Techniques to Reduce Flashing and Cavitation in Control Valves

Industrial control valves are pivotal in managing fluid flow in numerous applications across various industries, including oil and gas, chemical, and power generation. A common challenge in the operation of these valves is the phenomena of flashing and cavitation, which can severely damage valve components, reduce operational efficiency, and increase maintenance costs. Manufacturers have developed several port modification techniques to mitigate these issues, enhancing the longevity and reliability of control valves. One notable solution in this area is Flowserve Valtek's CavControl technology.

Flashing occurs when the pressure of a liquid drops below its vapor pressure, causing it to vaporize as it flows through the valve, leading to erosion and wear on valve components. Conversely, cavitation happens when vapor bubbles formed from flashing collapse downstream of the valve seat in a liquid phase, causing shock waves that can damage valve parts and connected piping systems. To address these challenges, engineers have devised various port modification techniques focused on controlling the flow within the valve to manage pressure drops more effectively and reduce the likelihood of flashing and cavitation.

One such technique involves the use of multi-stage trim designs. These designs distribute the pressure drop across several more miniature stages or steps within the valve, thereby preventing the pressure at any point from falling below the liquid's vapor pressure. This staged pressure reduction minimizes the energy available for vapor formation as the fluid progresses through the valve, effectively mitigating flashing and reducing the potential for cavitation.

Another port modification approach is incorporating specially designed flow paths that smooth fluid transition from high to low pressure. By carefully shaping these paths, engineers can ensure a more gradual pressure decrease, which helps maintain the liquid state of the fluid and reduces vapor bubble formation. This method not only combats cavitation but also optimizes the flow profile within the valve, enhancing performance and efficiency.

Flowserve Valtek's CavControl technology exemplifies applying advanced port modification techniques to combat cavitation. Cavcontrol utilizes a unique trim design that manages the pressure drop across the valve in a controlled manner, effectively mitigating the conditions that lead to cavitation. The technology incorporates a series of specially engineered notches or grooves in the valve's trim that create a series of pressure-reducing stages. As the fluid passes through these stages, the pressure decreases incrementally, preventing any sudden drop below the vapor pressure and thus avoiding the formation of vapor bubbles.

Furthermore, CavControl's design also focuses on energy dissipation. Controlling the flow path and dissipating the fluid's kinetic energy throughout the valve reduces the fluid's velocity and the impact of any vapor bubbles that may form, minimizing the potential for damage. This approach extends the valve's life and ensures smoother operation and improved control accuracy.

The challenge of flashing and cavitation in industrial control valves requires sophisticated engineering solutions. Port modification techniques, including multi-stage trim designs and controlled flow paths, effectively mitigate these issues. Flowserve Valtek's CavControl technology stands out in this field, demonstrating how advanced design and engineering can enhance the performance and reliability of control valves, ensuring their safe and efficient operation in industrial processes.

CTi Controltech
https://cti-ct.com
925-208-4250

Control Valve Application Notes

Using an incorrectly applied or sized control valve may have significant ramifications for operation, productivity, and, most importantly, safety. Here is a brief list of fundamentals to consider: 

Control valves are not isolation valves: 

Control valves do not isolate a process and do not offer a bubble-tight seal, and utilization in a shutoff capacity is unwise. 

Choose the suitable materials for the job: 

The valve body, seat, and wetted materials must all be compatible with the process under control. Before selecting a valve, evaluate the pressure ratings, operating temperatures, and material compatibility. 

Sensor placement: 

Place the flow sensor upstream of the control valve when configuring the control loop. When the flow sensor placement is downstream of the control valve, exposure to an unstable fluid (bubbles) created by the flashing and turbulence of the flow in the valve cavity is possible.

Control precision and mechanical constraints: 

Consider the degree of controllability you need and the inherent Deadband produced by your valve and associated components. Deadband is the built-in movement that occurs in a control valve between the signal change and the direction of the valve, which exacerbates by worn or poorly designed couplings between valve and actuator, mechanical sensor tolerances, friction in the valve stems and seats, or an undersized actuator. Due to opening/closing oscillations, too much deadband leads to poor controllability (hunting). 

Stiction: 

Stiction is the "stickiness" in valve action induced by packing gland, seat, or force against the disk friction. It may happen if the valve sticks in one position for a prolonged time or is constantly traveling in a minimal range for an extended period. The actuator must apply more force to break the disk free, resulting in overshoot and poor control. 

