Views: 6 Author: Site Editor Publish Time: 2023-01-29 Origin: Site
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.
Method of Control
The closure element of a butterfly valve consists of a metal circular disc or vane that pivots on an axis at right angles to the direction of flow in the pipe. When rotated on a shaft, the disc seals against seats in the valve body. The thin disc is always in the passageway but it offers little resistance to flow. These valves offer a rotary stem movement of 90 degrees or less, in a compact design. Unlike ball valves, butterfly valves do not have any pockets in which fluids may become trapped when the valve is closed. The valve operation time is short because the valving element is rotated a quarter turn to open or close the passageway.
butterfly valves can be used for on/off service or throttling. When a valve throttles or modulates the flow it is controlling the speed and capacity of media through the valve.
A butterfly valve for on/off services is usually line size and requires the lowest pressure drop available in the open position.
Control valves are an important part of a fluid handling system. Selecting a butterfly valve for this function requires more calculations and allow for system requirements. The user must be able to identify the maximum flow requirement, which is equivalent to the design flow, and maximum pressure drop allowed, which is provided by the consulting engineer and is usually three to five pounds maximum. This pressure drop should never exceed one half of the inlet pressure.
butterfly valve imageThe type of butterfly valve is noted by the design of the seat and disc.
Concentric designed valves have the center of rotation moved back from the centerline of the valve disc. The design relies on a frictional interference seal between the seal and seat which are designed conically and on center. It is best applied to soft seated valves.
Double eccentric designed valves are also known as high performance valves. The sealing plane of the disc is offset from the axis of rotation, this leads to uninterrupted circular sealing surface on the disc that makes it possible for a circular sealing element to be placed in the valve. The axis of rotation of the disc is laterally displaced from the center so it will move away from the seat in order to prevent jamming as the valve opens and closes. Double eccentric valves eliminate wear points around the disc at the top and bottom of the seat as well as extending the life of the valve's leak-free performance. Seats are available in metal or plastic. Metal seats are long lasting but do not provide as good a seal as soft plastic seats such as polytetrafluoroethylene (PTFE) and filled PTFE.
Based on body construction and connection design
A Wafer-style butterfly valves 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-style 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.
Flanged butterfly valves can be rotary, pneumatic, or hydraulic with applications in water distribution, chemical services, and ventilation, among others.
Butt-welded butterfly valves
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.