4. Selection of materials of ball valves
4.1 Materials of valve bodies and ball shafts
Titanium and titanium alloys have excellent properties such as low density, high specific strength, good corrosion resistance, high and low temperature resistance, cold and hot forming, and good welding performance. They especially have excellent corrosion and scouring resistance in seawater, and their corrosion rates in seawater is close to zero, which are preferred structural materials for marine environments. Therefore, the application of titanium alloy to ball valves can effectively improve the durability, reliability and usability of marine ball valves, and reduce repair and maintenance costs. Through the systematic comprehensive analysis of the mechanical properties and corrosion resistance of titanium alloy, titanium alloy ZTi2 is adopted to manufacture valve bodies by the investment casting and titanium alloy TC4 bars are used to make the ball shaft in this article.
Although titanium alloys have many excellent properties, they have the disadvantage of not being resistant to wear. This shortcoming will cause the wear resistance of the mating part composed of ball shafts and valve seats made of titanium alloys to decrease, thereby affecting the sealing performance and service life of the ball valve. The micro-arc oxidation technology is used to perform surface hardening treatment on valve bodies and ball shafts (Figure 4) to improve the wear resistance of ball valves. Meanwhile, it can effectively prevent the parts made from copper, steel, and copper-based alloys that are in contact with the titanium alloy from accelerating corrosion.
Figure 4 The schematic diagram of ball shafts treated by micro-arc oxidation
4.2 Materials for valve seats
Ball valve seats generally use polytetrafluoroethylene, which has excellent chemical stability, good lubrication, no stickiness, corrosion resistance, low friction coefficient, close static and dynamic friction coefficients. However, PTFE has poor mechanical strength, rigidity, hardness and low bearing capacity, and is prone to creep (cold flow) under external force; besides, it has low thermal conductivity coefficients, great thermal expansion, and poor wear resistance. Therefore, the new titanium alloy ball valve does not adopt pure PTFE, but uses reinforced PTFE filled with 25% carbon fiber. The above shortcomings will be greatly improved. The reinforced PTFE is used for ball valve seats, which has a long service and whose sealing effect is also greatly improved.
4.3 Hollow rubber rings
Hydrogenated nitrile rubber (HNBR) is a highly saturated elastomer obtained by special hydrogenation of nitrile rubber. It has good heat resistance, excellent chemical resistance, excellent ozone resistance and good resistance to compression and permanent deformation. Meanwhile, hydrogenated nitrile rubber also has the characteristics of high strength, good tearing performance, and excellent wear resistance, which is one of the rubbers with excellent comprehensive performance. The hydrogenated nitrile rubber used to make the hollow rubber ring can ensure that it can provide the minimum pre-tightening force required for the sealing of ball valves and reliable elastic compensation.
5. Testing
To verify the sealing effect and service life of the new marine titanium alloy ball valve, a marine titanium alloy ball valve with DN80 (PN3.0MPa) is designed, and two different sealing forms of PTFE spring group elastic valve seats and reinforced PTFE integral elastic compensation valve seats are compared on the testbed. The test shows that under the testing conditions of test pressure of 3.0MPa and a flow rate of 5.5m/s, the PTFE spring group elastic valve seat leaks at pressure between 0 to 3.0MPa after testing for 6000 times, while titanium alloy ball valves with reinforced PTFE integral elastic compensation valve seats don't leak at the pressure of 0 to 3.0MPa after operating for 100,000 times. It can be seen that the new marine titanium alloy ball valve has good sealing performance and long sealing life at high and low pressure. Its service life is 16 times longer than the ordinary ball valve.
6. Conclusion
(1) The use of titanium alloys to produce marine ball valves avoids corrosion and scouring of seawater, improves the reliability and service life of marine ball valves and saves maintenance costs for ball valves.
(2) The integral elastic compensation structure with soft sealing proposed in this article has excellent sealing performance, which successfully solved the problem of leakage of the current marine ball valve after a period of use and prolonged the service life of the ball valve.