Tuning the loop controller and/or positioner: 

A poorly configured loop controller or positioner is often the source of poor control and loop instability. Advanced auto-tuning capabilities in PI (proportional with integral), PD (proportional with derivative), and PID (proportional with integral and derivative) controllers have replaced human (often trial and error) loop tuning. 

Valve sizing should be correct: 

Control valves are often oversized, permitting maximum flow at just a tiny percentage of total travel. Minor adjustments in valve position have a significant impact on flow. A high valve-position-to-flow ratio promotes continual "hunting," which leads to excessive valve wear. A decent rule of thumb is to size a control valve at around 70% to 90% of its travel. 

What sort of flow characteristics does your valve produce: 

The flow characteristic of a control valve is the connection between the position of the valve disk, gate, or globe and the change in flow rate through the valve under normal circumstances. A linear flow characteristic is desirable. However, different valve designs have varying flow characteristics, some of which are linear and others that are not. Globe control valves have linear flow properties, while butterfly and gate valves have non-linear flow characteristics. Manufacturers will often create specifically shaped disks or orifices to "characterize" the valve's flow to improve linearity.

The above is a brief list of the most common things to consider when applying control valves. There are many other criteria to consider. It is suggested in the strongest terms to consult with an experienced application expert before selecting or using a control valve.

Flowserve Valve, Actuation and Instrumentation Product Portfolio

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Flowserve offers a comprehensive range of quarter-turn, rotary, linear, control, and specialty configurations. Every valve manufactured is built with unwavering performance, long service life, and safe operation in mind. Flowserve also provides precision actuation and instrumentation for all applications, including remotely controlled, fully-automated electric, hydraulic, and pneumatic actuators, as well as electronic positioners and level switches. In most hazardous environments, fail-safe isolation, on-off modulation, and precision process control ensure efficient and reliable operation. Every intelligent flow control solution provides industry-leading performance and reliability, with embedded technologies that make them simple to use and maintain. You'll get end-to-end intelligence that maximizes your uptime by combining our control solutions with real-time system analysis, predictive maintenance, and remote service capabilities.

The Flowserve Valtek FlowTop GS Control Valve

The Valtek FlowTop GS control valve (types V746 and V748) improves the reliability and uptime of production processes while minimizing operating and maintenance costs. Designed for general service applications, the FlowTop GS globe valve is a competitively priced, fully integrated valve-actuator-instrumentation package for continuous process flow loop control throughout the plant.

For more infomration about Flowserve Valtek products in Northern California and Western Nevada, contact CTi Controltech. Call them at 925-208-4250 or visit their website at https://cti-ct.com.

Reversing the Fail Action and Air Action on the Flowserve Valtek VL Actuator

The Flowserve Valtek VL series spring cylinder linear actuators are powerful, compact, high-performance pneumatic actuators that provide solid throttling or on-off operation for automatic control valves. The positioner supplies air to both sides of the piston, providing stiff, precise movement and high-frequency response and quick stroke speeds.

The VL series is the standard set of actuators for Valtek control valves, providing precise control and reliable performance. Providing maximum thrust from a compact lightweight package, the VL cylinder has set the industry standard for two generations.

For more information about Flowserve Valtek products, contact CTi Controltech by calling 925-208-4250 of by visiting https://cti-ct.com.

The Flowserve Logix 3800 Digital Valve Positioner

The Logix 3800 positioner features powerful diagnostics that identify field problems and expedite corrective actions to ensure reduced return-to-operation times.

Compatible with linear and rotary valves and actuators, the robust design of the Logix 3800 delivers high reliability in tough environments, reduces inventory costs and contributes to a lower total cost of ownership, and installs with an innovative, one-button quick calibration feature.

