Butterfly valves are simply thought to be named for their disc-shaped valve plates. The English name is "butterfly vlve", which translates to "butterfly valve". It is characterized by lower cost than ball valves and is suitable for large-diameter and low-pressure working conditions.
1. Classification of butterfly valves:
Butterfly valves can be mainly divided into three categories according to their structural forms: centerline butterfly valves, double eccentric butterfly valves, and triple eccentric butterfly valves. There are also single eccentric butterfly valves for special industries.
1. Centerline butterfly valve: the structure is completely symmetrical, the valve stem, valve plate, and sealing ring are completely symmetrical with respect to the valve body, the structure is simple, the processing difficulty is relatively low, and the cost is relatively low. , oil, gas and other media, the pressure resistance is less than or equal to PN16KG, and the service life is 30,000 to 50,000 times.
2. Double eccentric butterfly valve: The valve stem and valve plate are located off the center line relative to the valve body, which reduces the friction between the valve plate and the sealing ring and greatly improves the service life of the valve. It is often used in high-frequency switching conditions and has the highest service life. up to 2 million times. Its obvious appearance feature is that the sealing ring is embedded on one side of the valve body, and the sealing ring is pressed by the gland. The processing is difficult, and the cost is much higher than that of the centerline butterfly valve. At present, there are not many domestic manufacturers that can do well. (The structural feature of the double eccentricity is that the valve body is in a position off the center line, which reduces the friction between the valve plate and the sealing ring and improves the life of the valve. It can be seen from the figure that the contact surface between the double eccentric sealing ring and the valve plate of our structure Narrower, the smaller the friction, and we use O-rings to compensate, the service life is almost twice that of pure PTFE extrusion seal)
3. Triple eccentric butterfly valve: On the basis of double eccentric butterfly valve, there is an extra eccentric processing of the edge of the valve plate relative to the sealing surface of the valve body, which is often used in high temperature, or medium viscous, or even particles. Its appearance feature is that the sealing ring is embedded on the valve plate, and the sealing ring is pressed by the gland. There are many manufacturers of triple eccentric butterfly valves in China, but few of them can actually make them according to the eccentric standard.
4. Single eccentric butterfly valve: Only the valve stem is eccentric relative to the valve body, which is used in a specific industry (air separation system) and is relatively rare in the market.
two. Components and material selection of butterfly valve:
1. Valve body (BODY): The commonly used materials for the valve body are cast iron, ductile iron, carbon steel, stainless steel 304/316/316L, aluminum bronze, and duplex steel. Valve bodies of different materials are used according to different working conditions. Cast iron and ductile iron have the same performance, but nodular cast iron has a slightly stronger pressure bearing capacity. Carbon steel is often used in places with relatively high pressure and no corrosion. If corrosion resistance needs to be considered, stainless steel has outstanding performance, corrosion resistance 316L>316>304, price Also 316L is the highest.
2. Disc (DISC): The disc is the part of the butterfly valve that is most in contact with the medium, so choosing a disc with a suitable material will directly affect the use of the valve. Generally, the material of the valve plate will be higher than the material requirements of the valve body. When there is no need to consider the medium pollution, the valve plate of cast iron or ductile iron will be used. The valve plate generally has galvanized plate, nylon plate, stainless steel plate, and 2507 plate. , Aluminum bronze plate, rubber-lined plate, fluorine-lined plate, etc.
3. Valve seat (SEAT): The valve seat is the sealing ring, which is the most critical point in the selection of butterfly valves. Factors such as the chemical properties, temperature, and wear resistance of the applicable medium need to be considered.
Commonly used sealing ring materials for butterfly valves are: ① Soft seals: EPDM, NBR, VITON, PTFE, commonly used in centerline butterfly valves. ②Hard seal: 304 graphite, pure metal 304/316, etc., mainly used as sealing rings for butterfly valves with eccentric structure. In addition, silica gel, polyurethane rubber, neoprene rubber, low elastic rubber, etc. are also used.
Valve stem (STEM): The valve stem is generally made of a material with relatively high hardness, such as stainless steel SS410, 17-4 (SS630), its corrosion resistance is equivalent to that of the valve plate, and some manufacturers use the valve stem of the same material as the valve plate.
Of course, the most common material configuration of butterfly valves is: valve body FCD/valve plate 304/valve seat EPDM/valve stem 410.
3. Butterfly valve connection, pressure and diameter:
1. Butterfly valve connection: flange, wafer (including lugs), welding and thread (rarely used).
2. The pressure of the butterfly valve: PN10-PN16 is more.
3. The caliber of the butterfly valve: generally from DN40-DN3000, less than 40 butterfly valves are rarely used.