Ball valves can be classified into floating ball valves and trunnion ball valves due to the two types of their balls, that is, the floating balls and the fixed balls. Besides these two kinds of balls, 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 via the sealing pressure produced by the pressure of the medium. Floating ball valves are unsuitable for occasions with large diameters, or they will have poor sealing performance or even can not be sealed if the pressure of the medium is low and the ball itself is heavy. Under common circumstances, the combination of the pressure rating and the diameter for the floating ball valve is listed as follows.
A. Class150: DN150 and below DN150
B. Class300: DN100 and below DN100
C. Class600: DN50 and below DN50
 
If the valve body and the valve seat are reasonably designed with suitable size, the floating ball valve can also be used for the large diameter condition.
 
Floating ball valves can have either one-way sealed valve seats or two-way sealed valve seats depending on customs of manufacturers. The patent advantage of the one-way sealed valve seat is that the cavity of the valve can relieve the pressure 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 Fixed Ball
The trunnion 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.