The pipeline valve of city sub-high pressure and high pressure natural gas is required that the structure is simple, opening and closing quickly, low fluid resistance, good sealing performance, corrosion resistance, simple maintenance, reliable operation and long service life. Zhangjiagang Hong Kong and China Gas Co.Ltd adopted the directed buried fully welded ball valve which produced by the United States Cameron at the high pressure pipeline construction (2.5MPa). The directed buried fully welded ball valve solves the common problems of internal leakage, stem leakage, external surface corrosion and difficult to maintain
Ball valve body is welded after assembling six forged parts. It is characterized by tightly structure and reliable performance. At present, domestic large-diameter ball valves are most three pieces type split body. Parts are connected with bolts. Compared withe three pieces, the wall thickness of the fully welded ball valve forged pieces can be very thin at a same strength. The valve weight can reduced 1/4, and the resistance to pipe bending and extrusion is enhanced. Because the valve body flange and bolt are removed, the overall dimension is reduced and the potential external leakage is eliminated. In addition, the inner curve of the welding structure of the valve body is smooth, and the lubrication connection with the pipeline is maintained without dead angle, and the medium flow is good.
Ball valve i can be divided into fixed and floating ball n accordance with the form of ball. Domestic products usually adopt the floating ball structure. The ball under the pressure of media is tightly pressed on the sealing surface of export side to close the valve. Once the import and export pressure difference is large, it is often difficult to open because of a large torque. Fully welded ball valves are made of high strength forged steel stem and supported by stainless steel bearings, which are wrapped with PTFE to achieve the smooth and accurate rotation. Because the pivoted stem can absorb the thrust generated by the pressure of the pipe medium, and it can also reduce the friction between the ball and the seat, therefore, the torque of the operated recessed door is still very low and the ball valve switches freely under the whole process.
When the ball valve is near the fully closed or just open, narrow position between the ball and the seat (9 o'clock direction) , because of the natural gas is throttled into high turbulence and suffered high-speed erosion, and it becomes the most serious wear seat sealing zone., and finally cause leakage.CAMERON ball valves （DN≥300）adopt the unique design of rotating seat. When the valve opening or closing, the seat of the upstream and downstream of the valve seat is rotated 15°by the paddles degrees of the valve ball, so the sealing surface of the valve seat suffers erosion by natural gas in turn. After opening or closing the valve 12 times, the same position will be re-washed again, so that the seat is weared in average, and the service life is extended. Normal ball valve seat is fixed, after a long running,so the damage is on the same position and often leads to internal injuries. On the other hand, after the pipeline operates after a period of time, some harmful deposits will accumulate on the seat ring, then the valve seat will conventionally stuck or not seal properly, but the rotating seat can remove the seat surface when it is rotated.
The sealing material of the upper area of the stem: PTFE is a low friction and aging-resistance material with long service life. The stem of the fully welded ball valve(DN≥50) is also equipped with sealing grease injection system, once the sealing surface is damaged by the deposits which came into the valve seat, it also can maintain the sealing through the secondary seat grease injection. Common ball valves don’t have the sealing grease injection structure, if the sealing surface damaged and the leakage occurred, the only solution is replaced the whole ball valve.
Although the ball valve takes the upper and lower valve seat separation sealing, after a long running, the seat is inevitably weared. When the valve is closed, the upstream seat may have a internal leakage, and the leaking medium will come into the valve cavity. The unique design of downstream valve seat can effectively avoid excessive pressure in the valve ball. Once the valve pressure exceeds the downstream pressure by about 138KPa, the downstream seat is pushed away from the ball and the pressure is released into the downstream pipe. Ball valve installs with the pressure release tube and the cavity, and puts pressure pipe leads to the ground. Whether the valve is in a fully open or fully closed position, and it can open the valve chamber to discharge the pressure. Also the valve seat sealing performance can be tested through the release of pressure in the chamber.