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In a particular complex onshore gas plant, flare network purge is via continuous flow of N2 controlled through flow orifices, purge points being located at the ends of all major headers. There are also a few fuel gas purge connections but these are located close to the flare stack. Under normal operation fuel gas purge points are closed, ie no flow.
I would like to know what would be the risk of stopping all N2 purge gas and starting fuel gas purge. This would lead to the flare network being purged only close to the flare stack. Rest of the network will have to depend on control valves / other vents for a positive gas flow towards the stack.
We can assume for the sake of this discussion that the fuel gas rate is sufficient to safeguard the seal function of preventing air ingress through stack.
 
Answers
14/03/2009 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
In my opinion, best practices would favor sustaining injection of nitrogen at controlled slow rates to several points (via perhaps mini self contained flow control valves).
Injection of dry nitrogen---essentially free of oxygen + moisture and totally free of H2S at slow rates at the "far end spurs" of a refinery flare system helps to provide corrosion protection of the flare system itself and tends to block the potential for either ice formation or carbonaceous sludge formation when the ambient air temperatures are sub-freezing for extended periods. Too often, many design concepts ignore what can happen during extended sub-freezing ambient air temperature periods except with respect to "minimum metal design temperatures".
Ensuring an appropriate flow of gas through flare seal devices (in the final flare knock out drum and at the flare tip) is really a separate consideration.
14/03/2009 A: Hubert Mueller, Bayernoil, guhumueller@web.de
I agree with the previous answer but would like to add an additional safety measure.
If you install an oxygen analyzer in the flair stack, far enough from the tip to avoid noisy readings, you increase the safety of your flare system and maybe can control the fuelgas purge rate at a lower level.
The endpoint N2 purges should stay connected for emergency cases.
13/03/2009 A: Samuel Sinanan, IPSL, Samuel.Sinanan@ipsl.co.tt
There is no risk in stopping the nitrogen purge and starting the fuel gas purge. The only location where air can enter the system (during normal operation) is down the flare stack so the fact that the the fuel gas purge location is closer to the stack is a safer approach. Once the fuel gas purge rate is sufficient to prevent the ingress of air down the stack this should be sufficient. Utilizing nitrogen to purge is wasteful in any case.