Cavitation
Cavitation is a general term used to describe the behavior of voids or bubbles in a liquid. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the process where a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers etc.
nertial cavitation was first studied by Lord Rayleigh in the late 19th century when he considered the collapse of a spherical void within a liquid. When a volume of liquid is subjected to a sufficiently low pressure it may rupture and form a cavity. This phenomenon is termed cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating underwater with sufficient amplitude and acceleration. Other ways of generating cavitation voids involve the local deposition of energy such as an intense focussed laser pulse (optic cavitation) or with an electrical discharge through a spark. Vapor gasses evaporate into the cavity from the surrounding medium, thus the cavity is not a perfect vacuum but has a relatively low gas pressure. Such a low pressure cavitation bubble in a liquid will begin to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapor within will increase. The bubble will eventually collapse to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism, which releases a significant amount of energy in the form of an acoustic shock-wave and as visible light. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand kelvins, and the pressure several hundred atmospheres.
Inertial cavitation can also occur in the presence of an acoustic field. Microscopic gas bubbles which are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size, and then rapidly collapse. Hence, inertial cavitation can occur even if the rarefraction in the liquid is insufficient for a Rayleigh-like void to occur. High power ultrasonics usually utilize the inertial cavitation of microscopic vacuum bubbles for treatment of surfaces, liquids and slurries.
The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths which precede the formation of the vapor. Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid.
In order for cavitation inception to occur, the cavitation “bubbles” generally need a surface on which they can nucleate. This surface can be provided by the sides of a container or by impurities in the liquid or by small undissolved microbubble within the liquid. It is generally accepted that hydrophobic surfaces stabilize small bubbles. These pre-existing bubbles start to grow unbounded when they are exposed to a pressure below the threshold pressure, termed Blake’s threshold.
This is default description text on Padangan Themes, of course you can change this text via you profile administration.
August 25th, 2007 at 10:07 pm
A client recently asked me to advise them on the possibility of repairing a large hydraulic valve off a 400 ton excavator, used in open-cut mining.
The hydraulic valve in question was a spool-type directional control. It had been badly damaged as a result of cavitation, which had occurred over a long period in service.
What is cavitation?
Cavitation occurs when the volume of hydraulic fluid demanded by any part of a hydraulic circuit exceeds the volume of fluid being supplied.
This causes the absolute pressure in that part of the circuit to fall below the vapor pressure of the hydraulic fluid. This results in the formation of vapor bubbles within the fluid, which implode when compressed.
Cavitation causes metal erosion, which damages hydraulic components and contaminates the hydraulic fluid. In extreme cases, cavitation can result in major mechanical failure of pumps and motors.
While cavitation commonly occurs in the hydraulic pump, it can occur just about anywhere within a hydraulic circuit.
In the hydraulic valve described above, the metal erosion in the body of the valve was so severe that the valve was no longer serviceable. The valve had literally been eaten away from the inside, as a result of chronic cavitation.
In this particular case the cause of the cavitation was faulty anti-cavitation valves, which are designed to prevent this type of damage from occuring.
How can this type of failure be prevented?
This example highlights the importance of checking the operation and adjustment of circuit protection devices, including anti-cavitation and load control valves, at regular intervals.
As in this case, if the faulty anti-cavitation valves had been identified and replaced early enough, the damage to this hydraulic valve and the significant expense of its replacement could have been avoided.
August 26th, 2007 at 7:27 pm
to avoide cavitation you have select special material as stellite 6a material.
September 1st, 2007 at 2:09 pm
Abstract
The cavitation erosion behavior of Fe–Cr–C–Si–xMn (x = 5, 10 and 15 wt%) alloys were investigated for 50 h using 20 kHz vibratory cavitation erosion test equipment. Low-Mn alloys (<5 wt% Mn) and high alloys (>10 wt% Mn) exhibited the γ → α′ and γ → ε strain-induced martensitic transformation, respectively. Mn-addition above 10 wt% was observed to increase the cavitation erosion resistance of the Fe-based alloy. It was concluded that the γ → ε strain-induced martensitic transformation would be more beneficial than the γ → α′ strain-induced martensitic transformation due to the blocking of the dislocation motion, thus increasing the hardness of the matrix by effective work-hardening. The phase transformation was examined by X-ray diffraction before and after the cavitation erosion tests and the surface damage of the tested specimens was also investigated by scanning electron and optical microscopy.
