pilot Tube
The Pitot tube has two tubes:
1. Static tube (b): The opening of the static tube is parallel to the direction of flow. It measures the static pressure, since there is no velocity component perpendicular to its opening.
2. Impact tube (a): The opening of the impact tube is perpendicular to the flow direction. The point at the entrance of the impact tube is called as the stagnation point .At this point the kinetic energy of the fluid is converted to the potential energy. Thus, the impact tube measures the total pressure (also referred to as stagnation pressure) i.e. both static pressure and dynamic pressure (also referred to as impact pressure).
The two tubes are connected to the legs of a manometer or equivalent device for measuring pressure.
The relation between ps, pd and pt can be derived by applying a simple energy balance.
As mentioned earlier, in the case of a fluid or gas the potential energy is represented by the static pressure and the kinetic energy by dynamic pressure. The kinetic energy is a function of the motion of the fluid, and of course it's mass. It is generally more convenient to use the density of the fluid (r) as the mass representation.
\ K.E = pd = ½ m v2 = ½ r v2
The corresponding pressure balance equation is
In place of the pressure terms as used above, it is more appropriate to speak of the energy during pumping as the energy per unit weight of the liquid pumped and the units of energy expressed this way are foot-pounds per pound (Newton-meters per Newton) or just feet (meters) i.e. the units of head. Thus the energy of the liquid at a given point in flow stream can be expressed in terms of head of liquid in feet.
The pressure term can be converted to head term by division with the factor ‘r g’. For unit inter-conversions the factor ‘r g/gc’ should be used in place of ‘r g’.
Static pressure head
The head corresponding to the static pressure is called as the static pressure head.
Static pressure head = ps / r g
Velocity head
The head corresponding to dynamic pressure is called the velocity head.
Velocity head = pd / r g = (r v2 / 2) / r g = v2/2g
From the reading hm, of the manometer velocity of flow can be calculated and thus velocity head can be calculated. The pressure difference, dP (pt – ps) indicated by the manometer is the dynamic pressure.
dP = hm (rm - r) g = r v2 /2
Velocity head = dP / r g = hm (rm - r) /r
Vapor pressure, pv
Vapor pressure is the pressure required to keep a liquid in a liquid state. If the pressure applied to the surface of the liquid is not enough to keep the molecules pretty close together, the molecules will be free to separate and roam around as a gas or vapor. The vapor pressure is dependent upon the temperature of the liquid. Higher the temperature, higher will be the vapor pressure.
Mechanism of Cavitation |
The phenomenon of cavitation is a stepwise process as shown in Figure 2.
Figure 2: Phenomenon of Cavitation
Step One, Formation of bubbles inside the liquid being pumped.
The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor. But how does vaporization of the liquid occur during a pumping operation?
Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises, raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77 °F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77 °F, then the water quickly changes to a vapor.
Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.
NOTE: The vaporisation accomplished by addition of heat or the reduction of static pressure without dynamic action of the liquid is excluded from the definition of cavitation. For the purposes of this article, only pressure variations that cause cavitation shall be explored. Temperature changes must be considered only when dealing with systems that introduce or remove heat from the fluid being pumped.
To understand vaporization, two important points to remember are:
1. We consider only the static pressure and not the total pressure when determining if the system pressure is less than or greater than the liquid vapor pressure. The total pressure is the sum of the static pressure and dynamic pressure (due to velocity).
2. The terms pressure and head have different meanings and they should not be confused. As a convention in this article, the term “pressure” shall be used to understand the concept of cavitation whereas the term “head” shall be used in equations.
Thus, the key concept is - vapor bubbles form due to vaporization of the liquid being pumped when the local static pressure at any point inside the pump becomes equal to or less than the vapor pressure of the liquid at the pumping temperature.
How does pressure reduction occur in a pump system?
