This article takes a detailed look at butterfly valves.
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A butterfly valve is a type of quarter-turn rotary control mechanism used to regulate the flow of fluids in a pipeline system. Its operation is based on a rotating disc situated directly within the flow route. This disc is maneuvered by an external actuator, which is linked via a stem. When the disc aligns itself parallel with the flow, it fully obstructs fluid passage. Conversely, when it is set perpendicularly, the fluid can freely flow through. A 90-degree rotation of the disc is needed to open or seal the valve.
Similar to ball valves and plug valves, butterfly valves are quarter-turn valves. These devices exhibit a straightforward construction and operational principle and are designed compactly to fit between two pipe flanges. They can be manually operated or automated using an actuation system incorporated into the pipeline's process control. While ideal for simple on-off applications, their functionality for controlling flow throttling is somewhat restricted.
Available in a wide array of models and configurations, butterfly valves are built to sustain different temperature ranges, pressures, and flow volumes. Each valve type is precisely engineered to meet the unique needs of pipeline systems managing both liquid and gaseous flows.
The primary components of a butterfly valve include the following:
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 90° rotation from the closed position of the disc to allow full opening or closing of the valve. Flow is throttled when the disc is rotated less than 90°.
The butterfly valve disc is analogous to the ball for ball valves or a plug for plug valves.
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 temperature applications because they deform at elevated temperatures. Metal sealing rings are capable of performing at higher temperatures and have a longer seal ring life but are difficult to completely seal.
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).
Butterfly valves are designed to provide effective and dependable shut-off control in piping systems. They come in two main types: centric and eccentric. Centric butterfly valves feature a stem located centrally on the valve disc, whereas eccentric butterfly valves have one or more stems positioned away from the disc's center. Eccentric valves can be categorized based on the number of offsets in the stem, including single, double, or triple eccentric configurations.
Zero offset butterfly valves feature resilient soft seats, which rely on the flexibility and deformation of the seat to ensure a seal. During operation, the disc edges slide against the seat, creating significant friction that can shorten the valve's lifespan. Due to the use of polymeric or elastomeric materials for the seat, these valves are generally suited for lower pressure and temperature conditions.
Zero offset butterfly valves are commonly used in pipelines for liquids and gases with maximum pressure ratings of 250 psi and temperature limits of up to 400°F.
Similar to zero offset butterfly valves, double offset butterfly valves also utilize a soft seat. However, they are designed to handle moderate pressure and temperature conditions better than their zero offset counterparts, making them suitable for higher pressures and temperatures in liquid and gas pipelines.
Double offset butterfly valves are commonly employed in applications such as water purification, wastewater treatment, HVAC systems, and fire protection systems like fire sprinklers. To enhance temperature resistance, these valves often feature a metal backing that reduces the amount of soft seat material used.
Triple offset butterfly valves are designed with metal seats, which are more durable and rigid compared to soft seats. This makes them capable of handling higher temperatures and fluid pressures, thereby reducing valve wear. The metal seats ensure a reliable seal, preventing any fluid leakage from the valve. As a result, triple offset butterfly valves generally have a longer service life than both zero offset and double offset butterfly valves.
Triple offset butterfly valves are well-suited for managing high-pressure and superheated steam, as well as high-temperature liquids, gases, and corrosive chemicals that exceed the capabilities of valves with soft seats. These valves are frequently used in industries such as power generation (e.g., on-off valves for boilers), oil and gas processing, chemical production, pulp and paper manufacturing, and offshore pipeline systems.
Butterfly valves can also be categorized based on their connection design to the pipeline:
Wafer butterfly valves are known for their cost-effective connection design but are not suitable for isolation purposes or end-of-line applications. Maintenance on either side of the wafer butterfly valve requires a complete shutdown of the entire pipeline.
Wafer butterfly valves, like some other types of butterfly valves, are not recommended for high-temperature applications. The valve body can expand with heat, leading to increased stress on the valve bolts and potential failure.
Lug butterfly valves are mounted between two pipe flanges using bolts that pass through threaded inserts, or lugs, extending from the valve body. This design features separate sets of bolts on each side of the valve without the use of nuts, allowing the valve to function as an isolation valve or be installed in end-of-line applications. This setup enables maintenance on one side of the valve without affecting the other side, thereby reducing downtime.
However, lug butterfly valves must support additional loads, as the weight of the pipeline is distributed across the valve body. Typically, the pressure rating of these valves may be reduced when used in end-of-line service due to the lack of a downstream flange to help secure the seat in place. This can lead to seat deformation and potential leakage as the media may push past the disc and seat seal.
Double-flanged butterfly valves feature flanges on both sides of the valve body that align with the pipe flanges. They are secured with two sets of bolts and nuts on each side, ensuring a robust connection. This design is commonly used for larger-sized butterfly valves, offering enhanced stability and support.
