Part IV: Keys to effective valve sizing & selection

This is Part IV in a four-part series based on the contents of the new textbook, "Control Valve Application Technology, Techniques and Considerations for Properly Selecting the Right Control Valve."

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Part I: An Insider’s Guide to Valve Sizing & Selection

Part II: An Insider’s Guide to Control Valves & Process Variability

Part III: An Insider’s Guide to Installed Gain as a Control Valve Sizing Criterion

Selecting a properly sized control valve is essential to achieving the highest degree of process control. Today, the control valve sizing calculations are usually performed using a computer program. Most manufacturers of control valves offer control valve sizing software at no cost, though most are specific to that manufacturer’s valves only. One specific program includes a number of generic valves to choose from. The generic choices include typical equal percentage globe valves, linear globe valves, ball valves, eccentric rotary plug valves, high-performance butterfly valves and segment ball valves. These generic selections permit the user to investigate the applicability of different valve styles and sizes to a particular application, without showing a preference to a particular valve manufacturer. Additionally, there is a set of comprehensive Excel spreadsheets that follow the methods of ANSI/ISA-75.01.01 (IEC -2-1 Mod)- Flow Equations for Sizing Control Valves that are available at no cost at control-valve-application-tools.com. These spreadsheets are applicable to the valves of all manufacturers and are documented so the user can trace the calculations to the equations in the Standard. This article presents a brief review of some of the elements that must be considered to size and select the right control valve for a particular application.

Selection of control valve style

The choice of control valve style (e.g., globe, ball, segment ball, butterfly, etc.) is often based on tradition or plant preference. For example, a majority of the control valves in pulp and paper mills are usually ball or segmented ball valves. Petroleum refineries traditionally use a high percentage of globe valves, although the concern over fugitive emissions has caused some users to look to rotary valves because it is often easier to obtain a long lasting stem seal. Globe valves offer the widest range of options for flow characteristic, pressure, temperature, noise and cavitation reduction. Globe valves also tend to be the most expensive. Segment ball valves tend to have a higher rangeability and nearly twice the flow capacity of comparably sized globe valves and, in addition, are less expensive than globe valves. However, segment ball valves are limited in availability for extremes of temperature and pressure and are more prone to noise and cavitation problems than globe valves.

High performance butterfly valves are even less expensive than ball valves, especially in larger sizes (8" and larger). They also have less rangeability than ball valves and are more prone to cavitation. The eccentric rotary plug valve (a generic term commonly applied to valves with trade names like Camflex®, a registered trademark of Dresser Masoneilan, and Finetrol®, a registered trademark of Metso Automation) combines features of rotary valves, such as high cycle life stem seals and compact construction with the rugged construction of globe valves. Unlike the other rotary valves, which have a flow capacity approximately double that of globe valves, the flow capacity of eccentric rotary plug valves is on a par with globe valves.

Certainly the selection of a valve style is highly subjective. In the absence of a clear-cut plant preference, the following approach is recommended to select a control valve style for applications where the valve will be 6" or smaller. Considering pressure, pressure differential, temperature, required flow characteristic, cavitation and noise, one must first determine whether a segment ball valve will work. If a segment ball valve is not suitable, select a globe valve. Keep in mind that cage-guided globe valves are not suitable for dirty service. For applications where the valve will be 8" or larger, it is encouraged to first investigate the applicability of a high-performance butterfly valve because of the potential for significant savings in cost and weight.

Flow characteristic

As a general rule, systems with a significant amount of pipe and fittings (the most common case) are usually best suited for an equal percentage of inherent characteristic valves. Systems with very little pipe and other pressure-consuming elements (where the pressure drop available to the control valve remains constant and as a result the inherent characteristic of the valve is also the installed characteristic) are usually better suited to linear inherent characteristic valves.

Pipe reducers

Control valves are generally installed into piping that is larger than the valve itself. To accommodate the smaller valve, it is necessary to attach pipe reducers. Because the control valve size is usually not known at the time the pressure drop available to the control valve is being calculated, it is common practice to not include the reducers in the piping pressure loss calculations. Instead, the pressure loss in the reducers is handled as part of the valve sizing process by the inclusion of a "Piping Geometry Factor," F­P. All of the modern computer programs for control valve sizing include the F­P calculation. Because F­P is a function of the unknown Cv, an iterative solution is required.

Process Data

A valve sizing calculation will only be reliable if the process data used in the calculation accurately represents the true process. There are two areas where unreliable data enters the picture. The first involves the addition of safety factors to the design flow rate. The second involves the selection of the sizing pressure drop, DP. There is nothing wrong with judiciously applying a safety factor to the design flow. A problem can arise, however, if several people are involved in the design of a system, and each adds a safety factor without realizing that the others have done the same.