Partial List of Features:

• Rugged, certified SIL 3 capable modular design delivers maximum reliability in the toughest environments. 
• Installation and operation ease from innovative quick calibration feature to simplify commissioning. 
• Broad application versatility with industry-leading communication technologies, including analog, HART, Foundation Fieldbus, 4-20 or discrete I/O signals.
• Intrinsically safe, non-incendive and explosion-proof design from -52°C to 185°C (-62°F to 365°F) ensures safe, reliable operation in hazardous conditions for increased plant and personnel safety.
• Compatible with a variety of valve and actuation configurations including: rotary or linear valves; double- or single-acting; air- to-open or air-to-close.

https://cti-ct.com
925-208-4250

High Performance Butterfly Valves for Challenging Process Fluid Conditions

High performance butterfly valves handle challenging
 media and process conditions.
Image courtesy Flowserve - Valtek Control Products

Industrial process control applications can present stringent and challenging performance requirements for the physical equipment and components that comprise the process chain. The valves employed in fluid based operations need to be resistant to the impact of extreme fluid conditions, requiring careful design and selection consideration to assure proper performance and safety levels are maintained in a predictable way.

Industrial valves intended for extreme applications are generally referred to as severe service or high performance valves. While there are plenty of published and accepted standards for industrial valves, one does not exist to precisely define what constitutes a severe service valve.

So, how do you know when to focus valve selection activities on severe service or high performance valves, as opposed to those rated for general purpose? There are a number of basic criteria that might point you in that direction:

• Extreme media or environmental temperature or pressure
• High pressure drop operation that may cause cavitation
• Rapid or extreme changes to inlet pressure
• Certain types or amounts of solids contained in the fluid
• Corrosive media

Certainly, any of these criteria might be found in an application serviceable by a general purpose valve, but their presence should be an indicator that a closer assessment of the fluid conditions and commensurate valve requirements is in order. The key element for a process stakeholder is to recognize when conditions are contemplated that can exceed the capabilities of a general purpose valve, leading to premature failure in control performance or catastrophic failure that produces an unsafe condition. Once the possibility of an extreme or challenging condition is identified, a careful analysis of the range of operating conditions will reveal the valve performance requirements.

There are numerous manufacturers of severe service or high performance valves, each with specialized product offerings focusing on a particular performance niche. Flowserve, under their Valtek brand, manufactures the Valdisk high performance butterfly valves ranging from NPS 2-52 and ASME class 150, 300, 600, 900, 1500 pressure ranges. The valve design is ideal for manual or automated actuation, installed with a manual hand gear, electric actuator, or a pneumatic actuator. Seats are available as soft or metal on this double offset butterfly valve that provides tight closure for bi-directional flow. Construction materials include carbon steel and stainless steel. A range of options and variants are available to customize the valve build to suit a replacement or new installation.

There is more information available about the Valtek high performance valve offering. Share and discuss your special requirements with a valve specialist. They have application experience and access to technical resources that can leverage your own process knowledge and experience into an effective solution.

Eccentric Rotary Plug Control Valve

The MaxFlo 4 eccentric rotary plug control valve
Image courtesy of Flowserve Valtek

There is an extensive array of valves from which to select for a process control operation. Each candidate valve is targeted by its designers for a range of fluid applications, fortified with construction materials and design features specifically suited for meeting the challenges of that application range.

Flowserve, under their Valtek brand, developed a control valve that combines a number of useful design features. The MaxFlo 4 is an eccentric rotary plug valve intended for fluid control operations. The valve has some attractive design features.

• There is no shaft extending through the flow path, leaving flow unobstructed when the valve is fully open (see the illustration in the document included below).
• Valve trim provides tight bi-directional shutoff.
• Metal or soft seat construction is available to accommodate a wide range of applications.
• A variant provides flange to flange dimensions that allow the MaxFlo 4 to drop in as a replacement for standard size globe valves.
• High Cv rating may enable use of a smaller valve, when compared to other designs.
• Precise position control is delivered by the shaft form and plug mounting.
• Double offset eccentric plug eliminates sliding of plug across sealing surfaces, reducing wear and required seal maintenance.

More detailed information is provided in the document provided below. There is a revealing cutaway illustration showing the mounting and movement path of the plug. Share your industrial process control valve requirements and challenges with a valve selection and automation specialist to get the best match of control valve to application.

Eccentric Rotary Plug Control Valve from CTi Controltech

Valtek Mark One Valve Build - Piece by Piece

Here is a video produced by Flowserve that gives an inside look at the Valtek Mark One globe control valve. The Mark One is suitable for gaseous and liquid service, and the animation illustrates the ease with which the valve can be disassembled in place for easy maintenance.

Contact a product specialist for more information and share your fluid process control challenges with them. Combining your process knowledge with their product application expertise will yield an effective solution.

