With the widespread application of liquefied natural gas (LNG), the requirements for the ultra-low temperature special valves are gradually increasing. The ultra-low temperature ball valve adopts a top entry structure, and the sealing material is non-metallic. The commonly used nominal size is NPS 1/2”~24” (DN 15~600), and the pressure class is Class150~1500.
Due to the special working conditions, which belong to the ultra-low temperature state, the ultra-low temperature ball valve must be designed with a qualified structure and calculated based on the assembly clearance and bolt tightening torque in the low temperature state to ensure its good performance.
Cryogenic ball valves are mainly used in devices in the liquefied natural gas and air separation industries. The output liquid media include: liquefied natural gas, liquid oxygen, liquid hydrogen, liquefied ethylene, liquid nitrogen, and liquefied petroleum gas. Some of the media are not only flammable and explosive, but also prone to gasification. When gasified, the volume expands sharply and the pressure rises sharply. The valve cracked and exploded, causing serious casualties.
1.1 Conventional small-bore cryogenic ball valves are generally floating designs. This design is not much different from the conventional floating ball valve. When the valve is in the closed state, the medium exerts pressure from the upstream of the valve, and the ball valve can produce a certain displacement.
And backlog on the downstream sealing surface, resulting in outlet end seal. The low temperature ball valve mainly considers the shrinkage of the sealing ring in the low temperature environment, and the compression amount of the sealing ring is larger than that of the ordinary floating ball valve during design.
1.2 Design of large-bore cryogenic floating ball valve. For conventional floating ball valves, since the gravity of the valve body itself is directly borne by the sealing ring, when the gravity of the ball is too large, the force on different positions of the sealing ring will be severely uneven, resulting in irregular deformation of the sealing ring and affecting the valve performance.
Sealing performance. Therefore, when the caliber is large enough, no matter how low the pressure is, it is not suitable for the conventional floating ball valve structure. The design of the large-diameter low-temperature floating ball valve needs to retain the support of the lower valve stem to the ball in the fixed ball valve, but no bearing is installed at the contact between the lower valve stem and the ball. This ensures that the weight of the ball valve will not all be concentrated on the sealing ring. When the valve is closed, because there is a certain gap between the lower valve stem and the sphere, when the pressure pushes the sphere, the sphere can move slightly and press tightly on the sealing surface of the valve outlet end, so that the downstream valve seat is sealed. The upstream of the large-diameter low-temperature floating ball valve needs to design a spring, which can not only provide sufficient pre-tightening force, but also offset the impact of the load on the valve seat, thus ensuring the reliability of the seal. But this design results in higher torque.
1.3 Trunnion mounted cryogenic ball valve design. O-rings in conventional ball valves have a reliable sealing effect, but the sealing performance of O-rings is not reliable in low temperature or ultra-low temperature environments. In low temperature and ultra-low temperature environment, Lip seal not only can realize self-sealing like O-ring, but also its sealing performance increases with the increase of pressure. At this stage, the main seal of cryogenic ball valve must choose Lip seal. This design is similar to ordinary fixed ball valve, except that the O-ring is replaced by Lip seal. Lip seal is more expensive and easy to wear. The roughness of the mating surface of the parts is required to be high, and the machining accuracy of the mating surface is required to reach 0.1-0.2Ra. The installation of Lip seal is also relatively complicated. If the valve is used in harsh working conditions. In addition to using Lip seal at the valve seat, other leaking locations also need to be designed with Lip seals, which are sealed together with packing or wound graphite gaskets.
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