The Cavitations In Pumps Engineering Essay

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Introduction

Cavitation is one of the major problems confronting pumps and inducer designers. Cavitation takes place in liquids if the local pressure is decreased to the vapor pressure of the liquid.

The flow area at the eye or opening of the pump impeller, within centrifugal pump, is smaller than the flow area of the impeller vanes or of the pump suction piping. If the liquid that is being pumped enters the eye of the centrifugal pump, then the decrease in flow area leads to an increase in flow velocity along with a decrease in pressure. This drop is pressure is reliant on the pump flow rate; that is, greater the pump flow rate, greater is the pressure drop between the eye of the impeller and the pump suction. However, if the pressure or temperature is sufficiently high, then the pressure drop might be adequate for causing the liquid to vaporize when the local pressure drops below the saturation pressure for the pumped liquid (Gülich 2008) (Mc Nally Institute 2010). Furthermore, any vapor bubbles created by the pressure drop at the impeller eye will be swept across the impeller vanes by the flow of the liquid. Whenever the bubbles enter into a region wherein local pressure is higher than saturation pressure ahead of the impeller vane, then the vapor bubbles collapse abruptly. This process of creation and subsequent collapse of vapor bubbles in a pump is known as cavitation (Engineers Edge 2010) (Queensland Government 2006) (Robertson 2003).

Although cavitation is said to a common occurrence, it is the least understood among all pumping problems. A pump is said to be cavitating if vibrations and knocking noises are heard while it is operating. Other sign of cavitation include erratic power consumptions and reductions or fluctuations in pump output. If the pump is still continued to operate when it is cavitating, it will be damaged. Pump bowls as well as impeller surfaces wit pit and wear, resulting in mechanical destruction. Fewer centrifugal pumps are designed to operate and function under conditions wherein cavitation is unavoidable. Moreover, these pumps require special design and maintenance in order to withstand the amount of cavitation that occurs throughout their operation. Most of the centrifugal pumps are not designed for withstanding sustained cavitation (Mc Nally Institute 2010) (Neff 1989c).

The term cavitation entails that bubbles or cavities are formed in the fluid being pumped through the pump. These cavities are formed at the suction side of the pump at relatively low pressure, thereby causing various things to happen simultaneously (Queensland Government 2006). These outcomes are:

A loss in capacity is experienced

The bubbles or cavities collapse when they pass through the higher regions of pressure, vibrations, causing noise, and causes damage to several other components.

The efficiency of the pump drops

The pump is no longer capable of building the same head, pressure.

One of the most common working liquids, water, possesses a higher propensity to cavitate under commonly operating conditions. Inducers and pumps are usually damaged by the collapsing micro-bubbles or cavities that are formed through the cavitation process, thereby severely deteriorating the efficiency of the pump (Gülich 2008) (Mc Nally Institute 2010) (Neff 1989c) (Engineers Edge 2010).

Cavitation Parameters:

During the process of cavitation, one may imagine that vapor bubbles or cavities are produced as soon as the pressure in the fluid reaches the vapor pressure, pv, of the fluid at the operating temperature. Although many complicating elements discussed in previous section, deviate from this hypothesis, it is useful though to follow this criterion for the intention of the understanding the detailing and concepts of cavitations (Brennen 2000).

The static pressure, p, is usually non-dimensionalized as a pressure coefficient, Cp, as shown below:

Cp = (p - pi)/

Where, pi is a reference static pressure which uses the pump inlet pressure and U is the reference velocity for which inlet tip speed is used, i.e. ΩRT1. However, for the flow of an incompressible liquid around fixed boundaries, Cp is a function of the boundary geometry and the Reynolds number Re that may be defined as 2Ω R2T1 / v, where v is the kinetic viscosity of the liquid. Also, without any cavitation, the fluid velocities and the pressure coefficient are independent of the pressure level. Hence, for instance, as the inlet pressure p1 changes, all the other pressures will also change equally, so that Cp remains unaffected (Brennen 2000) (Gülich 2008) (Andrews 2007).