Subject-index terms: C0200; I0500; P0300; S1000
PACS classification codes: 28. 52.Fa
September 10th, 2007 at 10:56 pm
1. In a valve, a valve body having a valve chamber and inlet and outlet passages; a valve seat element in said chamber between said inlet passage and said outlet passage, said seat element having a bore formed therein; an end portion on said seat element closing said bore; a truncated conical seat surface on said seat element, said seat surface having a plurality of orifices extending therethrough and communicating with said bore; a valve element having a skirt portion and a pocket formed therein, said pocket having a truncated conical wall; and means movably securing said valve element in said valve chamber such that a portion of said seat element extends into said pocket in said valve element and said skirt portion of said valve element covers a portion of said plurality of orifices to form a plurality of tortuous flow paths.
2. A valve according to claim 1, said orifices extending through said seat surface comprising a plurality of axially aligned pairs of orifices, said pairs being spaced longitudinally of said seal element.
3. A valve according to claim 2, each said pair of axially aligned orifices having an axis extending perpendicular to the axis of said bore.
4. A valve according to claim 2, said end wall having an orifice extending therethrough for communicating said bore with said valve chamber, said end wall orifice being axially aligned with the bore formed in said seat element.
5. A valve according to claim 4, wherein the sum of the cross-sectional areas of the orifices extending through said seat surface and the orifice extending through said end wall is greater than the cross sectional area of the bore in the seat element.
6. A valve according to claim 5 wherein the sum of the areas of the orifices is at least about 20% greater than the cross sectional area of the bore.
7. A valve according to claim 1 wherein said plurality of orifices in said seat comprises a plurality of sets of orifices, each set comprising at least two orifices which are circumferentially spaced equidistance from each other and lie in a single plane.
8. A valve according to claim 7, each said axially aligned sets of orifices being spaced circumferentially of said seat element relative to each adjacent sets.
9. A valve according to claim 1 wherein the sum of the cross-sectional areas of the orifices extending through said seat surface is greater than the cross sectional area of the bore in the seat element.
10. A valve according to claim 8, said bore in said seat element having a variable cross sectional area along the length thereof and where the cross sectional area adjacent the end wall of said seat element being of a smaller diameter than the portion spaced away from said end wall.
11. A valve seat element for a flow control valve comprising a body portion having a bore formed therein; an end wall on said body portion closing one end of said bore; a truncated conical seat surface on said body adjacent said end wall; a plurality of pairs of longitudinally spaced orifices extending through said truncated conical seat surface, each pair of orifices communicating with said bore, said end wall having an orifice formed therein communicating with said bore; and connector means on said body connectable to a flow control valve.
12. A valve seat element according to claim 11 wherein the orifices of each pair are axially aligned with an axis perpendicular to the axis of said bore.
13. A valve seat element according to claim 12 said bore in said body having portions of different diameters adjacent said plurality of orifices.
14. A valve seal element according to claim 12 wherein the sum of the cross-sectional areas of the orifices extending through the seat surface and the orifice formed in said end wall is greater than the cross-sectional area of said bore.
15. A valve seat according to claim 14 wherein the cross-sectional areas of all of said orifices is at least about twenty percent greater than the cross-sectional area of said bore.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a valve seat design to reduce cavitation of liquid and erosion in a flow control valve generally used as a choke.
2. Description of the Prior Art
The removable internal parts of a valve are generally referred to as the “trim” and generally function to proportion the valve orifice area to control flow through the valve. Erosion of the valve trim is a major problem in valves controlling flow of high velocity liquid. Cavitation may occur if the pressure of liquid flowing through restricted passages becomes less than the vapor pressure of the liquid at the operating temperature. Flow control valves usually experience cavitation when a high pressure drop is created across a single point of pressure reduction.
Flow of fluid through an orifice is narrowed downstream of the orifice to form a vena contracta, which is the location of highest velocity of fluid through the valve and the location of the lowest pressure. As fluid velocity decelerates downstream of the orifice, pressure is regained and the vapor bubbles collapse or implode violently, expending energy which is absorbed by the valve causing cavitation related wear, fatigue and eventual failure.
