How does a butterfly valve work?

Butterfly valve is a quarter-turn valve,the disc connected to a rod. It closes when the rod rotates the disc by a quarter turn to a position perpendicular to the flow direction. When the valve opens, the disc is rotated back to allow the flow .

Butterfly valves are used for on-off or modulating services and are popular due to their light weight, small installation footprint, lower costs, quick operation and availability in very large sizes. These valves can be operated by handles, gears or automatic actuators.
 

Butterfly valve Principle of Operation

Types of Butterfly Valve

Butterfly valve can be concentric or eccentric depending on the location of the stem in relation to the disc and the seat surface angle on which the disc closes.
 

Concentric design

The most basic type of butterfly valve design is a centric or concentric butterfly valve. This means that the stem passes through the centerline of the disc which is in the center of the pipe bore and the seat is the inside diameter periphery of the valve body. This zero-offset valve design is also called resilient-seated because it relies on the flexibility of the seat rubber to efficiently seal the flow when closed. In this type of valve, the disc first comes into contact with the seat at around 85° during a 90° rotation. Concentric butterfly valve is commonly used for low pressure ranges.


Eccentric design

An eccentric butterfly valve means that the stem does not pass through the centerline of the disc, but instead behind it (opposite of flow direction) as seen in Figure 2 on the right. When the stem is located right behind the centerline of the disc, the valve is called single-offset. This design was developed to reduce the disc contact with the seal before full closure of the valve with the aim of improving service life of the valve. Today, single-offset valves have given way to double offset and triple offset butterfly valves.

In a double-offset or doubly eccentric butterfly valve, the stem is located behind the disc with an additional offset to one side. This double eccentricity of the stem enables the rotating disc to rub over the seat for only about one to three degrees.

A triple offset butterfly valve (TOV or TOBV) is often used in critical applications and is designed similar to a double offset butterfly valve. The third offset is the disc-seat contact axis. The seat surface takes a conical shape which coupled with the same shape at the ridge of the disc, results in minimal contact before full closure of the valve. A triple offset butterfly valve is more efficient and allows for less wear. Triple offset valves are often made of metal seats to create a bubble-tight shut-off. The metal seats allow butterfly valves to be used in higher temperature ranges.

High performance butterfly valve designs use the pressure in the pipeline to increase the interference between the seat and the disc edge. These butterfly valves have higher pressure ratings and are prone to less wear.