The reduction in local static pressure at any point inside the pump can occur under two conditions:
1. The actual pressure drop in the external suction system is greater than that considered during design. As a result, the pressure available at pump suction is not sufficiently high enough to overcome the design pressure drop inside the pump.
2. The actual pressure drop inside the pump is greater than that considered during the pump design.
The mechanism of pressure reduction in the external and internal suction system of a pump system is explored next.
· Pressure reduction in the external suction system of the pump
A simple sketch of a pump ‘external suction system’ is shown in Figure 3.
Figure 3: External Suction System
Nomenclature used for Figure 3 r - Liquid density in lbm / ft3 G - Acceleration due to gravity in ft / s2 Psn - p refers to local static pressure (absolute). Subscript s refers to suction and subscript n refers to the point of measurement. The pressure at any point can be converted to the head term by division with the factor - r g ps1 - Static pressure (absolute) of the suction vessel in psia hps1 - Static pressure head i.e. absolute static pressure on the liquid surface in the suction vessel, converted to feet of head (ps1/ r g/gc). If the system is open, hps1 equals the atmospheric pressure head. vs1 - Liquid velocity on the surface in the suction vessel in ft/s hvs1 - Velocity head i.e. the energy of a liquid as a result of its motion at some velocity ‘vs1’. (v2s1 / 2g). It is the equivalent head in feet through which the liquid would have to fall to acquire the same velocity, or the head necessary to accelerate the liquid to velocity vs1. In a large suction vessel, the velocity head is practically zero and is typically ignored in calculations. hs - Static suction head. . . . i.e. head resulting from elevation of the liquid relative to the pump centerline. If the liquid level is above pump centerline, hS is positive. If the liquid level is below pump centerline, hS is negative. A negative hS condition is commonly referred to as “suction lift”. hfs - Friction head i.e. the head required to overcome the resistance to flow in the pipe, valves and fittings between points A and B, inclusive of the entrance losses at the point of connection of suction piping to the suction vessel (point A in Figure 1). The friction head is dependent upon the size, condition and type of pipe, number and type of fittings, valves, flow rate and the nature of the liquid. The friction head varies as the square of the average velocity of the flowing fluid. ps2 - Absolute static pressure at the suction flange in psia hps2 - Static pressure head at the suction flange i.e. absolute pressure of the liquid at the suction flange, converted to feet of head - ps2 / r g/gc vs2 - Velocity of the moving liquid at the suction flange in ft/s. The pump suction piping is sized such that the velocity at the suction remains low. hvs2 - Velocity head at suction flange i.e. the energy of a liquid as a result of its motion at average velocity ‘vs2’ equal to v2s2 / 2g. pv - Absolute vapor pressure of the liquid at operating temperature in psia. hpv - Vapor Pressure head i.e. absolute vapor pressure converted to feet of head (pv / r. g/gc). Hs - Total Suction Head available at the suction flange in ft. Note: As pressure is measured in absolute, total head is also in absolute. |
The pump takes suction from a vessel having a certain liquid level. The vessel can be pressurised (as shown in the Figure 3) or can be at atmospheric pressure or under vacuum.
Calculation of the Total Suction Head, Hs
The external suction system of the pump provides a certain amount of head at the suction flange. This is referred to as Total Suction Head (TSH), Hs.
TSH can be calculated by application of the energy balance. The incompressible liquid can have energy in the form of velocity, pressure or elevation. Energy in various forms is either added to or subtracted from the liquid as it passes through the suction piping. The head term in feet (or meters) is used as an expression of the energy of the liquid at any given point in the flow stream.
As shown in Figure 3, the total suction head, Hs, available at the suction flange is given by the equation,
Hs = hps1 + hvs1 + hs - hfs + hvs2 | (1) |
For an existing system, Hs can also be calculated from the pressure gauge reading at pump suction flange,
Hs = hps2 + hvs2 | (2) |
Equations 1 and 2 above include the velocity head terms hvs1 and hvs2, respectively.
