Nitrogen Purge for Thermal Oxidizer Vent Collection Header

[noleche]

Hello, I am working on a project to add a nitrogen purge to a vent collection header that feeds a direct-fired thermal oxidizer. This is for a batch plant so there will be times when there are no process vapors being released to the header, only the purge. I cannot find a guide that tells me what flow rate / velocity of purge gas I should design for. According to the plant the TOX was assembled from available parts so there isn't a vendor to ask. I'd appreciate any help.

 

[vt2012]

What is the purpose of the nitrogen sweep? What value is it adding? Why will it be added when the system is down? With some more info about the objective of the project we could provide some insight.

 

[noleche]

Thanks for your reply. The purpose of adding nitrogen is to reduce the oxygen concentration to below the LOC, which in this case is about 12 vol% O2. The nitrogen won't be added when the system is down but in between batches when the TOX must remain in operation. My understanding is there must be a minimum flow to the TOX at all times to assure no flammable gases are accumulating in the header.

 

[georgeverghese]

There are values for flame propagation velocity for various fuels in Perry 7th edn pg 27-26, table 27-18. You could interpret these to be the min purge gas velocity at the burner feed nozzle.

 

[noleche]

That is some helpful information. Thanks very much.

 

[chemEcaleb]

I recommend using the Husa method with a safety factor of 6. Just google "Husa gas purge" and you'll get all the information you need. There have been numerous discussions on eng-tips regarding this. Also you may refer to API Standard 521 Section 7.3.3.3.3.