Cryogenic applications involve the application of extremely low temperatures, typically below -150°C, for a variety of industrial procedures. In such applications, valves are essential for controlling the passage of liquids or gases, and ball valves have become a popular option due to their unique features and advantages.
Typically, ball valves are constructed from stainless steel, which has exceptional resistance to the low temperatures of cryogenic applications. In addition, the ball and stem of the valve are frequently designed with extended or redesigned components to prevent frost formation, which can lead to valve malfunction or damage.
The primary advantage of employing ball valves for cryogenic applications is their capacity to provide good sealing, which is essential for maintaining the process's integrity and preventing leakage. Ball valves are constructed with a rotating ball that fits snugly within the valve body and provides a reliable closure against the seat. This design ensures that the valve can operate at exceptionally low temperatures without causing damage to the valve or the process.
Low operating torque is an additional benefit of using ball valves for cryogenic applications. Compared to other valve varieties, ball valves need less force to operate, making them ideal for applications requiring frequent valve operation. In cryogenic applications, the minimal operating torque of ball valves minimises the risk of causing damage to the valve or surrounding equipment.
Additionally, ball valves are extremely durable and require minimal maintenance, making them an economical option for cryogenic applications. They are less susceptible to wear and strain than other valve types and are less likely to be damaged by cryogenic processes' low temperatures.
Due to their ability to provide reliable sealing, minimal operating torque, and longevity, ball valves are an excellent choice for cryogenic applications. In industries where precision control of the flow of liquids or gases is essential, such as pharmaceuticals, biotechnology, aerospace, and cryogenic storage, they are utilised extensively. It is crucial to select the proper variety of ball valve for a given application to ensure optimal performance and avoid problems. Cryogenic applications necessitate the routine inspection and maintenance of ball valves to ensure that they remain in excellent working condition and provide long-lasting, dependable service.
Issue of thermal expansion
Ball valves are widely used in cryogenic applications, but their design must be modified for this purpose. The coefficient of thermal contraction of the seat ring material, which is typically greater than that of the stainless steel of the ball and valve body, is a primary consideration in the design of these valves. At low temperatures, the seat rings contract on the ball, causing the operating torque to increase. In extreme instances, the seat ring may crack due to excessive stress.
This effect of differential thermal contraction between the seats and the ball can be mitigated by reducing the installed pretension between the seats and the ball by a sufficient amount to guarantee the correct pretension at the cryogenic operating temperature. However, the sealing capacity of these valves at low fluid pressures may not be adequate if they must also operate at ambient temperatures.
Other ways to combat the effect of differential thermal contraction between the seats and the ball include supporting the seats on flexible metal diaphragms, selecting a seat-ring material with a significantly lower coefficient of contraction than virgin PTFE, such as graphite or carbon-filled PTFE, or constructing the seat rings from stainless steel with PTFE inserts containing a minimum amount of PTFE.
Due to the fact that plastic seat-ring materials become rigid at cryogenic temperatures, the surface texture of the seatings and the sphericity of the ball must be of high quality to ensure a high degree of seat tightness. Additionally, as with other types of cryogenic service valves, the extended bonnet should be positioned no further than 45 from the upright to ensure an effective stem seal.