Introduction to Butterfly Valves

Butterfly Valves are quarter-turn rotary valves in which a disc-shaped closure member is rotated through 90 degree or less to open or close or regulate the flow passage. Butterfly valves may be used for flow in both directions. Unlike a ball valve, the disc is always present within the flow, therefore a pressure drop is always induced in the flow, regardless of valve position.

Butterfly Valve Working Principle

A Butterfly Valve is from a family of valves called Quarter-Turn Valves. Butterfly valves have a relatively simple construction. The main components of a butterfly valve are the body, disc, stem and seat. 

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. The disc is positioned in the center of the pipe. A rod passes through the disc to the outside of the valve. Rotating the rod, through hand-wheel or actuator, turns the disc either parallel or perpendicular to the flow. When the disc plane is at right angle or perpendicular to the center line of pipe, butterfly valve is closed. Then when the disc is rotated a quarter turn to make the plane of disc in line with the center line of pipe, butterfly valve is fully open and it allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow.

Butterfly valves should normally be mounted with the stem horizontal since this allows debris in the pipe to be swept clear as the valve is closed. Where the stem is vertical solids can lodge under the disc at the spindle and cause damage to the seal. Furthermore, when opening the valve, the bottom of the disc should lift away from solids that may have accumulated on the upstream side of the disc. It’s important to note that pipelines that contain butterfly valves can’t be ‘pigged’ for cleaning.

Types of Butterfly Valves

Efforts to adapt butterfly valves to wider temperature and pressure ranges have led to the development of a family of butterfly valves that may be fitted with seating’s of a variety of construction materials to meet the operational requirements. Such seating’s may be metal-to-polymer or metal-to-metal, and may be designed to satisfy the requirements of fire-tested valves. There are different kinds of butterfly valves, each adapted for different pressures and different usage.

• As per Disc Closure Design
  • Concentric Butterfly Valve
  • Eccentric Butterfly Valve
• As per Piping Connection Design
  • Wafer Type Butterfly Valve
  • Lug Type Butterfly Valve
  • Double Flange Butterfly Valve

Eccentric Butterfly Valve

In Eccentric Butterfly Valves, also known as High Performance Butterfly Valves, the stem does not pass through the center-line of the disc, but instead located offset from the center according to the categories of double and triple offset valves. 

When the stem is located right behind the center-line of the disc, the valve is called Single-Offset Butterfly Valve. 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. Single-offset butterfly valves have given way to double offset and triple offset butterfly valves.

In a Double-Offset 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.

• 1^st offset : The axis of the shaft is behind the center-line of the sealing point of the disc to seat.
• 2^nd offset : The axis of the shaft is eccentric to the center of the valve/pipe line.

A Triple-Offset Butterfly Valve (TOV or TOBV) is often used in critical applications and is designed similar to a double offset butterfly valve with one more offset. 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 allowing the disc to seal against the seat with no friction. A triple offset butterfly valve is more efficient and allows for less wear. Triple offset butterfly 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.

The preferred body configuration for butterfly valves is the wafer, which is clamped between two pipeline flanges. An important advantage of this construction is that the bolts pulling the mating flanges together carry all the tensile stresses induced by the line strains and put the wafer in compression. This compressive stress is eased by the tensile stresses imposed by the internal fluid pressure. Flanged bodies, on the other hand, have to carry all the tensile stresses imposed by the line strains, and the tensile stresses from the line pressure are cumulative. This fact, together with the ability of most metals to handle compressive loads of up to twice their limit for tensile loads, strongly recommends the use of the wafer body.

Advantages and Disadvantages