Butterfly valve

Butterfly valve introduction

A Butterfly valve is a quarter-turn rotational motion valve, that is used to stop, regulate, and start flow.Butterfly valve is easy and fast to open. A 90° rotation of the handle provides a complete closure or opening of the valve. Large Butterfly valves are usually equipped with a so-called gearbox, where the handwheel by gears is connected to the stem. This simplifies the operation of the valve, but at the expense of speed.
A butterfly valve is a mechanism that regulates the flow of materials within large pipe diameters by the quarter-turn rotation of a disc.
A rod feeds through the center of a disc and this rod is connected to an actuator that controls the motion of the disc. The disc position is either parallel or perpendicular to the flow of media. A butterfly valve differs from other valves because the disc is always in the flow causing a pressure change for any position of the valve.

Butterfly valve working principle

Butterfly valve types

Concentric 

This is the most basic kind of butterfly valve. Sometimes called the centric valve or the zero offset butterfly valve, the stem passes through the centerline of the disc and through the center of the pipe. Also known as the resilient-seated butterfly valve, sealing depends on how efficient the seat is during closing. This kind of butterfly valve is used for low-pressure applications.
 

Eccentric

The stem of the eccentric type does not pass through the centerline of the valve. Rather, it is placed behind the face of the disc which is opposite of the flow direction. 
1 Single Offset
The single offset only has the stem attached at the back of the disc. This enables lesser disc-to-seal contact before full closure. This is to ensure a better and longer life service of the disc.
2 Double Offset
A double offset type has the stem located at the back of the disc. There is an offset present on the other side of the disc. This further limits the contact of the disc and the seat to about 1-3 degrees. The double offset butterfly valve is suitable for high-pressure applications that need a bubble-tight sealing.
3 Triple Offset
The triple offset butterfly valve has an additional offset located at the axis of the disc and seat, apart from the ones already mentioned in the double offset design. This creates a valve with a slightly tilting disc and seat. TOVs are often used for highly critical applications. It often has a metal-to-metal seat design.
 

Butterfly valve end connection

Flanged type :This type is the common end for the butterfly valve.
Wafer type  :This type is attached between two flanges using bolts that lock the entire valve body
Lug type :This type has lugs outside of the body. Bolts connect the valve to the pipe system. Commonly with low-pressure ratings, this is usually attached to dead-end services.

Butterfly valve actuation

Hand Lever: Is common for small butterfly valves and is capable of being locked into a position of open, partially open, or closed.
Gear: These are designed for slightly larger butterfly valve and utilize a gearbox to increase torque at the expense of decreased speed of opening/closing. Gear operated butterly valves are also self-locking (cannot be back driven) and can be equipped with position indicators.
Electric: Use an electric motor to turn the butterfly valve stem
Pneumatic: Requires compressed air to move a piston or diaphragm to open/close the butterfly valve.
Hydraulic: Requires a hydraulic pressure to move a piston or diaphragm to open/close the butterfly valve.