This article takes a detailed look at butterfly valves.
Read further and learn more about:
- What is a butterfly valve?
- How does it work?
- Components of a butterfly valve
- Types of butterfly valves
- Materials for the construction of butterfly valves
- Advantages and disadvantages of butterfly valves
- And much more…
Chapter 1: What is a Butterfly Valve?
A butterfly valve is a quarter-turn rotational motion device that utilizes a rotary disc to allow, obstruct, or control the flow of fluids in a piping system. It features a rotating disc that is situated on the passageway of the flowing media. The disc is rotated and controlled by an external actuating mechanism through the stem attached to it. When the disc is coplanar to the flow cross-sectional area, the flow is fully obstructed. Otherwise, the fluid is fully or partially allowed to pass through the butterfly valve. It takes a 900 turn to fully open a butterfly valve from a closed position, which means the disc should lie perpendicular to the flow cross-sectional area.
Butterfly valves are quarter-turn valves like ball valves and plug valves. They have a fairly simple construction and operation mechanism, and they have a compact size designed to fit two pipe flanges.
They can be operated manually or by an automatic actuating mechanism that is integrated into the process control system of the pipeline. They are ideal for on-and-off applications, but their applications to flow throttling are limited.
There are several types and designs of butterfly valves available, rated in varying temperatures, pressures, and flow rates to suit the needs of pipeline systems handling liquids and gases.
Chapter 2: Components of a Butterfly Valve
The main components of a butterfly valve are the following:
Made from a tough and rigid material, the valve body houses and protects the disc and other internal components of the butterfly valve. It links the valve to the piping system and to the external operating mechanism that controls the disc.
The disc is the main feature of butterfly valves that permits, regulates, and stops the flow of the fluid in the pipeline. Flow is controlled by the rotary motion of the disc. The discharge flow rate depends on the degree of disc opening. When the disc is perpendicular to the flow’s cross-sectional area, the fluid is fully obstructed from flowing out of the valve. Otherwise, the fluid is permitted to flow through the space between the seat and the disc. It takes a 900-rotation from a closed position of the disc to allow full opening of the valve and vice versa. Flow is throttled when the disc is rotated less than 900.
The butterfly valve disc is analogous to the ball for ball valves and the plug for plug valves.
The stem is a shaft that connects the disc to the external operating mechanism. It is sealed by O-rings and bushings to prevent fluid leakage. The stem can be made from a one-piece shaft or two-piece (split-stem) shaft. The placement of the stem axis and its connection to the disc depends on the type of butterfly valve.
The valve seat is a ring that provides sealing between the disc edge and the valve body when it is in a closed position. The sealing action is necessary to avoid leakage of any fluid to the discharge of the butterfly valve. Since the disc slides on the surface of the seat during valve opening, it must be made from a material with a low coefficient of friction.
The butterfly valve seat can be made a soft seat or a metal seat. The material of the seat limits the temperature and pressure rating of the butterfly valve. Soft seats, which are made from plastic and elastomeric materials, are limited to lower temperatures because they deform at elevated temperatures.
The external operating mechanism of a butterfly valve controls the fluid flow across the valve. It may be operated by manual rotation of the stem or by automatic actuation.
Manual operation of butterfly valves involves the application of torque to the lever or handwheel attached to the stem. Levers can set the valve into a closed, fully-opened, or partially-opened position. Larger butterfly valves are equipped with handwheels and gearboxes to increase torque and to aid in the opening and closing of the valve.
Automatic actuation may be used to control the butterfly valve situated in harsh environments and remote locations. It makes the opening and closing of butterfly valves faster, especially for larger valves requiring larger amounts of torque. The types of actuations used in butterfly valves to turn the valve stem are electromechanical actuation, (which uses an electric-powered motor), pneumatic actuation (which moves a piston or a diaphragm with compressed air), and hydraulic actuation, (which moves a piston or a diaphragm with hydraulic pressure).
Chapter 3: Types of Butterfly Valves
There are three main types of butterfly valves:
Zero Offset Butterfly Valves (Resilient Seat Butterfly Valves)
In zero offset butterfly valves, the stem passes through the centerline of the disc that is centered in the seat; all of this is centered inside the valve body. The valve body, seat, and disc lie concentrically when it is in a closed position. The disc rotates on the central axis; this allows a 3600 rotation. In a fully-opened position, the flow is divided into two halves on each side of the disc, which is now parallel to the flow. The advantage of this type is that the flowing media does not come in contact with the valve body because the seat is covering it.
