Double Block and Bleed versus Trunnion 2

[Zeroman]

Hi all,

I'm sorry for posting, but unfortunately the search isn't working.

Could someone please explain the difference between a floating ball valve and a trunnion mounted ball valve? And what is double block and bleed on a trunnion mounted ball valve?

Thanks in advance.

 

[zdas04]

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

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[Zeroman]

Thanks Dave. It finally makes sense.

 

[MaxPressure]

Zeroman & David,

Dave has just about covered the "how they work issue" although there are some variants. Piggable double block and bleed valves, or DBB's, are available so are floating ball versions, I've acquired them for projects before. Try a google/yahoo search for "alco dbb valves". or try

http://www.mono-flange.com

this has more details on the actual types and variations you can also refer to the EEMUA 182 standard for a deeper understanding.