Ball valves are classified according to the support mode of the ball.

1. Floating ball valve: Its main feature is that the ball has no support shaft, the ball system is supported by the valve seats at both ends of the valve inlet and outlet, and the valve stem and the ball are movably connected. The ball body of this kind of ball valve is clamped by two seats and is in a "floating state". The ball can rotate freely between the two seats by means of a handle or other driving device through the valve stem. When the flow passage hole of the sphere is aligned with the valve passage hole, the ball valve is in an open state, the flow is smooth, and the fluid resistance of the valve is the smallest. When the sphere is rotated at 90 C, the flow passage hole of the sphere is perpendicular to the valve passage hole, and the ball valve is closed. Under the action of fluid pressure, the sphere is pushed to the valve outlet end (hereinafter referred to as the back of the valve) seat to tighten and ensure sealing.

The main advantages of floating ball valve are simple structure, convenient manufacture, low cost and reliable operation. The sealing performance of floating ball valve is related to fluid pressure. Under the same other conditions, generally speaking, the higher the pressure, the easier to seal. However, consideration should be given to whether the seat material can withstand the load transferred to it by the sphere, since the force exerted by the fluid pressure on the sphere will be transferred to the back seat of the valve. In addition, for larger size floating ball valves, when the pressure is higher, the operating torque increases, and the sphere self-weight is larger. The pressure distribution generated by self-weight on the seating surface is uneven. Generally speaking, the pressure of the half-ring along the horizontal plane of channel diameter is smaller, and the pressure of the lower half-ring is larger, resulting in uneven wear and tear of the valve seat and leakage. Therefore, floating ball valves are generally suitable for valves less than PN10 and less than DN200. In order to make the floating ball valve have good sealing performance under lower working pressure, a certain pre-tightening force must be applied between the ball and the valve seat. The insufficient preloading force can not guarantee the seal, and excessive preloading force will increase friction torque, and may also lead to plastic deformation of seat material and damage the sealing performance. For low pressure ball valves, the sealing and reliability of moving ball valves can be limited by adjusting the thickness of sealing gaskets between flanges. Pre-tightening force should be paid attention to in design.

In order to ensure the tightness and reliability of floating ball valve when working, attention should be paid to the design:

A. Adequate sealing specific pressure should be provided on the contact surface between the sphere and the seat, but the sealing conditions should be satisfied.

B. Sealing specific pressure between seat and bottom of groove of valve body should be equal to that on contact surface of sphere. If the specific pressure of the seal is insufficient, the outer diameter of the bottom of the valve seat can be reduced or the inner diameter can be increased. Several triangular concentric grooves with a top angle of 60 mm and a depth of 1-2 mm can also be opened on the bottom.

C. The sphere should have correct geometry and appropriate surface roughness.

D. Rational selection of sealing materials

At present, the ideal way to exert preload is to use ball valve with elastic seat. The elastic seat can greatly increase the elastic deformation range of the seat itself, so that it can maintain good sealing performance under low or high pressure. As long as it works within the elastic deformation range of the elastic seat, it can basically keep its sealing specific pressure constant, so that the seat can not be damaged by overload.

2. Fixed ball valves: the ball is connected with the upper and lower stem, or made into a whole connecting ball, that is, the ball and the upper and lower stem are forged (welded) together and installed on the bearing. The ball can rotate freely along the axis perpendicular to the valve passage, but can not move along the passage axis. Therefore, when the fixed ball valve is working, all the force produced by the fluid pressure in front of the valve on the ball is transmitted to the bearing, and the ball will not move to the back seat of the valve, so the valve seat will not bear excessive pressure. Therefore, the fixed ball valve has small torque, small seat deformation, stable sealing performance and long service life, which is suitable for high pressure and large diameter occasions. The key to the sealing of fixed ball valve lies in the correct selection of the structure of elastic sealing seat, the rational design of the size of each part of the seat, and the ingenious use of fluid pressure or spring force to achieve the sealing requirements.