Zero offset butterfly valves have resilient soft seats because they depend on the flexibility and deformation of the soft seat during sealing. This causes the disc edges to slide onto the seat, which results in full friction between them during the operation. This reduces the service life of the valve. Since the design requires the seat to be made from a polymeric or elastomeric material, it is limited to lower pressure and temperature ratings.
Zero offset butterfly valves are used in liquid and gas pipelines, which have pressure and temperature ratings of 250 psi and 4000F, respectively.
Double Offset Butterfly Valve (High-Performance Butterfly Valves)
In double offset butterfly valves, the stem axis is offset behind the centerline of the seat and the body (first offset), then the stem axis is further offset from the vertical centerline of the valve (second offset). When the disc is opened, the seat is lifted from the seal; this reduces the friction during the first and last 10 degrees of the valve opening and closing, respectively. This results in a smoother valve operation, better sealing capability, and longer service life than the zero offset butterfly valve.
Like zero offset butterfly valves, double offset butterfly valves use a soft seat. They are available in moderate pressure and temperatures ratings, which are capable of withstanding higher pressures and temperatures than the zero offset butterfly valves in liquid and gas pipelines.
Double offset butterfly valves are typically used in water purification, wastewater treatment, HVAC, and fire protection systems (e.g., fire sprinklers). For increased temperature resistance, the amount of soft seat material is reduced by backing it with a layer of metal.
Triple Offset Butterfly Valve
In triple offset butterfly valves, an angular offset in the body sealing cone axis is made in addition to the first two offsets (third offset). This is accomplished by using the right-angled conical profile of the seat coupled with a matching profile at the disc edge. This offset eliminates the contact of the seat and the disc during valve opening and closing, hence, friction is also eliminated. Contact only occurs during the full closure of the butterfly valve; this also acts as a mechanical stop to prevent the disc from rotating further.
The design of triple offset butterfly valves requires the use of metal seats. Since metal seats are tougher and more rigid than soft seats, they can withstand high temperatures and high fluid pressures; this minimizes valve wear. The seats provide sealing that ensures no fluid will escape from the valve. Triple offset butterfly valves have longer service lives than the zero offset and double offset butterfly valves.
Triple offset butterfly valves are ideal in handling high-pressure and superheated steam, high-temperature liquids and gases, and corrosive chemicals that cannot be handled by butterfly valves with soft seats. These valves are commonly used in power plants (e.g., on-and-off valves for boilers), oil and gas processing, chemical manufacturing, pulp and paper manufacturing, and offshore pipelines.
Butterfly valves can also be classified according to their connection design in the valve body to the pipeline:
Wafer Butterfly Valve.
Wafer butterfly valves are placed between two pipe flanges, which are linked by long bolts across the valve body. The valve body may or may not have flange holes outside the valve body. O-rings and gaskets that are mounted in a flat valve face are placed on both sides between the valve body and the pipe flange to achieve a strong seal.
Wafer butterfly valves have the most economical connection design. However, they are not able to be isolation valves or be used in an end-of-line service. The entire pipeline must be shut down when maintenance must be performed on either side of the wafer butterfly valve.
Lug Butterfly Valve
Lug butterfly valves are placed between two pipe flanges by the bolts that pass through the threaded inserts (or lugs) protruding outside the valve body. The valve is supported by two separate sets of bolts on each side of the valve and no nuts are used in either lug. This enables the valve to become an isolation valve or to be installed in an end-of-line service. This design enables dismantling on one side of the valve without it affecting the other; this lessens the downtime during maintenance.
However, an additional load is carried by the valve as the weight of the pipeline is also distributed to the valve body. Generally, the pressure rating of butterfly valves is degraded to half of its original rating when it is installed in an end-of-line service because no companion flange shares the weight of the pipeline.
Double-Flanged Butterfly Valve.
Double-flanged butterfly valves have a pair of flanges built on each side of the valve body; these match the dimensions of the pipe flanges. Two sets of bolts and nuts are used to support the valve on each side; this produces a strong attachment. Double-flanged butterfly valves are more popular in butterfly valves with larger sizing.
