Among many valves, only the sealing performance of stainless steel ball valve is good, but in the performance of stainless steel ball valve, sealing performance is one of its main indicators. The sealing structure of the stainless steel ball valve is mainly composed of three parts, one is the sealing between the main and auxiliary valve bodies, which is realized by the sealing adjustment gasket; the other is the sealing between the main valve body and the valve stem, which is formed by the standard "V"-shaped packing Or 0-shaped sealing ring to form the outer sealing structure of the stainless steel ball valve. The third is the sealing between the ball and the main and auxiliary valve bodies, which is realized by the sealing seat, which constitutes the main sealing structure of the stainless steel ball valve. The tightness of the main sealing structure affects the working performance of the stainless steel ball valve, and is the key sealing structure in the design and manufacture of the stainless steel ball valve.

1. Sealing requirements of stainless steel ball valve

The tightness index of the stainless steel ball valve is determined by the internal leakage and external leakage requirements. The national standard stipulates that the tightness of the non-metallic sealed stainless steel ball valve shall be carried out according to Grade A of GB/T13927 "General Valve Pressure Test". The aerospace industry standard stipulates that the stainless steel ball valve used in the ground gas pipeline system "under the air pressure condition of 0.05MPa and 1.1 times the nominal pressure, keep the pressure for 3 minutes, and the internal leakage rate should not be greater than 3 × 10-5Pa·m3/s (without Bubble escape); under the air pressure condition of 1.1 times the nominal pressure, keep the pressure for 3min, the external leakage rate should not be greater than 1×10-5Pa·m3/s (no bubble escape).”

2. The main factors affecting the sealing performance of stainless steel ball valves

The main factors affecting the sealing performance of stainless steel ball valves are the roughness of the sealing surface and the parameters of spherical roundness. As we all know, no matter how advanced processing technology is adopted, it is impossible to completely solve the roundness error of the sphere and the microscopic defects of the sealing surface. Therefore, in order to achieve a good sealing effect and ensure that the processing process can be achieved, it is necessary to determine the appropriate sealing surface roughness. and the value of the sphere roundness parameter. The relevant data recommends that the maximum allowable value of the roughness Ra of the spherical sealing surface is 0.4µm; the maximum allowable value of the Ra value of the sealing seat roughness is 0.8µm when PTFE is used as the sealing material. The roundness tolerance of the spherical surface is more appropriate according to the 6-level accuracy in the annex 2 of GB/T1184.

3. Analysis of the main seal structure of the ball valve

In the early ball valves, metal materials were generally used as sealing structures (also known as hard seals). With the development of engineering plastics technology, especially the emergence of polytetrafluoroethylene, the ball valve sealing structure has an ideal sealing material. The soft sealing forms of the common ball valve main sealing structure include rigid sealing seat, sealing seat with elastic elements, PTFE lip sealing seat and combined sealing seat composed of elastic expansion ring and PTFE lip sealing seat, etc. form. The lip-shaped structure of the lip-type sealing seat makes it have better elasticity, which can ensure the pre-compression of the sealing surface and compensate for the pre-compression of the sealing surface in use, which is superior to the rigid structure.