Steam piping confusion

An F&T trap s probably completely unnecessary, and a distraction.

We have a steam main. The air needs to get out of that, so there are basically two ways to do that: either a vent at the far end of it, after the last radiaator runout, or a crossover trap allowing air to get into the related dry return, but not steam. Fair enough.

We also may have water in the steam main. Again, there are two ways to manage that: if the main is parallel flow, you need to have a drip to a wet return at the end of it. If the wet return really is -- that is, below the water line -- you don't need a trap or anything on that. If there is no matching wet return) you need a water seal -- a loop down to the floor and back up to the dry return. If the main is counterflow, you need a drip at or near the boiler.

Now next to the steam main we have a dry return. That receives condensate and air from the radiators. That return line needs to get rid of the condensate, so if it slopes down away from the boiler, it needs a drip to the handy wet return at that end. Otherwise it needs a drip near the boiler. In either case, it needs to be very well vented -- usually at the boiler. That vent is a main vent.