A word about the velocity head term:
There is a lot of confusion as to whether the velocity head terms should be added or subtracted in the head calculations. To avoid any confusion remember the following:
o Just like a static tube of Pitot, a pressure gauge can measure only the static pressure at the point of connection. It does not measure the dynamic pressure as the opening of the gauge impulse pipe is parallel to the direction of flow and there is no velocity component perpendicular to its opening.
In Figure 4 below, flow through a pipe of varying cross section area is shown.
Figure 4: Measuring Static Pressure
As the cross section at point B reduces, the velocity of flow increases. The rise in kinetic energy happens at the expense of potential energy. Assuming that there are no friction losses, the total energy (sum of potential energy and kinetic energy) of fluid at point A, B and C remains constant. The pressure gauges at point A, B and C measure only the potential energy i.e. the static pressures at respective points. The drop in static pressure from 10 psi (point A) to 5 psi (point B) occurs owing to rise the dynamic pressure by 5 psi i.e. increase in velocity at point B. However the gauge at point B records only the static pressure. The velocity decreases from point B to C and the static pressure is recovered again to 10 psi.
o At a particular point of flow, the total pressure is the sum of the static pressure and the dynamic pressure.
Thus, theoretically, the velocity head terms must always be added and not subtracted, in calculating Total Suction Head (TSH), Hs.
However, practically speaking, the value of these terms is not significant in comparison to the other terms in the equation.
o hvs1: In industrial scale suction vessels, the value of hvs1 is practically zero and it can be safely ignored.
o hvs2: It is good piping design practice to reduce the friction losses and prevent unnecessary flow turbulence by sizing the suction pipes for fluid velocities in the three to five feet per second range only. The velocity head corresponding to a velocity of 5 ft/s at the suction flange is only about 0.4 ft. Thus, for all practical purposes, in high head systems the velocity head at the suction flange is not significant and can be safely ignored. Only in low head systems does the factor need to be considered.
Therefore, neglecting the velocity head terms, Equations 1 and 2 simplify to:
Hs = hps1 + hs - hfs | (3) |
Hs = hps2 | (4) |
Two important inferences can be drawn from the above equations:
· The pressure reduction in the external suction system is primarily due to frictional loss in the suction piping (Equation 3).
· For all practical purposes, the total head at the suction flange is the static pressure head at the suction flange (Equation 4).
Therefore the pump’s external suction system should be designed such that the static pressure available at the suction flange is always positive and higher than the vapor pressure of the liquid at the pumping temperature.
For no vaporization at pump suction flange,
(ps2 > pv) or (ps2 - pv ) or (hps2 - hpv ) > 0 | (5) |
As the liquid enters the pump, there is a further reduction in the static pressure. If the value of ps2 is not sufficiently higher than pv, at some point inside the pump the static pressure can reduce to the value of pv. In pumping terminology, the head difference term corresponding to Equation 5 (hps2 - hpv) is called the Net Positive Suction Head or NPSH. The NPSH term shall be explored in detail in the next part of the article. For now, the readers should focus only on how the static pressure within the pump may be reduced to a value lower than that of the liquid vapor pressure.
· Pressure reduction in the internal suction system of the pump
The pressure of the fluid at the suction flange is further reduced inside the internal suction system of the pump.
Flow path of fluid inside the pump
The internal suction system is comprised of the pump’s suction nozzle and impeller. Figures 5 and 6 depict the internal parts in detail. A closer look at the graphic is a must in understanding the mechanism of pressure drop inside the pump.
Figure 5: Internal Pump Locations
Figure 6: Internal Pump Nomenclature
In Figure 7, it can be seen that the passage from the suction flange (point 2) to the impeller suction zone (point 3) and to the impeller eye (point 4) acts like a venturi i.e. there is gradual reduction in the cross-section area.
Figure 7: Pump Internal Suction System
In the impeller, the point of minimum radius (reye) with reference to pump centerline is referred to as the “eye” of the impeller (Figure 8).