Perhaps the most misunderstood area of control valve sizing is the selection of the pressure drop, DP, to use in the sizing calculation. The DP cannot be arbitrarily specified without regard for the actual system into which the valve will be installed. What must be kept in mind is that all of the components of the system except for the control valve (e.g., pipe, fittings, isolation valves, heat exchangers, etc.) are fixed and at the flow rate required by the system (e.g., to cool a hot chemical to a specified temperature, maintain a specified level in a tank), the pressure loss in each of these elements is also fixed. Only the control valve is variable, and it is connected to an automatic control system. The control system will adjust the control valve to whatever position is necessary to establish the required flow (and thus achieve the specified temperature, tank level, etc.). At this point, the portion of the overall system pressure differential (the difference between the pressure at the beginning of the system and at the end of the system) that is not being consumed by the fixed elements must appear across the control valve.

The correct procedure for determining the pressure drop across a control valve in a system that is being designed is as follows:

1. Start at a point upstream of the valve where the pressure is known, then at the given flow rate, subtract the system pressure losses until you reach the valve inlet, at which point you have determined P1.
2. Then go downstream until you find another point where you know the pressure, and at the given flow rate, work backward (upstream) adding (you add because you are moving upstream against the flow) the system pressure losses until you reach the valve outlet at which point you have determined P2.
3. You can now subtract P2 from P1 to obtain ΔP.
4. If you plan to perform sizing calculations at more than one flow rate (e.g. at both maximum and minimum design flows) you must repeat the calculation of P1 and P2 at each flow rate, since the system pressure losses (and pump head) are dependent on flow. This is illustrated in Figure 1.

In reality, there is a certain amount of rounding out of the graph at the DPchoked point as shown in Figure 2. This rounding of the flow curve makes predicting cavitation damage more complicated than simply comparing the actual pressure drop with the calculatedchoked pressure drop, which assumes the classical discussion of a sudden transition between non choked flow and choked flow. It turns out that both noise and damage can begin even before the pressure drop reaches DPchoked. Over the years, what this article refers to as DPchoked has gone by many names because it was never given a name in the ISA/IEC control valve standards. With the issuance of the Standard, for the first time it has been officially named "DPchoked."

Some valve manufacturers predict the beginning of cavitation damage by defining an incipient damage pressure drop, which is sometimes referred to as ΔPID, as shown in the formula in Figure 2. These manufacturers evaluate actual application experience with cavitation damage and assign what they believe to be meaningful values of KC to their valves. One manufacturer, for example, uses a KC for stem-guided globe valves that is equal to 0.7. There are other manufacturers who, based on the recommended practice, ISA–RP75.23–, use sigma (s) to represent various levels of cavitation. These valve manufacturers publish values of either smr (the manufacturers recommended value of sigma) or sdamage. Sigma is defined as "(P1 – PV)/ ΔP." smr and KC are reciprocals of each other and thus convey the same information. Higher values of KC move the point of incipient damage closer to DPchoked, where lower values of smr do the same.

A good method for predicting cavitation damage is based on the fact that the same element that causes damage also causes the noise, namely the collapse of vapor bubbles. The idea of correlating noise with cavitation damage got its start in . Hans Baumann published an article in Chemical Engineering magazine where, based on some limited damage tests, he established a maximum sound pressure level, SPL, of 85 dBA as the upper limit to avoid unacceptable levels of cavitation damage in butterfly valves.

To verify this premise, the valve manufacturer that the author was associated with for many years did a study of many applications. In some cases, cavitation damage was minimal, and in others it was excessive. The conclusion of the study was that it is possible to predict that damage will be within acceptable limits as long as the predicted noise level is below limits established in the study. In the case of 4" and 6″ valves, the limit turns out to be 85 dBA. The SPL limits established in the study (based on noise calculations using VDMA ), to avoid cavitation damage are: Up to 3" valve size: 80 dBA; 4" to 6": 85 dBA; 8" to 14": 90 dBA; and 16" and larger: 95 dBA. Note that regardless of the noise calculation, the actual pressure drop must be less than the choked pressure drop, because experience has shown that operating above the choked pressure drop is almost certain to result in damage.

It should be noted that although choked flow with gas does not cause valve damage, gas choked flow can result in high noise levels, but these will be revealed by any of the valve sizing programs. Many authorities warn against aerodynamic noise levels above 120 dBA (calculated with Schedule 40 pipe) due to the resultant high levels of vibration within the valve.

