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DRAG® V AL VES FO R VAL ALVES FOR CA VIT ATION CONTROL CAVIT VITA
The Cavitation Problem Cavitation is the process of formation and subsequent collapse of microscopic vapor bubbles in a flowing fluid. It is the foremost problem encountered in selecting and applying control valves for liquid service. In order for cavitation to occur, the fluid must flow through a low-pressure region, then into a region of higher pressure. In control valves, the low-pressure region is caused by local high velocities usually in the seating area or the vena contracta, immediately downstream. From Bernoulli’s Law, the total energy of the flowing liquid remains constant (neglecting losses), so where the velocity increases due to a restriction in area, pressure will decrease. Pressure and velocity profiles are seen in Figure 1.
immediately. For harder materials, repeated implosions fatigue the surface until particles break off. Although harder materials withstand it longer, no known metal can withstand cavitation damage indefinitely (see Figure 3). Another suggested mechanism is similar to fracturing concrete by impact. The surface fractures where the shock wave reaches the maximum tensile stress.
Figure 3
Inadequate Strategies for Cavitation Control HARDER MATERIALS
Figure 1
If the vapor pressure of the liquid is above PVC, vapor bubbles will form in the region near the restriction. Although molecular theory predicts a tensile strength for water on the order of 103 psi, minute particles and impurities form a nucleus around which vapor bubbles are created. If the vapor pressure is also above P2, a mixture of liquid and vapor will exit the valve. If the vapor pressure is less than P2, the bubbles will implode as they reach regions of higher pressure. The implosions are very violent, creating stresses above 100,000 psi on surfaces they implode on or near. For cavitation to occur, the pressure profile must be as depicted in Figure 2.
Valve designers have developed several methods to the cavitation problem. The first, and still popular method is to use hard materials, such as stellite #6, in areas of the valve where cavitation is likely. It is well recognized that hard materials can slow the damage, but not stop it. The true solution to the cavitation problem is to keep the velocities low enough to prevent the bubbles from forming in the first place. Tests have shown that the erosion action of cavitation varies as Vn, velocity raised to the n power, where n ranges from 5 to 10, with about 6 on the average. With this powerful an exponential function, the importance of velocity control can easily be seen.
DIRECTED BUBBLE COLLAPSE One of the strategies developed by valve designers involves controlling the location of bubble collapse. The fluid is directed at itself so that the bubbles don’t collapse near a metal surface, but with no effort to control velocity, and therefore bubble formation. This approach has several drawbacks. When the valve is partially closed, the bubbles may not collapse until they travel some distance down-stream, perhaps as far as the seat ring, where they may cause damage, as shown in Figure 4. The uncontrolled velocities lead to high erosion rates for the cage, and cavitation choking through the cage orifices. Since noise in generated in accordance with a strong velocity power function, extremely high noise levels can be generated in high velocity trim such as this one.
Figure 2
It is important to note that the restriction at the vena contracta need not be the seat area, but can be any restriction area in the valve, and is often the cage parts. Incorrectly sized valves may actually cavitate in the inlet and outlet nozzles. The exact mechanism of cavitation damage is not fully understood, but it is hypothesized that the imploding bubbles act like a shotpeen on the surface, but with very high stress levels. For weaker materials, the surface is stressed beyond the yield point and a minute particle of metal is removed ON THE CO VER COVER VER: Other manufacturer’s trim damaged by uncontrolled cavitation.
Figure 4
DRILLED DISK STAGES Another cavitation control strategy is to attempt to reduce the pressure in stages. One approach uses a set of drilled disks, arranged so that the flow is generally parallel to the valve stem, as shown in Figure 5. The plates are drilled in such a way as to cause the fluid to expand and contract through the orifices and distribute the pressure drop over a number of stages. As the valve is opened further, fewer and fewer stages remain to distribute the pressure drop. This means that for valves with a high pressure drop at full flow conditions, very little cavitation control exists with this design.
in the valve body limits the number of cylinders so that even if they all worked to reduce velocity, there would not be enough impedance for anything more than a moderate pressure drop. This type of trim will cavitate very easily.
Defining Cavitation Levels The readiness of these so called anti-cavitation trims to cavitate is reflected in the fact that these valves are allowed by their manufacturer to operate well into cavitating region. These manufacturers publish values of a cavitation index designated KC for their anti-cavitation trim. This value is calculated by the dimensionless expression:
Where: ∆P is a special pressure drop across the valve experimentally determined is the inlet pressure P1 PV is the vapor pressure
Figure 5
Where cavitation might occur at any (or every) point in the valve stroke, a radial flow trim must be used. With a radial flow trim, the fluid impedance is constant at any stroke position. The problem now is to package sufficient impedance to control velocities to the required level.
