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."

Control valves play a critical role in industrial systems by regulating the flow of gases, liquids, and steam to maintain stable and efficient operations. Selecting the correct valve type ensures safety by preventing leaks, system failures, and pressure issues that could harm equipment or personnel. It also drives efficiency, as well-matched valves improve flow control, reduce energy waste, and enhance overall process performance. From a cost perspective, proper selection reduces maintenance needs, downtime, and long-term operational costs. 

Key Factors to Consider Before Selecting a Control Valve Type

Selecting the appropriate control valve is crucial for ensuring optimal performance, safety, and efficiency in industrial systems. Several key factors must be evaluated to make an informed decision:

Fluid Characteristics

Understanding the properties of the fluid that will flow through the valve is essential:

• State: Determine whether the fluid is a liquid, gas, or steam, as this influences the valve type and materials required.​

• Composition: Identify if the fluid is corrosive, abrasive, or contains particulates. Corrosive fluids may necessitate valves made from specific alloys or with special linings to prevent degradation.​

• Viscosity: Highly viscous fluids demand valves designed to handle increased resistance to flow.​

• Temperature Sensitivity: Some fluids may change state or properties with temperature variations, affecting valve performance.​

Properly assessing these characteristics ensures the selected valve materials and design are compatible with the fluid, preventing premature failure and maintaining system integrity.​

Pressure and Temperature Requirements

Valves must withstand the operating pressures and temperatures of the system:

• Operating Pressure: Identify both the normal and maximum pressures the valve will encounter. Valves are rated for specific pressure ranges, and exceeding these can lead to mechanical failure.​

• Operating Temperature: Determine the temperature range, including extremes, to ensure the valve materials can endure thermal stresses without compromising performance.​

Selecting a valve with appropriate pressure and temperature ratings is vital for safety and longevity. According to Valin Corporation, understanding the application’s specific requirements helps in choosing a cost-effective option that fulfills the necessary functions. ​

Flow Rate and Control Precision Needs

Accurate control of flow rate is fundamental to process efficiency:

• Flow Rate (Cv): The valve’s flow coefficient (Cv) indicates its capacity to pass fluid. Proper sizing ensures the valve can handle the desired flow rate without excessive pressure drop.​

• Control Precision: Evaluate how precisely the flow needs to be regulated. Processes requiring fine adjustments may benefit from valves with high positioning accuracy and minimal hysteresis.​

Oversized valves can lead to poor control and instability, while undersized valves may not meet flow requirements. As noted by FluidFlow, selecting the correct valve size and type is crucial to avoid issues like cavitation and ensure optimal performance. ​

Industry-Specific Standards

Compliance with industry regulations and standards ensures safety and interoperability:

• Chemical Industry: Valves must resist aggressive chemicals and adhere to standards like those from the American National Standards Institute (ANSI) or International Organization for Standardization (ISO).​

• Oil & Gas: Valves should meet specifications from organizations such as the American Petroleum Institute (API) to handle high pressures and temperatures.​

• Pharmaceuticals: Hygienic design is critical, with valves often needing to comply with Food and Drug Administration (FDA) regulations or Good Manufacturing Practice (GMP) standards.

Control Valve Type: Applications and Use Cases

Design

Single-seat control valves feature a straightforward design comprising a single plug and seat. This configuration allows for precise control and tight shut-off capabilities, making them suitable for applications requiring accurate flow regulation and minimal leakage. The simplicity of the design also facilitates ease of maintenance and reliability in operation.​

Key Specifications

• Size Range: Typically available in sizes from DN25 to DN100, accommodating various pipeline diameters within this range.​

• Pressure Differential: Designed to handle pressure differentials up to 0.5 MPa, making them suitable for low to moderate pressure applications.​

Ideal Applications

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Single-seat control valves are particularly well-suited for scenarios involving:​

• Small Flow Rates with High Sealing Requirements: Their design ensures tight shut-off, making them ideal for processes where even minor leakage cannot be tolerated.​

• Precision Applications: Commonly employed in industries such as pharmaceuticals and fine chemicals, these valves provide accurate control necessary for applications like:​

  • Pharmaceutical Steam Lines: Ensuring precise regulation of steam flow in sterilization and process heating.​

  • Gas Regulation: Maintaining exact flow rates and pressures in gas delivery systems to ensure process consistency and safety.

Double-Seat Control Valves

Design

Double-seat control valves incorporate two plugs and two seats within the valve body. This dual-plug configuration balances the hydraulic forces acting on the valve stem, reducing the actuator force required for operation. Consequently, these valves are suitable for applications involving higher flow capacities and moderate pressure drops. ​

Key Specifications

• Size Range: Typically available from DN50 to DN400, accommodating medium to large pipeline diameters.​

• Pressure Differential: Designed to handle pressure differentials ranging from 0.5 MPa to 1.5 MPa, making them appropriate for moderate pressure conditions.​

Ideal Applications

Double-seat control valves are particularly suited for scenarios requiring:​

• High Flow Rates with Moderate Pressure Drops: The balanced design allows for efficient handling of substantial fluid volumes without necessitating large actuators.

