If your flow rates are small, I am not familiar with such a design practice. Typical minimum size industrially is 2" or 3" for the heat loss reasons. You must be careful about freezing the liquid, and pumping losses. These can be handled through appropriate tracing etc.
I suggest that you not rely on any anecdotal information from an engineering-tip bulletin board. I suggest that you contact the caustic soda vendor for his MSDS and Handling Guidelines ( I also assume that you are talking about 50% caustic... the concentration that is typically stored for water treatment usage...) Contact both PPG (
I don't know about caustic but I do know that for sulfuric acid, high fluid velocities will degrade the protective oxide coatings that form in pipes leading to increased corrosion.
The problem associated using a small bore pipe for caustic service is the welding. if the joints are socket, then u may have a rpoblem related to creavice corrosion. of course u can also do a butt weld but it becomes more of a prblem for size under 1".
The other thing to remember is PWHT depending on the concentration of the caustic and the temperature.