Butt-Weld Butterfly Valves
Butt-weld butterfly valves are valves directly welded between pipes; they are used for high-pressure services.
The butterfly valve may be a “non-wetted” or “wetted” valve:
Non-wetted valves have their valve body and stem isolated from the flowing media by lining them with plastic or elastomeric materials (e.g., rubber and Teflon). Hence, the material of construction is allowed to have lower corrosion resistance.
Wetted valves have their valve body and stem exposed to flowing media.
Chapter 4: Materials of Construction of Butterfly Valves
The valve body must possess high toughness and rigidity to protect the internal components of the butterfly valve. The stem and the disc must also possess the same characteristics to withstand fluid pressures. Butterfly valve components can be made from the following materials.
Stainless steel is a steel alloy that contains higher chromium content (10.5-30%), and trace amounts of nickel and molybdenum. Stainless steel is known for its high strength and toughness and corrosion and weather resistance. The corrosion resistance is attributed to its chromium content: a thin, inert chromium oxide layer is formed to prevent oxygen diffusion into the bulk of the material. When the surface is damaged or scratched, the chromium oxide layer reforms with the help of oxygen. This self-healing property disables rust formation. Resistance to pitting corrosion is attributed to molybdenum.
Stainless steel butterfly valves are excellent for handling acidic and corrosive fluids, a wide range of pressures, and a wide range of temperatures. Austenitic stainless steel is ideal for cryogenic processes, while duplex stainless steel is used for higher pressure. 316 Stainless Steel is the preferred stainless-steel grade.
Carbon steel is a steel alloy that has a higher carbon content of up to 2.5%. It has good mechanical properties and can be used for applications in which there are no special requirements. The common carbon steel grades available for butterfly valves are ASTM A216 WCB (Welded Cast B-grade) and LCC (Low Carbon Content). WCB grade is used for high-temperature applications while the LCC is used at low temperatures. The valve body and disc of the butterfly valves are produced by a sand-casting process.
Carbon steel butterfly valves are less expensive alternatives to stainless steel butterfly valves. However, they cannot be used in handling corrosive fluids.
Hastelloy is an alloy composed of nickel, chromium, and molybdenum; it was the first alloy formulated by Haynes International, Inc. It also has high strength, which is maintained at high temperatures, and it has excellent resistance to corrosion, cracking, and oxidizing and reducing agents. They are used to handle nitric, hydrochloric, and sulfuric acids at moderate temperatures.
Hastelloy butterfly valves are used in high temperature and pressure and corrosive applications, which are found in oil and gas processing and power generation.
Brass is an alloy of copper and zinc. It is a tough and durable material and can also withstand high temperatures. It is valued for its corrosion and biofouling properties, as well as for its anti-microbial properties that inhibit the growth of microbes and biofilms on its surface.
Brass butterfly valves are typically used in water purification, wastewater treatment, food, and pharmaceutical manufacturing. However, they are not applicable in the handling of water with high amounts of chlorine (e.g., seawater and swimming pool waters) because of the dezincification of brass by chlorine.
Nickel alloys are also known for their resistance to corrosion, harsh environments, and high temperatures. They have better performance than stainless steel in highly corrosive environments. However, nickel butterfly valves have high density and are more expensive.
Titanium alloys have a high strength-to-weight ratio. They are also known for corrosion and biofouling resistance and can be used in seawater environments and water treatment since they are non-toxic. However, titanium butterfly valves are also expensive.
Nickel Aluminum Bronze
Nickel Aluminum Bronze is a copper-based alloy that contains 10% aluminum, 5% nickel, and 5% iron. It is also known for its corrosion, antimicrobial, and biofouling properties. Nickel Aluminum Bronze butterfly valves are also used in seawater environments (e.g., offshore piping), water purification, and wastewater treatment.
The materials for butterfly valve seats are grouped into soft seats and metal seats. The temperature ratings of butterfly valves are limited based on the type of seat material.
Softs seats are made from plastic or elastomeric materials that have a low coefficient of friction and have self-lubricating properties. They generally have lower temperature and pressure ratings than metal seats but are less expensive.