Butt-weld butterfly valves are welded directly between pipes, making them suitable for high-pressure applications.
Top entry butterfly valves feature a design that permits repairs without dismantling the entire pipeline. The valve bonnet can be easily removed, providing direct access to the valve stem and butterfly plate. This accessibility allows for quick replacement or repair of the valve, minimizing downtime.
Butterfly valves can be classified as either “non-wetted” or “wetted”:
The valve body needs to have high toughness and rigidity to safeguard the internal components of the butterfly valve. Similarly, the stem and disc must exhibit these qualities to endure fluid pressures. Butterfly valve components can be constructed from the following materials:
Stainless steel is an alloy with a high chromium content (10.5-30%) and small amounts of nickel and molybdenum. It is renowned for its exceptional strength, toughness, and resistance to corrosion and weathering. The corrosion resistance is primarily due to the chromium, which forms a thin, protective chromium oxide layer that prevents oxygen from penetrating the material. If the surface is damaged, this layer can regenerate with the help of oxygen, thus preventing rust. Molybdenum contributes to resistance against pitting corrosion.
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 an alloy with a carbon content of up to 2.5%. It offers good mechanical properties and is suitable for applications that do not have specialized requirements. Common grades of carbon steel used for butterfly valves include ASTM A216 WCB (Welded Cast B-grade) and LCC (Low Carbon Content). The WCB grade is suited for high-temperature applications, while the LCC grade is appropriate for low-temperature environments. These valves are typically produced using a sand-casting process for both the body and the disc.
While carbon steel butterfly valves are more cost-effective compared to their stainless steel counterparts, they are not suitable for handling corrosive fluids.
Hastelloy is an alloy made of nickel, chromium, and molybdenum, and was the first alloy developed by Haynes International, Inc. It is known for its high strength at elevated temperatures and its excellent resistance to corrosion, cracking, and both oxidizing and reducing agents. Hastelloy is suitable for handling acids such as nitric, hydrochloric, and sulfuric at moderate temperatures.
Hastelloy butterfly valves are commonly used in applications involving high temperatures, high pressures, and corrosive environments, including those found in oil and gas processing and power generation.
Brass is an alloy composed of copper and zinc, known for its toughness, durability, and ability to withstand high temperatures. It is prized for its resistance to corrosion and biofouling, as well as its antimicrobial properties, which help prevent the growth of microbes and biofilms on its surface.
Brass butterfly valves are commonly used in applications such as water purification, wastewater treatment, and in the food and pharmaceutical industries. However, they are not suitable for handling water with high chlorine content, such as seawater or swimming pool water, due to the risk of dezincification caused by chlorine.
Nickel alloys are renowned for their excellent resistance to corrosion, harsh environments, and high temperatures. They typically outperform stainless steel in extremely corrosive conditions. However, nickel butterfly valves tend to have a higher density and come at a greater cost.
Titanium alloys offer a high strength-to-weight ratio and are well-regarded for their resistance to corrosion and biofouling. They are suitable for use in seawater environments and water treatment applications due to their non-toxic nature. Despite these advantages, titanium butterfly valves are also quite expensive.
Nickel Aluminum Bronze is a copper-based alloy that includes 10% aluminum, 5% nickel, and 5% iron. This alloy is notable for its resistance to corrosion, antimicrobial properties, and ability to prevent biofouling. Nickel Aluminum Bronze butterfly valves are commonly used in seawater environments, such as offshore piping, as well as in water purification and wastewater treatment applications.
Butterfly valves can also be constructed from rigid plastic materials. Typically, plastic butterfly valves are lighter and more cost-effective than their metal counterparts. However, plastic materials have lower melting points, which restricts their maximum operating temperature.
PVC is a strong and rigid material commonly used in pipes, fittings, and valves. PVC butterfly valves are known for their durability and resistance to corrosion. They are suitable for handling most chemicals, although they should not be used with hydrocarbons or aromatic compounds. PVC valves are hygienic, easy to clean, and simple to assemble, making them ideal for applications in food and beverage processing. They are also used in water supply distribution, irrigation, plumbing, and water and wastewater treatment. PVC butterfly valves have a maximum operating temperature of 60°C and a pressure rating of 150 psi.
CPVC contains more chlorine than PVC and is produced through the free radical chlorination of PVC. This modification allows CPVC to handle higher temperatures, up to 93.3°C, and enhances its flexibility. However, CPVC is generally more expensive than PVC. CPVC butterfly valves are commonly used in both cold and hot potable water systems, industrial liquid handling, swimming pools, fountains, and other applications.
PP (Polypropylene) butterfly valves are known for their exceptional strength and durability in harsh environments. They resist corrosion, biofouling, and UV degradation, making them suitable for handling a wide range of chemicals and corrosive agents. However, they have selective compatibility with certain organic substances and solvents. PP butterfly valves are commonly used in aquaculture, water and wastewater treatment, and chemical manufacturing. They have a maximum operating temperature of 80°C.
