Butterfly Valves

What is butterfly valve?

Butterfly valve, also known as flap valve, is a regulating valve with simple structure, which can be used for on-off control of medium in low-pressure pipeline. Butterfly valve refers to a valve whose closing part (valve disc or butterfly plate) is a disc and rotates around the valve shaft to open and close.
This type of valve can be classified based on several parameters:

  • Design: center butterfly valve, double eccentric butterfly valve, triple offset butterfly valve;
  • End connection type: wafer, lug (half or full), flange and double flange;
  • Seat materials: soft (e.g. PTFE, Buna rubber, rubber, etc.) or metal to metal (SS304, SS316), body and disc materials (from cast iron to high nickel alloy);
  • Drive: Manual (lever, gear, worm gear) and drive (electric, pneumatic, hydraulic and hydro pneumatic types).

Material of butterfly valve

Valve body

Like any other type of valve, butterfly valves have a variety of materials and material combinations (body, disc, seat).


The selection of materials is based on the fluid to be closed, the pressure and temperature of the application at hand. The main materials used for butterfly valves, usually cast grades, are listed below.
Butterfly valve accessories

Carbon steel

Carbon steel is an alloy of iron and carbon (no other alloying elements are added to control the properties of the material).
For butterfly valve structures, carbon steel is most commonly used to form the body and disc by sand casting process.
The most common grades used for butterfly valve bodies and discs are astma216wcb and LCC (low carbon) steel (cast steel). They are used for high temperature and low temperature services respectively.
The main advantages of carbon steel (CS) butterfly valves are relatively low cost and acceptable performance (especially for low pressure applications). The weakness of carbon steel valves is their poor corrosion resistance (which can be improved by coating external coating or lining material as Teflon)

Stainless steel

Stainless steel is an alloy of iron, nickel and chromium (the minimum content of Cr is 10.5%). The addition of chromium forms a self-healing oxide layer on the surface of the material to protect it from rust.
When the surface of stainless steel products is scratched, chromium reacts with oxygen and prevents it from reacting with iron and rusting. Many different types of stainless steel are available, commonly referred to as 300 and 400 series.
Stainless steel can be divided into ferrite, austenite, martensite, duplex and precipitation hardening (14ph).
The classification is based on the microstructure of materials by changing the composition of mixed elements. The most common SS types used to construct valves are usually austenitic and duplex grades.

Austenitic stainless steel 

Austenitic stainless steels contain (except chromium) nickel, which has the effect of maintaining a high temperature face centered cubic austenite structure at a temperature that has usually been transformed into a ferrite centered cubic structure.
Compared with ferritic stainless steel, this face centered cubic structure makes the material more ductile and ductile.
According to the percentage of nickel contained, the tough austenitic structure can be maintained even at very low temperatures, making the material suitable for low temperature applications.
The addition of molybdenum enhances the resistance to pitting corrosion of the valve.

Duplex stainless steel

Duplex stainless steels (UNS S32205 and UNS S31803) have a well-balanced structure, including austenitic face centered cube and ferrite central iron structure.
The composition is obtained by controlling alloying elements and heat treatment on the alloy to produce a structure composed of 50% austenite and 50% ferrite.
Duplex stainless steel combines the high strength of ferrite composition and the remarkable toughness of austenite.
Super Duplex (UNS S32750 and UNS s32760) contain high levels of chromium and molybdenum to enhance corrosion resistance.

Nickel alloy steel

Nickel alloys are used in valves subject to very severe service conditions in terms of fluid, temperature and pressure.
These alloys are particularly suitable for highly corrosive applications, which can destroy the protective oxide layer of low-grade steel (such as stainless steel).
Inconel, Incoloy and Hastelloy are the most commonly used nickel alloys for valve structures. The main disadvantage of nickel alloys is their heavy weight and high cost (multiple stainless steel valves).

