Control Valves Should Always Be in the What Position?

What Are Control Valves?

Control valves, a key component in any pipeline or industrial automation setting, direct the flow of fluids such as gases, liquid, or a mixture of particles and fluid by varying the size of the pathway through which the fluids flow based on a signal from a controller. The question “control valves should always be in the what position” arises more often than one would think, and the answer heavily depends on the valve type, operational requirements, and system pressures.

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Control Valves Should Always Be in the What Position 1

Control valves operate as the final control element in a control system and directly influence flow rate, which, as a result, impacts other process quantities such as pressure, temperature, and liquid level. The correct valve position, whether closed or open, directly affects the differential pressure and flow of a fluid, which is crucial for efficient operation.

Types of Control Valves

As we journey into exploring the question, “control valves should always be in the what position”, it’s important to understand that different types of control valves are best suited to particular positions and applications due to their distinct properties and functionalities.

Control Valve TypeKey FeaturesGlobe ValveMost common and versatile type of control valve, characterized by a movable disc-type element and a stationary ring seat in a generally spherical body.Ball ValveAllows for tight shut-off and low pressure drop operation for a wide range of applications. Notable for its spherical closure unit.Butterfly ValveFeatures a simple construction, characterized by a disc that rotates within the valve body to open or close the flow path.Gate ValvePrimarily used to permit or prevent the flow of liquids, featuring a flat closure element that slides into the flow stream to provide shut-off.Diaphragm ValveUtilizes a diaphragm as the throttling element. Suitable for handling corrosive fluids and provides excellent sealing capabilities.Plug ValveKnown for its cylindrical or conical plug with one or more horizontal ports. Ideal for on-off stop valves and can provide tight shut-off.Pinch ValveUsed in slurry applications and feature a design where the flow is pinched off to control the flow rate.Pressure Relief ValveDesigned to protect systems against overpressure by releasing excess pressure when a preset limit is reached.Check ValveAutomatically opens with forward flow and returns to the closed position to prevent backflow when the fluid reverses direction.Needle ValveOffers precise control of flow rate and features a high-pressure resistance due to its unique design.

Based on the pressure drop profile

There are high recovery valves and low recovery valves. High recovery valves, including butterfly, ball, plug, and gate valves, typically recover most of the static pressure drop from the inlet to the outlet, characterized by a lower recovery coefficient. Conversely, low recovery valves, such as globe and angle valves, regain little of the static pressure drop, resulting in a higher recovery coefficient.

Based on the movement profile of the controlling element

Control valves can be categorized as sliding stem or rotary based on their movement profile. Sliding stem valves — such as globe, angle and wedge-type gate valves — have valve stems/plugs that move in a linear or straight-line motion. On the other hand, rotary valves (e.g., butterfly and ball valves) have valve discs that rotate.

Based on functionality

The functionality of control valves can also be diverse, accommodating different needs. Control valves manage flow parameters proportionately to an input signal from the control system. The shut-off/On-off valves function either fully open or closed. Check valves allow flow only in a single direction, and Steam conditioning valves regulate the pressure and temperature of the inlet media to fit the required parameters at the outlet.

Based on the actuating medium

Lastly, control valves can also be sorted based on their actuating medium, which can be manual, pneumatic, hydraulic, or electric. Manual valves function by the turn of a hand wheel, pneumatic valves use a compressible medium like air, hydrocarbon, or nitrogen with a spring diaphragm, piston cylinder or piston-spring type actuator, hydraulic valves apply a non-compressible medium such as water or oil, and electric valves engage an electric motor for operation.

How to Operate Control Valves?

The operation of control valves is determined by several considerations, including the system’s requirements, the type of controlling element, the actuating medium, and the pressure drop profile.

Control Valves Should Always Be in the What Position 2

The opening or closing of automatic control valves is usually achieved by electrical, hydraulic or pneumatic actuators. Typically, with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening. Proper installation and understanding the minimum flow, maximum flow, minimum pressure, and control signal are necessary for the process, contributing greatly to prolonging valve life and preventing valve failure.

