Types Of Butterfly Valves Pdf Free
Christal Rasband
Butterfly valves are essential in applications where tight shut-off is required. The disc opens and closes with a low-torque rotation of 90 degrees and works for any compatible application. They tend to cost less and are lighter weight, so they are preferred over other types of valves. Butterfly valves are often distinguished by the three main types: zero, double, or triple offset.
These valves are designed to solve the tightening problems that can occur between the disc and seat of the butterfly valve. The stem and disc of the valve are situated in the center, and it creates a seal when the disc deforms the soft seat, causing full friction during the whole operating cycle. This valve can handle chemicals, coatings, food, some solids, and abrasive applications. One advantage is that most of the media flowing through the valve will not contact the body since the seat of the valve covers the body.
The double-offset structure features the stem center deviated from the center of the disc and valve. With this structure, the valve disc can leave the valve seat quickly, greatly reducing unnecessary over-compression and scraping between them. This structure also helps decrease the wearing process and prolongs the operational life of the valve. The reduction in scraping also allows the seats of these valves to be made from metal, expanding the application range of butterfly valves to high-temperature applications.
The triple offset design eliminates the rubbing between the seat and seal ring through the flow path, reducing seat and seal wear and extending cycle life. They are used in applications similar to gate valves, where a metal seat is required, and tight shutoff and/or quarter-turn actuation is desired, but triple offset butterfly valves can open and close more quickly and can be frequently operated, even if there is an emergency shutoff. This valve has low torque and is recommended for both high and low-temperature applications.
Butterfly valves are among the most popular industrial quarter-turn valves that control different fluid media flow-through systems. They come in different designs, offering varied advantages, such as the quarter-turn control, small dimensions for use in areas with limited space, and excellent control capabilities. They are also light, cheaper, and allow for a high flow coefficient, making them useful in many applications. However, their designs make them difficult to clean and prone to cavitation and choke. Throttling is limited to low differential pressure. This article discusses the various design options available for butterfly valves.
A butterfly valve is a quarter-turn valve that works similarly to a ball valve. As the name suggests, the butterfly is a disk connected to a rod or spindle that opens and closes in a way reminiscent of a butterfly's flapping wings. The butterfly valve closes when the rod rotates the disc 90 to a position perpendicular to the flow direction, thus restricting flow. It opens when the disc rotates back to allow the flow of media. Butterfly valves are used across many industries for throttling, on-off, or modulating services to control the flow of media (gas, liquid, or slurries) through industrial systems. They allow fluid flow in only one direction. Read our article on butterfly valves for more information.
Butterfly valves have different designs, each serving specific applications and pressure ranges. They are classified in different ways, including their disc closure design, function, connection design, control or actuation method, actuator speed, and seat material.
Concentric butterfly valves are suitable for low-pressure applications. A concentric butterfly valve is the most basic type. The spindle passes through the centerline of the disc, located at the center of the pipe bore. The valve seat sits in the inside periphery of the valve body. The disc contacts the seat at around 85 during a 90 rotation. This design is known as resilient-seated because it relies on the rubber seat to efficiently seal the flow when closed.
A wafer-style butterfly valve presents the most economical design. It is sandwiched between two pipe flanges to allow flow and sealing against bidirectional differential pressures and backflow in universal flow systems. It's light, offering a lower initial and installation cost.
These valves may or may not have flange holes outside their body (Figure 3, left). The pipe flanges are connected to piping using long bolts that run through the entire valve body. Gaskets, o-rings, and flat valve faces achieve the sealing between the valve and pipe flanges.
Flanged butterfly valves use a disc or vane centrally mounted on a shaft, stem, or flange, which acts as the axis of the rotating disc. These valves have two flanges on either end (Figure 3, middle), giving them a larger face-to-face than lugged or wafer valves. The position and direction of the disc relative to the flanges determine the flow direction.