GFPP (Glass-Filled Polypropylene) is an advanced modification of PP that incorporates glass fibers to enhance its rigidity, tensile strength, flexural strength, and dimensional stability. Similar to standard PP valves, GFPP butterfly valves are resistant to chemicals and corrosion. They are designed for more demanding environments and heavy-duty applications.
PVDF (Polyvinylidene Fluoride) butterfly valves are lightweight and offer exceptional corrosion resistance. They are highly abrasion-resistant, making them suitable for handling media with suspended solids. PVDF valves are ideal for high-purity applications and are compatible with most chemicals, including alcohols, hydrocarbons, and halogenated solvents, which makes them commonly used in chemical manufacturing. They also provide reliable, leak-free handling of substances. The maximum operating temperature for PVDF butterfly valves is 120°C.
ABS (Acrylonitrile Butadiene Styrene) is a thermoplastic polymer known for its strength, rigidity, and impact resistance. It is commonly used for constructing valve discs and is compatible with housings made from GFPP, PP, and PVC. ABS is relatively cost-effective. The maximum operating temperature for ABS is 60°C.
Butterfly valve seats are classified into two main types: soft seats and metal seats. The temperature ratings of butterfly valves are determined by the material used for the seat.
Soft seats are crafted from plastic or elastomeric materials known for their low friction and self-lubricating properties. While they typically have lower temperature and pressure ratings compared to metal seats, they are also more cost-effective.
Polytetrafluoroethylene (PTFE), commonly known as Teflon, is a synthetic fluoropolymer distinguished by its low friction, high durability, and exceptional resistance to chemicals and heat. Its non-toxic properties make it ideal for applications in the food and pharmaceutical sectors. PTFE valve seats operate efficiently in temperatures ranging from -100°F to 450°F.
Reinforced Polytetrafluoroethylene (RTFE) is an advanced form of PTFE that includes 15% glass fiber. This reinforcement enhances the wear resistance and compressive strength of the material. However, RTFE is not suitable for use with hydrofluoric acid or caustic soda due to its reaction with glass. RTFE seats are effective within a temperature range of -320°F to 450°F.
Ethylene-Propylene Diene Monomer (EPDM) is a synthetic rubber known for its exceptional elasticity and resistance to abrasion. EPDM valve seats are well-suited for handling acids, alcohols, and alkaline solutions, as well as chlorinated water, bleach, and concentrated sulfuric acid. It is also resistant to ozone, though it should not be used with petroleum-based solvents and oils. EPDM seats are rated for temperatures between -20°F and 230°F.
Buna-N, or nitrile rubber, is a versatile rubber with high durability and resistance to wear. Buna-N seats are effective for gases, water, fuels, petroleum, and alcohol. However, they are not suitable for use with acetones, ketones, chlorinated hydrocarbons, nitro hydrocarbons, or ozone. Buna-N seats operate within a temperature range of -35°F to 250°F.
Viton, also known as FKM, is a fluorocarbon elastomer compatible with a wide range of chemicals but is less effective for steam and water applications. Viton seats function well within temperatures from -20°F to 400°F.
Neoprene, or chloroprene rubber, is a synthetic rubber with good flexibility and resistance to greases, oils, fats, and most chemicals, except for strong oxidizers. It is flame-resistant and used in handling refrigerants and in pulp and paper manufacturing. Neoprene’s black color makes it less suitable for utility water in bleached paper production. Neoprene seats operate between -35°F and 225°F.
Metal seats are preferred for high-temperature applications as they retain strength and rigidity even at elevated temperatures. They also support higher pressure ratings due to their robust nature compared to soft seats.
Inconel, a nickel-chromium-molybdenum alloy, is known for its superior mechanical properties and resistance to corrosion, oxidation, and scaling in extreme conditions. Inconel seats can handle temperatures up to 90°F, with grades such as Inconel 625 and 600 being commonly used in metal seat construction.
Stainless steel is also an effective material for metal seats, offering a more cost-effective alternative to Inconel while still providing strong performance.
Stellite is a corrosion-resistant, non-magnetic cobalt-based alloy that includes chromium and tungsten. This alloy can be customized to suit different application requirements. Incorporating Stellite into valve seats has markedly enhanced their strength and longevity, providing substantial improvements in durability and performance.
Nitriding is a heat treatment technique designed to harden metal surfaces through the diffusion of nitrogen. This process is commonly employed for manufacturing valve seats and can be applied to materials such as low alloy steels, aluminum, molybdenum, and titanium. The nitriding process, which typically lasts several hours, enhances hardness, wear resistance, corrosion resistance, and high-temperature performance.
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