In any fluid flow with predefined fluid velocities, Reynolds number and geometry, the specific location at which the pressure p1 is minimal and the difference between this minimum pressure p and the inlet pressure p min is indicated as:

C pmin = (p min - p1)/

Where, C pmin is a negative value which is a function of the geometry of the pump and the Reynolds number. The value of the inlet pressure p1 can be obtained if the value of C pmin is obtained either theoretically or experimentally. At this point the cavitation would appear as p1 is reduced, assuming that this takes place when pmin= pV . (Brennen 2000)

(p1) cavitation appearance = pv + (- C pmin)

Which will be function of the velocity, U, for a given fluid, device, and fluid temperature (Andrews 2007).

Inception and 3% head loss cavitation values plotted against a Reynolds number for four flow rates, on the basis of wT1 and blade chord length (Brennen 2000).

Types of Cavitations

Suction Cavitation:

Suction cavitation is caused by insufficient net positive suction head available (NPSHA). Typically, the concept is that NPSHA must be at least equal to and potentially greater that of net positive suction head required (NPSHR) in order avoid Suction Cavitation. Hence, a question arises as to how much amount of NPSHA above NPSHR is sufficient? The only solution to this question, considering the nature of the pump and cavitation, is that the amount of NPSHA produces the minimum amount of damage to the pump. As far as complexity is concerned, larger margin of NPSHA over NPSHR usually produces more damage to a pump than lower margins, particularly when dealing with cool water, i.e. less than approximately 150 degree F (Neff 1989a).

Recirculation Cavitation

Recirculation Cavitation is caused due to the low flow rate through the pump. Two types of recirculation cavitation may occur either separately or together: Suction Side & Discharge Side. Both these types of recirculation are obtained from the similar principles of reverse fluid flows in related proximity to each other. Whenever two flow paths in a fluid move in opposite directions and with close proximity to each other, then vortices are formed between the two independent directions of the flow, which causes turbulence and high fluid velocities. This phenomena result in lower pressure where occurrence of cavitation is possible. However, in general, pumps that have low pump specific speed (NS) as well as low suction specific speed (NSS) are more tolerant to recirculation cavitation (Robertson 2003) (Neff 1989a).

Suction Recirculation Cavitation

In suction recirculation cavitation, direction of the fluid entering the pump suction nozzle is totally reversed. This results into high velocity vortexes in or around the impeller eye, within the suction nozzle, or inside the pipe near to the suction nozzle. High velocities produce low localized pressures, wherein local pressures may fall below the vapor pressure of the fluid, leading to cavitation. Damage caused due to cavitation, at the pressure sides of the inlet vanes, close to the impeller eye, is an indication of suction recirculation, and thus this observation is necessarily diagnostic. Observing from the impeller eye, the inlet vanes' pressure side is located on the underside of the vane, and hence can only be observed through a mirror. Noise produced because of suction recirculation cavitation may be distinctive from other forms of cavitation noise, and is also diagnostic. Suction recirculation cavitation noise is described to be crackling, loud popping, knocking, or hammering sound, produced with highest intensity at the suction nozzle (Andrews 2007) (Neff 1989a).

Discharge Recirculation Cavitation

In discharge recirculation cavitation, direction of the fluid passing at low flow rates, from the pump discharge nozzle, can be reversed. This results into production of vortexes of high velocities between the two individual flow directions, thereby causing localized low pressure regions. Damage caused due to recirculation cavitation also happens at the impeller periphery's discharge side, within the discharge nozzle, at the cutwater(s), or within the pipe near the discharge nozzle. Noise produced due to discharge recirculation cavitation is commonly less noisy than that of suction side recirculation. Discharge recirculation cavitation noise can be heard generally at the pump discharge nozzle, however, there won't be any crackling noise heard at the occurrence of suction recirculation (Neff 1989a).

The type and location of damage caused due to Discharge Recirculation Cavitation is shown above.