If pressure drop and flow rate measurements are performed under a range of conditions, a curve such as shown in Figure 7 can be generated. At low values of ∆P, the relationship between the square root of ∆P and the flow rate is linear. As ∆P is increased, cavitation begins. Further increases in ∆P increase the rate of bubble formation and the severity of cavitation. When the formation of bubbles is great enough that they begin to block the flow of liquid, the linear behavior of the flow curve ends. The pressure drop in which this occurs is used to calculate the value of KC.
Figure 6
CONCENTRIC DRILLED CYLINDERS Another approach uses a series of concentric drilled cylinders, with part of the pressure drop occurring at each cylinder, as shown in Figure 6. A problem exists, however, when it comes to putting enough cylinders into a valve body. As a result, the trim is limited to 3 or 4 cylinders. For drilled cylinders, the velocity through an orifice is approximately as follows: Where: ∆P is the pressure drop across the orifice K is the loss coefficient of the orifice (about 1.5) If the area expansion between cylinders is too rapid, the affect of later cylinders is drastically diminished. In effect, the entire pressure drop is taken through the first cylinder. Space limitations
Figure 7
The control valve manufacturer allows the valve to be placed in applications where the K value of the application is less than the published KC value. This approach can be misleading. Cavitation initiates at K values well below the point where it affects the flow rate. In addition, much of the cavitation may occur in the shear area between a high velocity flow stream and stagnant regions, where flow will not be affected. In chapter six of its Handbook of Control Valves, the Instrument Society of America concludes: “Therefore, the cavitation index, KC, is useful in control valve sizing and analysis work, but caution should be exercised in applying this index to any decision regarding whether or not a valve application will limit cavitation to an ‘acceptable’ level.”
The DRAG V alve Solution to Cavitation Valve The solution to reducing cavitation is controlling the fluid velocity. Specifically, controlling the ratio of the difference between the fluid pressure and the vapor pressure to the velocity pressure head at all points in the valve. This dimensionless ratio, Ci, is also defined in cavitation literature as the cavitation index.
English
Metric
At the other end of velocities, a lower limit of 75ft/sec. (22m/s) is used for most industrial applications. Experience has shown that controlling velocities to this level will provide excellent protection against cavitation. Once the required velocity constraint is established, the proper number of stages or turns are selected to meet the requirements of each application. Large pressure drops and outlet pressures close to the vapor pressure require the most turns. Liquid valves should flow from the outside toward the inside.
Where: P PV
ρ
V
is the fluid pressure measured at any point in the valve, psia (kPa) is the fluid vapor pressure, psia (kPa) is the fluid density, lbm/ft3 (Kg/m3) is the fluid velocity at the point P is determined, ft/s (m/s)
If Ci is one, or smaller, cavitation will occur. CCI selects a value for Ci equal to two, as the design criteria. This allows a conservative margin of one velocity head above the theoretical incipient point of cavitation (see Figure 8), and assures that
Figure 8
cavitation occurring on the periphery of the flow stream is also controlled. The exit velocity of the flow control orifice which meets this criteria is found by the equation:
English
Metric
Two other experience-based constraints are applies to ensure the optimal flow solution. On the high end, the disk exit velocity is never designed to be greater than 100ft/sec. (30m/s), even if the backpressure is sufficient to eliminate any chance of cavitation. This value is chosen to prevent erosion and vibration of trim and body parts.
Figure 9
Controlling velocity through multiple turns is highly effective in eliminating cavitation. However, CCI has found that if the velocity is slowed even further as the fluid reaches the more cavitation-prone, low-pressure area, cavitation resistance is improved. The flow area of each passage expands from the inlet to the outlet of the passage (see Figure 9). This is contrary to what common sense indicates is appropriate for liquids. However, through expansion, the pressure drop across the valve can be taken in the first few turns of the passage where pressure is well above the vapor pressure and cavitation is impossible. Toward the exit of the passage, the rate of pressure drop at each turn is reduced, as the velocity is reduced. The expanding passage design allows very low velocities in the low-pressure, cavitation-prone region of the passage exit while maintaining economical dimensions. Through analysis and experimentation at our cavitation test facility, we have found the optimum expansion ratio for the fluid path. If it expands too rapidly, the effect of the downstream turn is diminished and cavitation will occur at the passage entrance. If it expands too slowly, more turns are required to meet the velocity requirements. By designing the proper rate of area expansion between turns, a disk can be chosen with the optimum cavitationreducing characteristics. CCI has been engineering and building state of the art control valves for demanding applications since 1961. Let us help solve your control valve problems.