Butt-welded butterfly valves are double- or triple-offset that are resistant to extreme temperatures, pressure, and corrossion. This makes them suitable for use in heating systems, natural gas, energy, chemical, refinery, and chemical services.
These valve types have threaded inserts (lugs) outside the valve (Figure 3, right). They have protruding legs providing bolt holes matching those in the pipe and flanges, and the valve is installed between two flanges using a set of bolts for each flange. The installation allows for dead-end service or eliminating downstream piping without affecting the other side. A comparison between lug-style and wafer-style butterfly valves is in the table below.
The seat is what the disc seals against to allow or shut off flow. Butterfly valves have soft or metal seat materials, depending on application and cost. Inexpensive valves might have irreplaceable or unrepairable seats molded into the body. These are usually metal. Most precision valves, on the other hand, have soft seats. These seats are not permanently affixed to the body and are repairable and replaceable.
These valves have both sides of the seal made of a metal material such as steel, copper, graphite, combinations, alloys, or any other hard material. Although these seals are resistant to high temperatures, abrasion, and corrosion, they have relatively poor sealing performances.
One side of soft seated butterfly valves is metal material like steel. The other has a soft, elastic, non-metallic material such as rubber or tetrafluoroethylene, providing excellent sealing. However, this seal isn't resistant to high temperatures, wears quickly, and has poor mechanical properties.
The butterfly valve actuator operates the stem and disc, controlling the opening and closing of the valve. Actuation can be manual (lever or hand-operated) or automatic (electric, pneumatic, or hydraulic).
Manual butterfly valve actuators are operated manually with a hand-lever or a gear to open or close the valve. They're cheap and are widely applicable in remote systems that may lack access to power. However, gear-operated valves are self-locking and are used for slightly larger butterfly valves. A butterfly valve with a hand lever can be seen in Figure 4 labeled A.
These are power-operated actuators that enable remote control of butterfly valves. They're more expensive than manual actuators and enable the rapid operation of larger valves. Automatic actuators can also be designed to fail-open or fail-close to keep the valve closed or open in case of actuator failure.
Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves are generally favored because they cost less than other valve designs, and are lighter weight so they need less support. The disc is positioned in the center of the pipe. A rod passes through the disc to an actuator on the outside of the valve. Rotating the actuator turns the disc either parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present within the flow, so it induces a pressure drop, even when open.
A butterfly valve is from a family of valves called quarter-turn valves.[1] In operation, the valve is fully open or closed when the disc is rotated a quarter turn. The "butterfly" is a metal disc mounted on a rod. When the valve is closed, the disc is turned so that it completely blocks off the passageway. When the valve is fully open, the disc is rotated a quarter turn so that it allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow.
There are different kinds of butterfly valves, each adapted for different pressures and different usage. The zero-offset butterfly valve, which uses the flexibility of rubber, has the lowest pressure rating. The high-performance double offset butterfly valve, used in slightly higher-pressure systems, is offset from the center line of the disc seat and body seal (offset one), and the center line of the bore (offset two). This creates a cam action during operation to lift the seat out of the seal resulting in less friction than is created in the zero offset design and decreases its tendency to wear. The valve best suited for high-pressure systems is the triple offset butterfly valve. In this valve, the disc seat contact axis is offset, which acts to virtually eliminate sliding contact between disc and seat. In the case of triple offset valves the seat is made of metal so that it can be machined such as to achieve a bubble-tight shut-off when in contact with the disc.
The wafer style butterfly valve is designed to maintain a seal against bi-directional pressure differential to prevent any backflow in systems designed for unidirectional flow. It accomplishes this with a tightly fitting seal; i.e., gasket, o-ring, precision machined, and a flat valve face on the upstream and downstream sides of the valve.the drawback is that wafer butterfly valves only have a small flow control range. The pressure drop across wafer butterfly valves may be greater. Wafer butterfly valves are prone to clogging due to their design.[2]
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