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Calculation of an atmospheric vent line length

Calculation of an atmospheric vent line length

Calculation of an atmospheric vent line length

(OP)
Dear Sirs,

we need your support to calculate the right lenght of an atmospheric vent line.
The context is a desulfuration plant on a refinery.
By using a gaschromatograph to measure H2S and H2, I discovered that the actual lenght of the vent line probably does not permit the gas sampling at atmospheric pressure. This means that the GC analyze a gas volume more great than the nominal volume that is 360 microliters.
To equalize the sample during the sampling phase of the gaschromatograph, there're two valves named ATM VLV and X1 VLV. The first valve is installed between the sampling out line from the GC and the process return line. The second valve permits the gas return to the process line (look at the attached images). The X1 VLV moves opposite respect to the ATM VLV:
-when X1 VLV is open, the ATM VLV is closed. This is fast loop mode
- contrariwise, when the X1 VLV is closed, the ATM VLV is open. This the sampling mode. In this way the GC needs atmospheric sample.
E.g.: if the calibration gas pressure from a bottle is set at 0.5barg, the measure overestimates the expected value. Trying to set the cal gas pressure below or above the above mentioned pressure value, the result is the same and also not stable or not passable.
Considering this phenomenon, I disconnected the vent pipe from the ATM VLV, that is a 6mm steel line of three meters lenght. Thus I obtained a good result. By keeping this configuration all the GC measures are close to the bottle concentrations.
If you agree with my interpretation please help me to calculate the better atmospheric vent circuit or vent line size.

Thanks in advance
Niccomasso

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