Various trims which reduce cavitation are shown in ISA HANDBOOK OF CONTROL VALVES, 2nd Ed, 1976 Instrument Society of America. In cage hole designs to reduce cavitation, a cylindrical cage member with circumferentially opposed opening directs flow to impinge upon itself. These designs do reduce cavitation; however, the cylindrical trim elements with openings therein tend to foul and create localized erosion and flow cutting. In cascading designs trim is designed to create a series of smaller pressure drops to reduce cavitation. However, these cascading type trims tend to reduce flow capacity and are expensive to manufacture.
In addition, heretofore it has been necessary to select valves having trim of various designs depending upon the velocity of fluid and field conditions under which a flow control valve is to be used for oil field service.
SUMMARY OF THE INVENTION
The valve disclosed herein and illustrated in the attached drawings incorporates improved trim elements including a valve seat element having a central bore formed therein and a valve element having a skirt portion to control flow to the bore in the seat element. A truncated conical seat surface is formed on the seat element and a plurality of orifices extend through the seat surface to communicate with the central bore. This seat element has an end portion closing one end of the bore, the end portion being provided with an orifice which is axially aligned with the bore formed in the seat element. The orifices formed in the seat surface generally comprise spaced pairs of aligned orifices having axes which extend perpendicular to the axis of the bore in the seat element. The pairs of orifices extending through the seal element are spaced longitudinally of the seat element and each pair is circumferentially spaced from each adjacent pair to form a plurality of tortuous flow paths to form a plurality of flow streams which impinge against each other to minimize cavitation in the valve.
DESCRIPTION OF DRAWINGS
Drawings of a preferred embodiment of the invention are annexed hereto so that the invention may be better and more fully understood, in which:
FIG. 1 is an elevational view of a flow control valve incorporating the improved valve seat design to reduce cavitation;
FIG. 2 is an enlarged fragmentary cross sectional view illustrating details of construction of the valve element and the valve seat element; and
FIG. 3 is a view similar to FIG. 2 showing an alternative embodiment.
Numeral references are employed to designate like parts throughout the various figures of the drawings.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIG. 1 of the drawing, the numeral 10 generally designates a flow control valve body having an inlet connector 12 and an outlet connector 14 having inlet passage 16 and outlet passage 18 formed therein. Valve body 10 has a valve chamber 20 with which inlet passage 16 and outlet passage 18, respectively, communicate to receive fluid flowing from inlet passage 16 to outlet passage 18.
As will be hereinafter more fully explained, outlet passage 18 has an internally threaded portion 19 in which a seat element generally designated by the numeral 30 is mounted. A valve element generally designated by numeral 60 is secured to a rising stem 22 having a hand wheel, not shown, mounted thereon. Stem 22 has a threaded portion 25 which extends through a threaded opening in a bonnet 26. A bonnet retaining sleeve 28 extends into a threaded opening in valve body 10 and urges seals on bonnet 26 into sealing engagement with valve body 10. A packing cup 29 is secured to stem 22 adjacent bonnet 26. Valve bonnets of the type hereinbefore briefly described are well known to persons skilled in the art and further description of obviousness is not deemed necessary.
Valve seat element 30 comprises a valve seat element body 32 having a threaded end 34 which engages threaded portion 19 of outlet passage 18 in valve body 10. A truncated conical seat surface 35 is formed on the opposite end of body 32. A bore 36 is formed in seat body 32 and extends through the threaded end portion 34 and communicates with outlet passage 18. The opposite end of bore 36 is closed by an end wall 38.
As best illustrated in FIG. 2 of the drawing, an orifice 40 extends through end wall 38 and is axially aligned with bore 36.
A plurality of orifices 42-52 extend through seat surface 35 and communicate with bore 36 in the valve seat body 32. It is important to note that the first pair of orifices 42 and 44 lie on a common axis which is perpendicular to the axis of bore 36. The second pair of orifices 46 and 48 have a common axis which is perpendicular to the axis of bore 36 and spaced longitudinally of body 32 from the first pair of orifices 42 and 44. Orifices 42 and 44 are circumferentially spaced about the truncated cylindrical seat surface 35 relative to the second pair 46 and 48 of orifices. As illustrated in FIG. 2, a common axis of orifices 42 and 44 is spaced and perpendicular to the common axis of orifices 46 and 48.
A third pair of orifices 50 and 52 have a common axis spaced from, but parallel to the axis of the first pair of orifices 42 and 44.
The bore 36 in seat body 32 has portions 54, 56 and 58 of varying diameter spaced longitudinally along the portion of bore 36 adjacent end wall 38 such that the cross section area of the bore increases progressively from the closed end portion adjacent orifice 40 toward the open end of the bore.