B
We Solve Control Valve Problems Telephone: (949) 858-1877 • FAX: (949) 858-1878 22591 Avenida Empresa • Rancho Santa Margarita • California 92688 USA E-Mail: [email protected] • Web Site: http://www.ccivalve.com MC-CC-3/97
Sales & Service Offices throughout the World
DRAG® V AL VES FO R VAL ALVES FOR CA VIT ATION CONTROL CAVIT VITA
The Cavitation Problem Cavitation is the process of formation and subsequent collapse of microscopic vapor bubbles in a flowing fluid. It is the foremost problem encountered in selecting and applying control valves for liquid service. In order for cavitation to occur, the fluid must flow through a low-pressure region, then into a region of higher pressure. In control valves, the low-pressure region is caused by local high velocities usually in the seating area or the vena contracta, immediately downstream. From Bernoulli’s Law, the total energy of the flowing liquid remains constant (neglecting losses), so where the velocity increases due to a restriction in area, pressure will decrease. Pressure and velocity profiles are seen in Figure 1.
immediately. For harder materials, repeated implosions fatigue the surface until particles break off. Although harder materials withstand it longer, no known metal can withstand cavitation damage indefinitely (see Figure 3). Another suggested mechanism is similar to fracturing concrete by impact. The surface fractures where the shock wave reaches the maximum tensile stress.
Figure 3
Inadequate Strategies for Cavitation Control HARDER MATERIALS
Figure 1
If the vapor pressure of the liquid is above PVC, vapor bubbles will form in the region near the restriction. Although molecular theory predicts a tensile strength for water on the order of 103 psi, minute particles and impurities form a nucleus around which vapor bubbles are created. If the vapor pressure is also above P2, a mixture of liquid and vapor will exit the valve. If the vapor pressure is less than P2, the bubbles will implode as they reach regions of higher pressure. The implosions are very violent, creating stresses above 100,000 psi on surfaces they implode on or near. For cavitation to occur, the pressure profile must be as depicted in Figure 2.
Valve designers have developed several methods to the cavitation problem. The first, and still popular method is to use hard materials, such as stellite #6, in areas of the valve where cavitation is likely. It is well recognized that hard materials can slow the damage, but not stop it. The true solution to the cavitation problem is to keep the velocities low enough to prevent the bubbles from forming in the first place. Tests have shown that the erosion action of cavitation varies as Vn, velocity raised to the n power, where n ranges from 5 to 10, with about 6 on the average. With this powerful an exponential function, the importance of velocity control can easily be seen.
DIRECTED BUBBLE COLLAPSE One of the strategies developed by valve designers involves controlling the location of bubble collapse. The fluid is directed at itself so that the bubbles don’t collapse near a metal surface, but with no effort to control velocity, and therefore bubble formation. This approach has several drawbacks. When the valve is partially closed, the bubbles may not collapse until they travel some distance down-stream, perhaps as far as the seat ring, where they may cause damage, as shown in Figure 4. The uncontrolled velocities lead to high erosion rates for the cage, and cavitation choking through the cage orifices. Since noise in generated in accordance with a strong velocity power function, extremely high noise levels can be generated in high velocity trim such as this one.
Figure 2
It is important to note that the restriction at the vena contracta need not be the seat area, but can be any restriction area in the valve, and is often the cage parts. Incorrectly sized valves may actually cavitate in the inlet and outlet nozzles. The exact mechanism of cavitation damage is not fully understood, but it is hypothesized that the imploding bubbles act like a shotpeen on the surface, but with very high stress levels. For weaker materials, the surface is stressed beyond the yield point and a minute particle of metal is removed ON THE CO VER COVER VER: Other manufacturer’s trim damaged by uncontrolled cavitation.
Figure 4
DRILLED DISK STAGES Another cavitation control strategy is to attempt to reduce the pressure in stages. One approach uses a set of drilled disks, arranged so that the flow is generally parallel to the valve stem, as shown in Figure 5. The plates are drilled in such a way as to cause the fluid to expand and contract through the orifices and distribute the pressure drop over a number of stages. As the valve is opened further, fewer and fewer stages remain to distribute the pressure drop. This means that for valves with a high pressure drop at full flow conditions, very little cavitation control exists with this design.
in the valve body limits the number of cylinders so that even if they all worked to reduce velocity, there would not be enough impedance for anything more than a moderate pressure drop. This type of trim will cavitate very easily.
Defining Cavitation Levels The readiness of these so called anti-cavitation trims to cavitate is reflected in the fact that these valves are allowed by their manufacturer to operate well into cavitating region. These manufacturers publish values of a cavitation index designated KC for their anti-cavitation trim. This value is calculated by the dimensionless expression:
Where: ∆P is a special pressure drop across the valve experimentally determined is the inlet pressure P1 PV is the vapor pressure
Figure 5
Where cavitation might occur at any (or every) point in the valve stroke, a radial flow trim must be used. With a radial flow trim, the fluid impedance is constant at any stroke position. The problem now is to package sufficient impedance to control velocities to the required level.
