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.
Because the disc of butterfly valves moves into the seat with a wiping motion, most butterfly valves are capable of handling fluids with solids in suspension and, depending on the robustness of the seating’s, also powders and granules. Butterfly valves are generally favored because they cost less than other valve designs, and are lighter weight so they need less support.
In high-performance butterfly valves, the shutoff may be provided by an interference-fit seat design or a line-energized seat design, where the pressure in the pipeline is used to increase the interference between the seat and disk edge. The advent of Elastomers, PTFE, Metal Seating’s etc has initiated the rapid development of tight shut-off butterfly valves in which these elements serves as the sealing element between the rim of the disc and the valve body.
Early use of butterfly valves focused on water applications but new designs and component materials have allowed them to be utilized in growing industrial fluid applications. Presently, butterfly valves can be found in almost every chemical plant handling a variety of diverse fluids.
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
Concentric Butterfly Valve
Concentric Butterfly Valve, also know as Zero Offset Butterfly Valve, is the most basic type of butterfly valve design. The stem passes through the center-line of the disc which is further placed in the center of the pipe bore and the seat is the inside diameter periphery of the valve body. The disc is in constant contact with the seat.
This zero-offset valve design is also called “Resilient Seated” or “Rubber Seated” because it relies on the flexibility of the seat rubber to efficiently seal the flow when closed. Concentric butterfly valves are commonly used for low pressure ranges. An advantage of concentric butterfly valve is that the media going through the valve will not contact the body, since the seat is constructed as a sleeve or cartridge inside of the body.
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.
- 1st offset : The axis of the shaft is behind the center-line of the sealing point of the disc to seat.
- 2nd 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.
Wafer Type Butterfly Valve
A wafer-style butterfly valve is the most economical version and it is sandwiched between two pipe flanges and the flange bolts surround the valve body. The pipe flanges are connected through long bolts that cross the entire valve body. The sealing between the valve and pipe flanges is accomplished by using a tightly fitted seal, such as an O-ring, gasket, precision machined, along with a flat valve face on the downstream and upstream sections of the valve. This type of connection is designed for sealing against bi-directional differential pressures and to prevent backward flow in systems designed for universal flow.
Lug Type Butterfly Valve
The lug-style butterfly valve has threaded lugs outside the valve body. Two sets of bolts connect pipe flanges to each side of the bolt inserts without nuts. This design enables the disconnection of one side without affecting the other for dead-end service. Lug-style butterfly valves used in dead end service generally have a lower pressure rating. The lug-style butterfly valves, unlike the wafer-style, carry the weight of the piping through the valve body.
Wafer and Lug butterfly valves have own advantages and disadvantages over each other.
- Wafer type butterfly valve is less expensive than a lug type butterfly valve.
- Wafer type butterfly valve do not transfer the weight of the piping system directly through the valve body. The lug-style butterfly valves, unlike the wafer-style, carry the weight of the piping through the valve body.
- Lug type butterfly valve allows dead-end service or removal of downstream piping. Lug type design is preferred if a valve may be required to remain in place in closed position on removal of the pipe on one side for a temporary operation, it must be flanged for bolting to a pipe flange on the other side.
Double Flange Butterfly Valve
The Double Flange Butterfly Valve have flanged end on both sides of valve. The valve is connected to piping flanges through bolts. The double flange butterfly valve body configuration ensures precise installation in piping and can also be used as an end to the piping line, if required. The unique feature of this valve is its adaptability for lined pipes due to negligible disc protrusion beyond the body laying length. This also ensures installation close to other pipe fittings.
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
- Butterfly valves are easy and fast to open and close.
- Butterfly valves are relatively inexpensive and cost effective.
- Butterfly valves have less space requirements for installation.
- Butterfly valves are generally associated with reduced maintenance because there are a minimal number of moving parts and there are no pockets to trap fluids.
- Butterfly valves are especially well suited for handling of slurries or liquids with large amounts of suspended solids.
- The disc of butterfly valve is always present within the flow, therefore a pressure drop is always induced in the flow, regardless of valve position.
- When fully open, the disc of butterfly valve creates little turbulence or resistance to flow.
- Throttling service is limited to low differential pressure.
- Cavitation and choked flow are two potential concerns.
- Disc movement is unguided and affected by flow turbulence.