The Sealing of Fully Welded Forged Ball Valves for Natural Gas Pipelines (Part two)

2. Reasonable design of key components
(1) Sealing
In the reasonable design of the sealing structure of fully welded ball valves in natural gas long-distance pipelines, attention should be paid to the design content of key components. The most important thing is to accurately calculate the sealing force to improve the service life of the ball valve’s sealing structure. In general, when calculating the sealing force, it is necessary to give a highly reliable calculation result of sealing force according to the following calculation formula, to ensure that the sealing effect of the valve seat meets the actual requirements of the long-distance natural gas pipeline. The formula for calculating the sealing force Q1 of the front seat of the valve is: Q1 equals  (d1²-0.4D2²-0.6D1²), where d1 represents the outer diameter of the front seat of the ball valve, and D1 represents the inner diameter. As for the sealing force Q2 of the valve seat behind the valve, it should be calculated according to the following formula: Q1 equal to  (0.6D2²+0.4D1²-d2²).
 
(2) The width of the sealing surface
When determining the width of the sealing surface, corresponding research should be carried out in combination with the specific characteristics and geometry of the ball valve’s sealing structure. Normally, the ball and its seat sealing structure can be regarded as an elastic body, and when calculating the contact force between elements, it also means calculating the contact stress between the ball and the seat. According to the characteristics of the contact surface and the semi-elliptical distribution law, it can be known that the width of the sealing surface is closely related to the radius of the sphere, the material coefficient of the valve seat’s sealing structure, and the elastic modulus of the sphere material.
 
3. Properly selection of the pipeline ball valve spring
From the selection process of the pipeline ball valve spring, it can play a role in enhancing the sealing of the ball valve. Generally, the seat of the ball valve is elastic, and the disc spring is selected to minimize the space occupied by the ball valve. The structure of this spring is more uniform than the cylindrical helical spring. Relevant personnel should use the following calculation formula when determining the preload force required for the spring: preload force equals to 4π x minimum sealing specific pressure. The square value of the inner diameter of the valve seat minus the square value of the outer diameter of the valve seat.
 
4. Determining the spherical sealing structure of the ball valve
(1) Double sealing
To ensure that the sealing performance of fully welded cast ball valves in long-distance natural gas pipelines plays an important role during natural gas transportation, it is necessary to adopt a double sealing to strengthen the sealing. Double sealing refers to embedding a soft sealing ring in the metal valve seat groove, so that the overall sealing effect is good in fire resistance. When this double sealing structure encounters a fire or in a high-temperature environment, although the soft sealing structure is damaged, there is still a hard sealing structure, thereby achieving the purpose of preventing fire. Therefore, the double sealing has outstanding practical value in the design of the sealing structure of the natural gas fully welded casting ball valve.
 
(2) Resilient valve seats
When designing the elastic valve seat, it is also necessary to use the double-piston effect, and install elastic valve seats on the front and rear sides of the ball valve, so that when the ball valve seat is subjected to spring force and medium pressure, it can be transferred to the ball and the valve seat can maintain normal operation, thereby enhancing its sealing. When the sealing structure of the valve seat fails, the medium will push the pressure on the ball back to the ball through the middle cavity, thereby achieving a similar sealing effect. Therefore, when designing the elastic valve seat, it is necessary not only to obtain the pressure value correctly, but also to compensate the medium pressure accordingly to achieve the sealing effect, and also to promote the ball valve to protect for the long-term natural gas pipeline for a longer time.
 
(3) The structure of the stem
When designing the structure of the valve stem, the following issues should be paid attention to ① Filling materials: the valve stem needs to rotate in the ball valve, so the generally used filling material is an O-ring, and the upper and lower seals are adopted at the same time. This kind of filling material is easy to install and has good sealing performance. ②Grease injection structure: the ball valve stem will wear after long-term use, so the grease injection structure can prevent the valve stem from leaking after long-term wear, thus affecting the ball valve seal. Therefore, attention should be paid to the layout of the valve stem structure when designing the sealing structure of fully welded cast ball valves for long-distance natural gas pipelines.
 
5. Conclusions
To sum up, through the reasonable design of the sealing structure of the fully welded cast ball valve of the long-distance natural gas pipeline, the ball valve can be made to have a certain degree of safety and fire resistance, ensuring that natural gas can play a real role in energy supply. In addition, the double sealing structure can make the valve sealing effect better. Therefore, relevant personnel should scientifically design the ball valve’s sealing structure in combination with the actual situation, to provide a reference for later product development.
 

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