Titanium and titanium alloys

Titanium alloy has excellent strength and weight and excellent corrosion resistance.
Compared with any other metal, titanium alloy has the highest strength to weight ratio. Similar to stainless steel, the surface of titanium alloy has protective oxide layer, which can prevent the product from rusting.
Titanium has strong resistance to seawater corrosion, especially in the presence of hypochlorite.
The main disadvantages of titanium alloy are high price, multiple of stainless steel and the complexity of production process, because of the high reactivity of metal (special casting process is required to prevent titanium from reacting with oxygen during melting and pouring).

Nickel aluminum bronze

This is a copper alloy containing about 10% aluminum, 5% nickel and 5% iron. It has strong corrosion resistance, especially in seawater application, and can resist the formation of biofilm, which is a common cause of corrosion problems of stainless steel.
Butterfly valves API 609 can have soft seats or metal seats. Valves with soft seats, also known as “soft seat” or “elastic” butterfly valves, belong to the central type. Eccentric design (double offset or three offset) usually adopts metal seat design, or it can be fireproof.

Seat material

Soft (elastic butterfly)

Soft seat butterfly valve (elastic) has EPDM, Viton, NBR, Buna-N and similar soft seats. Soft seated butterfly valves are usually concentric and suitable for low pressure and non critical applications.


Metal seal butterfly valve
In this case, the valve body and the disc body have a metal to metal seal.


High performance butterfly valves (double eccentric) provide soft and metal seats. Triple eccentric butterfly valves usually have metal seats and ATEX fire protection devices.

Type of butterfly valve

Butterfly valves can be used in a wide range of applications, such as water supply, wastewater treatment, fire prevention, chemical and petroleum industry, fuel treatment system and power generation industry.
With the introduction of double eccentric and three eccentric design, butterfly valves are more and more used in the oil and gas industry, and begin to compete with more traditional valves, such as ball valves and ball valves.

Butterfly valves can be used in a wide range of applications, such as water supply, wastewater treatment, fire prevention, chemical and petroleum industry, fuel treatment system and power generation industry.
With the introduction of double eccentric and three eccentric design, butterfly valves are more and more used in the oil and gas industry, and begin to compete with more traditional valves, such as ball valves and ball valves.

Working principle of butterfly valve

The opening and closing part of butterfly valve is a disc-shaped butterfly plate, which rotates around its own axis in the valve body, so as to open, close or adjust the valve, which is called butterfly valve. The butterfly valve is usually less than 90 degrees from full opening to full closing. The butterfly valve and the butterfly rod are not self-locking. In order to position the butterfly plate, a worm gear reducer should be installed on the valve rod. The use of worm gear reducer can not only make the butterfly plate have self-locking ability and stop the butterfly plate at any position, but also improve the operation performance of the valve.

The industrial butterfly valve is characterized by high temperature resistance, high applicable pressure range, large nominal diameter of the valve, carbon steel body and metal ring instead of rubber ring. Large high temperature butterfly valve is made of steel plate welding, which is mainly used for smoke and air duct and gas pipeline of high temperature medium. Concentric butterfly valve the structural feature of this kind of butterfly valve is that the valve stem axis, butterfly plate center and body center are in the same position. The utility model has the advantages of simple structure and convenient manufacture. Common rubber lined butterfly valves belong to this category. The disadvantage is that the butterfly plate and valve seat are always in the state of extrusion and scraping, large resistance distance and fast wear. In order to solve the extrusion problem between the butterfly plate and the valve seat of the concentric butterfly valve, the single eccentric butterfly valve is produced. Its structural feature is that the valve stem axis deviates from the center of the butterfly plate, so that the upper and lower ends of the butterfly plate no longer become the rotation axis, disperse and reduce the excessive extrusion between the upper and lower ends of the butterfly plate and the valve seat.
The double eccentric butterfly valve is further improved and formed on the basis of the single eccentric butterfly valve, which is the most widely used double eccentric butterfly valve at present. The utility model is characterized in that the axis of the valve rod deviates from both the center of the butterfly plate and the center of the body. The double eccentric effect enables the butterfly plate to quickly separate from the valve seat after the valve is opened, greatly eliminates the unnecessary excessive extrusion and scraping between the butterfly plate and the valve seat, reduces the opening resistance, reduces the wear and improves the service life of the valve seat. The triple offset butterfly valve must be resistant to high temperature and must be hard sealed, but the leakage is large; To achieve zero leakage, a soft seal must be used, but it is not resistant to high temperature. In order to overcome the contradiction of double eccentric butterfly valve, the butterfly valve is eccentric for the third time. Butterfly valve is a kind of valve which uses circular butterfly plate as the opening and closing part and rotates with the valve stem to open, close and adjust the fluid channel. Butterfly valve not only has simple structure, small volume, light weight, low material consumption, small installation size, small driving torque, simple and rapid operation, but also has good flow regulation function and closing sealing characteristics. Butterfly valve is the leading valve form in the application field of large and medium diameter and medium and low pressure. With the application of anti-corrosion synthetic rubber and polytetrafluoroethylene, the performance of butterfly valve can be improved and meet different working conditions.