Figure 8: Impeller Eye
How pressure reduction occurs as the fluid flows inside the pump?
According to Bernoulli’s principle, when a constant amount of liquid moves through a path of decreasing cross-section area (as in a venturi), the velocity increases and the static pressure decreases. In other words, total system energy i.e. sum of the potential and kinetic energy, remains constant in a flowing system (neglecting friction). The gain in velocity occurs at the expense of pressure. At the point of minimum cross-section, the velocity is at a maximum and the static pressure is at a minimum.
The pressure at the suction flange, ps2 (Point 2) decreases as the liquid flows from the suction flange, through the suction nozzle and into the impeller eye. This decrease in pressure occurs not only due to the venturi effect but also due to the friction in the inlet passage. However, the pressure drop due to friction between the suction nozzle and the impeller eye is comparatively small for most pumps. However the pressure reduction due to the venturi effect is very significant as the velocity at the impeller increases to 15 to 20 ft/s. There is a further drop in pressure due to shock and turbulence as the liquid strikes and loads the edges of impeller vanes. The net effect of all the pressure drops is the creation of a very low-pressure area around the impeller eye and at the beginning of the trailing edge of the impeller vanes.
The pressure reduction profile within the pump is depicted in Figure 9.
Figure 9: Pressure Profile in a Pump
As shown in Figure 9, the impeller eye is the point where the static pressure is at a minimum, p4. During pump operation, if the local static pressure of the liquid at the lowest pressure becomes equal to or less than the vapor pressure (pv) of the liquid at the operating temperature, vaporization of the liquid (the formation of bubbles) begins i.e. when p4 £ pv.
It is at the beginning of the trailing edge of the vanes near the impeller eye where the pressure actually falls to below the liquid vapor pressure. The region of bubble formation is shown in Figure 10.
Figure 10: Impeller Cavitation Regions
In summary, vaporization of the liquid (bubble formation) occurs due to the reduction of the static pressure to a value below that of the liquid vapor pressure. The reduction of static pressure in the external suction system occurs mainly due to friction in suction piping. The reduction of static pressure in the internal suction system occurs mainly due to the rise in the velocity at the impeller eye.
Step Two, Growth of bubbles
Unless there is no change in the operating conditions, new bubbles continue to form and old bubbles grow in size. The bubbles then get carried in the liquid as it flows from the impeller eye to the impeller exit tip along the vane trailing edge. Due to impeller rotating action, the bubbles attain very high velocity and eventually reach the regions of high pressure within the impeller where they start collapsing. The life cycle of a bubble has been estimated to be in the order of 0.003 seconds.
Step Three, Collapse of bubbles
As the vapor bubbles move along the impeller vanes, the pressure around the bubbles begins to increase until a point is reached where the pressure on the outside of the bubble is greater than the pressure inside the bubble. The bubble collapses. The process is not an explosion but rather an implosion (inward bursting). Hundreds of bubbles collapse at approximately the same point on each impeller vane. Bubbles collapse non-symmetrically such that the surrounding liquid rushes to fill the void forming a liquid microjet. The micro jet subsequently ruptures the bubble with such force that a hammering action occurs. Bubble collapse pressures greater than 1 GPa (145x106 psi) have been reported. The highly localized hammering effect can pit the pump impeller. The pitting effect is illustrated schematically in Figure 11.
Figure 11: Collapse of a Vapor Bubble
After the bubble collapses, a shock wave emanates outward from the point of collapse. This shock wave is what we actually hear and what we call "cavitation". The implosion of bubbles and emanation of shock waves (red color) is shown in a small video clip available here.
In nutshell, the mechanism of cavitation is all about formation, growth and collapse of bubbles inside the liquid being pumped. But how can the knowledge of mechanism of cavitation can really help in troubleshooting a cavitation problem. The concept of mechanism can help in identifying the type of bubbles and the cause of their formation and collapse. The troubleshooting method shall be explored in detail in the next part of the article.
