In my experience most low pressure applications request 2 piece floating ball valves and when not specified I've always sold the 2 piece floater. What instances would a 150# or 300# trunnion be the preferred design. Also what is the advantage of a 3 piece vs a 2 piece body?

There are a variety of rules of thumb for when to switch from floating to trunnion mounted. Myself, I specify trunnion mounted at 12" in Class 150 and 8" in Class 300. However, there is no hard and fast reference and different people have different criteria.

I generally accept either 2 piece or 3 piece bolted body designs.  2 piece is less steel and so lower weight and potentially lower cost. However, most manufacturers have their own criteria for when they build 2 piece vs. 3 piece and so as end user you often don't get a choice.  Certain sizes and pressure classes are one or the other.

For really low pressure operations (i.e., from deep vacuum to about 10 psig) in gas I specify trunnion down to 4-inch because I've had problems with floaters being able to shift the ball enough to seal at very low dP after they've been in service a couple of years.  For normal pressures I look at the service to decide if I want trunnion or floater at 6-inch ANSI 300--if company policies allow me to use a trunnion for double block and bleed I use trunnions for 6-inch, if not I use floaters.  For 8-inch and above (ANSI 300) I use trunnion like rneil.  I don't do much in ANSI 150 so I haven't thought about that one enough.

If I'm installing a true double block and bleed valve (one that does not have self relieving seats), I use trunnion regardless of the size. I also specify trunnion for ESD valves.

eems like a good idea to provide an extra level of protection for the downstream system.  In full liquid systems also a good idea to prevent overpressuring of the body cavity if the valve heats up (solar radiation).

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