CTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
Butterfly valves are lighter, smaller and lighter than other types of control valves, making them the best choice for regulating flow in many applications.Traditionally, standard butterfly valves have been used in automatic on/off applications and they are perfect for this role.However, some engineers consider them unacceptable when it comes to regulating flow in a closed-loop system.
Butterfly valves use rotating discs to control flow through pipes.Discs typically operate 90 degrees, so they are sometimes referred to as angle rotary valves.Usually, they are used when economy is considered.When tight shutoff is required, butterfly valves with soft elastomeric seals and/or coated discs can be used to provide the required performance.High Performance Butterfly Valves (HPBVs) – or Double Offset Valves – are now the industry standard for butterfly control valves and are widely used for throttling control.They do well for applications with relatively constant pressure drops or slow process cycles.
The advantages of HPBV include a straight flow path, high capacity, and the ability to pass solid and viscous media with ease.They generally have the lowest installed cost of any valve type, especially in NPS 12 and larger sizes.Their cost advantage increases significantly compared to other types of valves in sizes over 12 inches.
They provide good shut-off performance over a wide temperature range and are available in different body designs including wafer, lug and double flanged.They are much lighter in weight and more compact than other types of valves.For example, a 12-inch ANSI Class 150 double-flanged segmented ball valve weighs 350 pounds and has a face-to-face dimension of 13.31 inches, while a 12-inch lug butterfly valve equivalent weighs only 200 pounds and has a 3-inch face-to-face dimension.
Butterfly valves do have some limitations that make them unsuitable for flow control in certain applications.These include limited pressure drop capability compared to globe ball valves that have a greater potential for cavitation or flash evaporation.
Because the large surface area of the disc acts like a lever, applying the dynamic force of the flowing medium to the drive shaft, standard butterfly valves are generally not used for high pressure applications.When they are, actuator size and selection becomes critical.
Butterfly control valves can sometimes be oversized, which can negatively impact process performance.This may be due to the use of line-sized valves, especially high-capacity butterfly valves.It can increase process variability in two ways.First, oversizing can give the valve too much gain, leaving less flexibility in adjusting the controller.Second, an oversized valve may operate more frequently at lower valve openings, and seal friction may be greater in a butterfly valve.Because an oversized valve produces a disproportionately large flow change for a given increment of valve travel, this phenomenon greatly exaggerates the process variability associated with friction-induced deadband.
Specifiers sometimes use butterfly valves to be economical or to fit a given line size regardless of their limitations.There is a trend to oversize butterfly valves to avoid pinching the pipes, which can lead to poor process control.
The biggest limitation is that the ideal throttle control range is not as wide as a ball valve or segmented ball valve.Butterfly valves typically do not perform well outside the control range of about 30% to 50% open.
Generally, the loop is easiest to control when the control loop operates in a linear fashion and the process gain is close to unity.Therefore, a process gain of 1.0 becomes a target for good loop control, with an acceptable range of 0.5 to 2.0 (a 4:1 range).
Performance is best when most of the loop gain comes from the controller.Note that in the gain curve of Figure 1, the process gain becomes quite high in the region below about 25% of valve travel.
Process gain defines the relationship between process output and input change.A stroke where the process gain remains between 0.5 and 2.0 is the optimum control range for the valve.When the process gain is not in the 0.5 to 2.0 range, poor dynamic performance and loop instability can occur.
Butterfly disc design has a significant effect on valve flow as the valve goes from closed to open.Discs with inherent equal percentage characteristics can better compensate for pressure drops that vary with flow.Equal percentage trims will provide linear mounting characteristics for varying pressure drop, which is ideal.The result is a more accurate, one-to-one variation between flow and valve travel.
Butterfly valves have recently introduced discs with inherent equal percentage flow characteristics.This provides an installation feature that allows installation process gain in the desired 0.5 to 2.0 range over wider strokes.This significantly improves throttle control, especially in the lower travel range.
This design provides good control, with an acceptable gain of 0.5 to 2.0, from approximately 11% open to 70%, a nearly three-fold increase in control range compared to a typical high performance butterfly valve (HPBV) of the same size.Therefore, equal percentages of disks provide an overall lower process variability.
Butterfly valves with inherent equal percentage characteristics, such as the Control-Disk valve, are ideal for processes requiring precise throttling control performance.They can be controlled closer to the target set point regardless of process disturbances, reducing process variability.
If the butterfly valve is not performing well, simply replacing it with a properly sized valve will solve the problem.For example, a paper company is using two oversized butterfly valves to control moisture removal from pulp.Both valves were operated with less than 20% travel, resulting in process variability of 3.5% and 8.0%, respectively.Most of their lifespan is spent in manual mode.
Two appropriately sized NPS 4 Fisher Control-Disk butterfly valves with digital valve controller were installed.The loop is now running in automatic mode with process variability from 3.5% to 1.6% for the first valve and 8% to 3.0% for the second valve without any special loop tuning.
Poor water pressure and flow control in the steel mill’s cooling system resulted in inconsistent end products.The nine installed HPBVs were not able to effectively control the water flow as required.
The plant wanted to install valves that would better control the process and needed to minimize installation costs.The plant will spend $10,000 to replace the piping for each valve to switch from HPBV to segmented ball valves.Instead, Emerson recommends using a Control-Disk butterfly valve that meets the face-to-face dimensions of current HPBVs.
A Control-Disk valve was tested side-by-side with one of the nine existing HPBVs and it performed to the specified requirements.The plant replaced the remaining eight HPBVs within a year, each equipped with a Control-Disk valve, eliminating the need to replace the $90,000 plumbing for the segmented ball valve, and the ball valve cost approximately 25% more than the butterfly valve.
Control-Disk valves provide precise control and help eliminate variability in the end product.The mill estimates the installation of nine Control-Disk valves will save about $1 million annually.
Compared to most other valve types, HPBVs with digital positioners have lower initial installation costs and, when properly sized, provide adequate control range.They have high capacity and minimal flow restrictions.Butterfly valves with inherent equal percentage trims offer the opportunity to extend the control range, similar to globe or ball valves, and take up only the space of the HPBV.
When selecting valves, especially HPBVs, make sure they are the correct size, otherwise they may be manually controlled by the control room.It is also important to consider valve style, inherent characteristics and valve size, which will provide the widest range of control for the application.
Mark Nymeyer is the global marketing communications manager for Emerson Automation Solutions, responsible for traffic control.
This is not a paywall.This is a free wall.We don’t want to get in the way of your purpose here, so it only takes a few seconds.
Post time: Jan-20-2022