Six Modes of Control Valves Fail Positions

Control valves can fail in various modes, which can impact the system’s safety and functionality. Understanding these modes can help to better answer the question “Control valves should always be in the what position?”. Here are six common fail positions:

Fail Open(FO)

When a control valve is configured to fail open, it means that the valve will be in its fully open position in the event of a failure, such as loss of air supply or control signal. This position is usually adopted in processes where the primary concern is the flow of the fluid, preventing potential damage from overpressure or ensuring the availability of essential resources like water and steam. For example, in an engine cooling system, the control valves should always be in the fail-open position so that the engines don’t overheat.

Fail Closed(FC)

Fail closed is the opposite of fail open, as it configures the control valve to remain fully closed when a failure occurs. This mode is suitable for situations where stopping the flow of a fluid is essential for safety or process considerations. An example could include a hazardous chemical plant where cutting off the flow of a toxic substance upon failure is crucial to prevent any potential spills or accidents.

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Fail Locked(FL)

When a control valve is designed to fail locked, it will maintain its last set position in the event of a failure. This mode is often employed in processes where maintaining a specific flow rate or fluid temperature is critical to ensuring product quality or process efficiency. This can be particularly relevant in applications like temperature control systems for reactors, where a precise balance between cooling and heating is necessary to maintain stable conditions.

Fail Indeterminate

In a fail indeterminate mode, the final position of the control valve upon failure is unspecified. This situation typically occurs when designers have not considered the appropriate fail position during the design process, or when an appropriate fail position cannot be reliably predicted. While less common in well-designed systems, it still serves as a reminder that proper installation and selection of control valves are crucial for ensuring a truly fail-safe system.

Fail Last/Drift Open(FL/DO)

In the fail last/drift open mode, the control valve initially remains in its last position upon a failure. However, as fluid continues to flow, the force of the fluid eventually pushes the valve into the fully open position. This mode is commonly used when processes tolerate increased flow, such as temperature control systems, where some excess heat exchange may not be severely detrimental to the process.

Fail Last/Drift Closed(FL/DC)

As the counterpart to the FL/DO mode, the fail last/drift closed mode dictates that a control valve will remain in its last set position upon failure but will drift to its fully closed position with continued fluid flow. This mode is relevant when complete fluid cutoff is necessary, such as in potential leak situations or when uninterrupted flow is dangerous.

Factors to Consider When Determining The Ideal Settings for Control Valves

Several factors must be considered when answering the question “Control valves should always be in the what position?”.

Flow Conditions and Valve Sizing

Incorrect sizing or position can lead to valve issues and system inefficiencies. Proper valve sizing for minimum, maximum, and normal flow conditions is vital for optimal valve performance and minimal maintenance efforts.

Piping Design

When designing the piping system, it is crucial to ensure a smooth, non-turbulent flow in all cases. The positioning of the control valve is heavily influenced by the design and placement of other components such as check valves, diaphragms, controllers and actuators. Striving for smooth flow conditions ensures that the control valve performs optimally and requires minimal maintenance. Thus, determining the control valves should always be in the what position must align with the overall piping design and system performance goals.

Control Valves Should Always Be in the What Position 3

Materials and Gaskets

Valve body material must match the pipe and flange material, while trim materials should be corrosion resistant and suitable for the specific application. Gaskets must be compatible with temperature, pressure, and chemical compatibility needs, ensuring tight seals and reduced leakages.

Cavitation and Flashing

Cavitation and flashing are common issues in control valve systems that can lead to valve wear, damage, erosion and inefficient performance. Proper valve positioning helps mitigate these issues and maintain overall system efficiency.

Position of Control Valve

VerticalControl valves are commonly mounted in a vertical position, ideally with the actuator in an upright position. This ensures optimal thermal insulation between process fluid and gland packing, preventing ice formation on valve stem. However, caution should be exercised when installing cryogenic service valves (-100°C and below), as they must not be mounted with the actuator in a horizontal position.HorizontalAlthough control valves can operate in any orientation, the preferred installation is with the actuator in a vertical position. When installing a control valve with the actuator in a horizontal position, special attention must be given to steam jacketing, ensuring a good condensate drain and accessory installation.

Control Valve Installation Guidelines

1. Follow the manufacturer’s guidelines for valve installation.
2. Consider piping design for smooth, non-turbulent flow conditions.
3. Size the valve properly according to system requirements and flow conditions.
4. Select appropriate materials for the valve body, trim, and gaskets.
5. Maintain recommended distances between control valve and adjacent components, such as pumps, elbows, expansion joints.
6. Install the control valve with the actuator in the preferred vertical position, if possible.
7. In horizontal installation, pay close attention to steam jacketing and proper accessory orientation.

