What are the characteristics and structure of ball valves？

According to the structural form, ball valves can be divided into two types: floating ball valves and fixed ball valves.

The ball valve is a valve developed on the basis of the plug valve. It has some advantages of the plug valve.

① Medium and small diameter ball valves have simple structures, small dimensions and light weight.

② The fluid resistance is small, and the ball valve has the smallest fluid resistance among all types of valves. This is because the cross-sectional areas of the valve body channel, ball channel and connecting pipe are equal and connected in a straight line when fully open.

③ Opening and closing is quick and convenient, and the flow direction of the medium is not restricted.

④ The opening and closing torque is smaller than that of the plug valve. This is because the contact area of the sealing surface of the ball valve is smaller, and it takes less effort to open and close than the plug valve.

⑤ The sealing performance is good because the material of the ball valve sealing ring is mostly made of plastic, which has a small friction factor; when the ball valve is fully open, the sealing surface will not be eroded by the medium.

The disadvantage of the ball valve is that the ball is difficult to process and grind.

Ball valves are mainly composed of valve body, ball, valve seat, valve stem and transmission device.

① Valve body According to the valve body channel form, ball valves can be divided into straight-through ball valves), three-way ball valves (Figure 1-1) and four-way ball (Figure 1-2). There are four types of valve body structures: integral, two-piece, three-piece and split. The integral valve body is generally used for smaller diameter ball valves; the two-piece and three-piece valve bodies are suitable for medium and large diameter ball valves. , the split valve body is mainly used for hard-sealed ball valves in the coal chemical industry.

Figure 1-1 Three-way ball valve

Figure 1-2 Four-way ball valve

② The ball is the opening and closing part of the ball valve, and its sealing surface is the surface of the ball. The surface roughness of the sphere requires high accuracy. For straight-through ball valves, the channel on the ball is straight; for three-way ball valves, the ball channels are L-shaped, T-shaped and Y-shaped, and their distribution form is the same as that of plug valves. According to the way the ball is fixed in the valve body, ball valves can be divided into two types: floating ball valves and fixed ball valves.

a. Floating ball valve. The ball is held by two valve seats and can float. Under the action of medium pressure, the ball is pressed against the sealing ring on the outlet side to seal it. This kind of structure is simple, single-sided sealing, and good sealing performance, but the sealing surface has a large bearing capacity, so the opening and closing force is also large. Generally suitable for medium and low pressure, medium and small diameter valves (≤DN200)

b. Fixed ball valve. The ball is supported and fixed by the upper and lower valve stems. It can only rotate and cannot move horizontally. In order to ensure sealing, it must have a floating valve seat that can generate thrust to press the sealing ring against the ball. This kind of structure is complex, with large dimensions and small opening and closing torque. Suitable for high pressure and large diameter ball valves (≥DN200).

③The lower end of the valve stem is movablely connected to the ball, which can drive the ball to rotate. The opening and closing action of the sphere uses a wrench according to the pressure and diameter, or pneumatic, hydraulic, electric or various linkage transmissions.

For ball valves used for regulation, users mostly prefer split structures. Because when used as a regulating valve, each component of the ball valve is under heavy operating conditions, so frequent maintenance and repairs may be required. It is best to integrate the valve stem and the ball into one body, so as to avoid damage caused by high partial pressure. The dynamic load caused by the medium wears the joints.

For ball valves used as cut-offs, it is more economical to use an integral welded structure. Because its working conditions are not as heavy as those during adjustment, the maintenance of parts is also smaller, and the valve stem and ball are subject to less dynamic loads. Therefore, even if the connection is loose, the wear will not be too great, and the sealing But it has great benefits.

From the perspective of floating balls and fixed balls, it is more inclined to use fixed balls, that is, balls with trunnion support. Generally speaking, when the pressure is lower than PN40, from economic and technical considerations, it is more appropriate to use a floating ball, because in this case the ball is pushed to the outlet end and can hold well under a small pressure drop. of seal.

When the diameter is larger than DN150, fixed ball valves are most commonly used. This is because the ball of this caliber valve has a large mass, about 50kg or more. If a floating ball is used, the ball will press on the sealing seat, which will cause great wear to the sealing seat during operation. On the contrary, the fixed ball seals axially along the flow channel, so it will not cause such great wear on the seal seat.

Ball valves with a pressure of PN50 and a diameter of DN50 or above also mostly use fixed balls, because if a floating ball is used, the force exerted by the medium on the outlet sealing seat through the ball is calculated based on the medium pressure on the area formed by the diameter of the flow channel. of. If a fixed ball is used, the pressure of the medium acts on the end face of the sealing seat, and the pressing force between the ball and the sealing seat generated by this force is much smaller than the former. In this way, the friction force is reduced, the normal component force on the connection surface between the ball and the valve stem is reduced, the torque is reduced, the reliability is improved, and the service life is extended.

It is also worth noting that foreign ball valves used in oil, natural gas and flammable medium transportation pipelines are required to have fireproof structures and have been verified by fireproof tests. Ball valves used in vulcanized environments are also required to use materials resistant to sulfide stress cracking (SSC). The performance parameters of commonly used ball valves are as follows: nominal size DN10~1400; nominal pressure PN16~420; operating temperature t≤600℃.