Subject: Butterfly valves are lighter, smaller and weigh less than other kinds of control valves—making them the best choice for regulating flow in many applications. This article looks at the advantages and limitations of high performance butterfly valves, and then describes developments making them more suitable for flow control.
Standard butterfly valves have traditionally been used for automated on/off applications, a role for which they are well suited. But when it comes to regulating flow in a closed-loop system, some engineers may consider them unacceptable. The article, Butterfly Valves Take Control in the June 2018 issue of Control Engineering Europe magazine, describes the advantages of high performance butterfly valves (HPBVs), and shows how they can be used for flow control:
Standard butterfly valves are typically used in services where economy is a consideration. When tight shutoff is needed, butterfly valves with soft elastomer seals and/or coated disks can be used to deliver the required performance. High performance butterfly valves—or double offset valves—are now the industry standard for butterfly control valves.
HPBVs are used in every process industry as control valves in one application or another. In power stations they are used in air extraction, while a pulp and paper mill often uses them for dryer steam and condensate return. You can find them all over refineries controlling regenerator inlet air, pressure swing adsorption or hydrogen recycle. HPBVs are widely used for throttling control, and do a good job for applications with a relatively constant pressure drop, and/or with slow process loops. Here’s why they’re used so frequently:
Advantages of HPBVs include a straight through flow path, very high capacity, and the ability to easily pass solids and viscous media. They generally have the lowest installed cost of any valve type, especially in NPS 12 and larger sizes. The cost advantage of HPBVs compared to other types of valves increases dramatically in sizes over 12 inches.
They also offer great shutoff performance over a wide range of temperatures, and are available in different body designs including wafer, lugged and double flanged. Finally, butterfly valves weigh much less than other types of valves and are more compact. For example, a 12-inch ANSI class 150, double flanged segmented ball valve weighs 350 lbs and has a 13.31-inch face to face dimension, while a 12-inch lugged butterfly valve lugged equivalent weighs only 200 lbs and has a 3-inch face to face.
But they have a few limitations:
Butterfly valves do have certain limitations that make them unsuitable for flow control in some applications. These include a limited pressure drop capability compared to globe ball valves with greater potential for cavitation or flashing. Because the large surface area of the disk acts like a lever, applying the dynamic forces of flowing media to the drive shaft, standard butterfly valves are generally not used in high pressure applications. When they are, actuator sizing and selection becomes crucial.
Another consideration when applying butterfly valves for flow control is process gain, which defines the relationship between changes in process output and input. The travel over which process gain stays between 0.5 to 2.0 is a valve’s optimal control range. When process gain is not within 0.5 to 2.0, poor dynamic performance and loop instability can occur.
Butterfly valve disk design has a significant effect on valve flow rate as the valve travels from closed to open. Recent developments solve these problems:
Butterfly valves recently became available with disks having an inherent equal percentage flow characteristic. This results in significantly improved throttling control, especially in the lower travel ranges. This newer design provides good control with acceptable gain of 0.5 to 2.0 from about 11 percent open to 70 percent, a control range improvement of nearly threefold when compared to a typical high-performance butterfly valve (HPBV) of the same size.
Butterfly valves with an inherent equal percentage characteristic, such as Fisher’s Control-Disk Valve, are ideal for processes requiring precise throttling control performance. They can control closer to the target set point regardless of process disturbances, which results in a reduction in process variability.
The article wraps up with two examples of how the Fisher Control-Disk valve solved flow control problems at a paper company and a steel mill, resulting in savings of $1 million per year:
HPBVs with digital positioners have a lower initial installed cost than most other valve types, and can provide an adequate control range when sized properly. They have high capacity and minimal flow restriction. Butterfly valves with inherent equal percentage trim offer an opportunity for an expanded control range, similar to that of a globe or ball valve and only take up the space of a HPBV.