Abstract: This paper discusses the sealing structure of stainless steel ball valve. Focus on the analysis of the main valve seal structure, factors affecting the sealing performance of the ball valve and the use of performance, reliability and sealing material selection, the elastic ring and poly four chloride lip seat combination seal structure.

Ball valve with its characteristics of fast shut-off, as everyone knows, change the media flow and automatic control is widely used in all kinds of piping system, can use manual, electric or pneumatic drive mode. Sealing requirements of ball valve

Ball valve sealing index by internal leakage and external leakage requirements to determine, the national standard A GB/T13927 " seal according to general valve pressure test " execution of non metal sealing ball ", for the duration of the experiment no visible leakage "; aerospace industry standard ball " in the 0.05MPa and 1.1 times the nominal pressure pressure conditions for the ground gas pipeline system, pressure 3min, internal leakage rate should be less than 3 ×10-5Pa ·m3/s ( no bubbles escape ) in air condition; 1.1 times the nominal pressure, holding pressure 3min, the leakage rate should be less than 1 ×10-5Pa ·m3/s ( no bubbles escape ). "

2 Effect of ball valve sealing mainly because

Prime factors affecting ball valve sealing of the sealing surface roughness and ball roundness parameters. As everyone knows, no matter how advanced processing technology, and can not completely eliminate the roundness error and surface of sphere micro defects, sealing and therefore, in order to achieve good sealing effect and ensure the realization of the process, must determine the sealing surface roughness and ball roundness parameters suitable. Information recommendation, the sphere sealing surface roughness Ra value of the maximum allowable value for 0.4µ m; sealing seat roughness Ra value in PTFE sealing materials to the maximum allowed value of 0.8µ M. The roundness tolerance of spherical surface, according to the GB/T1184 of schedule 2 of 6 grade precision more appropriate.

3 ball valve seal structure analysis

Early ball valve, generally using metal materials as sealing structure ( also called hard seal ), along with the development of engineering plastics technology, especially poly four chloride appear, the ball valve sealing structure sealing material is an ideal. Common ball valve seal structure of soft seal forms a rigid sealing seat, with elastic element sealing seat, PTFE lip seal seat and the elastic expansion ring and PTFE lip seal seat combination type sealing seat form. Lip lip seal seat with, so that it has good flexibility, in use can ensure the sealing surface of the pre-compression and the sealing surface of precompression compensation and is better than the rigid structure. But in operation, PTFE elastic-plastic deformation capacity and stability in temperature change is not very ideal, and it has good cold flow tendency; at the same time, because of the use of pressure range of ball valve is larger, therefore, the use of PTFE as the sealing lip type material of the sealing seat, in the low pressure or long-term use is easy to generate leakage. To solve this problem is using elastic ring and PTFE lip seal seat combination type sealing seat, it can rely on the elastic metal ring good elastoplastic deformation ability, enhance the overall elastoplastic deformation ability and compensation ability, improve the ball valve sealing ability.

3.1 ball valve sealing conditions

To ensure that the valve can meet the requirements of sealing provisions, shall promote the ball with the medium pressure ( floating ball valve or sealing seat ( fixed ball valve to produce small displacement, the sealing seat surface of a certain plastic deformation, the formation of the necessary sealing pressure ratio. Seal than the pressure is too high will lead to seal the pressure more than the material allowable specific pressure and damage, the seal failure, can lead to operating torque increased to influence performance; is too low to make the sealing seat processing error surface elastoplastic deformation are not enough to offset the sphere, leading to the main seal can not meet requirements sealing provisions.

Ball valve sealing condition is: qMF < Q < [q]

In the formula: qMF - ensure the seal must sealing surface than the pressure; [q] - allowable specific pressure sealing surface; Q - Calculation of the actual pressure ratio.

The sealing mechanism of 3.2 floating ball valve

As shown in Figure 1 (a ), because the sphere is no fixed bearing, floating ball valve is closed, in the medium under pressure, the ball will produce certain floating amount ( displacement ), left the population of end sealing seat is pressed on the export side sealing seat, make the medium in the outlet end sealing seat by blocking. The floating ball valve is in fact only outlet seal seat play sealing action, and the population of end sealing seat and the ball keeps a very small gap between the floating.

The sealing mechanism of 3.3 fixed ball valve

High pressure or nominal diameter ball valve DN ≥200mm, often using a fixed structure. Fixed ball valve the ball does not produce floating amount, can ensure the two-way seal, the valve operating torque reduction; sealing amplitude load fluctuation seat. Fixed ball valve can be divided into the ball before sealing ( population sealed ) and the ball seal ( outlet sealing type ) valve