When dealing with steam vapor heating, there is much to be learned from an understanding of the radiator valve itself. The original vapor heating systems were called atmospheric vapor systems because the radiators were really open to the atmosphere on their discharge sides since there were not yet any radiator traps. They either used water seals or were just connected to the return pipes with what would appear to be nothing to stop the steam from getting into the return pipes. To a degree that was very true, as the water seal would only stop 'wisps' of steam from getting thru to the return pipes. The water seal is by no means a trap. It actually traps nothing.
That leads us back to the radiator valve at the inlet of the radiator.
If there is not a thermostatic radiator trap on the outlet of the radiator, then the inlet valve was the important part of the radiator. See, there are two ways to allow a radiator to heat and still keep the steam out of the return pipes. First is to have a thermostatic radiator trap on the outlet of the radiator which stops the steam from exiting the radiator, (I call this controlling the exit door), or secondly, by stopping the steam from getting into the return pipes by allowing only so much steam into a radiator in the first place. I call that controlling the entrance door. Think of the valve as a turnstile. Only so much is going to get in.
In the original vapor heating systems the radiator valve was the turnstile that controlled how much steam could get into a radiator. If the valve was accurately set, then the steam would 'die' before it actually heated all the way across the radiator. See how important this valve was. A modern valve opens too far for most of these systems. There are techniques that can be employed to deal with this, but what I'm trying to impress upon you is that these original vapor valves were of a special design and were very important to the proper functioning of the system.
Some of the best descriptions of how a vapor steam radiator inlet valve operates are the catalog page from Federal Radiator of Lavigne's radiator valve and the patent that Joseph Lavigne submitted to the government. The control capability of these inlet valves becomes quite apparent after reading these, and I hope it helps your understanding of the problems that can be created when too much steam is allowed into a vapor radiator by a modern valve that opens too far.