Valve element 60 is a cylindrical member having an internally threaded opening 62 formed in one end thereof and a pocket 64 having a truncated conical wall 65 formed in the opposite end thereof, pocket 64 being surrounded by skirt portion 68 of valve elements 60. When the valve element is in the full closed position, the lower edge 70 of skirt 68 is urged into sealing engagement with surface 37 on valve seat body 32.
A truncated conical seat surface 35 on seat element 30 has the same taper as the inner wall 65 of pocket 64 in valve element 60 such that movement of valve element 60 to the closed position moves surfaces 65 and 35 to adjacent spaced relationship.
As the end 70 of skirt portion 68 moves away from surface, 37, truncated conical wall 65 slowly moves away from tapered seat surface 35 to form an annulus 75. It will be appreciated that when the lower end 70 of skirt 68 engages surface 37, all of the orifices 40-52 are enclosed within the skirt portion of the valve element, thus, blocking fluid flow from inlet passage 16 to outlet passage 18 in the valve body. However, as valve element 60 is moved longitudinally away from seat element 30, the edge 70 of skirt 68 progressively moves past orifices 42 and 44, and then past orifices 46 and 48 as valve element 60 reaches the full open position.
At a mid-stroke position of valve element 60, surface 70 on the end of skirt 68 would enclose a portion of the area of orifices 42 and 44, the other portion of the surface area of the orifices lying outside of pocket 64 in valve element 60. Thus, a portion of fluid flow from inlet passage 16 would flow directly into the uncovered portion of orifices 42 and 44. A portion of fluid flow from inlet passage 16 would flow around the end surface 70 on skirt 68 into the annular passage 75 to reach the covered portion of orifices 42 and 44. Additional flow through annular passage 75 would reach the second pair of orifices 46 and 48 and the third pair of orifices 50 and 52 inside the skirt portion 68 of valve element 60 and also orifice 40 extending through end wall 38.
It will be appreciated that since the orifices of each pair of orifices have a common axis, flow through the orifices will be in opposite directions and streams flowing therethrough will impinge against each other in bore 36. For example, flow through orifices 46 and 48 will impinge in bore 36 and will be mixed with flow of fluid diffused from impingement of flow streams through the other pairs of orifices spaced along the length of bore 36.
In the preferred embodiment of the invention, the sum of the cross sectional area of orifices 40-52 is greater than the cross sectional area of bore 36 such that minimal restriction of flow through the valve will be observed when valve element 60 is in the full open position. In addition, these valves are designed such that in their normal operating range the flow area of annular passage 75 is always less than the sum of the covered portions of the orifices 42-52 plus orifice 40 at all valve positions. As edge 70 of valve element 60 moves toward surface 37 on valve seat element 30, flow will be steadily reduced. It will be observed that the pair of orifices 42-52 extending through seat surface 35 and spaced longitudinally and circumferentially relative to each other form a plurality of tortuous flow paths which form a plurality of flow streams which impinge against each other causing flow to be diffused immediately upon passage through the orifices, thereby minimizing cavitation.
In FIG. 3, an alternative embodiment 10′ of the present invention is shown. The valve has an inlet passage 16′ and an outlet passage 18′ through which the flow is regulated. The valve consists of frustoconical-shaped seat and valve elements, 30′ and 60′ respectively. The valve element 60′ is axially movable toward and away from the seed element 30′. The seat element 30′ can be formed from a hardened tungsten carbide material to increase life. The bore 36′ in the seat element 30′ can be likewise frustoconical-shaped to improve valve performance. Valve element 60′ can likewise have a frustoconical-shaped tungsten carbide insert 61′ pressed into the element.
It is understood, of course, that the foregoing specification discloses exemplary embodiments of the present invention and that numerous modifications, alterations and changes can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.
September 10th, 2007 at 10:59 pm
When the pressure drop is too large or the downstream pressure is too low, a valve can cavitate, a phenomenon that produces noise, vibrations and valve or pipe damage, not to mention headaches for plant operators and maintenance personnel. There is no need to tolerate this nuisance when you can easily predict the intensity of cavitation and reduce or eliminate its effects.