Essential Maintenance Tips for Optimal Valve Performance

1. Adhere to manufacturer recommendations for valve maintenance schedules.
2. Thoroughly clean valves during maintenance to ensure optimal performance.
3. Avoid using grease and lubricants for valve maintenance unless specified by the manufacturer.
4. Follow proper torque values during valve reassembly to prevent leaks and malfunctions.
5. Regularly inspect valve components for wear and replace as needed.
6. Keep the valve installation area clean and free of debris to avoid potential damage.
7. Consult the manufacturer for any specific maintenance requirements unique to the valve type or system application.

Conclusion

The optimal position of control valves depends on several factors like proper sizing, piping design, material selections, and the specific needs of the system. Ultimately, control valves should be in a position that satisfies the system requirements, ensuring that it operates safely and efficiently.
The landscape of industrial process control is continuously evolving, necessitating ongoing updates to control valves and instrumentation systems to allow for smooth integration with new automation technologies. As a reputable manufacturer of control valves, Dombor is committed to producing better products for our customers. Should you have any queries or concerns, please don’t hesitate to get in touch with us.

Control Valve Installed Gain



The green line in the left graph of Figure 1 represents an ideal linear installed flow characteristic where the ideal linear installed characteristic is a straight line, and changes in relative valve travel (Δh) cause equal changes in relative flow (Δq). In the figure, a change in valve position of 1 percent causes a change in flow of 1 percent. Because the slope of the green line is constant, it follows that this valve’s installed gain will be constant, and because a change in position of 1 percent causes a change in relative flow of 1 percent, its installed gain is 1, (gain = 1%/1% = 1.0). So just as the green line in the installed characteristic graph represents the ideal linear installed flow characteristic, the green line in the installed gain graph with a constant value of 1.0 represents the ideal installed gain.

It’s impossible to get the exact ideal installed characteristic and installed gain because: (1) real valves do not have exact linear or equal percentage inherent characteristics; and (2) the interaction between the equal percentage inherent characteristic and the system characteristic do not exactly cancel each other. However, one strives to get as close as possible. This is why the perfectly linear installed characteristic and the constant installed gain of 1.0 are the benchmarks.

In Figure 1 there are two other valves with straight line installed flow characteristics. One has a very steep slope and the other a very shallow slope.

The valve with the blue graph, whose installed characteristic has a steep slope, is a very sensitive valve. Its installed gain graph has a constant, but large value.

The valve with the red graph, whose installed characteristic has a shallow slope, is a very insensitive valve. (In the figure, only the portion of the valve’s characteristic that we are concerned with is shown.) Its gain graphs as a constant, but small value. Neither of these valves would make a very good control valve. The low gain valve would not make a good control valve because when the valve stem moves, the flow hardly changes at all. A control valve that, when it moves, does not change the flow is not much of a control valve. The valve with the steep slope has a very high gain, meaning that small changes in valve position cause very large changes in flow. It is less obvious why this valve would not be a good control valve. When two parts (such as a ball and a seat, or a valve shaft and packing) are in contact with each other, they exhibit two kinds of friction. When the parts are not moving, they tend to stick together and the friction is high. When they are moving the friction becomes much lower. The interaction between static and dynamic friction makes it very difficult to position a valve exactly where it needs to be. From the definition of gain, the change in flow is equal to the change in position multiplied by the installed gain (Δq = Δh * Gain). If the high gain valve (installed gain of 4) could only be positioned in 1 percent increments, the most accurately that flow could be controlled would be in 4 percent increments, which might not be accurate enough



At Point 1, a line has been drawn that is tangent to the installed characteristic to represent the instantaneous slope of the installed characteristic (and thus the installed gain) at Point 1. This tangent is not as steep as the ideal linear installed characteristic, and therefore the gain is less than the ideal 1.0. A point has been placed on the installed gain graph (Point 1) that is less than the ideal gain of 1.0.

At Point 2, if one were to draw a tangent to the installed characteristic graph, it would be parallel to the ideal linear graph. This means, at Point 2, the instantaneous gain is 1.0, and a corresponding Point 2 is placed on the installed gain graph at a gain of 1.0.

