Floating vs the trunnion ball valve
As generally known, in a floating ball design the ball is pushed against the downstream seat by the in-line pressure, causing the tightness. When operated from closed to the open position, the ball is to be rotated against both the in-line pressure (∆p) AND the friction of the seats. In other words: the torque needed to operate the valve is created by both in-line pressure and the nature of the valve seats. This amount of torque needed increases significantly when operating pressure (∆p) and/or valve size increase, or whenever the nature of the seat is made more robust (i.e. metal seat).
In a trunnion design, the ball is inserted in a central bottom shaft (the trunnion). The ball is fixed between the stem and the trunnion, which inclines that the ball is not floating but fixed. The inline pressure presses the seats against the ball, causing the tightness. This inclines that during operation, the ball does not have to be rotated against the in-line pressure (∆p) AND the valve seats, but that is solely needs to be rotated against the pressure of the seats.
As a result, the required torque of a trunnion mounted ball valve is generally lower than a comparable floating ball valve. For example: a DN200 metal-seated floating ball valve would require a significantly bigger actuator than DN200 comparable trunnion valve, leading to significantly lower costs of the overall package. Also, the trunnion seat design in general is much more stable which makes it more suitable for extreme conditions.
The trunnion-mounted ball valve is more suitable for high pressure applications and bigger dimensions compared to the floating ball. Another advantage of the trunnion design vis-à-vis the floating design is the fact that a trunnion generally is included with a drain or bleed connection, making it suitable to function as a dual safe device. Furthermore, it functions as an relief valve automatically whenever the pressure in the central cavity is higher than the spring force of the seats. When this happens, the seat springs relieve automatically in order to drain the excess pressure back into the main line. Because of these reasons, the trunnion is commonly used in oil & gas applications, where extreme conditions pose the standard.
Off course, a big disadvantage of the trunnion compared to the floating design is associated with its costs; which are significantly bigger. Because of these costs, trunnions are used solely when they HAVE to be used.
Due to the above considerations, it is depending on the exact application which design is preferred and advised. Our specialists happily offer their advice on this matter.