When use pipeline to transmit liquid or gas, the cost is only 1/3 of railway transportation and 2/3 of marine transportation. So, pipeline has already been highly developed for 100 years. The valves used for pipeline are called as pipeline valves, including ball valve
, gate valve
, check valve
and plug valve
, which are dedicated valves meeting special requirements for pipeline transportation and having special functions. The ball valve used for pipeline is pipeline ball valve. Technical standards specifying pipeline ball valve are API 6D issued by American Petroleum Institute.
Long distance pipeline is global energy supplying line from Arctic Circle to equator, mountain to seabed, plateau to desert, crossing through seismic belt, marshland, tundra, river, lake and hillside. It is erected or directly buried under ground. The pipeline is operated in field having difficulties for repairing. So, the pipeline is required to have service life of 30 years. In general, transmitted oil or gas still contains sulfur, rust and metal particles though having been processed, which requiring zero level sealing. Wicked actual service environment and importance of energy supply have strict technical requirements of pipeline ball valve.
1. Strength and toughness
Besides bearing pressure of internal media, the ball valve also should sustain load of foundation. Axial tension and pressure caused by temperature variation, external load caused by landslide, land subsidence and flood should be considered. If the valve is used in frigid or cold zone, low temperature impact toughness also should be considered in case of low temperature brittle rupture. For fully-welded ball valve, fracture toughness should be taken into consideration based on theory of fracture mechanics, due to welded joint and heat affected zone.
2. Zero level sealing
Valve needs zero level sealing, ensuring effective truncation on downstream pipeline. Considering the influence of metal particles in media on zero level sealing, metal-to-metal sealing is used as primary sealing and PTFE/rubber-to-metal is used as secondary sealing. Once sealing losing efficacy, urgent sealing measures should be taken.
3. Having fire safety and anti-static performance
Fire safety should be considered when designing pipeline ball valve. Once catching fire, the out leakage and inner leakage of valves should not exceed than leakage standards specified by API 607. The ball is clamped by non-metal materials, which may cause static. So, it must contact with body. Under 24VDC, resistance value should not be higher than 10 Ohm.
4. DBB (Double Brock & Bleed)
When chamber is discharging, the upstream seat and downstream seat should be cut off automatically at the same time, ensuring safety during discharging.
5. In case of being clamped by pressure in chamber
No matter the valve is opened or closed, the media in chamber should be prevented from being clamped. If the media are clamped, gas or liquid media should maintain the pressure inside chamber. Discharging large value of pressure in chamber should not exceed than 1.33 times of rated pressure value. The caliber of external pressure relief value is DN15 or larger.
Media in chamber can be discharged. On-line sealing test can be carried out on valve through drain hole.
7. On-off indication
No matter the valve is manual operation or power driven. The valve should have obvious dial indicator, showing the valve is opened or closed.
8. Transmission chain
The design torque of transmission chain should be 2 times of large torque of ball valve at least.
9. Working conditions of vulcanization
Pressure containing members and materials of bolt should have stress cracking resistance, meeting requirements of NACE and MR 0175.
10. Urgent cutting off
Long distance pipeline transmission system adopts SCADA System. When pressure drop rate and duration time of pipeline reach a set value, the valve should be cut off urgently.
11. Potential corrosion and stress corrosion of underground water
When using full welding structure, cathode of pipeline should contact with ground and the surface should have corrosion resistance, in case of potential corrosion and stress corrosion.
Pipeline ball valve manufacturing
1. Body processing
High sealing performance and low torque are the two objectives for designers of ball valve. The two objectives depend on the advancement of ball crusher. Body processing in China commonly adopt turning, adding a rotating knife rest on common lathe. The feature is simple, convenient for operation. However, due to abrasion of cutting blade and the difference of cutting speed, body has poor machining precision and circular degree. Particularly, large caliber ball processing cannot meet requirements of design. That is the reasons for Chinese ball valve having high torque and poor sealing. The solution is to use milling and grinding or use CNC ball crusher having compensation, equipped with precision online inspection device.
2. Body welding
For full welding body, welding is carried out after products assembling. It is the later process. The valve is not allowed to be installed after welding. So the welding has to face with the following problems:
Prediction and control of temperature field during welding;
Prediction and control of axial deformation and radial deformation during welding;
Control of residual stress during welding.
A welding method can be used to figure the problems. Optimize welding technology including chosen of materials, wire for welding and soldering flux, optimization for parameter of welding technology, manufacturing automatic welding equipment.
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