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

3) If it is correct, does it meet the intent of trunion mounted valve.

4) In which standard can i find the details of trunion mounting design.

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.

Every trunion ball valve I've ever seen has a body bleed.

Satisfying double-block-and-bleed is an individual company decision.  Since the seats are spring loaded, each seat in a trunion ball valve is a more positive seal than the downstream seat in two floating ball valves, so company's should accept it, but the size of the vent is so small that some don't (and some require blowing the body down and then removing the plug to give a larger vent).  The regulations describe the concept and then require you to meet it without specifying technologies.  I prefer to get double-block-and-bleed with a trunion to using a pair of valves separated by (sometimes) miles of pipe, but not everyone agrees.

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