If pressure drop and flow rate measurements are performed under a range of conditions, a curve such as shown in Figure 7 can be generated. At low values of ∆P, the relationship between the square root of ∆P and the flow rate is linear. As ∆P is increased, cavitation begins. Further increases in ∆P increase the rate of bubble formation and the severity of cavitation. When the formation of bubbles is great enough that they begin to block the flow of liquid, the linear behavior of the flow curve ends. The pressure drop in which this occurs is used to calculate the value of KC.
Figure 6
CONCENTRIC DRILLED CYLINDERS Another approach uses a series of concentric drilled cylinders, with part of the pressure drop occurring at each cylinder, as shown in Figure 6. A problem exists, however, when it comes to putting enough cylinders into a valve body. As a result, the trim is limited to 3 or 4 cylinders. For drilled cylinders, the velocity through an orifice is approximately as follows: Where: ∆P is the pressure drop across the orifice K is the loss coefficient of the orifice (about 1.5) If the area expansion between cylinders is too rapid, the affect of later cylinders is drastically diminished. In effect, the entire pressure drop is taken through the first cylinder. Space limitations
Figure 7
The control valve manufacturer allows the valve to be placed in applications where the K value of the application is less than the published KC value. This approach can be misleading. Cavitation initiates at K values well below the point where it affects the flow rate. In addition, much of the cavitation may occur in the shear area between a high velocity flow stream and stagnant regions, where flow will not be affected. In chapter six of its Handbook of Control Valves, the Instrument Society of America concludes: “Therefore, the cavitation index, KC, is useful in control valve sizing and analysis work, but caution should be exercised in applying this index to any decision regarding whether or not a valve application will limit cavitation to an ‘acceptable’ level.”
The DRAG V alve Solution to Cavitation Valve The solution to reducing cavitation is controlling the fluid velocity. Specifically, controlling the ratio of the difference between the fluid pressure and the vapor pressure to the velocity pressure head at all points in the valve. This dimensionless ratio, Ci, is also defined in cavitation literature as the cavitation index.
English
Metric
At the other end of velocities, a lower limit of 75ft/sec. (22m/s) is used for most industrial applications. Experience has shown that controlling velocities to this level will provide excellent protection against cavitation. Once the required velocity constraint is established, the proper number of stages or turns are selected to meet the requirements of each application. Large pressure drops and outlet pressures close to the vapor pressure require the most turns. Liquid valves should flow from the outside toward the inside.
Where: P PV
ρ
V
is the fluid pressure measured at any point in the valve, psia (kPa) is the fluid vapor pressure, psia (kPa) is the fluid density, lbm/ft3 (Kg/m3) is the fluid velocity at the point P is determined, ft/s (m/s)
If Ci is one, or smaller, cavitation will occur. CCI selects a value for Ci equal to two, as the design criteria. This allows a conservative margin of one velocity head above the theoretical incipient point of cavitation (see Figure 8), and assures that
Figure 8
cavitation occurring on the periphery of the flow stream is also controlled. The exit velocity of the flow control orifice which meets this criteria is found by the equation:
English
Metric
Two other experience-based constraints are applies to ensure the optimal flow solution. On the high end, the disk exit velocity is never designed to be greater than 100ft/sec. (30m/s), even if the backpressure is sufficient to eliminate any chance of cavitation. This value is chosen to prevent erosion and vibration of trim and body parts.
Figure 9
Controlling velocity through multiple turns is highly effective in eliminating cavitation. However, CCI has found that if the velocity is slowed even further as the fluid reaches the more cavitation-prone, low-pressure area, cavitation resistance is improved. The flow area of each passage expands from the inlet to the outlet of the passage (see Figure 9). This is contrary to what common sense indicates is appropriate for liquids. However, through expansion, the pressure drop across the valve can be taken in the first few turns of the passage where pressure is well above the vapor pressure and cavitation is impossible. Toward the exit of the passage, the rate of pressure drop at each turn is reduced, as the velocity is reduced. The expanding passage design allows very low velocities in the low-pressure, cavitation-prone region of the passage exit while maintaining economical dimensions. Through analysis and experimentation at our cavitation test facility, we have found the optimum expansion ratio for the fluid path. If it expands too rapidly, the effect of the downstream turn is diminished and cavitation will occur at the passage entrance. If it expands too slowly, more turns are required to meet the velocity requirements. By designing the proper rate of area expansion between turns, a disk can be chosen with the optimum cavitationreducing characteristics. CCI has been engineering and building state of the art control valves for demanding applications since 1961. Let us help solve your control valve problems.
B
We Solve Control Valve Problems Telephone: (949) 858-1877 • FAX: (949) 858-1878 22591 Avenida Empresa • Rancho Santa Margarita • California 92688 USA E-Mail: [email protected] • Web Site: http://www.ccivalve.com MC-CC-3/97
Sales & Service Offices throughout the World
