【Daily Question 20090306】Discussion on the Use of Butterfly Valves

The main issues associated with the use of butterfly valves include broken valve stems, poor sealing, and the choice of packing materials. In China, butterfly valves can currently reach ANSI 1500LB standards, and they all utilize metal hard seals. Due to the high pressures involved, a triple-eccentric design is generally used, which enables zero leakage. With advances in technology, the application of butterfly valves is becoming increasingly widespread.

The main maintenance issue for butterfly valves is leakage. Compared to control valves, it is not very large. The choice of sealing material for soft seals is extremely important; hard seals require high precision in manufacturing. As for the filler, PTFE is generally used, while graphite is chosen in high-temperature situations, although this increases friction and thus the required torque as well.

Use Nalese butterfly valves for regulation; they feature a metal hard seal and perform very well.

1. Since the multi-layered soft and hard stacked seal ring is fixed on the valve plate, when the valve plate is in the open position, the medium exerts direct erosion on its sealing surface; as a result, the soft seal strip located in the metal interlayer is damaged by this erosion, which directly affects the sealing performance. II. Due to structural constraints, this design is not suitable for valves with a diameter of DN200 or less, as the overall structure of the valve disc is too thick, resulting in high flow resistance. III. Due to the principle of the three-eccentric structure, the sealing between the sealing surface of the valve disc and the valve seat is achieved by the torque of the actuation mechanism, which presses the valve disc against the valve seat. In the forward flow state, the higher the medium pressure, the tighter the sealing compression. When the flow medium flows in reverse, as the pressure of the medium increases and the unit normal pressure between the valve plate and the valve seat becomes less than the pressure of the medium, leakage in the seal begins.

The operating element of a butterfly valve is a disc-shaped butterfly plate that rotates around its own axis within the valve body, thereby enabling the valve to be opened, closed, or adjusted. Such a valve is called a butterfly valve. The rotation range of a butterfly valve from fully open to fully closed is usually less than 900 degrees. The butterfly valve and its shaft do not possess sufficient self-locking capability; therefore, a worm gear reducer must be installed on the shaft in order to position the butterfly disc. The use of a worm gear reducer not only enables the butterfly valve disc to have self-locking capability, allowing it to stay in any position, but also improves the operating performance of the valve. Industrial butterfly valves are characterized by their ability to withstand high temperatures, a wide pressure range, a large nominal diameter. The valve body is made of carbon steel, and metal rings are used as seals for the valve disc instead of rubber rings. Large high-temperature butterfly valves are manufactured by welding steel plates, and are primarily used in flue gas ducts and gas pipelines handling high-temperature media. The advantages of butterfly valves are as follows: 1. They can be opened and closed easily and quickly, with minimal effort, and present low fluid resistance, allowing for frequent operation. 2. Simple structure, small size, and light weight. 3. It can transport mud, with minimal liquid accumulation at the pipe outlet. 4. Good sealing can be achieved at low pressures. 5. Good adjustment performance. The disadvantages of butterfly valves are as follows: 1. The range of operating pressure and working temperature is limited. 2. The sealing performance is poor. Butterfly valves can be classified by structure into offset plate type, vertical plate type, inclined plate type, and lever type. Based on the sealing type, they can be divided into the semi-sealed type and the hard-sealed type. Soft-sealed types generally use rubber rings for sealing, while hard-sealed types usually use metal rings for sealing. Based on the type of connection, they can be divided into flange connections and clamp connections ； Based on the transmission method, they can be classified into manual, gear-driven, pneumatic, hydraulic, and electric types. The following points should be noted for the installation and maintenance of butterfly valves: 1. During installation, the valve disc must be in the closed position. 2. The opening position should be determined based on the rotation angle of the butterfly valve disc. 3. For butterfly valves equipped with a bypass valve, the bypass valve should be opened first before turning on the valve. 4. It should be installed in accordance with the manufacturer’s installation instructions; for butterfly valves with heavy weights, a solid foundation should be provided.

Butterfly valves have a simple structure, are small in size and light in weight, consisting of only a few components. Moreover, it can be quickly opened and closed by just a 90° rotation, making it easy to operate; at the same time, this valve boasts excellent fluid control properties. When the butterfly valve is in the fully open position, the thickness of the butterfly disc is the only resistance to the flow of the medium through the valve body; as a result, the pressure drop across this valve is very small, giving it excellent flow control characteristics. Butterfly valves come in two types of sealing: elastic sealing and metallic sealing. Elastic-sealing valves, where the sealing ring can be embedded in the valve body or attached around the butterfly disc. Valves with metal seals generally have a longer lifespan than those with elastic seals, but it is difficult to achieve complete sealing. Metal seals can withstand higher operating temperatures, while elastic seals have the drawback of being limited by temperature. If a butterfly valve is to be used for flow control, the key is to select the correct size and type of valve. The structural principle of butterfly valves makes them particularly suitable for manufacturing large-diameter valves. Butterfly valves are widely used not only in general industries such as petroleum, gas, chemicals, and water treatment, but also in the cooling water systems of thermal power plants. The commonly used butterfly valves are wafer-type butterfly valves and flange-type butterfly valves. A wafer butterfly valve is connected between two pipe flanges using bolts, while a flanged butterfly valve has flanges on it, with the flanges at both ends of the valve being connected to the pipe flanges via bolts.

