I recently read a thread on the steam car forum, which had a conversation going about using the same pump as a circulation pump as well as the feed pump. The idea sounds interesting.
https://steamautomobile.com:8443/ForuM/ ... #msg-27437
I don't fully understand the valve described there, but I suspect it serves a similar purpose as the 3-way valve I have in mind. Let's say the pump is adjusted to keep track of the average steam consumption or higher to keep the monotube wet. In order to keep the water level within designed limits, one could avoid the readjustment of the pump alltogether and use a 3-way valve.
In case of the water level reaching the high water mark, a water level control would actuate a 3-way valve to shut off the water feed and link the pump to the water inside the steam dryer flask. (If the monotube design HAS a water-holding vessel) So, the liquid water inside the flask gets recirculated until the water level reaches a low water mark, which in turn actuates the 3-way valve to return to the original position, linking the monotube with the feed water.
The advantages I see are: Even in case of excess water inside the flask, the liquid water being removed from the flask won't exit the pressurized system. It simply gets recirculated. This means the energy being invested in pumping that amount of water into the pressurized system won't get lost. The same with the thermal energy already inserted into the water. The recirculation pumping doesn't swallow as much energy as the feed pumping, as the former just needs to overcome the friction losses inside the monotube and not the pressure differential between the hotwell and the pressure vessel on top of that.
As an example: A pump could be adjusted to deliver 4 times the average needed water. It would not consume 4 times the energy. It would always only consume the energy used for the water needed for the engine. All the excess water delivery would just need to overcome the friction losses inside the monotube. So, if a pump works at 4 times the need for the engine, it would only work at 1/4th of the time against the full pressure. The 3-way valve would be switched to recirculation mode the other 3/4ths of the time. A pump delivering 4 times the needed average is just exaggerated to bring the point home.
The drawback in comparison to the aforementioned method of using a small orifice is the need for a sensor to read the water level and use the signal to actuate the 3-way valve. Other than that, it definitely isn't a complex system to plumb. And this method of boiler water level control should work with all kinds of boilers having a water level.