If one continues drawing tangent lines at Points 3, 4 and 5, the corresponding Points 3, 4 and 5 on the installed gain graph are achieved.
Typically, the installed characteristic and installed gain graphs of equal percentage valves in systems with a lot of pipe (and/or other pressure consuming elements), which is the most common case, will have shapes similar to those in Figure 2, but not necessarily as symmetrical as shown in the figure.

Control Valve Installed Gain Recommendations

Below are recommendations (and the rules that Nelprof, the Metso control valve sizing and selection software, uses when selecting the best valve size for an application) for gain magnitude and variation.

Within the specified control range:

1.      Gain > 0.5
2.      Gain < 3.0
3.      Gain (max) / Gain (min) < 2.0
4.      As constant as possible
5.      As close to 1.0 as possible
    

Within the specified control range (by definition the system will not be controlling outside this range so one is not concerned with what happens there), that is between the minimum and maximum required flow rates, the gain should not be less than 0.5 or greater than 3.0. Remember the definition of gain is the change in flow equals the change in valve position multiplied by the gain (Δq = Δh * Gain). If the gain is too low, when the valve moves, the flow hardly changes. This means the valve will not be effective in controlling flow. If the gain is too high, small errors in valve position will result in large errors in flow, making it difficult or impossible to control accurately.

Typically, if the gain changes by not much more than a 2-to-1 ratio, it will be possible to come up with one set of PID tuning parameters that will result in good control and stability throughout the required flow range. As the variation in gain within the specified flow range becomes larger, it will become more difficult to tune the system for both stable and good control.

When selecting the best valve out of several that meet the first three criteria, criteria 4 and 5 should be considered. The gain should be as constant as possible. The more constant the gain, the more aggressive the PID tuning can be without the danger of instability. The gain should also be as close to 1 as possible. Usually, when comparing the installed gain of different valves for the same application, as the gain becomes more constant, it also comes closer to 1.

Selecting the Best Valve Based On Installed Gain



Using this software one can demonstrate how an analysis of installed gain can help select the best control valve for a particular system. The demonstration is based on the system shown in Figure 3. The graph in Figure 3 shows how P1, P2 and ΔP vary with flow as modeled by the software. The goal is to select a valve whose installed gain does the best job of meeting the installed gain recommendations given above. For this example, things like choked flow, noise and velocity do not affect the selection, allowing one to concentrate on installed gain.


Figure 6 shows the installed gain of the same two valves. Again, emphasis has been added to the portion of the graphs that are within the specified flow range of 80 GPM to 550 GPM. Examining the two graphs, it is immediately clear that the 3” valve is the best choice because it meets all of the above installed gain recommendations, and the 6” valve does not. The 6” valve has a maximum gain of about 3.5. This means at that point, a 1 percent valve position error would cause a 3.5 percent flow error. In contrast, a similar position error with the 3” valve would result in a 2 percent flow error. The change in gain within the specified flow range is about 2-to-1 for both valves. The gain of the 3” valve is clearly closer to 1 than the 6” valve. Had a 4” valve been analyzed, it would have been found to be better than the 6” valve, but not as good as the 3” valve. Note that on the gain graph, 1.0 on the q/qmaz axis will always be the maximum required flow.

NOTE: The program cannot actually show the results for two valves at one time. The graphs shown above were produced by combining the results from the two calculations into a single graph. When using the sizing program, it is possible to quickly step through the graphs for each of several valves to easily compare them.

