Overview
The butterfly valve is an important device in the water supply and drainage pipeline system. With the progress of industrial technology, different requirements are put forward for the structure and performance of the butterfly valve. Therefore, in the design and selection, the type, material, and connection form should be reasonably selected according to the working conditions.
Design
2.1 Structure
The closing part (disc) of the butterfly valve is in the middle of the medium, and its influence on the flow resistance should be considered in the design.
- For the structure of the large-diameter butterfly valve disc, AWWA C504 (American Water Engineering Association Standard) stipulates that the disc should not have transverse ribs, and its thickness should not be greater than 2.25 times the diameter of the valve stem.
- The water-facing and outlet surfaces of the slab should be streamlined.
- The internal screws cannot protrude outside the disc, so as not to increase the water-facing area. In the butterfly valve produced by an international company, the disc is made into a discus shape in order to reduce the flow resistance. There are also companies that make the disc of a small butterfly valve very thin, and the butterfly valve used in the water and electricity network has a flow guide when the butterfly valve is in the fully open position. All to reduce flow resistance. The Chinese JB/T 5317121999 standard also includes the flow resistance and flow coefficient (Table 1) of the international standard ISO/TC 153/SC1GT 8237, which shows that flow resistance is an important indicator.
Table 1 C(Kv), K(ξ) values of butterfly valve
2.2 Rubber sealing ring
Sometimes the rubber butterfly valve has a short service life, which is related to the quality of the rubber and the width of the sealing surface. The sealing ring of the rubber-sealed butterfly valve should be made of high-quality rubber materials, and the process rules should be followed when pressing and forming. The metal sealing surface matched with the rubber sealing ring should have sufficient width, otherwise, the rubber sealing ring is not easy to embed. In addition, the shape and position tolerance, symmetry, precision, smoothness of the valve body and disc, and the amount of compression of the sealing ring also affect the service life of the rubber sealing ring.
2.3 Structural strength of butterfly valve
Structural strength is an important issue in butterfly valve design, which is related to factors such as butterfly plate, valve shaft and connection.
- Shaft Dimensions Shaft dimensions are specified in AWWA C504. If the size of the valve shaft does not meet the requirements, there may be insufficient rigidity, reverse seal leakage, and large opening torque. The stiffness of the shaft is related to 1/EI, that is to say, in order to increase the stiffness and reduce the deformation, we should start by increasing the EI. E is the modulus of elasticity. Generally, the difference between steels is not large, and the selected material has little effect on the stiffness. I am the moment of inertia, which is related to the cross-sectional dimension of the shaft. The size of the valve shaft is generally calculated according to the combination of bending and torsion, which is not only related to the torque but also mainly related to the bending moment. In particular, the bending moment of large-diameter butterfly valves is much larger than the torque.
- Shaft hole fit As specified in the old version of AWWA C504, the butterfly valve shaft is a straight-through shaft. After the 1980 version, it was proposed that two short shafts could be made. AWWA C504 and GB12238 stipulate that the embedded length of the shaft and the hole should be 115 d. In the axial dimension of the Japanese butterfly valve, the clearance (C value) between the inner edge of the valve body and the support end of the disc is specified, which is generally related to the size of the diameter and is between 25 and 45 mm, that is, the distance between the shaft supports should be minimized as much as possible. (C value) , thereby reducing the bending moment and deformation of the shaft.
- Butterfly plate structure The butterfly plate structure is directly related to stiffness, so in addition to the flat plate shape, it is mostly made into a pot shape or a truss shape. In order to increase the moment of inertia of the section to increase the stiffness.
- Body structure There is also a stiffness problem in the design of the large-diameter butterfly valve body. Generally, there are ring bars and horizontal bars. In fact, the horizontal bars only increase the stability and should not be too many, mainly the ring bars. If the ∩-shaped rib can be added, it is more beneficial to the rigidity, but there is a problem of poor manufacturability.
