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The Optimized Design for Cryogenic Ball Valve

May 22, 2018

Abstract:

As a key component of fluid medium storage and transportation equipment, cryogenic ball valves express excellent performance and have great application prospects. SINOVAL analyzes the performance characteristics of the various components of the ball valve from the material and structure aspects, elaborates the relevant design ideas, and discusses its necessity. 

Introduction:

With the continuous development of the petrochemical industry, the demand of the storage and transportation of flammable, explosive and cryogenic medium have become more and more larger, and the performance requirements for fluid control valves are also increasing more higher. Because of its special structure and material, cryogenic ball valve has many advantages, such as low resistance, reliable sealing and low noise, which has attracted wide attention of valve companies and opened up a new path for the study of valves under cryogenic conditions. Starting from the practical application, SINOVAL summarized the design features and sealing requirements of cryogenic ball valves with different structures, and pointed out the direction for the optimization design of the new Cryogenic ball valves. According to the design criteria, the relationship between the various cryogenic ball valve sealing modules was elaborated, which provided references for related research.

1. Structure Design for Cryogenic Ball Valve

The on-off valve of the ball valve is a ball with a circular passage, which rotates around a central axis perpendicular to the passage so as to achieve the purpose of controlling the on-off of the passage and the medium flow. The ball is in a fixed state and does not shift when there is pressure. The complete ball valve is usually composed of a valve body, a valve cap, a drip plate, a bracket, a valve stem and a drive device, and the ball valve of the full-diameter structure is shown in FIG. 1 . The design of the valve body adopts a one-piece structure, which reduces the leakage of the ball valve compared with the two-piece type and the three-piece type. The structural design features of each part will be described in detail below.

1.1 Valve Bonnet Design

The temperature of the fluid in the valve chamber must be taken into account for the design of valve bonnet. Therefore, an extended cap structure is used to keep the installation position of the packing and the valve control device away from the low temperature region. On the one hand, it is ensured that the filler of the valve works in an acceptable temperature range, reducing the impact of cryogenic frost on the filler, and conducive to the normal operation of the valve stem and other components. On the other hand, the cold burn to the ball valve operator by the low temperature medium is avoided.

A drip plate is usually provided on the extended bonnet to prevent the outside condensate from entering the insulation and causing the bolts to rust. At the same time, the location of the drip plate indicates the highest position of the insulation medium for the ball valve. 

1.2 Pressure Relief Components and Sealed Structure Design

Some medium will swell after high-temperature vaporization. If the valve is closed at this time, the pressure in the chamber will rise sharply, which will not only destroy the function of the various components of the ball valve, but also cause the valve to fail, affecting the normal operation of the entire conveyor line.  Therefore, it needs to be sure to install pressure relief components at the valve ports.

The design of the pressure relief part is usually in the form of a pressure relief hole to ensure proper communication between the cavity and the pipe port. The seal structure is the core part of the entire ball valve to determine the quality of the entire valve, generally includes two components of the stem seal and the valve seat seal. The stem seal must meet the requirements of easy maintenance and low leakage. Usually two or more seals are designed according to the specific application to meet the Class B seal standard of IS015848-2. The C01 class standard of IS015848-1 can also be selected according to the actual application. Effectively reduce the penetration of water droplets into the flexible filler, causing device damage. Under Cryogenic conditions, the valve seat seal mainly uses a lip-type seal ring, which has high precision,  works strictly for the performance requirements of the relevant parts  with good sealing effect. According to the need of pressure relief, the upstream valve seat usually adopts a one-way sealing ring, while the downstream valve seat usually adopts a two-way sealing ring. The most widely used sealing ring material is polychlorotrifluoroethylene (PCTFE), which has excellent Cryogenic characteristics. Secondly, increasing the amount of flexible material filling and increasing the number of sealing rings will also improve the sealing performance of the ball valve. 

1.3 Fire Protection and Anti-Static Structure Design

When the temperature, pressure and other factors change drastically, media leakage may occur and cause fire, so the ball valve design must have a fireproof structure. The commonly used fire protection design is to set the sealing structure at the joint between the valve cap and the valve body. The double channel design of the flexible medium winding gasket and the lip seal ring is adopted. The sealing of the valve stem position also adopts various structures such as flexible medium filling and sealing ring. collective effect. The most important feature of this kind of fireproof design is that it can avoid the problem of melting failure of the sealing structure. In addition, there is a double fireproof design using a combination of non-metallic gaskets and metal seats. This structure can prevent leakage of the valve caused by internal leakage of the delivery medium, but it is relatively expensive to use. Because of the special nature of cryogenic ball valves, fire protection design should implement APl6FA or APl607 fire safety standards. 

1.4 Actuator Structure Design

When the valve stem and the vertical line are at an angle of 30 degrees, the normal operation of the valve can be achieved. In order to realize the free lock of the Cryogenic ball valve in the fully opened and fully finished state, the manual ball valve device such as the gear drive and the actuator should be provided with a lock. The actuator can not only maintain operations under harsh conditions, but also can realize online maintenance, replacement, and other operations, providing convenience for the sustainable use of cryogenic ball valves. 

3. Conclusion

In order to meet the ever-increasing transport needs of liquefied medium, the performance structure of cryogenic ball valves is continuously optimized. SINOVAL analyzes the structural design features of various components of the cryogenic ball valve, discusses the material selection and implementation standards for related components, and summarizes the shortcomings of traditional ball valves. 

If you have any question about ball valve, please feel free to contact SINOVAL- Your Ball Valve Expert of Proven Quality & Trustworthy Sevice!

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