Vacuum, cryogenic and ultra-cryogenic, high temperature ball valves

(1) Vacuum ball valves:

Ball valves used in vacuum situations are vacuum ball valves. In structure, the vacuum ball valve and the general ball valve have no special features, but in the selection of materials, processing and test requirements must meet the vacuum technical requirements. As a vacuum ball valve, the material of the parts must be compact, machined or polished, the parts must be cleaned strictly before assembly, and the assembled valves must be cleaned strictly to ensure vacuum pumping and vacuum degree.

Because the ball valve operates in vacuum, it can not rely on fluid pressure to push the ball purple seat or fluid pressure to push the ball stop to seal, but can only rely on preloading force to achieve seal. To compensate for the wear and tear of the seat and keep the sealing force constant, the sealing structure of the seat is designed.

Vacuum ball valves should be designed and tested according to ZBJ78005 "Technical Conditions for Vacuum Valves" and ZBJ78006 "Performance Method for Vacuum Valves".

(2) Low and ultra-low temperature ball valves

Ball valves used in low temperature and ultra-low temperature medium conditions are called cryogenic and ultra-low temperature ball valves. Cryogenic and ultra-cryogenic ball valves shall meet the following requirements:

1. Low heat loss

2. It works reliably in low temperature environment.

3. Small heat capacity;

4. Simple structure, low fluid resistance;

The valve body is suitable for installation in the insulating box, but the opening and closing mechanism of the valve should be exposed outside the insulating box.

The most prominent problem of cryogenic valves is packing sealing. At low temperatures, the elasticity of the filler disappears, the sealing ability is greatly reduced, and the fluid is easy to leak out from the filler box, resulting in the freezing of the part, affecting the normal operation of the valve. In order to increase the temperature gradient between the filler box and the body cavity, the structure of lengthened stem is often adopted in cryogenic ball valves, which can keep the temperature above 0 degrees Celsius, improve the working conditions of the filler box, prolong the service life of the filler and reduce the cold loss. At the same time, it is convenient to install the valve in the vacuum shell or the shell with insulation layer. For the low temperature ball valve that must be installed outside the incubator, the whole valve should be insulated with vacuum jacket.

(3) High temperature ball valve

Ball valves used in high temperature medium conditions are called high temperature ball valves. The service temperature of the ball valve usually depends on the heat resistance of the seat material. When the ball valve seat is made of polytetrafluoroethylene or nylon, the service temperature is 200'C. For high temperature ball valves (usually above 250'C), the key to their design is:

1. Correct selection of sealing materials suitable for use at high temperatures.

2. How to compensate the wear and tear of the seat in service and coordinate the temperature difference between the body, seat and sphere due to different assembly temperature and service temperature.

Generally, high temperature ball valves use metal-to-metal sealing pairs, namely metal sealing ball valves, which are characterized by the ball and body are made of high temperature resistant metal materials, and the seat is made of metal or heat resistant alloy (such as cobalt, chromium, tungsten carbide).

The advantages of metal-to-metal sealing pair high temperature ball valves are: wide temperature range, little difference in thermal expansion coefficients of various metal materials, as long as properly selected, they can achieve basic consistency, which is conducive to coordinating temperature difference deformation. The disadvantage is that the hardness of metal material is high and the specific pressure required to achieve the seal is also high. It is difficult to achieve the seal by fluid pressure when the diameter is small and the pressure is low. Even if the seal is achieved by pre-tightening force, the wear between the valve seat and the sphere must be very large because of the high specific pressure of the seal. Moreover, the friction coefficient between metal and metal is relatively large, and the opening and closing torques in the ball are also large.