2.4 Self-lubricating bearings
Most of all (reverse) medium pressure on the disc is transmitted to the bearing through the shaft, so the bearing plays a very important role. Some foreign butterfly valves rotate very lightly, and small-diameter valves can rotate with one finger, while some domestic butterfly valves rotate very heavily. This is not only related to coaxial, symmetry, machining accuracy, smoothness, and packing quality, but also very important. An important factor is the lubricity of the bushing material. The AWWA C504 standard requires that the shaft sleeve or bearing installed in the valve body should be a self-lubricating material, and the shaft sleeve has the problem of anti-friction lubrication and is not allowed to rust. Without bushing, even though the valve shaft is stainless steel, the valve body has problems with rust and glue sticking. The use of bushings also increases rigidity.
2.5 Connection between shaft and disc
The shaft and the disc of the small-diameter butterfly valve are preferably connected by a key or spline, or a polygonal shaft connection or a pin connection can be used, and the shaft and the disc of the large-diameter butterfly valve are mostly connected by a key or a taper pin. At present, the shaft and the butterfly plate are mostly connected by pins. The connecting pin is damaged under harsh working conditions, which is mainly due to manufacturing reasons, among which the accuracy of the fit is not good, the size of the pin is not selected properly, the hardness of the pin is not enough or the material used is not suitable, etc., should be paid attention to. The shaft and disc of the large butterfly valve can be connected by a special method.
2.6 Structure length
The structural length of the butterfly valve is developing towards a short series, but this approach should be done with caution. Because the length of the structure is too short, it has an effect on strength. International standards stipulate the length of the short series of flanged butterfly valves, but the structural length of valves with higher pressure should not be shortened, otherwise, problems will occur, especially for brittle materials such as cast iron.
How to choose butterfly valve
3.1 Choose by type
Butterfly valve has different structures such as single eccentric, inclined plate type, centerline type, double eccentric, and triple eccentric. The medium pressure acts on the valve shaft and bearing through the disc, so when the flow resistance of the high-pressure small diameter is large, the shaft diameter and disc thickness will increase accordingly. If a gate valve, ball valve, or globe valve is used, from the aspects of sealing and flow resistance The analysis is more suitable than using a butterfly valve. In recent years, the emergence of soft-sealed gate valves in water systems has improved the shortcomings of gate valves that are prone to sedimentation. Its flow resistance is relatively small, and it can be repaired online. It is very suitable for small and medium calibers, so it has gradually been used.
3.2 Choose according to the sealing structure
The main advantages of metal-sealed butterfly valves are reliable sealing, long life, high-temperature resistance, not ease of damage and tear, and high pressure bearing capacity. Therefore, it has been widely used in recent years, especially for key parts of engineering systems. If the rubber soft sealing butterfly valve is well designed and manufactured, it can get a good seal, and the large diameter is easy to change the ring online. However, rubber has aging problems and can only be used under low-temperature conditions, so it is widely used in tap water and water supply systems. Foreign metal-sealed butterfly valves are also used in pump stations, condensers, steam extraction systems, and heat exchange systems in thermal power plants, nuclear power plants are also used in pressure shell isolation, spray systems, and brine, etc., oil storage isolation, steam in petroleum refining systems Supply valve, desulfurization system and exhaust gas treatment, thermal cracking and catalytic units. There are also systems for petrochemical plants, cryogenics, slurry, and papermaking.
It is better to choose a metal-sealed butterfly valve for medium temperature and medium pressure conditions. Metal-sealed butterfly valves should also be used for places that require high reliability and long service life of the valve, and are not suitable for a long-term replacement. For the valve used for normal temperature water, if the requirements are not very high, it will not be replaced for a long time or it is inconvenient to replace, then the butterfly valve with a soft sealing structure should be used. If the rubber quality can be guaranteed, the large-diameter butterfly valve for water should still be dominated by rubber-seat butterfly valves.
Epilogue
The continuous emergence of new processes and new materials will improve the structural performance of the butterfly valve, expand the scope of use of the butterfly valve, and put forward updated requirements for the design and selection of the butterfly valve.
