Tudor Vapor System with tall drop loop

They allow condensate to pass from the steam main to the dry return, but block steam.

Assuming the pressure in the steam main doesn't get too high…

In most systems like this there will be only one vent to the atmosphere and it will be close to that loop on the return side so if steam pushes through the loop it will close the vent and allow pressure to build in the return. the mains usually will have crossover traps to vent them in to the returns.

Slow down. Let us consider the situation at the end of a parallel flow main in a vapoor (or other low pressure steam) system. There will be a dry return next to it at a slightly lower elevation which may or may not slope the same way as the main (we won't worry about its slope for the moment.

And then let us suppose that the system has steamed long enough to come up to operating pressure — perhaps 3 or 4 ounces per square inch — but steam is only just starting down the mains.

As steam starts down the mains, condensate forms (probably a lot of it!) which flows down slope to the far end, where our tall loop is located and quite naturally drops into the tall loop. Eventually enough water gets in there so that the it fills both the side of the loop connected to the steam main and the side connected to the dry return. But… there is a little pressure in the steam main, and none in the dry return, so the water stands a little higher on the dry return side (at 4 ounces, about 7 inches) and so it overflows into the dry return and makes its way back to the boiler.

Neither steam nor air can get through the loop from the steam main to the dry return unless the pressure differential is great enough to force the water so far down on the steam side as to allow it to blow through to the dry return side.

What happens to the air in the steam main? Depends on the system. On many systems, that air is allowed to escape to the dry return through a crossover trap — a pretty ordinary radiator trap, but pipe above the steam main so it doesn't handle condensate at all. Very effective. In these systems, all the air — mains, risers, radiators, whatever — is released at or near the boiler through a main vent or main vent cluster on the dry return. In some systems — usually newer or modified — there will be a main vent at the end of the steam main instead. There still has to be the main vent cluster for the dry returns.

At a slight risk of confusing things, note that there is a variation where the steam main is parallel flow and the dry return follows it, and is thus counterflow. in that arrangement rther than there being a loop at the end of the steam main both the steam main and the dry return have drips (separate) which go to a wet return which carries the condensate back. The principle is very similar — the water in the dry return drip stands higher than that in the steam main drip. This arrangement, has a subtle trap for the unwary. The water in the steam main side stands at the water level in the boiler, or only slightly higher. The trap is when a new boiler is installed the water line may be below the elevation of the wet return at that point, and there will be no seal. In any system of this sort it is essential to match the water line of the new boiler to that of the original.