Butterfly valves are fast acting (1/4 turn) rotary motion valves. This type of valve can be used to start, stop and regulate flow (throttle flow).
The butterfly valve has found widespread applications in many industries due to its low number of moving parts, low maintenance costs and simple design. This type of valve is particularly well suited to high flow, low pressure systems. As the butterfly valve has a large seating area, it is also able to seat well (seal tight) even when used within systems that have a large amount of suspended solids.
A butterfly valve’s main components consist of stem, body, sealing arrangement, upper bearing, lower bearing, bonnet, actuator, seat and disc.
The seat of the butterfly valve is very important as any damage to this area will lead to passing (leaking) of the valve; the seat is also sometimes referred to as the ‘valve liner’. The seat is often manufactured from elastomeric materials with similar properties to rubber, although it is also possible to use O-rings.
A gearbox can be installed between the actuator and stem so that larger butterfly valves can be operated without needing to apply significant torque, however, it should be noted that a butterfly valve with a gearbox requires a longer period of time to cycle.
The disc is bored throughout its entire length and the stem penetrates through the bored hole. The stem is supported using bearings and bushings placed into inserts within the valve body. It is possible to fix the stem directly to the disc using bolts, pins or rivets, but it is also possible to fit the stem within a square recess within the disc (this is referred to as the ‘float’ configuration as it allows the valve to self-align when seating).
The body design of the valve is either wafer or lug wafer type, both designs allow for easy installation of the valve. Compared to other types of valve, the butterfly valve body is considered ‘thin’ and occupies little space within the pipework.
The space between the stem and valve body is sealed using either O-rings or traditional fibrous packing material.
A locking pin or spring loaded locking handle is often used to prevent the valve changing position when left un-attended.
Flanges are installed on the suction and discharge side of the valve so that piping can be easily connected.
How Butterfly Valves Work
Butterfly valves utilise a circular shaped disc that pivots around the valve stem for opening and closing of the valve. The disc is approximately the same diameter of the pipe to which it is connected and can pivot horizontally or vertically depending upon the orientation of the stem.
When the disc is positioned perpendicular to the flow direction, the valve is in the fully closed position and there is no flow. When the disc is positioned parallel to the flow direction, the valve is in the fully open position and there is almost unrestricted flow.
A stream lined butterfly disc allows the flowing medium to pass over the disc without a large pressure drop or flow restriction occurring.
Butterfly valves are fast acting and the stem only needs to be rotated 90 degrees (1/4 of a turn) in order to change the valve from the fully open to fully closed position.
Unlike the gate and ball valve designs, the butterfly valve is well suited for flow regulation (throttling) and does not create significant turbulent flow when partially opened/closed.
Butterfly valves are similar in design to ball valves, but are more suitable for larger applications as they are generally cheaper and weigh less.
Butterfly valves have a low pressure drop when fully open, but not as low as gate and ball valves.
Butterfly valves are generally not maintained in situ; they are replaced entirely then overhauled in a workshop. Other types of valves can be serviced in situ if necessary.
It is not possible to insert pigs (devices which clean and inspect the internal surface of piping) through butterfly valves.
3D Model Details
This 3D model shows all major components associated with a typical butterfly valve, these include:
- Nuts and Bolts
- Indicator Dial and Handle Locking Plate