Digital Valve Positioner Delivers Extended Functionality

Digital Valve Positioner
Flowserve

A valve positioner works in concert with an actuator to regulate the movement of an industrial process control valve in a predictable manner. Evolving from mechanical based designs, current state of the art positioners no longer employ mechanical means, such as cams, to establish positioner output. The latest models are software based and provide additional information and functionality to the user.

• Loop powered device
• Accommodates 4-20 mA input, as well as HART protocol for two way remote communications
• Suitable for use with single or double acting actuators, linear or rotary motion
• Certificates of approval for use in wide range of hazardous industrial environments internationally

Either of the input signal paths, analog or digital, can be utilized as the command source. When analog is selected as the input source, the 4-20 mA signal is considered the command signal. Selecting the digital source causes the level of the 4-20 mA input to be ignored and the signal sent via HART serves as the input command signal.

The software based units deliver enhanced functionality for in-field setup and calibration, as well remote monitoring and setup via HART. Multiple operating modes are accessible to the user to properly characterize valve position to desired curves, one of which can be customized. An analog feedback signal of the stem position is provided as an output.

There is more to learn. The document below goes into more depth about the features, specifications and options, with a good illustration of the user interface and screen shots taken from the SoftTools software suite that works with the digital positioner. Reach out to a valve control specialist with your requirements and challenges. The combination of your process knowledge and their product application expertise will produce effective solutions.

Digital valve positioner for industrial process control valves from CTi Controltech

Benefits of an Intelligent Control Valve System

Intelligent Control
Valve and Loop System

An intelligent control valve system, the Flowserve StarPac can stand alone or be integrated into a system, and is available for a variety of applications including oil and gas, chemical, paper, mining, industrial gases and power generation.

Advantages for using this type of device are the following:

Process Diagnostics – Evaluation of process equipment is possible with the StarPac intelligent control system. By generating an initial signature of the process and then identifying a standard process signature, a pump, compressor, heat exchanger, reactor, etc., can be discovered to be defective and in need of servicing. Process diagnostics can be per- formed with a personal computer or DCS connected to the unit via its serial data port.

Logging / Tuner Operation – The StarPac will take a 'snapshot' of process conditions at user-specified intervals (1 to 9999 seconds) and store them in its memory for later retrieval to aid in diagnosing process upsets. The personal computer operator interface will trend process variable, set point, and stem position to aid in tuning the control loop. This data is in a form that can be read by loop auto-tuning software to calculate the optimum PID constants.

Process Transmitters – The StarPac has sensors in the valve body to measure process conditions including upstream and downstream pressure, temperature, and flow. Flow is indicated both as the current value and a totalized amount. For gases, flow is reported in mass units, such as pounds per hour (since gas flow is always compensated for changes in pressure and temperature). Other sensors measure stem position and pressures in the top and bottom chambers of the cylinder actuator. All of these values are available over the serial link to a computer or DCS.

Connections to Operator Interface – The StarPac can be connected to an operator interface (personal computer, DCS console or hand station and recorder) through its analog, discrete and digital channels. The StarPac houses two analog inputs and one analog output. Up to six analog outputs are available using an external converter module. Two discrete inputs and two discrete outputs are also available at the StarPac. The user can define how these channels will be defined. Normally one analog input is a command signal, one discrete input sets the control mode, and the discrete outputs signal alarm conditions. The digital channel can be connected to computers and most DCSs through gateways or Modbus ports. All of the process and configuration information is available through the digital channels.

High Interchangeability – Since the Valtek Mark OneTM, MaxFloTM and ShearStreamTM control valves are the basis of the StarPac system, most valve parts are interchangeable with other Valtek valves, reducing parts inventory.

Local Display – Eight variables (P1, P2, temperature, gas flow, liquid flow, total flow, setpoint, stem position) can be displayed on a liquid crystal display in the electronics module. Each variable can be set to display continuously, or the StarPac can be set to scroll through the chosen variables at ten-second intervals. An error indicator will show if the StarPac system should fail any internal diagnostic or if the process is out of range.

Multiple Failure Modes – StarPac technology allows multiple failure modes to be set, including: power, air supply, command signal, or process failure. This results in greater reliability and consistency of the process during emergency shutdowns, protecting the process and people.