Incipient Cavitation

In order to understand the working of incipient cavitation, one should be familiar with the term NPSHi, that is, Net Positive Suction Head Inception, and NPSHR

NPSHR: It is the fluid pressure, measured at the pump suction nozzle, wherein a 3 % drop in the precise calculation of ∆P occurs.

NPSHi: It is that fluid pressure, measured at the pump suction nozzle, wherein all cavitation within the pump is suppressed.

Differential Pressure (dP or ∆P): It is the pressure differential that rises across a pump, when measured at the discharge and intake nozzle (Brennen 2000) (Andrews 2007) (Neff 1989a).

Therefore, incipient cavitation is defined as the cavitation occurring in a pump, from 3 % value of NPSHR, till the incipient point. In most pumps, incipient cavitation occurs at al times. Furthermore, the cause for this type of cavitation is the turbulence produced by the impeller that results into localized pressure lower than the vapor pressure of the pumpage. Typically, in the common pump market, omnipresence of incipient cavitation causes minor loss of performance and damage, thus the concept is not discussed commonly. Even though this fact might partly be due to it under-description, the fact still remains that damage from incipient cavitation is not a commonly discussed topic except in certain markets. Incipient cavitation discussion is concerning to those markets wherein high energy suction pumps are utilized (Brennen 2000) (Neff 1989a). Most common examples are boiler feed pumps, chilled water systems, HVAC cooling towers that have severe problems of incipient cavitation. Higher margins of NPSHA over NPSHR may produce increasingly serious incipient cavitation destruction. For higher the margins, more damage occurs till the NPSHi value is obtained, which is usually unattainable. Cooler the water, more damaging is the cavitation.

The Hydraulic Institute and other organizations have established universal recommended margins for NPSHA for particular markets.

Following are the factors that indicate incipient cavitation might be a problem:

Some ranges of Pump Specific Speed

Heavy weight liquids like water, and particularly when such liquids are at cooler temperatures. The temperature for water would be 150° F, or less. In reality, water is among the worst players regarding the cavitation damage in general.

System having high ∆P values

Systems having large margins of NPSHA over NPSHR

High Suction Specific Speeds, approx Nss > 9500, and what is called as "High Energy" pumps by the Hydraulic Institute (HI) that are determined by a chart issued by HI.

Standards set by the Hydraulic Institute for the operation of turbines and pumps

Significantly, incipient cavitation is strongly related to the Suction Specific Speed of a centrifugal pump, i.e. for high suction specific speed, greater is the possibility of incipient cavitation being a problem. Furthermore, high Suction energy pumps demand higher margins of NPSHA over NPSHR. Issued opinions describe this margin as ranging from 2 to 20 times NPSHA over NPSHR. Therefore, extra margin of NPSH are required, and if too much of NPSH is supplied, then incipient damage turns into a problem. In certain pumps, a small margin operates well, while for other pumps larger margins are required (Neff 1989a). The main reason behind this dilemma is the methods by which NPSHR values are measured (Andrews 2007). The standard HI test method for NPSHR involves setting NPSHR at a point at which a 3 % drop in accurate dP throughout the pump occurs as the pump inlet pressure decreases. For pumps with low Nss and Ns values, this 3 % fall of in dP value indicates a minor but noticeable amount of cavitations. Similarly, pumps with high Nss and Ns values show more effectiveness in sweeping water through the impeller, by which a 3% drop in dP represents a large amount of cavitation which may damage the pump quickly and severely (Brennen 2000). Therefore, NPSHR does not entail the same value for all pumps (Neff 1989a).

Van Passing Syndrome Cavitation

Van Passing Syndrome Cavitation results when the impeller vane tips till the cutwater clearance is too small. This cavitation also results into excessive turbulence every time a vane passes the cutwater, thereby producing cavitation and pulsation. The position of damage caused by this type of cavitation is diagnostic. Moreover, typical cavitation type damage can be detected on the discharge edge of the impeller cover, on the center of the cutwater, at the impeller vane tips, as well as to the pump casing downstream and immediately behind the water (Mc Nally Institute 2010).