Next let us explore the general symptoms of cavitation and its affects on pump performance
General Symptoms of Cavitation and its Affects on Pump Performance and Pump Parts
Perceptible indications of the cavitation during pump operation are more or less loud noises, vibrations and an unsteadily working pump. Fluctuations in flow and discharge pressure take place with a sudden and drastic reduction in head rise and pump capacity. Depending upon the size and quantum of the bubbles formed and the severity of their collapse, the pump faces problems ranging from a partial loss in capacity and head to total failure in pumping along with irreparable damages to the internal parts. It requires a lot of experience and thorough investigation of effects of cavitation on pump parts to clearly identify the type and root causes of cavitation.
A detailed description of the general symptoms is given as under.
· Reduction in capacity of the pump:
The formation of bubbles causes a volume increase decreasing the space available for the liquid and thus diminish pumping capacity. For example, when water changes state from liquid to gas its volume increases by approximately 1,700 times. If the bubbles get big enough at the eye of the impeller, the pump “chokes” i.e. loses all suction resulting in a total reduction in flow. The unequal and uneven formation and collapse of bubbles causes fluctuations in the flow and the pumping of liquid occurs in spurts. This symptom is common to all types of cavitations.
· Decrease in the head developed:
Bubbles unlike liquid are compressible. The head developed diminishes drastically because energy has to be expended to increase the velocity of the liquid used to fill up the cavities, as the bubbles collapse. As mentioned earlier, The Hydraulic Standards Institute defines cavitation as condition of 3 % drop in head developed across the pump. Like reduction in capacity, this symptom is also common to all types of cavitations.
Thus, the hydraulic effect of a cavitating pump is that the pump performance drops off of its expected performance curve, referred to as break away, producing a lower than expected head and flow. The Figure 12 depicts the typical performance curves. The solid line curves represent a condition of adequate NPSHa whereas the dotted lines depict the condition of inadequate NPSHa i.e. the condition of cavitation.
Figure 12: Pump Performance Curves
· Abnormal sound and vibrations:
It is movement of bubbles with very high velocities from low-pressure area to a high-pressure area and subsequent collapse that creates shockwaves producing abnormal sounds and vibrations. It has been estimated that during collapse of bubbles the pressures of the order of 104 atm develops.
The sound of cavitation can be described as similar to small hard particles or gravel rapidly striking or bouncing off the interior parts of a pump or valve. Various terms like rattling, knocking, crackling are used to describe the abnormal sounds. The sound of pumps operating while cavitating can range from a low-pitched steady knocking sound (like on a door) to a high-pitched and random crackling (similar to a metallic impact). People can easily mistake cavitation for a bad bearing in a pump motor. To distinguish between the noise due to a bad bearing or cavitation, operate the pump with no flow. The disappearance of noise will be an indication of cavitation.
Similarly, vibration is due to the uneven loading of the impeller as the mixture of vapor and liquid passes through it, and to the local shock wave that occurs as each bubble collapses. Very few vibration reference manuals agree on the primary vibration characteristic associated with pump cavitation. Formation and collapsing of bubbles will alternate periodically with the frequency resulting out of the product of speed and number of blades. Some suggest that the vibrations associated with cavitation produce a broadband peak at high frequencies above 2,000 Hertz. Some suggest that cavitation follows the vane pass frequency (number of vanes times the running speed frequency) and yet another indicate that it affects peak vibration amplitude at one times running speed. All of these indications are correct in that pump cavitation can produce various vibration frequencies depending on the cavitation type, pump design, installation and use. The excessive vibration caused by cavitation often subsequently causes a failure of the pump’s seal and/or bearings. This is the most likely failure mode of a cavitating pump,
· Damage to pump parts:
o Cavitation erosion or pitting
During cavitation, the collapse of the bubbles occurs at sonic speed ejecting destructive micro jets of extremely high velocity (up to 1000 m/s) liquid strong enough to cause extreme erosion of the pump parts, particularly impellers. The bubble is trying to collapse from all sides, but if the bubble is lying against a piece of metal such as the impeller or volute it cannot collapse from that side. So the fluid comes in from the opposite side at this high velocity and bangs against the metal creating the impression that the metal was hit with a "ball pin hammer". The resulting long-term material damage begins to become visible by so called
Pits (see Figure 11), which are plastic deformations of very small dimensions (order of magnitude of micrometers). The damage caused due to action of bubble collapse is commonly referred as Cavitation erosion or pitting. The Figure 13 depicts the cavitation pitting effect on impeller and diffuser surface.