Butterfly valves are becoming increasingly widely used. The main problem encountered in their use at present is poor sealing. For example, in systems used to control water or compressed air, inadequate sealing performance makes it impossible to isolate the equipment from the system during maintenance, resulting in difficulties; maintenance can only be carried out during shutdowns for major repairs. Furthermore, in valves used in utility systems, due to the low number of operations, it often becomes more difficult to operate them over time, and replacing the valves involves system shutdowns and high costs.

There are two types of seat installation configurations for three-eccentric butterfly valves, with the majority being installed on the butterfly disc for convenience； Another type adopts a body seat design, with the seat mounted on the body; its advantage is that, compared to butterfly valve seats, it **reduces the chances of the seat coming into direct contact with the medium, thereby minimizing erosion of the seat and extending its service life.

Firstly, the sealing performance of butterfly valves is inferior to that of other types of valves; they are generally used as control valves and can meet the requirements of the process. The main problem is the poor sealing of the valve stem, which leads to leaks (used in circulating water pipes and at the pump inlets and outlets) ； When used as a automatic control valve, the connection between the actuator and the valve is prone to failure due to corrosion and other factors (the control valve fails to operate)!

Based on their structural design, butterfly valves can be classified into center-line butterfly valves, single-eccentric butterfly valves, double-eccentric butterfly valves, triple-eccentric butterfly valves, etc. The differences between them are as follows: First, for center-line butterfly valves (concentric butterfly valves), their structural feature is that the axis of the valve stem, the center of the butterfly plate, and the center of the valve body are all in the same position. It has a simple structure and is easy to manufacture. Common rubber-lined butterfly valves fall into this category. The disadvantage is that the butterfly plate and the valve seat are constantly under compression and scraping, resulting in high resistance and rapid wear. To overcome compression and scraping and ensure sealing performance, the valve seat is generally made of elastic materials such as rubber or polytetrafluoroethylene; however, the sealing materials are limited by temperature in terms of their usability, which is why butterfly valves are traditionally considered unsuitable for high-temperature environments. II. Single-eccentric butterfly valve: The structural feature of the single-eccentric butterfly valve is that the axis of the valve stem is offset from the center of the butterfly plate; as a result, the upper and lower ends of the butterfly plate are no longer the rotation axis, which reduces the excessive pressure exerted on these ends by the valve seat. This approach solves the problem of compression between the butterfly plate and the valve seat that occurs in concentric butterfly valves. However, due to the fact that the scraping between the butterfly disc and the valve seat does not disappear throughout the entire opening and closing process of the valve in a single-eccentric design, and its application range is similar to that of concentric butterfly valves, it is not widely used. III. Double-eccentric butterfly valve: The double-eccentric butterfly valve is an improved version of the single-eccentric butterfly valve, and it is the most widely used type. Its structural feature is that the axis of the valve stem is offset from both the center of the butterfly plate and the center of the body. The effect of dual eccentricity enables the butterfly disc to quickly disengage from the valve seat once the valve is opened, significantly reducing unnecessary excessive compression and scraping between the butterfly disc and the valve seat, decreasing the opening force, minimizing wear, and extending the lifespan of the valve seat. Scratching is significantly reduced, and this also allows dual-eccentric butterfly valves to use metal seat inserts, thereby expanding the application of butterfly valves in high-temperature environments. However, because its sealing principle relies on a positional sealing mechanism – that is, the sealing surfaces of the butterfly disc and the valve seat are in line contact, and sealing is achieved through the elastic deformation resulting from the pressure exerted by the butterfly disc on the valve seat – high requirements are placed on its closed position (especially for metal valve seats), and its pressure resistance is low. This is why traditionally it has been believed that butterfly valves cannot handle high pressures and suffer from high leakage rates. IV. Three-eccentric butterfly valves: They need to be able to withstand high temperatures, thus require hard-sealing, but they have high leakage rates ； To achieve zero leakage, soft seals must be used, but they cannot withstand high temperatures. To overcome the contradiction of the double-eccentric butterfly valve, a third eccentricity was applied to the butterfly valve. Its structural feature is that, in addition to the double eccentricity of the valve stem axis, the conical axis of the butterfly valve’s sealing surface is also tilted relative to the cylindrical axis of the valve body; in other words, after the third level of eccentricity, the sealing cross-section of the butterfly valve is no longer circular but elliptical, and as a result its sealing surface becomes asymmetric – one side being inclined relative to the center line of the valve body while the other side remains parallel to it. The most significant feature of this third type of eccentric design is that it fundamentally changes the sealing mechanism: it is no longer a position-based seal but rather a torque-based seal. In other words, sealing is achieved not by relying on the elastic deformation of the valve seat, but entirely through the pressure at the contact surface between the valve components. This approach solves the problem of zero leakage in metal valve seats. Moreover, since the contact surface pressure is proportional to the medium pressure, high pressure and high temperature conditions can also be handled easily.

We have 6 Fisher butterfly valves with a DN300 size for use as anti-surge valves in our air compressors; they feature a hard seal and are made of stainless steel. Question two: 1. It cannot be shut off completely; after prolonged use, gas erosion causes wear on the sealing surfaces. Air leakage occurs, and the more leakage there is, the more wear takes place – a vicious cycle. (Wasting energy). 2. It cannot be opened; sometimes it fails to function or gets stuck when it is needed to be used. (It relates to the safe operation of air compressors.) I take this opportunity to see if anyone has any good suggestions. :handshake

The butterfly valves used in our company are of small diameter, ranging from DN25 to DN250. They are available in white steel and cast iron versions. Repair parts such as sealing rings wear out over time; the valve disc deforms and bends after being opened and closed many times, which often leads to internal leakage

The main problem with the butterfly valves we use is the sealing issue.