Ball Valve Type

I was just going thru a General Specification for Piping and Valve Classification which had a note indicating use of Trunion Mounted Ball Valve for sizes >NPS 8" and use of Floating Ball Design for 900 & 1500# for sizes>=NPS 4"
Can anybody tell me the diffrence between these two type of ball valve and why such a classification as per pipe sizes and pressure rating. Can u please also tell me where i can find c/sectional drawing for both these type of valves.

 

[chuchcatlanta]

A floating ball valve is captured between the seats and is litterally floating---this type of valve is generally of smaller size and of lower pressure class since a higher pressure can press the ball into the downstream seat and "unseat" the upstream side. Also really heavy large balls can depress the seats and shift downward just from the shear weight of the ball.

A trunion mounted ball had a "post" added to both the top and bottom of the ball and some recess in the body to "capture" the ball so it can not move within the body. A stem is then attached to the top or bottom post via a slot for turning

Charles