Valve Diagnostics – A maintenance feedback package allows StarPac intelligent systems to display the condition of the valve packing, seat, actuator, and O-rings with only a brief process interruption without removing the valve from the line. PC software displays the actuator or process signature for analysis. Optional valve diagnostic software can determine specific valve failures.

User Friendly Software – When the user is not connecting the StarPac unit to a DCS, StarTalk software can configure, operate, and diagnose the StarPac unit through a personal computer. The software has a graphical user interface, allowing most users to operate their StarPac systems with minimal training.

Environmental Considerations – Locating pressure and temperature sensors in the valve body eliminates several process connections. All pressure sensors have at least two seals to reduce the possibility of leaks to the atmosphere. Remote-mounted pressure sensors can be fitted with isola- tion and/or purge valves to aid maintenance of the sensors.

Check out this video for further explanation:

CTi Controltech
22 Beta Ct.
San Ramon, CA 94583
Phone: 925-208-4250
Fax: 925-208-4251
Email: contactus@cti-ct.com
www.cti-ct.com

Large Control Valves for Higher Pressures and Bubble-tight Shutoff

High performance
rotary control valve
(courtesy of Valtek)

High performance control valves, providing a bubble-tight seal along with a wide range of sizes and pressure classifications are used in many industrial applications. Examples are:

• Fire protection
• Gas service
• Condenser cooling
• Gas scrubbing
• Mine dewatering
• Pipeline service,
• Cooling water

This type of control valve should achieve bi-directional, bubble-tight shutoff while maintaining low breakout torque - at both high and low pressure drops.

Often, these valves are available in carbon steel, 316 stainless steel and other alloys and is available in ANSI Classes 150 through 2500, sizes 2 through 30-inch. 

Rotary disc, high performance control valves offer a double offset designed into the disc that lifts the disc out of the seat immediately upon actuation. This avoids wear on the seat and disc, reducing leakage and parts replacement. It also improves throttling by eliminating friction.

For more information, review the tech manual below:

Valtek Valdisk Control Valves from CTi Controltech

A Look Inside the Valtek Mark One Control Valve

This video provides a look inside the Valtek Mark One Control Valve and highlights its major components and unique design.

When reviewing industrial control valves, the Valtek Mark One globe control valve offers stands out because of it's excellent performance controlling liquids and gases, while at the same time allowing fast, easy and inexpensive maintenance.

The unique spring-cylinder actuated Mark One valve provides stiffness which contributes to the valve's high positioning accuracy, controlled high speed, repeatability, and response.

Control Valve Flow Characteristics

Valtek ControlValve

Flow characteristics, the relationship between flow coefficient and valve stroke, has been a subject of considerable debate. Many valve types, such as butterfly, eccentric disk and ball valves, have an inherent characteristic which cannot be changed (except with characterizable positioner cams). Flow characteristics of globe valves can be determined by the shape of the plug head.

Control Valve Flow
Characteristics

The three most common types of flow characteristics are quick opening, equal percentage and linear. The adjacent figure shows the ideal characteristic curve for each. These characteristics can be approximated by contouring the plug. However, inasmuch as there are body effects and other uncontrollable factors, plus the need for maximizing the flow capacity for a particular valve, the real curves often deviate considerably from these ideals. When a constant pressure drop is maintained across the valve, the characteristic of the valve alone controls the flow; this characteristic is referred to as the “inherent flow characteristic.” “Installed characteristics” include both the valve and pipeline effects. The difference can best be understood by examining an entire system.

Equal Percentage

Equal percentage is the characteristic most commonly used in process control. The change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made. While the flow characteristic of the valve itself may be equal percentage, most control loops will produce an installed characteristic approaching linear when the overall system pressure drop is large relative to that across the valve.

Linear

An inherently linear characteristic produces equal changes in flow per unit of valve stroke regardless of plug position. Linear plugs are used on those systems where the valve pressure drop is a major portion of the total system pressure drop.

Quick Open

Quick open plugs are used for on-off applications designed to produce maximum flow quickly.

This information provided courtesy of Flowserve Valtek. Further technical information can be found at this link.

Great Animated Movie Showing the Internals of a Control Valve

Here is a great video of the internal construction of a control valve (in this case its the Valtek MaxFlo 3 by Flowserve).

The animation is first class and reveals all the major internal parts of the valve and actuator. For anyone just learning about control valves, its well worth the 2 minutes to view.