Jon F. Monsen, Ph.D., P.E., is a control valve technology specialist at Valin Corporation, with more than 30 years’ experience. He has lectured nationally and internationally on the subjects of control valve application and sizing, and is the author of the chapter on “Computerized Control Valve Sizing” in the ISA Practical Guides book on Control Valves. He is also the author of the book "Control Valve Application Technology: Techniques and Considerations for Properly Selecting the Right Control Valve."

Process plant operations involve many rigorous tasks requiring the highest level of measurement and control performance. Control valve technology, in particular, plays a vital role in production processes. Valves are the most important single element in any fluid handling system because they regulate the flow of fluid to the process.

This article describes the importance of proper control valve sizing and selection to manufacturing efficiency, reliability, quality and safety. When choosing a valve to meet a specific application requirement, and considering key factors such as sizing and trim materials, it is wise to consult with a qualified valve engineer capable of analyzing the application to ensure that the right device is chosen and deployed appropriately.

Background

Process industry manufacturers are trying to stay on top of industry changes more than ever — maintaining utmost product quality and meeting and exceeding increasingly stringent safety regulations are just two of the challenges they face. They must implement effective manufacturing techniques, which are cost-efficient, time-saving and reliable.

Control valves are employed in many different ways in a typical plant. They are controlled devices that regulate the flow of a liquid or gas in a system. The varying resistance that the valve introduces into the system as it is stroked accomplishes this regulation. As the valve modulates to the closed position, the system pressure drop shifts to the valve and reduces the flow in the system (See Image 2).

Modern control valve designs allow them to be used simply as an on-and-off device, or for any combination of controlling to include regulation, modulation, mixing or even isolation. They are a highly engineered product and should not be treated simply as a commodity. Addressing control valve performance has a dramatic effect on process plant efficiency, overall profitability and asset life cycle costs.

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Need for proper valve sizing

When engineers talk about control valve sizing, they generally refer to the entire process of selecting equipment that will provide an optimal solution for a specified measurement and control function. Indeed, choosing a properly sized valve is essential to achieving the highest degree of control for the liquid, gas or multiphase fluid.

The style of control valve is usually determined by the user’s requirements, past experiences or plant preference. Valve selection can be a tricky process, but sizing the valve can be even more difficult. Valves are often incorrectly specified at the time of installation.

The most important variables to consider when sizing a valve include:

• What medium will the valve control?
• What effects will specific gravity and viscosity have on the valve size?
• What will the inlet pressure be under maximum load demand? What is the inlet temperature?
• What pressure drop (differential) will exist across the valve under maximum load demand?
• What maximum capacity should the valve handle?
• What is the maximum pressure differential for closing the valve?

The required flow rate the valve must pass and the pressure drop that can be allowed across the instrument typically govern its sizing for a particular duty. For example, an undersized valve does not have the capacity to pass the required flow, and thus cannot offer good control performance. It will typically lead to saturation-type non-linearities. The valve can only control the process in one direction (closing the valve) and it is likely that the process variable will not attain set point.

Control valves are frequently sized based on a future maximum process design plus a safety factor. This leads to specifying, buying and maintaining a larger device than is needed for the flow rate, and results in imprecise control and poor production outcomes. An oversized valve is very sensitive to operating conditions. Even the smallest changes in valve position will cause significant changes in flow. This makes it difficult or even impossible for the valve to exactly adjust to the required flow.

When sizing a control valve, the general rule is to size it so that it operates between 20 to 80 percent open at maximum required flow rate and whenever possible, only slightly less than 20 percent open at the minimum required flow rate. This approach is intended to use as much of the valve’s control range as possible while maintaining a reasonable (but not excessive) safety factor. Properly sized globe valves, for instance, are usually one size smaller than the line.

Experience shows there is no substitute for working with a knowledgeable expert to ensure the correct valves are specified for a given installation. The problem with just filling out the specification sheet is that optimal valve or process performance is not guaranteed, even if the spec sheet is filled out correctly. When valves misbehave and the result is poor process control, the root cause of the problem is likely an inadequate selection process.

Importance of trim material

After a high level of performance is achieved through proper valve sizing, how can it be maintained? A control valve behaves much like other mechanical devices. Over time, wear gradually decays control performance. If left unchecked, this decay can eventually lead to failure, downtime on production lines, and unanticipated costs for spare parts and repair.

The internal elements of a control valve (collectively referred to as its "trim") are a crucial consideration in the valve selection process. Valve trim typically includes a disc, seat and stem, as well as the sleeves needed to guide the stem. The disc and seat interface, along with the relation of the disc position to the seat, normally determines a valve’s performance.

