SUPPORT REQUEST TO EVALUATE VENT LINE SIZE AND LENGHT

[Niccomasso]

Hi All,

I need a 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 analyses 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 the "fast loop mode"
- contrariwise, when the X1 VLV is closed, the ATM VLV is open. This the "sampling mode" (in this way the it's needed atmospheric pressure to avoid the phenomenon above described).
E.g.: if the calibration gas pressure from a bottle is set at 0.5barg, the measures are overestimated respect to the expected value. Trying to set the cal gas pressure below or above the previous 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.
Please help me to calculate the better atmospheric vent circuit or vent line size and lenght!!!

 

[MikeHalloran]

The 6mm OD of the tubing is absolutely the wrong number to use in pressure drop or flow calculations. The OD is useless; you need to know the ID, inside diameter.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

It also occurs that the atmospheric vent valve should be located at the end of a pipe, and should not have any pipe or tubing 'downstream' of it if at all possible, else you couldn't call it a vent valve. Four plus meters of pipe does not have negligible pressure drop unless there is zero flow, or the pipe is large relative to the valve. Why so much tube between the ATM valve and the actual ATM?





Mike Halloran
Pembroke Pines, FL, USA

 

[Niccomasso]

Lines 6mm OD have 4mm ID.
To avoid risks for humans, the vent line is 4 mt lenght because the sample contains H2S and H2 and the ATM VLV is installed at about 2 mt height from ground on the cabinet rear.
My further doubt is that the vent circuit should be installed on the SAMPLE IN line and not on the SAMPLE OUT. This could permit a better sample equalization on the sampling circuit of the V1-1 inside the GC.
I don't know if you agree with me..

 

[Niccomasso]

The tests executed last week have highlighted that the vent circuit is not correct.
Waiting for comments and or support!!!