I need to create a variable 3-7 psi / 0.2-0.5 bar in a low density gas running through some piping. I can calculate the required Cv factor to do this with a valve would be in the 30-200 range. Looking at some valve spec sheets, a typical valve to get to the lower end of that range would have an effective orifice size of about 1".
The trouble is that the gas is supersonic going through that size orifice. Similarly, if I use a 4" valve (such that the velocity would be reasonable in the connecting pipe) the valve needs to be over 90% closed, making the effective orifice size only an inch anyway.
I've thought about just running through a long length of pipe, but its difficult and expensive to make the pressure drop variable. I've also thought about running the gas through a vertical liquid column to create a hydrostatic head on the gas, but if the density is near 1 kg/L, I'd need a 16' column, which would be both expensive and impractical.
Has anyone every solved this problem before? Is there a good way to create substantial pressure drops in gases without creating high velocities?