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Butterfly valves are lighter, smaller, and lighter than other types of control valves, making them the best choice for regulating flow in many applications. Standard butterfly valves are traditionally used for automatic opening/closing applications, and they are well suited for this role. However, when it comes to regulating flow in a closed-loop system, some engineers consider them unacceptable.
The butterfly valve uses a rotating disc to control the flow through the pipe. Discs can usually be operated through 90 degrees, so they are sometimes referred to as quarter-turn valves. Usually, they are used when considering economy. When tight closing is required, butterfly valves with soft elastic seals and/or coated discs can be used to provide the required performance. The high performance butterfly valve (HPBV)-or double offset valve-is now the industry standard for butterfly control valves and is widely used for throttling control. They perform well for applications with relatively constant pressure drop or slow process loops.
The advantages of HPBV include straight-through flow path, high capacity, and the ability to easily pass solid and viscous media. Their installation cost is usually the lowest of all valve types, especially NPS 12 and larger valves. Compared with other types of valves, their cost advantage increases significantly when the size exceeds 12 inches.
They can provide good closing performance in a wide temperature range, and provide different valve body designs, including wafer type, lug type and double flange. They are much lighter than other types of valves and are more compact. For example, a 12-inch ANSI Class 150 double flange segmented ball valve weighs 350 pounds and has a face-to-face dimension of 13.31 inches, while the equivalent 12-inch lug butterfly valve weighs only 200 pounds and a face-to-face dimension of 3 inches.
Butterfly valves do have some limitations that make them unsuitable for flow control in certain applications. These include limited pressure drop capabilities compared to ball valves, with greater cavitation or flashing potential.
Because the large surface area of the disc acts as a lever to apply the dynamic force of the flowing medium to the drive shaft, standard butterfly valves are usually not used for high-pressure applications. If so, the size and selection of the actuator becomes critical.
Sometimes it happens that the butterfly control valve is oversized, which will have a negative impact on process performance. This may be due to the use of pipeline-sized valves, especially large-capacity butterfly valves. It can increase process variability in two ways. First of all, oversize brings too much gain to the valve, and thus lacks flexibility in adjusting the controller. Secondly, oversized valves may operate more frequently at lower valve openings, while the sealing friction of butterfly valves may be greater. Because for a given valve stroke increment, an excessively large valve will produce disproportionately large flow changes. This phenomenon will greatly exaggerate the process variability associated with the dead zone due to friction.
Code-setters sometimes use butterfly valves for economic reasons or to adapt to a given pipeline size, regardless of their limitations. There is a trend to oversize the butterfly valve to avoid squeezing the pipe, which can lead to poor process control.
The biggest limitation is that the ideal throttling control range is not as wide as a stop valve or a segmented ball valve. Butterfly valves generally do not perform well outside the opening control range of approximately 30% to 50%.
Generally, when the control loop is operating in a linear manner and the process gain is close to 1, the loop is easiest to control. Therefore, a process gain of 1.0 becomes the target for good loop control, and the acceptable range is 0.5 to 2.0 (range 4:1).
The 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 the valve stroke.
Process gain defines the relationship between process output and input changes. The stroke where the process gain is maintained between 0.5 and 2.0 is the optimal control range of the valve. When the process gain is not in the range of 0.5 to 2.0, poor dynamic performance and loop instability may occur.
When the valve is closed to open, the butterfly valve disc design has a significant impact on the valve flow. The disc with inherent equal percentage characteristics can better compensate the pressure drop that changes with the flow rate. Equal percentage internals will provide linear installation characteristics to change the pressure drop, which is ideal. The result is a more accurate one-to-one change between flow rate and valve stroke.
Recently, butterfly valves can use discs with inherent equal percentage flow characteristics. This provides an installation feature that results in the installation process gain within the required range of 0.5 to 2.0 over a wider range of travel. This will significantly improve throttling control, especially in the lower range of travel.
This design provides good control, with an acceptable gain of 0.5 to 2.0 from approximately 11% opening to 70%, and the control range is nearly three times higher than that of a typical high-performance butterfly valve (HPBV) of the same size. Therefore, equal percentage discs provide overall lower process variability.
Butterfly valves with inherent equal percentage characteristics, such as control disc valves, are ideal for processes that require precise throttling control performance. Regardless of process disturbances, they can be controlled closer to the target set point, thereby reducing process variability.
If the butterfly valve does not work well, just replace the valve with the right size to solve the problem. For example, a paper company uses two oversized butterfly valves to control the removal of moisture from the pulp. The two valves operate at less than 20% of the stroke, resulting in process variation of 3.5% and 8.0%, respectively. Most of their service life is spent in manual mode.
Two appropriately sized NPS 4 Fisher Control-Disk butterfly valves with digital valve controllers were installed. The loop is now running in automatic mode, the process variability of the first valve has increased from 3.5% to 1.6%, and the process variability of the second valve has increased from 8% to 3.0%, without any special loop adjustments.
The poor water pressure and flow control of the cooling system in the steel plant led to inconsistencies in the final product. The HPBV installed in Jiutai could not effectively control the water flow as required.
The plant hopes to install valves that can better control the process and needs to minimize installation costs. The factory will spend $10,000 to replace the pipes of each valve to switch from HPBV to segmented ball valves. Instead, Emerson recommends that its Control-Disk butterfly valve fits the current HPBV face-to-face size.
A Control-Disk valve was tested together with one of nine existing HPBVs, and its performance met the specified requirements. The factory replaced the remaining 8 HPBVs within a year, and each HPBV was equipped with a Control-Disk valve. This eliminated the need to replace $90,000 pipelines for segmented ball valves, and the cost of ball valves increased by approximately 25% compared to butterfly valves.
Control-Disk valves provide precise control and help eliminate variability in the final product. The steel plant estimates that installing nine Control-Disk valves can save approximately US$1 million per year.
Compared with most other valve types, HPBV with a digital positioner has a lower initial installation cost and can provide adequate control range when the size is right. They have high capacity and minimal flow restrictions. The butterfly valve with inherent equal percentage of internal parts provides an opportunity to expand the control range, similar to a globe valve or a ball valve, and only takes up the space of the HPBV.
When selecting valves, especially HPBV, make sure they are the correct size, otherwise they may be manually controlled by the control room. It is also important to consider the valve type, inherent characteristics, and valve size that provide the widest control range for the application.
Mark Nymeyer is the global marketing communications manager for flow control at Emerson Automation Solutions.
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Post time: Oct-11-2021