Limitations

While advantageous in many respects, double-seat control valves have certain drawbacks:​

• Sealing Performance: Achieving a tight shut-off is more challenging compared to single-seat valves due to the complexity of sealing two plugs simultaneously. ​

• Replacement by Sleeve Valves: In some applications, double-seat valves are being replaced by sleeve (cage-guided) valves, which offer improved sealing capabilities and are better suited for higher pressure drops.

Sleeve Control Valves

Design

Sleeve control valves, also known as cage-guided valves, utilize a cylindrical sleeve to guide the valve plug. This design enhances stability and minimizes vibration during operation, leading to improved control accuracy and longevity. The sleeve’s structure also facilitates streamlined fluid flow, reducing turbulence and associated noise. ​

Key Specifications

• Size Range: Commonly available in sizes from DN50 to DN400, accommodating medium to large pipeline diameters.​

• Pressure Differential: Capable of handling pressure differentials between 0.5 MPa and 4 MPa, making them suitable for applications with moderate to high-pressure drops.​

Ideal Applications

Sleeve control valves are particularly well-suited for:​

• Medium-to-Large Systems with High-Pressure Drops: Their robust design allows for efficient operation in systems experiencing significant pressure variations.​

• Petrochemical Refining and Liquid Transfer: Commonly employed in industries such as petrochemical refining, where precise flow control of various liquids is critical.

Advantages Over Double-Seat Valves

Compared to double-seat valves, sleeve control valves offer:

• Enhanced Sealing: The design provides improved shut-off capabilities, reducing leakage risks.​

• Increased Durability: Reduced vibration and balanced flow characteristics contribute to longer service life and reliability.

Multi-Stage Control Valves

Design

Multi-stage control valves are engineered to manage extreme pressure drops by incorporating multiple stages of pressure reduction within the valve trim. This design effectively controls the velocity of the fluid, thereby minimizing cavitation and reducing noise levels. The labyrinth structure forces the fluid through a complex pathway, dissipating energy gradually and preventing the detrimental effects associated with high-pressure differentials. ​

Key Specifications

• Size Range: Typically available in sizes from DN100 to DN600, accommodating large-scale industrial applications.​

• Pressure Differential: Designed to handle pressure drops of 4 MPa (approximately 580 psi) or greater, making them suitable for severe service conditions.​

Ideal Applications

These valves are particularly well-suited for high-energy systems where extreme pressure reductions are necessary, such as:​

• Boiler Feedwater Systems: Ensuring precise control and preventing cavitation in high-pressure water applications.​

• High-Pressure Gas Regulation: Managing the flow and pressure of gases in systems with substantial pressure differentials.​

Comparative Table Overview: Control Valve Type at a Glance

Industry-Specific Recommendations

Selecting the appropriate control valve is crucial for optimizing performance, ensuring safety, and maintaining efficiency across various industries. Below are tailored recommendations for valve types suited to specific sectors:

Oil & Gas: Sleeve Valves for Refinery Processes

In the oil and gas industry, particularly within refinery operations, sleeve (cage-guided) control valves are preferred for their ability to handle high-pressure drops and large flow capacities. Their robust design provides enhanced stability and reduces vibration, making them ideal for the demanding conditions of refining processes. For instance, these valves are effectively utilized in controlling the flow of heavy cycle oil back to fractionation towers, ensuring precise regulation and system efficiency. ​

Pharmaceuticals: Single-Seat Valves for Precision Gas Control

The pharmaceutical industry requires meticulous control over processes to maintain product integrity and comply with stringent hygiene standards. Single-seat valves are commonly employed for their precise flow regulation and excellent sealing capabilities, which are essential for applications like gas control in sterile environments. Manufacturers such as Alfa Laval offer single-seat valves designed to meet high hygiene standards, ensuring process safety and minimizing contamination risks. 

Power Generation: Multi-Stage Valves for Boiler Systems

In power generation, particularly within thermal power plants, managing extreme pressure drops and high-energy systems is critical. Multi-stage (labyrinth) control valves are designed to handle such conditions by providing gradual pressure reduction, thereby minimizing cavitation and noise. These valves are essential in applications like boiler feedwater control, where precise regulation is necessary for safe and efficient plant operation. ​

Chemical Processing: Sleeve or Double-Seat Valves for Corrosive Fluids

Chemical processing industries often deal with corrosive fluids that require valves offering both durability and reliable sealing. Depending on the specific application, sleeve (cage-guided) valves or double-seat valves may be appropriate choices. Sleeve valves provide robust construction suitable for handling significant pressure drops, while double-seat valves offer modular solutions for managing the simultaneous flow of different products without the risk of cross-contamination. For example, Alfa Laval’s double-seat valves are designed to provide exceptional operation and enhanced cleanability, meeting the stringent requirements of chemical processing applications.

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