Selection Principles of Ball Valves Construction Types
Ball valves based on ball construction have two types: floating ball valves and trunnion mounted ball valves. Due to the two types of their balls function, the floating balls and the trunnion mouted balls. Additionally these two kinds of ball construction, ball valves also have some other ball types such as the hemisphere type, the V-shaped type, the eccentric type and the orbital type (the ball taking the swing action), which are the patented types of some manufacturers.
The Floating Ball
The floating ball valve has the simple structure and is sealed by force from the sealing pressure generated by the pressure of the pump. Floating ball valves are unsuitable for occasions with large pipeline, or they will be too heavy to operation or even can not be sealed if the pressure of the medium is low to push the ball to seal. Under common circumstances, the combination of the pressure rating and the diameter for the floating ball valve is listed as follows.
A. Class150: Up to DN300
B. Class300: Up to DN250
C. Class600: Up to DN150
If the ball valve body and the valve seat are properly designed with suitable size, the floating ball valve can also be used for the large diameter condition up to DN300.
Floating ball valves can have either one direction sealed design or bi-direction sealed seat design depending on application purpose. The advantage of the one direction sealed design ball valve seat is that the pressure in the cavity of valve can be relieved automatically.
The above combination of the pressure rating and the diameter for the floating ball valve is not the default choice of all valve manufacturers. When it is needed to adopt other ball types, it should be indicated in the valve data sheet.
The Trunnion mounted Ball
The trunnion mounted ball valve is sealed through the sealing pressure generated by the valve core and the floating valve seat supported by the spring. Composed of the valve seat, the sealing ring, the supporting spring, etc., the floating valve seat has the complex structure and the large size. However, the trunnion ball valve has the apparent superiority that it can be sealed without the pressure of the medium, and can have the reliable sealing performance. It can also be easily two-way sealed. All these make them frequently used for the large diameter conditions.
If there are not special requirements on trunnion ball valves, they can not relieve pressure in the cavities by itself. Therefore, when there are some specific requirements, it should be indicated in the valve data sheet.