Engineering specifications can attempt to preclude this trouble by not letting pump manufacturers to provide pumps of the largest impeller diameter that are available for a given class of pumps. However, this is not urged practice for engineers since it presumes that a pump manufacturer supplies a pump having vane passing syndrome without the manufacturer being aware of the problem, or if they are aware, they are not notifying the customer. This practice of not permitting a pump manufacturer to supply the largest impeller diameter is apprehended from the viewpoint of the behavior of certain pump manufacturers (Andrews 2007) (Mc Nally Institute 2010) (Neff 1989a).

Pump Performance:

The pump performance when represented non-dimensionally takes the generic form as shown in the diagram above. The non-cavitating performance ψ (φ) contains the head coefficient ψ which is a function of the flow coefficient φ, wherein particular point on the curve indicates the design conditions are identified. Also, the non-cavitating characteristic must be independent of the speed Ω, although there might be deviating due to Reynolds number or viscous effect, at lower temperatures.

The cavitating performance, ψ (φ, σ), indicated a family of curves each for a certain flow coefficient, on a graph of head coefficient vs. cavitation number, σ. The NPSH is usually used instead of the cavitation number that represents the abscissa for the cavitation performance graph (Gülich 2008).

Causes of Cavitations

Essentially, cavitation takes place when the net positive suction head; i.e. NPSH, pressure at the inlet of the pump drops below the fluid's vapor pressure that is the minimum pressure needed for keeping a fluid in that state and for preventing it from turning into vapor. In other words, the NPSH is said to be the actual pressure which is experienced by the fluid within the pump. In case the NPSH drops below the vapor pressure corresponding to that fluid, vapor bubbles are formed. With the flow of bubbles through the impeller, the pressure exerted on the liquid increases to a point over the liquid's vapor pressure, thereby causing the bubbles to collapse in a violent manner. Furthermore, this action tends to attack the surfaces within the pump, and results in significant damage to the pump, including loss of efficiency, loss of hydraulic performance because of impeller wear, etc. Anybody who stands near the cavitating pump might easily distinguish between cavitation and other faults, since it sounds like rocks traversing through the pump or popping of popcorn (Martino 2006) (Queensland Government 2006) (Reeves) (Neff 1989d) (Engineers Edge 2010).

However, industrial and chemical plants with huge amount of pumps in crucial applications cannot position any personnel besides them in order to detect intermittent cavitation. Hence a better alternative for detecting cavitation is by automatically monitoring vibration levels (Neff 1989c).

Since noise is among the several indications that a centrifugal pump is suffering from cavitation, a cavitation pump sounds like a shaking container of marbles. Whereas, other indications observed from a remotely situated operating station, are fluctuations in flow rate, pump motor current, and discharge pressure (Queensland Government 2006) (Reeves).

Effects of Cavitations

Cavitation in pumps has significant effects on the pump performance. Cavitation not only degrades the performance of a pump that results in fluctuating discharge pressure and flow rates, but also can destruct pumps' internal components. Whenever cavitation is detected in a pump, vapor bubbles are formed in the low pressure area immediately behind the rotating impeller vanes. Then, these vapor bubbles move towards the oncoming impeller vane and collapse and cause a physical shock at the leading edge of the impeller vane. Small pits are created by this physical shock at the leading edge of the impeller vane (Engineers Edge 2010). Every individual pit is microscopic in size however; the cumulative effect caused by millions of such pits that form over a period of days or even hours is capable of destroying a pump impeller. Other effects of cavitations are excessive pump vibrations that cause damage to pump bearings, wearing of seals and rings (Queensland Government 2006) (Martino 2006) (Neff 1989b) (Neff 1989c).