Cavitation is a pipeline phenomenon that forms vapor cavities, which then go unstable and collapse violently in low-pressure, turbulent, flow-separation regions inside valves, elbows, pipe expansions and other fluid handling hardware. The energy release associated with the nearly instantaneous collapse manifests itself as noise, vibration and metal being ripped from the inside surfaces of cavitating devices or downstream pipe. Cavitation also restricts the maximum valve flow rate for a given upstream pressure.
Cavitation’s intensity and its effect on hardware can vary from insignificant to devastating. In its least violent form, cavitation produces only a light crackling sound, about the same intensity as popcorn popping. This harms neither the valve nor the system. At more advanced stages, however, cavitation noise becomes objectionable. For certain valve types, cavitation can sound like gravel rumbling through the pipeline. The noise intensity can exceed 100 db, a level that constitutes a risk of hearing damage.
Vibration from cavitation can increase to a point at which it jeopardizes the mechanical integrity of the system or component. Eventually, material loss can occur inside the valve and piping components located downstream from the valve (Figure 1). Cavitation can cause seat leakage, leakage to atmosphere and eventual valve failure. Under extreme conditions, cavitation has been known to destroy valves after only a few days of operation.
In its most advanced form, cavitation limits the system’s maximum flow capacity, a situation referred to as choked flow or super-cavitation. At this condition, a large vapor cavity extends several pipe diameters immediately downstream from the valve. Extremely high noise levels, severe vibrations and material damage usually occur at the first significant obstruction, such as an elbow, tee or flowmeter. The exact location of the cavitation noise sometimes can create confusion. One might not suspect the valve is cavitating because the noise, vibrations and damage occur some distance downstream.
Quantify the problem
It’s possible to predict cavitation intensity if you know the valve’s flow and pressure conditions. The analysis requires condensing the system’s operating conditions into a single number — the cavitation index — that you compare to experimentally determined cavitation limits. For some valves, experimental data are available on four levels of cavitation, each representing a different potential impact on a valve and its system. If cavitation is excessive, specific methods can be used to reduce or eliminate it.
Analyzing valve cavitation requires a parameter to quantify the cavitation potential. Researchers have developed a variety of cavitation indices. One such index is SIGMA.
A valve’s potential for cavitation depends on the downstream pressure (P[-]d[-]), the barometric pressure (P[-]b[-]), the absolute vapor pressure (P[-]v[-]) and the pressure differential across the valve (DELTA P). The sigma cavitation index is defined as:
SIGMA = (P[-]d[-] + P[-]b[-] – P[-]v[-])/ DELTA P
Cavitation is less likely to occur at larger values of SIGMA. For example, increasing the pressure drop across a valve or reducing the downstream pressure reduces the value of SIGMA and thus increases the likelihood or severity of cavitation.
It’s usually not difficult to determine whether a valve is cavitating. One merely has to listen. However, to determine if the cavitation intensity is high enough to cause damage requires quantifying the intensity and comparing it with available experimental cavitation reference data for the valve of interest. Cavitation intensity can be quantified relative to four levels.
Incipient cavitation refers to the onset of audible, intermittent cavitation. At this lower limit, cavitation intensity is slight. The operating conditions that foster incipient cavitation are conservative and seldom used for design purposes.
Critical cavitation, the next stage, describes the condition when the cavitation noise becomes continuous. The noise intensity is often hard to detect above the background flow noise. Critical cavitation causes no adverse effects and commonly defines the “no cavitation” condition. This level is referred to as critical because cavitation intensity increases rapidly with any further reduction in SIGMA.
Incipient damage refers to the conditions under which cavitation begins to destroy the valve. It’s usually accompanied by loud noise and heavy vibration. The potential for material loss increases exponentially as SIGMA drops below the value that initiates incipient damage. Consequently, this is the upper limit for safe operation with most valves. Unfortunately, it’s the limit that’s most difficult to determine, and experimental data are available only for a few valves.
Choking cavitation refers to a flow condition in which the mean pressure immediately downstream from the valve is the fluid’s vapor pressure. This represents the maximum flow condition through a valve for a given upstream pressure and valve opening. It’s a condition that damages both valve and piping. Choking cavitation is an interesting and complex operating condition. Even though the pressure at the valve outlet is at vapor pressure, the downstream system pressure remains greater. Reducing the downstream pressure increases the length of the vapor cavity, but doesn’t increase the flow rate. The noise, vibration and damage occur primarily at the location where cavity collapse occurs.
