NFPA 68 venting gas deflagration

[anousone]

Hi,
I need to design venting area for a gas deflagration in an empty tank (no obstacles).
As specified in the NFPA 68, the Kg values are dependent on the volume of the vessel. If I used the Kg value corresponding to my volume (read from the figure Kg vs volume) the required venting area calculated is too high for practical purpose. And I didn't even consider initial turbulence and potential inertia effects...
Does anyone know about papers or articles that have dealt with the Kg variation with wolume or that have shown that it is acceptable to use the Kg values measured in the 5L sphere?
Thanks.

 

[psafety]

Anousone,
If I understand your question, I think you misunderstood NFPA 68 reference to Kg determination. The Kg value "standard" is the 20-liter spherical test bomb. Other sizes such as 5-liter may be used but the data "adjusted" to correspond to an equivalent value from the 20-liter apparatus.

Your particular vessel venting project does not affect Kg.

 

[anousone]

Thanks for your answer, but maybe my request was not very clear.
I have checked NFPA 68 again. I understand that the recommended volume for testing is 20L (as per annex B section B.4). In my case testing cannot be made. So section 6.3.3.8 specifies that when testing cannot be made, Kg can be approximated by ratioing from the Kg of propane.
Now Kg value presented in table D.1 of annex D are measured in a 5L sphere (Bartknecht data), and all new Kg value should be adjusted to the Bartknecht data (section D2 of appendix D) which somehow make the 5L sphere a reference I think.
So in fact my question is:
Is it justified to use the Kg data of table D.1, even if the vessel I have to design is bigger than 5L (having in mind that there is a strong variation of Kg with volume as shown by figure B1 of annex B for example)?
Maybe I have miss some part of the NFPA 68, but so far that is what i have understood.
Thanks.

 

[psafety]

anousone,

Kg of 100 for propane is correct, although values are adjusted (note Table D.2) its measured value is 76.

If you follow the directive referenced @ 6.3.3.8 it informs to use 100 bar-m/sec. Which comes from the y=1.5(x)-14, where (x) =76.

Kg determination of your gas should then be by measured values (adjusted to the 20-liter standard) - or - approximated from both gases burning velocities. Once your unique gas Kg is determined it is a "constant" in the venting equation, not a varible based on the size of your vessel.

NOTE: There are several very good testing labs to determine explosivity characteristics of gases and dusts. This is the SAFE way to approach the problem.

 

[psafety]

anousone,
Another direction you may want to investigate instead of venting is controlling the MOC (Minimum Oxygen Concentration)and verifying that you never have available O2 to support combustion. I've used this before, the controls usually become elaborate, but a viable solution.

Don Coffman

 

[25362]

I'm almost a year too late. Anyway, the MOC estimation method based on multiplying the stoichiometric (oxygen to fuel) molecular ratio by the LFL, doesn't apply for gases such as ethylene oxide and acetylene, since for both gases the measured MOC is zero.

BTW, published data says that for hydrogen it is 5%, and for H₂S 7.5%. For most of the hydrocarbons it is higher than 9%.