Saturday, May 25, 2019

Pressure Regulators and Other Fluidsy things

So close friends know that I am working on a major rocket project right now.
It involves pressurants, solenoids, regulators, check valves, diffusers, tanks, COPVs, and so many damn more things that *aren't* electronics or guidance algorithms.
So I am learning a lot.

Things to look for in selecting regulators:

  • Inlet maximum pressure: this is the most obvious parameter. You need to select a regulator that will work with the system you are designing. So if it is hooked directly to a helium K-bottle, you're probably dealing with 3000+psi ratings here.
  • Outlet maximum pressure: this is another gimme. If you need to press a tank to 400psi, don't find a regulator that varies to 10psi output.
These two are hard enough to spec out for high pressure rocket systems. Then on top of that you have:
  • C_v Flow Coefficient: this one is super important and is probably the most overlooked thing in my experience. Your device must be able to flow a lot of fluid to keep up with the dynamics of the system. If the effective orifice size is small, then it will not have a high mass flow rate. 
It is defined as such:  $C_v = Q \sqrt{\frac{SG}{\Delta P}}$.  You can think of Q as your $\dot{m}$. SG is your specific gravity, which is normally unity (1 for water) on data sheets (I think). And $\Delta P$ is the pressure drop across the system. So basically the higher this coefficient, the higher the flow rate, and the lower the pressure drop.

This equation is meant for incompressible fluids, and the gas equation is more complicated. Swagelok has some good info here:

The flow coefficient is important for all fluid devices that constrict flow. So just be careful when you're attempting to procure random devices on eBay. Finding stuff for reasonable prices is hard, so I always watch out for these brands:
  • Marotta
  • Aqua Environment
  • Tescom
  • Swagelok
YOU CAN get away with low flow regulators if you attach their output to the membrane of a dome loaded reducing regulator. This seems like the usual way to do things, rather then flow directly from a single hand-turn regulator. Grove Mity Mites were apparently the way to go back in the day. There is a new company that makes them with the same design you might want to watch out for.

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