Cavitation

Cavitation not only affects the pump performance, but also destroys valves and pumps. It causes a loss of efficiency in pumps instantly, as it erodes the pump components thereby degrading the machinery due to cavitation. Thus, it is essential to understand this phenomenon adequately in order to predict and minimize cavitation and its effects, and also to be able to diagnose and discover practical solutions to problems of cavitation (Gülich 2008) (Martino 2006) (Robertson 2003).

Apart from loud popping and crackling sounds, the collapsing vapor bubbles produce distinct vibration patterns. The key to recognizing the occurrence of cavitation is the ability to discover high frequency energy at a pump as well as determine whether or not this energy is increasing or decreasing (Reeves) (Neff 1989b) (Neff 1989d) (Engineers Edge 2010).

The figure below indicates the vibration data gathered from a pump that experiences cavitation. The diagram shows that the similarity in the data for both pump bearings must be expected, since pump cavitation vibrations are present at both the bearings, while bearing problems are typically separated on one specific bearing. However, if energy increases on only one of the bearings, then the vibration is possibly concerned to either lack of lubrication or a bearing fault, but not as the result of cavitation (Reeves) (Neff 1989b).

Cavitation enhanced chemical erosion:

Pumps that operate under the cavitation conditions are more vulnerable to chemical attacks and corrosion. Commonly, metals develop a layer of oxide or passivated layer that is capable of protecting the metal from more corrosion. Cavitation eliminates this oxide layer on an uninterrupted basis and may make the metal vulnerable to farther oxidation. Moreover, the two processes, that is, cavitation and oxidation, work together for rapid removal of the metal from the pump casing as well as the impeller. Nevertheless, stainless steel is not vulnerable to this process (Martino 2006) (Neff 1989d).

Materials Selection:

No plastic, metal, or other material is capable of resisting the high levels of energy produced by cavitation process in the form of pressure and heat. Practically, however, materials are selected bearing in mind the long life and customer value and their ability to withstand energies produced by cavitation, so that care towards pump construction materials is productive and valuable. Commonly, in the cases where cavitation may neither be a problem nor predicted to be one, materials like bronze, cast iron are best suitable for pump constructions (Neff 1989d). Following are the factors that indicate usage of materials with higher resistance to cavitation:

Corrosive Pumpage: Water having chlorine, salt water, or any other oxidizers. A metal which usually does not have any problem with erosion through a specific chemical may become vulnerable to that chemical when cavitation occurs. Furthermore, cavitation may eat away the oxide or passivated layer which normally prevents the metal from corroding. Moreover, stainless steels undergo chemical erosion when passivated layer surface of the stainless steel is constantly eliminated by cavitation, thereby exposing fresh and vulnerable metal layer to the oxidizing agent (Neff 1989d).

Low NPSHA: Long-term operations along with inadequate or marginal NPSHA

Low Flow Rate: Lengthy or long-term operations at low flow rates may result into both kinds of recirculation cavitation.

Heavy Weight / High Density Fluids: Heavy fluids such as water produce more damage during cavitation situations. Since water molecules are dense and small, density is at highest point at cooler temperatures, therefore, water and similar fluids become more of a problem at temperatures below 150° F.

Systems with greater margins of NPSHA over NPSHR: during such situations, reduction of NPSH may either reduce or practically remove the cavitation damage.

High Suction Specific Speeds (Nss > 9500)

Systems with high dP values throughout the pump

High Specific Speed Pumps (Ns > 9000) (Neff 1989d)

Henry's Law

Henry's law states that the solubility of a gas in a fluid is reliant on temperature, the partial pressure of the gas on the fluid, the nature of the gas and the nature of the solvent. Water is the most common solvent.

Solubility of gas is always curbed by the equilibrium between the gas and a saturated solution of the gas. Moreover, the dissolved gas always follows the Henry's Law. The strength or concentration is dissolved gas is dependent on the partial pressure exerted by the gas. This partial pressure is capable of controlling the number of collisions of gas molecules with the solution surface. If the partial pressure is doubled, then the number of collisions with the solution surface will also double, as a result the concentration of the dissolved gas is also doubled (Volland 2005).