Figure 13: Photographic Evidence of Cavitation
Cavitation erosion from bubble collapse occurs primarily by fatigue fracture due to repeated bubble implosions on the cavitating surface, if the implosions have sufficient impact force. The erosion or pitting effect is quite similar to sand blasting. High head pumps are more likely to suffer from cavitation erosion, making cavitation a “high-energy” pump phenomenon.
The most sensitive areas where cavitation erosion has been observed are the low-pressure sides of the impeller vanes near the inlet edge. The cavitation erosion damages at the impeller are more or less spread out. The pitting has also been observed on impeller vanes, diffuser vanes, and impeller tips etc. In some instances, cavitation has been severe enough to wear holes in the impeller and damage the vanes to such a degree that the impeller becomes completely ineffective. A damaged impeller is shown in Figure 14.
Figure 14: Cavitation Damage on Impellers
The damaged impeller shows that the shock waves occurred near the outside edge of the impeller, where damage is evident. This part of the impeller is where the pressure builds to its highest point. This pressure implodes the gas bubbles, changing the water’s state from gas into liquid. When cavitation is less severe, the damage can occur further down towards the eye of the impeller. A careful investigation and diagnosis of point of the impeller erosion on impeller, volute, diffuser etc. can help predict the type and cause of cavitation.
The extent of cavitation erosion or pitting depends on a number of factors like presence of foreign materials in the liquid, liquid temperature, age of equipment and velocity of the collapsing bubble.
o Mechanical deformations
Apart from erosion of pump parts, in bigger pumps, longer duration of cavitation condition can result in unbalancing (due to un-equal distribution in bubble formation and collapse) of radial and axial thrusts on the impeller. This unbalancing often leads to following mechanical problems:
· bending and deflection of shafts,
· bearing damage and rubs from radial vibration,
· thrust bearing damage from axial movement,
· breaking of impeller check-nuts,
· seal faces damage etc.
These mechanical deformations can completely wreck the pump and require replacement of parts. The cost of such replacements can be huge.
o Cavitation corrosion
Frequently cavitation is combined with corrosion. The implosion of bubbles destroys existing protective layers making the metal surface permanently activated for the chemical attack. Thus, in this way even in case of slight cavitation it may lead to considerable damage to the materials. The rate of erosion may be accentuated if the liquid itself has corrosive tendencies such as water with large amounts of dissolved oxygen to acids.
Frequently cavitation is combined with corrosion. The implosion of bubbles destroys existing protective layers making the metal surface permanently activated for the chemical attack. Thus, in this way even in case of slight cavitation it may lead to considerable damage to the materials. The rate of erosion may be accentuated if the liquid itself has corrosive tendencies such as water with large amounts of dissolved oxygen to acids.
Cavitation – heart attack of the pump
Thus fundamentally, cavitation refers to an abnormal condition inside the pump that arises during pump operation due to formation and subsequent collapse of vapor filled cavities or bubbles inside the liquid being pumped. The condition of cavitation can obstruct the pump, impair performance and flow capacity, and damage the impeller and other sensitive components. In short, Cavitation can be termed as “the heart attack of the pump”.
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