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.

High Pressure Valves for Industrial Processes and Operations

Industrial operations present substantial
challenges to engineers and equipment

I am convinced that there is a valve out there for every conceivable application. Of course, that is not literally true, but there is an enormous array of manufacturers producing countless valve variants to meet specific requirements of the many industrial fluid processing applications.

A valve installed in a fluid process needs not only to perform its intended control function, but to stand up to the impact of several physical challenges.

Cutaway view of high pressure angle valve
Courtesy Flowserve - Kammer

• Temperature
• Pressure
• Corrosion

Any combination of these factors in the extreme can call for the use of a severe service valve. A good match between the valve ratings or capabilities and the demands imposed by the process conditions is essential for achieving safe operation and a reasonable useful valve lifespan.

Valves designed to handle very high pressure will exhibit specific attributes designed to accommodate the imposed physical stress. Body construction, assembly hardware, seats, and trim will all be noticeably heavier, stronger.

Rely on a valve specialist to contribute product expertise to the valve selection process. Combine your own process knowledge and experience with their product application expertise to develop an effective solution.

High Pressure Control Valves - Kammer from CTi Controltech

Preventing Cavitation in Industrial Process Control Valves

Example of multi-stage valve trim
designed to eliminate cavitation
Courtesy Flowserve - Kammer

In process control valves, cavitation results from a rapid drop in pressure as liquid passes through the valve. It results in the formation of vapor spaces or bubbles within the valve cavity. When the bubbles move downstream into a larger cross-sectional area, velocity decreases and pressure increases. The higher pressure now surrounding the bubbles causes them to implode, producing shockwaves which propagate through the liquid. These shockwaves can cause metal fatigue and excessive wear on the internals of the valve. The collapsing bubbles also make a discernible sound with accompanying vibration. The cumulative effects of cavitation can cause rapid deterioration of a valve, resulting in reduced control function, frequent need for service, or premature failure.

There are ways to mitigate cavitation. Some involve changes in the process, others, incorporating a properly designed and selected valve with trim that reduces or prevents the conditions that cause cavitation. The paper below, authored by Flowserve, provides an in depth examination of the causes of cavitation, then continues with explanation of how their specialty valves are designed to overcome the conditions that promote it.

There are detailed illustrations showing the specific valve trim features that impede cavitation. Share your process control valve challenges with application experts, combining your process knowledge with their product application expertise to develop effective solutions.

Controlling Cavitation in Industrial Control Valves from CTi Controltech

Severe Service Valves - More Than Just Heavy Duty

Severe Service Industrial Control Valves
Courtesy Flowserve - Kammer

Industrial process control applications can be associated with some very stringent and challenging performance requirements for the physical equipment and components that are part of the process chain. In fluid based operations, the control valves can be a point of significant 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 that are intended for application at the extremes are generally referred to as severe service valves. While there are plenty of published and accepted standards for industrial valves, one does not exist to precisely define a severe service valve.

So, how do you know when to focus valve selection activities on severe service valves, as opposed to general purpose valves?

There are a number of basic criteria that might point you in that direction:

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

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 in evidence that might overrun 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 a severe service condition is identified, a careful analysis of the possible operating conditions will reveal the performance requirements for the valve.

There are numerous manufacturers of severe service valves, each seeming to concentrate on a particular niche. Kammer, a brand in the Flowserve family, has an interesting approach to accommodating the highly varied requirements of severe service applications. Their product line consists of a number of standard configurations, but also includes semi-modular components that can be combined to meet the many custom requirements of severe service applications.

I have included a technical bulletin (you know it's technical when it's not in color) that describes and nicely illustrates how the company employs various interchangeable body styles and packings with their multi-stage plug to meet a wide range of severe service challenges. Browse the bulletin, as it has some good cutaway views of valve interiors. 

You can always get more information, or discuss your special requirements, with a product application specialist. They have access to technical resources that can help with selecting the right valve components to meet your severe service applications.

Kammer Severe Service Valves For Industry from CTi Controltech