Excess Flow Valve Setpoint

[carltogr]

Folks,

I have been asked to specify an excess flow valve (EFV) for a high purity hydrogen system with 1/2" tubing/piping. The system will be at 165 PSIG, ambient temperature. We would like to run the system at a max flow of 420 standard liters per minute (SLPM), or approximately 15 SCFM. The vendor states that the EFV must be set at 3 times the max flow, 1260 SLPM, or 45 SCFM. Imagine if there were a line break downstream of the EFV, how does one determine the flow that the system would be at to relieve this pressure? or how does one determine the maximum flow that the system could handle? I know where we want our system to run at (max 420 SLPM), but what if the system doesn't allow, or doesn't reach the 1260 SLPM that the EFV would need to shut-off? Any clarifications?

thanks....

 

[BigInch]

I assume the vendor of the EFV knows the fluid is hydrogen.

You can determine the maximum possible flow of H2 from 165 psig to atmospheric pressure through a conservatively short length of 1/2" tubing.

Excess flow valves activate when the set excess trip setting is reached. Before that figure is reached, there should not be a need to shut off, the theory being that the resulting explosive limit concentration would never be reached at any downstream location receiving the H2. As long as the actual flowrate remained below that trip setting flowrate, dispersion into the atmosphere or wherever should carry away enough H2 such that it will never reach the ignition point. So, theoretically, if the system cannot reach that trip point, you should still be able to flow at a lesser rate with a high probability of maintaining safe conditions. If not, the maximum possible flowrate and the correspoinding trip flowrate setting needs to be reduced to such a point where it is possible to maintain safe conditons.

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