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In order to get good control with stability throughout the full range of required flow rates, one must use a control valve that has an installed flow characteristic that is linear, or at least as close to linear as possible in most systems. This was discussed in detail in Part I of this series in the February issue (pages 29–32). It is often difficult to compare the control capability of two valves with less than perfectly linear installed characteristics by simply studying their installed characteristic graphs, and one can learn more about how well they will control a particular system by examining their installed gain.The left graph in Figures 1 and 2 is a control valve’s hypothetical installed flow characteristic, and the right graph is the corresponding installed gain. The gain of a device is defined as the ratio of the change in output to the corresponding change in input. In the case of a control valve, the output is the flow in the system (q) and the input is valve travel (h), meaning its installed gain is defined as: Gain = Δq/Δh. The graphical interpretation of the installed gain is the SLOPE of the installed flow characteristic, and the mathematical interpretation is that the installed gain is the first derivative of the installed flow characteristic.The green line in the left graph of Figure 1 represents an ideal linear installed flow characteristic where the ideal linear installed characteristic is a straight line, and changes in relative valve travel (Δh) cause equal changes in relative flow (Δq). In the figure, a change in valve position of 1 percent causes a change in flow of 1 percent. Because the slope of the green line is constant, it follows that this valve’s installed gain will be constant, and because a change in position of 1 percent causes a change in relative flow of 1 percent, its installed gain is 1, (gain = 1%/1% = 1.0). So just as the green line in the installed characteristic graph represents the ideal linear installed flow characteristic, the green line in the installed gain graph with a constant value of 1.0 represents the ideal installed gain.It’s impossible to get the exact ideal installed characteristic and installed gain because: (1) real valves do not have exact linear or equal percentage inherent characteristics; and (2) the interaction between the equal percentage inherent characteristic and the system characteristic do not exactly cancel each other. However, one strives to get as close as possible. This is why the perfectly linear installed characteristic and the constant installed gain of 1.0 are the benchmarks.In Figure 1 there are two other valves with straight line installed flow characteristics. One has a very steep slope and the other a very shallow slope.The valve with the blue graph, whose installed characteristic has a steep slope, is a very sensitive valve. Its installed gain graph has a constant, but large value.The valve with the red graph, whose installed characteristic has a shallow slope, is a very insensitive valve. (In the figure, only the portion of the valve’s characteristic that we are concerned with is shown.) Its gain graphs as a constant, but small value. Neither of these valves would make a very good control valve. The low gain valve would not make a good control valve because when the valve stem moves, the flow hardly changes at all. A control valve that, when it moves, does not change the flow is not much of a control valve. The valve with the steep slope has a very high gain, meaning that small changes in valve position cause very large changes in flow. It is less obvious why this valve would not be a good control valve. When two parts (such as a ball and a seat, or a valve shaft and packing) are in contact with each other, they exhibit two kinds of friction. When the parts are not moving, they tend to stick together and the friction is high. When they are moving the friction becomes much lower. The interaction between static and dynamic friction makes it very difficult to position a valve exactly where it needs to be. From the definition of gain, the change in flow is equal to the change in position multiplied by the installed gain (Δq = Δh * Gain). If the high gain valve (installed gain of 4) could only be positioned in 1 percent increments, the most accurately that flow could be controlled would be in 4 percent increments, which might not be accurate enoughUnderstanding the meaning of installed gain, how does one apply this concept to an equal percentage valve installed in a system with a lot of pipe (and/or other pressure-consuming elements), where the installed characteristic is nearly linear but slightly “S” shaped, as shown in the left graph of Figure 2? The dashed lines represent the hypothetical ideal linear installed characteristic and the resulting ideal installed gain with a constant value of 1.0. Here the shape of the installed characteristic graph is constantly changing, and its slope is as well. Consider the instantaneous slope (and thus the installed gain) at several points.At Point 1, a line has been drawn that is tangent to the installed characteristic to represent the instantaneous slope of the installed characteristic (and thus the installed gain) at Point 1. This tangent is not as steep as the ideal linear installed characteristic, and therefore the gain is less than the ideal 1.0. A point has been placed on the installed gain graph (Point 1) that is less than the ideal gain of 1.0.At Point 2, if one were to draw a tangent to the installed characteristic graph, it would be parallel to the ideal linear graph. This means, at Point 2, the instantaneous gain is 1.0, and a corresponding Point 2 is placed on the installed gain graph at a gain of 1.0.If one continues drawing tangent lines at Points 3, 4 and 5, the corresponding Points 3, 4 and 5 on the installed gain graph are achieved.Typically, the installed characteristic and installed gain graphs of equal percentage valves in systems with a lot of pipe (and/or other pressure consuming elements), which is the most