A control valve’s trim can be selected to create a variety of passage shapes that control the flow in deliberate ways. The valve opens the gap by moving the plug, disc or valve away from the seat. The length of the stroke determines the opening size and how much liquid, gas or vapor passes the seat. By altering the size of the internal gap, the control valve increases, decreases or holds steady the flow through itself. The valve alters the opening whenever the process parameter, or variable, being controlled does not equal the value it is meant to be (i.e., the set point).

Erosion, or the gradual reduction and weakening of valve bodies or trim components resulting from severe process conditions, is a significant problem in modern manufacturing plants. Typical damage includes seal ring and gasket loss; stem, body and trim retainer wear on the seat ledge; plug, seat ring and cage wear; and packing leakage.

Several common reasons for premature trim wear in control valves exist. For example, flashing occurs when the pressure of a fluid falls below its vapor pressure, changing from a liquid to a vapor. During this process, small vapor cavities form that grind away at the outlet of the valve and its trim components. Cavitation is similar to flashing, except the fluid pressure recovers to a pressure that is above its vapor pressure. This causes the previously formed vapor cavities to implode, producing impinging jets with the potential to cause severe erosive damage. Outgassing occurs when the pressure of a fluid drops below the saturation pressure of a dissolved gas. Once this point is reached, the gas separates from the solution and produces high-velocity, erosive vapor droplets.

Depending on the type of supply, the globe valve’s disc is moved by a hydraulic, pneumatic, electrical or mechanical actuator. The valve modulates flow through movement of a valve plug in relation to the port(s) located within the valve body. The valve plug is attached to a valve stem, which is connected to the actuator.

Some globe valve designs feature a bolted bonnet and post-guided inner valve. They are well-suited for modulating control of liquids and vapors in environments where compact size, coupled with the ability to withstand high temperature and pressure, are essential (See Image 3).

Globe valves meet demanding process application requirements because of the quality and precision tuning of their trim components. For example, valves are available with pre-formed diaphragm and multisprings to ensure extremely linear travel versus input signal performance. Plus, valves utilizing a single O-ring and Nylatron guide bushing provide minimum hysteresis. Technicians can adjust the spring preload to suit specific closing force requirements and make use of adjustable travel stops.

A significant improvement in control valve technology is the implementation of 316 stainless steel for trim material such as the valve body, bonnet and inner valve. This ensures longer trim life, and as such, less downtime and lower device repair and replacement costs. The most common stainless steel on the market, 316 is an austenitic grade with the addition of 2 to 3 percent molybdenum, which further improves corrosion resistance. It is often referred to as a marine-grade stainless steel because of its effective resistance to chloride corrosion in comparison to other stainless steel grades. The material also has superior welding and forming qualities.

Many users choose to mate globe valves to high-accuracy, electropneumatic I/P positioners to position the device based on a 4-20 mA control signal. The latest generation of I/P positioners delivers fully automatic determination of the control parameters and adaptation to the final control element.

Benefits to plant operators

A demanding business environment calls for the most reliable and accurate control of production processes possible. Failure to meet specific operating standards can have serious consequences for quality and safety, while running an inefficient operation can significantly affect the financial margins for the product. In both cases, optimal control valve performance is vital.

Industrial organizations will benefit from working closely with their manufacturer representative or instrumentation supplier to specify an appropriate measurement and control solution. This collaboration can meet important performance criteria such as:

• Precise flow and pressure control resulting in stable and consistent production results
• Lower repair and maintenance costs resulting from longer valve trim life
• Fewer unplanned shutdowns and increased plant availability
• Efficient energy usage and reduced costs

Control valves must withstand the erosive effects of the flowing fluid while holding an accurate position to maintain the process variable. A valve will perform these tasks satisfactorily if it is sized correctly for the application, and designed and built in a way that is appropriate for the process service conditions.

Conclusion

There is no doubt that enhanced control valve technology helps all kinds of manufacturers continually improve process efficiency and product quality, while safeguarding people, plant assets and the environment. The right solution can support a comprehensive system to track every step of the manufacturing process.

Key to the outcome of any control valve project is the assistance of qualified engineers, who analyze the application to ensure the right instruments are selected and sized correctly. Valve manufacturers that understand control performance can share those capabilities and show they can conform to a user’s performance specifications.

Brian Kettner is marketing manager for Badger Meter. He has more than 14 years of experience in fluid system technology, combining marketing, sales and product development roles, with a focus on the processing industry. Currently, Kettner manages Research Control Valve and Preso differential pressure product lines for Badger Meter.

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