Typical flow-versus-differential pressure relationships (C[-]d[-], C[-]v[-] and the like) aren’t valid once the valve begins to choke because increasing DELTA P doesn’t increase flow. The only way to increase the flow rate through a choking valve is by increasing the upstream pressure. Choking cavitation may be an acceptable design point for pressure relief valves because their operating cycle is limited.
The values of SIGMA associated with incipient, critical, incipient damage and choking, referred to here as the reference sigma data, vary with valve opening and are determined experimentally. The reference sigma data are typically presented as a function of the valve discharge coefficient, C[-]d[-].
C[-]d[-] = V/sqrt(2 DELTA P/RHO + V^2)
where RHO is the fluid density and V is the average flow velocity at the valve inlet. C[-]d[-] is dimensionless and independent of valve size. Other discharge coefficients, such as C[-]v[-], which are commonly used in the water works industry, aren’t dimensionless and vary with valve size. For this reason, C[-]d[-] is used to quantify cavitation and make comparisons between valves.
Do the numbers
Evaluating the cavitation intensity for a given valve opening and flow rate proceeds as follows. Plug the valve opening and system flow conditions into Equations 1 and 2 to determine the system sigma and discharge coefficient. Then, identify the cavitation intensity by comparing the system sigma value to the SIGMA values corresponding to the four cavitation limits. The examples below are simplified because of scale effect adjustments that must be made to experimental data to account for differences in valve size and operating pressure. See Tullis (1989) and Tullis (1993) for more details on scaling cavitation data.
Figures 2 and 3 present experimental C[-]d[-] and SIGMA data for a 6-in. butterfly valve. These data will be used to demonstrate cavitation analysis. Obtain similar data for a specific valve from the valve manufacturer or the literature.
Assume that a line-size 6-in. butterfly valve is cavitating. System flow conditions include a valve opening of 63%, which corresponds to C[-]d[-] = 0.43. The flow rate is 9.36 cubic feet per second (cfs), P[-]u[-] = 60 psi upstream, P[-]b[-] = 12.65 psi, P[-]v[-] = 0.25 psi and P[-]d[-] = 33 psi.
Using Equation 1, SIGMA = (33 + 12.65 - 0.25)/(60 - 33) = 1.68.
The intersection of C[-]d[-] = 0.43 and SIGMA = 1.68 in Figure 3 falls between incipient damage (SIGMA[-]id[-] = 2.4) and choking (SIGMA[-]ch[-] = 1.2). This suggests that cavitation is damaging the valve.
Tame the beast
If a valve is cavitating excessively, you have several options to control or eliminate the problem. The first is to reduce the cavitation intensity by modifying the system operating conditions. This requires reducing the flow, reducing the pressure drop or increasing the downstream pressure. If system conditions can’t be altered, consider other options. A common solution is simply to tolerate the cavitation and replace the valve when it no longer performs its intended function. Better options include installing a valve having better cavitation performance, using a valve with hardened parts or installing a downstream device to increase outlet pressure and decrease pressure drop across the valve. In some cases, injecting air into the cavitation region can suppress cavitation damage.
For example, cavitation intensity can be reduced below the damaging level by installing a second 6-in. butterfly valve downstream from the existing valve to divide the total pressure drop between two valves. For valves in series, the downstream valve can provide about 1/3 of the total pressure drop. For the preceding example, this means the downstream valve can provide a 9-psi drop and the upstream valve 18 psi.
Calculate the discharge coefficient and system sigma for each valve using the corresponding downstream pressure and associated pressure drop. Critical sigma values (SIGMA[-]cr[-]) and incipient damage sigma values (SIGMA[-]id[-]) for the upstream valve and downstream valve are scaled from Figure 3. The results are displayed in Table 1.
The system sigma values for both valves now fall between critical and incipient damage. Consequently, the valves will produce cavitation noise but no damage will occur. This would provide a long-term solution.
Another way to suppress cavitation is by using a valve with better cavitation characteristics. A number of styles of cavitation control valves are available. Three options are globe valves with cavitation trim, sleeve valves and stack valves. Each uses a combination of small jets discharging into sudden expansions or several energy dissipation stages in series. Additional information on cavitation control valves is presented in Tullis (1989), (1993) and (2005).
Cavitation intensity can be predicted and, in most cases, reduced to an acceptable level through reasonable means. Cavitation doesn’t have to be tolerated