Our client has specified Trunion mounted ball valve.

As per my understanding in trunion mounting, at the bottom of the ball, there will be a central rod (Which is called trunion) which is similar to the dimension of the stem, and trunion is supported to the bottom boss.

I now have a couple of valve manufacturer's brouchers and Cross section drawings which does not have the bottom central trunion, instead there is a step in the side (Just after the seat). The manufacturer still terms this as turnion mounted,  they claim that it is not seat supported and the ball is supported by the body (The step i mentioned)and hence trunion mounted.

1)Can some body tell me the correct definition of trunion mounted ball valve.

2) Are the claims made by the valve manufacturer correct.

A trunnion mounted valve means that the ball is constrained by bearings and is only allowed to rotate.  The bearings act on the trunions which may integral to the ball, or may be separate depending on the valve design.  The key feature is that the ball does not shift as it does in a floating valve to press the ball into the downstream seat.  Instead, the line pressure forces the upstream seat onto the ball to cause it to seal. As the area on which the pressure acts is much lower, the amount of force exerted on the ball is much less, leading to lower friction values and smaller actuators or gear boxes.

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