common case, will have shapes similar to those in Figure 2, but not necessarily as symmetrical as shown in the figure.Below are recommendations (and the rules that Nelprof, the Metso control valve sizing and selection software, uses when selecting the best valve size for an application) for gain magnitude and variation.Within the specified control range:Within the specified control range (by definition the system will not be controlling outside this range so one is not concerned with what happens there), that is between the minimum and maximum required flow rates, the gain should not be less than 0.5 or greater than 3.0. Remember the definition of gain is the change in flow equals the change in valve position multiplied by the gain (Δq = Δh * Gain). If the gain is too low, when the valve moves, the flow hardly changes. This means the valve will not be effective in controlling flow. If the gain is too high, small errors in valve position will result in large errors in flow, making it difficult or impossible to control accurately.Typically, if the gain changes by not much more than a 2-to-1 ratio, it will be possible to come up with one set of PID tuning parameters that will result in good control and stability throughout the required flow range. As the variation in gain within the specified flow range becomes larger, it will become more difficult to tune the system for both stable and good control.When selecting the best valve out of several that meet the first three criteria, criteria 4 and 5 should be considered. The gain should be as constant as possible. The more constant the gain, the more aggressive the PID tuning can be without the danger of instability. The gain should also be as close to 1 as possible. Usually, when comparing the installed gain of different valves for the same application, as the gain becomes more constant, it also comes closer to 1.There is a control valve sizing program which, based on a database of actual valve inherent characteristics, along with some user-supplied information about how the system pressure drop changes with flow, can calculate and graph the installed flow characteristic of a particular type and size valve in its particular system. Next, the program calculates the first derivative of the installed flow characteristic and graphs it as the installed gain. In order for the program to define the process model, at least two flow points (maximum and minimum required flow) are required along with the associated values of pressure upstream of the control valve, P1, and the pressure drop across the control valve, ΔP.Using this software one can demonstrate how an analysis of installed gain can help select the best control valve for a particular system. The demonstration is based on the system shown in Figure 3. The graph in Figure 3 shows how P1, P2 and ΔP vary with flow as modeled by the software. The goal is to select a valve whose installed gain does the best job of meeting the installed gain recommendations given above. For this example, things like choked flow, noise and velocity do not affect the selection, allowing one to concentrate on installed gain.Figure 4 shows the installed flow characteristics of two valves being considered for the application, a 6” segment ball valve and a 3” segment ball valve. Emphasis has been added to the portions of the installed flow graphs that are within the specified flow range of 80 to 550 gallons per minute. Because segment ball valves have an inherent equal percentage flow characteristic and because the system is one where valve pressure drop decreases with increasing flow, it is not surprising that the installed flow characteristics are very nearly linear (especially within the specified flow range). This computer-generated graph shows how linear the installed characteristics are and how much safety factor is at each end of the required flow range. However, because the vertical scale is normalized to show actual flow divided by the fully open flow for each valve, when comparing several valves for the same application, one cannot see the differences in the steepness (slope) of the graph and thus the sensitivity to changes in valve position. If the data in Figure 4 were plotted on a GPM scale, instead of flow divided by fully open flow, it would look like Figure 5 where the relative steepness of different valves can be clearly seen. The program uses a graph like Figure 5, where the gain is calculated based on the maximum required flow, qmax, that is Gain =Δ (q/qmax) / Δh. Note that this graph on a GPM scale is not visible to the user.Figure 6 shows the installed gain of the same two valves. Again, emphasis has been added to the portion of the graphs that are within the specified flow range of 80 GPM to 550 GPM. Examining the two graphs, it is immediately clear that the 3” valve is the best choice because it meets all of the above installed gain recommendations, and the 6” valve does not. The 6” valve has a maximum gain of about 3.5. This means at that point, a 1 percent valve position error would cause a 3.5 percent flow error. In contrast, a similar position error with the 3” valve would result in a 2 percent flow error. The change in gain within the specified flow range is about 2-to-1 for both valves. The gain of the 3” valve is clearly closer to 1 than the 6” valve. Had a 4” valve been analyzed, it would have been found to be better than the 6” valve, but not as good as the 3” valve. Note that on the gain graph, 1.0 on the q/qmaz axis will always be the maximum required flow.The program cannot actually show the results for two valves at one time. The graphs shown above were produced by combining the results from the two calculations into a single graph. When using the sizing program, it is possible to quickly step through the graphs for each of several valves to easily compare them.Part I: An Insider's Guide to Control Valves & Process Control Part II: An Insider’s Guide to Control Valves & Process Variability Part IV: Keys to Effective Valve Sizing & Selection

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