This concept is illustrated diagrammatically as shown below:

Low concentration with

low pressure equilibrium

Double the concentration with

double the pressure equilibrium

Equilibrium is the dissolving process for gases. The solubility of a gas directly relies on the gas pressure. Also, the number of molecules that exit the gas phase to enter the solution is equal to the number of gas molecules that leave the solution. When the temperature remains constant, increase in pressure increases the amount of dissolved gas (Volland 2005).

O2 (g) ↔ O2 (aq)

Gas Solubility Gas Partial Pressure

Pgas = kC at constant T

The Henry's law constant, k, is different for all gases, solvents and temperatures. The unit of k depends on the unit used for pressure and concentration. The value of k is the same for same gas, solvent and temperature. This entails that the ratio of concentration to pressure is same if the pressures change. The following equation related pressure and concentration changes: (Gülich 2008) (Volland 2005)

Minimizing effects of Cavitations

Since cavitation is associated with the suction side of the pump, every prevention measure must be directed towards this area. As suction lifts that are too high only promote cavitation, as a common rule, pumps situated at a position less than 4 meter above the water level must not experience cavitation. Following guidelines can be applied to prevent the cavitation problem:

Using eccentric reducers, and not concentric

Minimizing the number of bends and valves in the suction line

Ensuring that the eccentric reducer's straight side is accurately installed along the top of the suction line

Suction pipe must be at least the same diameter as that of the pump inlet connection.

Length of the suction must be as short as possible.

Using long radius bends

Preventing air from entering the suction line

Increasing the size of the pipework and valves.

Ensuring sufficient submergence above the foot valve. This submergence must be at least 5.3 times the suction diameter (Queensland Government 2006).

Secondly, the requirement that must be fulfilled in order to avoid cavitation is that the value of NPSHA must be equal to or greater than the NPSHR. This condition may be stated mathematically as follows: (Mc Nally Institute 2010)

NPSHA ≥ NPSHR

Mathematical equation of NPSHA can be represented as the following:

NPSHA = P suction - P saturation

Whenever a centrifugal pump takes suction from a reservoir or a tank, the pressure existing on the suction of the pump is measured as the sum of the absolute pressure on the fluid surface in the tank and the pressure caused by the elevation difference between the fluid surface in the tank plus the pump suction minus the head losses due to friction within the suction line running from the tank to the pump.

Mathematically, it is shown as: (Gülich 2008)

NPSHA = Pa + P st - h f - P sat

Where,

NPSHA = net positive suction head available

Pa = absolute pressure at the surface of the fluid

P st = pressure created due to elevation between the fluid surface and pump suction

h f = head losses at the pump suction piping

P sat = saturation pressure of the pumped fluid (Mc Nally Institute 2010).

Also,

Suction Specific Speed = rpm

Where,

rpm = Speed of the pump

Capacity = Liters per second or gallons per minute of the largest impeller

Head = NPSHR, (Net positive suction head required) in meters or feet at that rpm (Mc Nally Institute 2010)

When a centrifugal pump is undergoing cavitation, various changes within the system operation or design are essential in order to increase the NPSHA above the NPSHR to terminate cavitation. One technique of increasing the NPSHA is by increasing the suction pressure of the pump. A good example of this method is, when a pump is taking suction from an enclosed reservoir or tank, the suction pressure can be increased either by lifting the level of the fluid in the tank or by increasing the pressure at the space above the liquid (Gülich 2008). Additionally, it is possible to raise the value of NPSHA by lowering the temperature of the fluid being pumped. Reducing the temperature of the fluid decreases the saturation pressure, thereby causing NPSHA to increase. Moreover, if the head losses within the pump suction piping could be decreased, the NPSHA would be increased. Several methods of reducing head losses are increasing the pipe diameter, reducing the number of valves, elbows, and fittings inside the pipe, and lastly, reducing the length of the pipe (Queensland Government 2006) (Mc Nally Institute 2010).

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