A floating ball valve relies on a differential pressure to shift the ball a few microns into the downstream seat. This takes the ball off of the upstream seat and allows upstream pressure to fill the ball cavity.
A trunnion ball valve uses springs to push both of the seats against the ball (which is held rigidly against lateral movement by the trunion bearings). A trunnion ball valve has two seating surfaces against the process fluids and has a body bleed to evacuate the cavity in the ball when the valve is shut. This is similar to having two floating ball valves with a vent valve in between.
In cases where safety (or the government) dictates that you have "double block and bleed" (e.g., energy isolation for hot work in a situation where no one is working in a confined space), a trunnion ball valve meets the definitions (it is up to each company to satisfy themselves if it is truly a safe practice, but it does meet the definitions).
On the other hand, some manufacturers sell "double-block-and-bleed" valves which are not trunnion ball valves. These valves typically have two complete valves and a vent in one casting. They use various technologies and probably work as advertized. I'm not terribly familiar with them, but one that I looked at was not piggable and I went back to Trunnion Ball Valves.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The Plural of "anecdote" is not "data"