  1. The key component of the butterfly valve is the disc, and the metal ring is located on the centerline of the pipe and connected to the external actuator (manual level or actuator) through the rod.
  2. Compared with ball valves, the disc of butterfly valves is always in contact with the fluid transported by the pipeline, resulting in pressure drop.
  3. In the closed position, the disc blocks the hole of the valve, and in the open position, the disc allows fluid to pass through.
  4. A quarter turn of movement is required to switch the valve from the fully open position to the fully closed position (and this movement can be performed as quickly as a ball valve).

Butterfly valve symbol in P & ID drawing

Concentric butterfly valve

The structural feature of the butterfly valve is that the valve stem axis, the center of the butterfly plate and the center of the body are in the same position. The utility model has the advantages of simple structure and convenient manufacture. Common rubber lined butterfly valves belong to this category. The disadvantage is that the butterfly plate and valve seat are always in the state of extrusion and scraping, large resistance distance and fast wear. In order to overcome extrusion, scraping and ensure sealing performance, the valve seat is basically made of elastic materials such as rubber or polytetrafluoroethylene, but its use is limited by temperature. This is why people traditionally think that the butterfly valve is not resistant to high temperature.

Single eccentric butterfly valve

In order to solve the extrusion problem between the butterfly plate and the valve seat of the concentric butterfly valve, a single eccentric butterfly valve is produced. Its structural feature is that the valve stem axis deviates from the center of the butterfly plate, so that the upper and lower ends of the butterfly plate no longer become the rotation axis, disperse and reduce the excessive extrusion between the upper and lower ends of the butterfly plate and the valve seat. However, due to the single eccentric structure in the whole opening and closing process of the valve, the scraping phenomenon between the butterfly plate and the valve seat has not disappeared, and the application range is similar to that of the concentric butterfly valve, so it is rarely used.

Double eccentric butterfly valve

Based on the single eccentric butterfly valve, the double eccentric butterfly valve, which is currently used Zui widely, is further improved. The utility model is characterized in that the axis of the valve rod deviates from both the center of the butterfly plate and the center of the body. The double eccentric effect enables the butterfly plate to quickly separate from the valve seat after the valve is opened, greatly eliminates the unnecessary excessive extrusion and scraping between the butterfly plate and the valve seat, reduces the opening resistance, reduces the wear and improves the service life of the valve seat. At the same time, the double eccentric butterfly valve can also adopt metal seat, which improves the application of butterfly valve in high temperature field. However, because its sealing principle belongs to the position sealing structure, that is, the sealing surface of the butterfly plate and the valve seat is in line contact, and the sealing effect is produced by the elastic deformation caused by the extrusion of the valve seat through the butterfly plate. Therefore, it has high requirements for the closing position (especially the metal valve seat) and low pressure bearing capacity. This is why people traditionally think that the butterfly valve is not resistant to high pressure and has large leakage.

