Look into the Venturi type desuperheater.

Good Luck,
Latexman

The water pressure has to be a little higher or there will be no flow.
Typically the steam would be slightly throttled into the de-superheater.
The resulting pressure drop would be enough to allow water flow.

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P.E. Metallurgy, consulting work welcomed

Attemperator desuperheater is a common industry name for such a device.

https://www.s-k.com/desuperheaters/attemperator-de...

Feedwater will always be available at higher than any steam system pressure. The desuperheater systems I have seen in marine propulsion all use a tube bundle in the mud drum called the control desuperheater to provide the water for the attemperator. I believe there was a pressure reducing valve for the superheated steam before the attemperator.

You must also consider steam quality. The attemperator type units allow the water flow rate to be varied in order provide a small amount of superheat over a wide range of conditions in order to provide very high quality steam.

Sorry Maybe I should explain the problem better.

We have (1) superheated steam mixing with (2)two phase saturated steam and water.

We need to fully mix the two streams to remove all the superheat. The remaining water phase is later removed and we have just saturated steam left. This works OK when we have high two phase flow (2) and good mixing. But if flow (2) is low we are not mixing enough and we still have superheat after the water has been removed.
We need a better way to mix the water (2) with the superheated steam (1)
Thoughts are;
a) flow the water in from the top over trays to increase contact area
b) fill a vessel with the water and flow the SH steam (1) from below.

We do not want to increase back pressure on either the steam or the water. This would require pumping or a reduction in flow.

Maybe annular tubes with a perforated center tube. Two phase in the center tube and SH in the outer. Even consider attacking a spiral wire to the inside of the inner tube to help more evenly distribute liquid water around the inside of the tube. You will need to size your tubes so the SH velocity is always higher to create a favorable pressure gradient for water to flow through the perforations.

You really want to maximize contact with the water and minimize contact with the saturated steam. Any extra water in the end is wasted heat.

With the limitations you've listed, try a static mixer for limited turndown performance, since a desuperheater requires single phase feedwater at higher pressure.

Was going to suggest static mixers, George beat me to it.