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.
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.
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.
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.
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:
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.
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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."
A more modern version of the globe valve design uses a piston-shaped plug inside a surrounding cage with ports cast or machined into it.
These cage-guided globe valves throttle flow by uncovering more or less of the port area in the surrounding cage as the plug moves up and down. The cage also serves to guide the plug so the stem need not be subjected to lateral forces as in a stem-guided valve design.
A photograph of a cut-away control valve shows the appearance of the cage (in this case, with the plug in the fully closed position). Note the “T”-shaped ports in the cage, through which fluid flows as the plug moves up and out of the way:
An advantage of the cage-guided design is that the valve’s flowing characteristics may be easily altered just by replacing the cage with another having different size or shape of holes.
By contrast, stem-guided and port-guided globe valves are characterized by the shape of the plug, which requires further disassembly to replace than the cage in a cage-guided globe valve.
With most cage-guided valves all that is needed to replace the cage is to separate the bonnet from the rest of the valve body, at which point the cage may be lifted out of the body and swapped with another cage.
In order to change a globe valve’s plug, you must first separate the bonnet from the rest of the body and then de-couple the plug and plug stem from the actuator stem, being careful not to disturb the packing inside of the bonnet as you do so.
After replacing a plug, the “bench-set” of the valve must be re-adjusted to ensure proper seating pressure and stroke calibration.
Cage-guided globe valves are available with both balanced and unbalanced plugs. A balanced plug has one or more ports drilled from top to bottom, allowing fluid pressure to equalize on both sides of the plug. This helps minimize the forces acting on the plug which must be overcome by the actuator:
Unbalanced plugs generate a force equal to the product of the differential pressure across the plug and the plug’s area (F = PA), which may be quite substantial in some applications.
Balanced plugs do not generate this same force because they equalize the pressure on both sides of the plug, however, they exhibit the disadvantage of one more leak path when the valve is in the fully closed position (through the balancing ports, past the piston ring, and out the cage ports):
Thus, balanced and unbalanced cage-guided globe valves exhibit similar characteristics to double ported and single-ported stem- or port-guided globe valves, and for similar reasons.
Balanced cage guided valves are easy to position, just like double-ported stem-guided and port-guided globe valves.
However, balanced cage-guided valves tend to leak more when in the shut position due to a greater number of leak paths, much the same as with double-ported stem-guided and port-guided globe valves.
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