Abstract: Aiming at the two-way sealing problem of metal hard seal butterfly valve, based on the principle of oil cylinder piston in the design of hydraulic components, a new metal hard seal butterfly valve with moving seal and non inducing steel ring under the action of dynamic water is designed. The basic structure is analyzed and the design of double eccentric structure is determined; The forward specific pressure, reverse specific pressure and moving distance of metal seal ring are calculated, which effectively solves the reverse pressure problem of double eccentric metal hard seal butterfly valve; The selection of sealing pair materials is analyzed, which effectively ensures the sealing effect and specific pressure requirements.

Key words: metal hard seal butterfly valve; Movable seal; Hard seal; design calculation 

Butterfly valve is widely used in medium and low pressure pipelines because of its simple structure, light overall product quality, small floor area, simple operation and rapid opening and closing. With the development of national economy and the progress of industrial technology revolution, the requirements for butterfly valves are higher and higher. Metal hard seal butterfly valves suitable for high pressure, high temperature, wear and corrosion conditions are developing rapidly, and the market demand is increasing year by year. This paper introduces a new type of metal hard sealing butterfly valve designed for the water supply of an urban pipe network.

1 design of sealing pair

The sealing pair structure of metal hard sealing butterfly valve is the key of product design. When the valve is closed, the sealing surface of the valve plate is consistent with the elastic valve seat, and the sealing pair produces small elastic deformation under the action of medium pressure, so as to realize sealing. Therefore, the basic requirements for the sealing pair are: good sealing performance, flexible operation and small torque.

In order to meet the basic requirements for sealing pair, the designed butterfly valve adopts double eccentric structure, as shown in Figure 1. The structure 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 moment, reduces the wear and improves the service life of the valve seat. The scratch is greatly reduced, so that the double eccentric butterfly valve can also adopt metal seat, which expands the application of butterfly valve in the field of high wear medium. However, the sealing principle of the previous metal hard sealing butterfly valve 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 through the elastic deformation caused by the extrusion of the valve seat by the butterfly plate. Therefore, the requirements for the closing position are very high (especially the metal valve seat) and the pressure bearing capacity is low, especially when the valve needs two-way pressure bearing, the reverse pressure bearing is difficult to meet the positive pressure bearing requirements.

Fig. 1 double eccentric structure (1) valve plate; 2. Valve rod; 3. Elastic valve seat; 4 gland; 5 valve body; M center line of valve seat sealing; N center line of valve body channel; P center line of valve shaft; Q running track of sealing surface of butterfly plate; E) radial eccentricity; L axial eccentricity;)

The basic idea of hydraulic design is introduced in the design, and the sealing valve seat is designed as a piston type that can move under the action of dynamic water in a certain range. In the positive direction, the sealing is ensured by the position of valve plate and valve seat; In reverse, the greater the hydrodynamic pressure, the greater the force acting on the sealing valve seat, the greater the compression force between the valve seat and the valve plate, and the better the sealing performance. Its structural form is shown in Figure 2.

Fig. 2 sealing structure (1) butterfly plate; 2. Metal sealing ring of valve body; 3. High strength wear-resistant rubber ring; 4 gland; 5 valve body; 6 stainless steel surfacing layer;)

As can be seen from Figure 2, the sealing ring on the valve body is a special-shaped stainless steel piston ring, which is located in the sliding groove of the valve body. When the valve is closed in the positive direction, the stainless steel sealing ring is located in the middle position determined by the design position, and the valve plate squeezes the stainless steel metal sealing ring. At the same time, the high-strength wear-resistant rubber sealing ring between the stainless steel sealing ring and the valve body is extruded, which not only has the function of sealing medium, but also supports the stainless steel ring in its elastic deformation. The stainless steel ring has both the hardness and stiffness of metal and the elasticity of rubber. The sealing surface of the valve plate is made of stainless steel overlaid on nodular cast iron or carbon steel substrate. At this time, the sealing specific pressure is formed on the sealing pair composed of the valve plate sealing surface and the sliding sealing ring to achieve the sealing effect and requirements. When the valve is closed reversely, the medium pressure will cause the fitting clearance between the valve plate and the valve shaft, the fitting clearance between the valve shaft and the valve body bearing, the fitting clearance between the bearing and the valve body shaft hole and the machining dimensional error to accumulate on one side, that is, the side away from the metal sealing ring, so that the sealing specific pressure formed by the sealing pair composed of the valve plate sealing surface and the sliding sealing ring is reduced. At this time, under the action of medium pressure, the stainless steel metal sealing ring moves in the same direction with the valve plate along the diameter axis of the valve body and close to the valve plate, so as to achieve the sealing specific pressure required by the sealing and meet the requirements of reverse sealing.