Polytetrafluoroethylene, popularly known as Teflon, is a synthetic fluoropolymer and is the most popular valve seat material. It has low friction, high resilience, and is highly resistant to chemicals and fire. Due to its non-toxicity, it is used in food and pharmaceutical manufacturing. PTFE seats operate between -1000F and 4500F.
Reinforced Polytetrafluoroethylene (RTFE)
Reinforced Polytetrafluoroethylene is a modification of PTFE that contains 15% glass fiber. The glass fiber improves the wear and abrasion resistance of PTFE and increases the compressive strength and pressure rating of the seat. It is not compatible with hydrofluoric acid and caustic soda because of its degrading reaction to glass. RTFE seats operate between -3200F and 4500F.
Ethylene-propylene Diene Monomer (EPDM)
Ethylene-propylene Diene Monomer is a kind of synthetic rubber that has high resiliency and abrasion and wear resistance. EPDM valve seats can handle acids, alcohols, and alkaline solutions and are ideal for handling chlorinated water, bleach, and concentrated sulfuric acid. EPDM is also resistant to ozone. However, it is not compatible with petroleum, hydrocarbon solvents and oils, or turpentine. EPDM seats operate between -200F and 2300F.
Buna-N, also known as nitrile rubber, is a multi-purpose rubber that has high strength and abrasion resistance. Buna-N seats are used in the handling of gases, water, fuels, petroleum, and alcohol, but they are not compatible with acetones, ketones, chlorinated and nitro hydrocarbons, or ozone. Buna-N seats operate between -350F and 2500F.
Viton is a fluorocarbon elastomer that is compatible with most chemicals, but it has poor performance in handling steam and water. Viton seats operate between -200F and 4000F.
Metal seats overcome the limitation of soft seats in operation at high temperatures since they can withstand and retain their strength at elevated temperatures. They also have higher pressure ratings due to their toughness and rigidity than soft seats.
Inconel is a nickel-chromium-molybdenum alloy that has excellent mechanical properties and good corrosion, oxidation, and scaling resistance that is maintained in extreme environments and high temperatures. Inconel seats can operate up to 9000F. Inconel 625 and 600 are the commonly used Inconel grades in the construction of metal seats.
Stainless steel also performs well as a metal seat material. It is a less expensive alternative to Inconel.
Chapter 5: Advantages and Disadvantages of Butterfly Valves
The advantages of butterfly valves are the following:
- Butterfly valves have low pressure drop, which benefits the pumping system of the pipeline.
- Butterfly valves are quick-acting and have a simple operating mechanism. There are various types of automatic actuation systems that can be utilized in operating butterfly valves.
- Butterfly valves are more compact and have a simpler construction and lighter weight than other valve types. Hence, they generally consume less space and material. This is because they are designed to fit between two pipe flanges. They are cheaper alternatives to globe valves and gate valves. They also require less maintenance.
- Butterfly valves can operate at a wider range of temperatures and pressures than ball valves. Soft seats can be backed with metal seats to increase their temperature and flame resistance for fire prevention applications.
- Butterfly valves can handle large fluid flows at relatively low pressures as well as liquids containing suspended solids.
The disadvantages of butterfly valves are the following:
- Throttling in butterfly valves is limited to low pressure differentials.
- Butterfly valves have a poor sealing function. Effective sealing is achieved at low pressures.
- Butterfly valves require high torque during operation.
- Cavitation and choking are potential concerns in butterfly valves.
- Butterfly valves are a type of quarter-turn valve that feature a rotating disc element that allows, controls, and obstructs the fluid flow in a pipeline. They are valued for their compact size and simple construction and operating mechanism and for inducing a low pressure drop across the valve.
- The main components of butterfly valves are the valve body, disc, stem, seat, and operating mechanism.
- Butterfly valves can be classified according to the number of offsets, which divide them into three classifications: zero offset, double offset, and triple offset butterfly valves.
- Butterfly valves can have a wafer, lug, double-flanged, or welded connection design. The wafer type is the only connection design that does not allow isolation or end-of-line service.
- The butterfly valve body must be made from a tough and rigid material to protect its internal components.
- Butterfly valve seats are categorized according to their material type. Soft seats are made from elastomeric or plastic materials; these have a low coefficient of friction and self-lubricating properties. Metal seats overcome the limitation of soft seats to operate at high temperatures. The material of construction of the valve seats limits the temperature rating of the butterfly valve.