Triple eccentric butterfly valve

To be resistant to high temperature, hard seal must be used, but the leakage is large; To achieve zero leakage, a soft seal must be used, but it is not resistant to high temperature. In order to overcome the contradiction of double eccentric butterfly valve, the butterfly valve is eccentric for the third time. The utility model is characterized in that the conical axis of the sealing surface of the butterfly plate deviates from the cylindrical axis of the body while the double eccentric valve rod axis position is eccentric, that is, after the third eccentricity, the sealing section of the butterfly plate is no longer true circle, but ellipse, and the shape of the sealing surface is asymmetric, one side is inclined to the centerline of the body and the other side is parallel to the centerline of the body.
The biggest feature of this third eccentricity is that it fundamentally changes the sealing structure, no longer the position seal, but the torque seal, that is, the sealing effect is achieved not according to the elastic deformation of the valve seat, but completely according to the contact surface pressure of the valve seat. Therefore, the problem of zero leakage of the metal valve seat is solved in one fell swoop, and because the contact surface pressure is directly proportional to the medium pressure High pressure and high temperature resistance can also be easily solved.

Double eccentric butterfly valve VS triple eccentric butterfly valve

The image shows the design difference between double eccentric butterfly valve and triple eccentric butterfly valve (on the left and right of the figure respectively).

Design of butterfly valve

The first key classification is between concentric or eccentric butterfly valves:

  • Concentric butterfly valve: the valve stem is located in the center of the valve disc, and the valve disc is located in the center of the hole
  • Eccentric butterfly valve (double offset and three eccentric): it has one or more short shafts (valve stems) which are located offset from the center of the valve. Eccentric butterfly valves are also defined as double or triple offset.
Concentric butterfly valve Double eccentric butterfly valve Three eccentric butterfly valve

Advantages and disadvantages of butterfly valve

In recent ten years, metal sealed butterfly valves have developed rapidly. With the application of high temperature, low temperature, strong corrosion, strong erosion and high-strength alloy materials in butterfly valves, metal sealed butterfly valves have been widely used under working conditions such as high temperature, low temperature and strong erosion, and partially replaced stop valves, gate valves and ball valves.

Advantages of butterfly valve

  • 1. The butterfly valve has the advantages of convenient and rapid opening and closing, labor saving, small fluid resistance and regular operation.
  • 2. Butterfly valve has the advantages of simple structure, small volume and light weight.
  • 3. The butterfly valve can transport mud, and the liquid accumulated at the pipe mouth is the least.
  • 4. Butterfly valve can achieve good sealing under low pressure.
  • 5. Butterfly valve has good regulation performance.

Disadvantages of butterfly valve

  • 1. The butterfly valve has a small range of operating pressure and temperature.
  • 2. The sealing performance of butterfly valve is poor.
  • 3. Butterfly valves can be divided into offset plate, vertical plate, inclined plate and lever type according to the structural form. According to the sealing form, it can be divided into relatively dense
  • 4. Seal type and hard seal type. The soft seal type is generally sealed with rubber ring, and the hard seal type is usually sealed with metal ring.
  • 5. According to the connection type, it can be divided into flange connection and wafer connection; According to the transmission mode, it can be divided into manual, gear transmission, pneumatic, hydraulic and electric.

The installation and maintenance of butterfly valve shall pay attention to the following matters:

  • 1. When the butterfly valve is installed, the valve disc shall stop at the closed position.
  • 2. The opening position of butterfly valve shall be determined according to the rotation angle of butterfly plate.
  • 3. For butterfly valve with bypass valve, the bypass valve shall be opened before opening.
  • 4. The butterfly valve shall be installed according to the installation instructions of the manufacturer, and the heavy butterfly valve shall be provided with a solid foundation.
  • 5. Butterfly valve has the characteristics of simple structure, small volume, light weight, low material consumption, small installation size, fast switching, 90 ° reciprocating rotation, small driving torque, etc. it is used to cut off, connect and adjust the medium in the pipeline, and has good fluid control characteristics and closing sealing performance.

China Butterfly Valve Manufacturer www.sfutube.com supplies Industrial Butterfly Valves, Butterfly Valves for industrial use, in kinds of materials such as carbon steel, stainless steel, alloy steel, etc.

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