Control Valve Type: Selection Guide

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. 

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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

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.​

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• 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

Valve Type Size Range (DN) Max Pressure Differential (MPa) Best Applications Limitations Single-Seat Valve 25 to 100 ≤ 0.5 Small flow rates requiring high sealing integrity; precision tasks like pharmaceutical steam lines and gas regulation. Limited to low pressure differentials; not suitable for large flow capacities. Double-Seat Valve 50 to 400 0.5 to 1.5 High flow rates with moderate pressure drops; applications where tight shut-off is not critical. Inferior sealing compared to single-seat valves; may experience higher leakage rates. Sleeve (Cage-Guided) Valve 50 to 400 0.5 to 4 Medium-to-large systems with significant pressure drops; industries like petrochemical refining and liquid transfer. More complex design can lead to increased maintenance requirements. Multi-Stage (Labyrinth) Valve 100 to 600 ≥ 4 Extreme pressure drops and high-energy systems; suitable for boiler feedwater and high-pressure gas regulation. Larger size and complexity; higher initial cost and maintenance demands.

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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Conclusion

ruwl,

With respect to air failure (fail open/closed) and mechanical failure (flow opens/closes) in globe valves, when all else is equal then my opinion is that these two failures should generally be consistent if possible (i.e. specify fail close + flow closes, etc). The problem arises in many cases that globe valves do not have the desired characterisics or functionality in a flow closes configuration. In this case the flow direction is not something you are really free to specify for that type valve, and the configuration will be flow to open for that valve. The more common (and possibly plant wide) situation of air failure is then considered by itself and set to the desired failure mode resulting in the very common imstallation of: flow opens, but fail close (on loss of air pressure).

Of course every installation is a custom job, so if all types of failures must be controlled in a critical application, then alternative valve designs or shutoffs may be required.

best wishes,
sshep If you have a process where there is tight control required, and you have to operate anywhere near the seat, flow under the seat is preferable to flow over the seat.

Flow over the seat tends to try to shut the valve, and if the actuator is air operated, the actuator has to constantly fight to balance the compressability of the air in the diaphragm against the constantly varying closing forces of the flow over the seat trying to slam the valve shut.

rmw Thank you all for the inputs. They all are very helpful.

So, it appears that selecting a the flow-to-open or flow-to-close valve has an effect in selecting the type of actuator also?

If so, when do I specify a "piston" type actuator or "diaphragm" type actuator? Is there a general rule of thumb to specify what instrument air pressure requirement for a specific type of actuator also (3 to 15 psig or something else)?

Again, thank you all for the inputs. Diaphragm actuators are very common to control valves. Most use air/gas in the 3-30 psi range.

Piston actuators are used more in on/off valves. They can also be used for control, but is less common. Piston actuators usually can take higher pressures, up to 80 psi. This gives them better performance for quick closing in an on/off service.

jmw said: Don't assume the fail to open/close is the only option.
Valves can be built up with the actuator energise open/energise close (spring return) or with air to open and air to close in which case an air failure means fail in position.

I am not quire sure what you are saying - it seems you are saying the same thing in both sentences.

A fail open valve needs "something" (instrument air, instrument gas, hydraulic, etc) to close it since usually a spring holds it in the open position. A fail close valve has the spring doing the opposite.

A flow open valve has the flow direction "pushing" the valve open (ie upwards in a globe valve). A flow close valve has the flow direction "pushing" the valve close (ie downwards in a globe valve).

The only other type of fail position is fail unknown. Examples are air-to-open-and-close valves, and MOVs. Because there is no stored energy device to move the valve to one position or another, the fail position is hence unknown. Thanks for all your replies.

I was just curious because I had seen some some old specs that have: 1) Flow to close - Failed closed 2) Flow to open - Failed Closed.

I've also noticed from the old spec that item 1) was used with angle valves where the pressure drop is large ( psig) for a letdown applications. The old spec had actually called out for a piston type actuator.

I was trying to understand the reasoning for the old spec.