Looking for a high pressure, cryogenic regulator 1

[eric2718]

I have an accumulator supplying 8000 psi, -320°F nitrogen. After the accumulator, the nitrogen then passes through a mass flow meter and into a reaction chamber. This is a batching process, lasting 0.5-1.0 seconds and repeats every 6.0 seconds. The problem I'm having is that the flow moves so fast that the mass flow meter can not close the supply valve fast enough, and the system overshoots the batching mass into the reaction chamber by 300%. I tried a non-cryogenically rated regulator by Tescom that is rated to 8000 psi on the inlet, and 200-5000 psi on the outlet. It worked well for two cycles, then leaked. I have tried replacing the seal where the leakage is occuring without success.

The regulator has (1) viton o-ring with a PTFE back-up ring. Unsuccesfully I've tried a 100% teflon o-ring, a FEP encapsulated o-ring, a spring energized u-seal, and chevron PTFE packing. Next, I will try a 100% silicone o-ring, a viton o-ring with a spriral PTFE back-up ring, a PFA encapsulated o-ring, and a PTFE spring energized u-seal located on a different piston.

I do not have high hopes for sealing this regulator, so I have the following brainstormed ideas:
1. find the all perfect regulator, which would be the ideal solution
2. try a metal diaphragm instead of an elastomeric seal
3. slow down the system so that the mass flow meter has more time to close:
a.by either chocking the flow with an orifice
b.I could have the mass flow meter close the valve at the exit of the accumulator, instead of the valve at the inlet to the reaction chamber. This would reduce the the cooling time of the regulator to 4 out of 6 seconds.
4. Use a throttling valve based on an I/P and a pressure probe downstream of the mass flow meter to control the pressure by throttling, but I don’t think this will happen fast enough.
5. I could ignore the leak in the regulator.
6. Abuse a cryo rated pressure regulator, and overpressurize it.
7. Add heat, with a band heater to the regulator.
8. Plumb the regulator leak to a vent line.
9. Run the accumulator's supply pump with a variable frequency drive to always keep the accumulator within an ideal pressure range.

 

[iainuts]

8000 psi? -320 F? I've been in the industry for almost 20 years, but haven't heard of a reg that will do this.

I'd be most concerned about safety using unrated components. If the regulator is used, how can the safety of people in the area be ensured? I'd be less concerned about valves that were all 300 series stainless, aluminum, or copper alloy. If the pressure retaining components are made from suitable materials, I'd be less worried about leakage.

I personally wouldn't think you could get a leak tight seal with any kind of elastomer or Teflon U-cup, though many of the Teflon type seals are rated for low temp. I'm assuming you're using a piston type regulator. Heating isn't really an option either, just too much refrigeration in the LN2. A metal diaphragm would work if it were rated for the pressure, but that will still be diffcult to find.

Reducing flow with an orifice should help. In fact, you might try that with a cryogenic shut off valve.

Could the overshoot have anything to do with heat entering the LN2 after flow has stopped?

 

[Jacquo]

Material of the moving and static components of your regulator are important if different shrinkage opens gaps for seal extrusion.