4.1 Materials of valve bodies and ball shafts
Titanium and titanium alloys have excellent properties such as low density, high specific strength, good corrosion resistance, high and low temperature resistance, cold and hot forming, and good welding performance. They especially have excellent corrosion and scouring resistance in seawater, and their corrosion rates in seawater is close to zero, which are preferred structural materials for marine environments. Therefore, the application of titanium alloy to ball valves can effectively improve the durability, reliability and usability of marine ball valves, and reduce repair and maintenance costs. Through the systematic comprehensive analysis of the mechanical properties and corrosion resistance of titanium alloy, titanium alloy ZTi2 is adopted to manufacture valve bodies by the investment casting and titanium alloy TC4 bars are used to make the ball shaft in this article.
Although titanium alloys have many excellent properties, they have the disadvantage of not being resistant to wear. This shortcoming will cause the wear resistance of the mating part composed of ball shafts and valve seats made of titanium alloys to decrease, thereby affecting the sealing performance and service life of the ball valve. The micro-arc oxidation technology is used to perform surface hardening treatment on valve bodies and ball shafts (Figure 4) to improve the wear resistance of ball valves. Meanwhile, it can effectively prevent the parts made from copper, steel, and copper-based alloys that are in contact with the titanium alloy from accelerating corrosion.
Figure 4 The schematic diagram of ball shafts treated by micro-arc oxidation
4.2 Materials for valve seats
Ball valve seats generally use polytetrafluoroethylene, which has excellent chemical stability, good lubrication, no stickiness, corrosion resistance, low friction coefficient, close static and dynamic friction coefficients. However, PTFE has poor mechanical strength, rigidity, hardness and low bearing capacity, and is prone to creep (cold flow) under external force; besides, it has low thermal conductivity coefficients, great thermal expansion, and poor wear resistance. Therefore, the new titanium alloy ball valve does not adopt pure PTFE, but uses reinforced PTFE filled with 25% carbon fiber. The above shortcomings will be greatly improved. The reinforced PTFE is used for ball valve seats, which has a long service and whose sealing effect is also greatly improved.
4.3 Hollow rubber rings
Hydrogenated nitrile rubber (HNBR) is a highly saturated elastomer obtained by special hydrogenation of nitrile rubber. It has good heat resistance, excellent chemical resistance, excellent ozone resistance and good resistance to compression and permanent deformation. Meanwhile, hydrogenated nitrile rubber also has the characteristics of high strength, good tearing performance, and excellent wear resistance, which is one of the rubbers with excellent comprehensive performance. The hydrogenated nitrile rubber used to make the hollow rubber ring can ensure that it can provide the minimum pre-tightening force required for the sealing of ball valves and reliable elastic compensation.
5. Testing
To verify the sealing effect and service life of the new marine titanium alloy ball valve, a marine titanium alloy ball valve with DN80 (PN3.0MPa) is designed, and two different sealing forms of PTFE spring group elastic valve seats and reinforced PTFE integral elastic compensation valve seats are compared on the testbed. The test shows that under the testing conditions of test pressure of 3.0MPa and a flow rate of 5.5m/s, the PTFE spring group elastic valve seat leaks at pressure between 0 to 3.0MPa after testing for 6000 times, while titanium alloy ball valves with reinforced PTFE integral elastic compensation valve seats don't leak at the pressure of 0 to 3.0MPa after operating for 100,000 times. It can be seen that the new marine titanium alloy ball valve has good sealing performance and long sealing life at high and low pressure. Its service life is 16 times longer than the ordinary ball valve.
6. Conclusion
(1) The use of titanium alloys to produce marine ball valves avoids corrosion and scouring of seawater, improves the reliability and service life of marine ball valves and saves maintenance costs for ball valves.
(2) The integral elastic compensation structure with soft sealing proposed in this article has excellent sealing performance, which successfully solved the problem of leakage of the current marine ball valve after a period of use and prolonged the service life of the ball valve.
Next: The Operation of Valves
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