Oxygen concentrations after inerting

[RogerH]

I have a question with respect to residual oxygen concentration in a process unit, vessel etc after inerting.
The usual limit of oxygen (in nitrogen) I have usd in refineries are 0.5 or 1.0 vol% after an inerting step. The the inerting is regarded to be complete and accepted. However looking at figures such as LOC combined with LEL/HEL this figures is about 11-12 vol% oxygen.
In my past I have been accustomed to use the 1.o vol% limit without giving it much consideration. however I cannot find a god explanation why such a low number should be used. Pratically I can buy into using a margin from e.g. LOC to cover for uncertanities in sampling/analyses and that all part of a unit/system that has been subject for the inerting havent mixed ideally. Naturally if hydrogen can be present I an see a reason for a lower limit, but in the case there is no hydrogen present, should the limit be this low?
The 1.0 vol % seems to be an industry practice. Is there anything I forget, overlook here.
Would be very much interested to hear others view on this subject.
Thanks

 

[25362]

You may find some answers by visiting thread798-33615 and thread135-116595 and the links therein.

 

[GL431]

My view is that other than causing fire and explosion within the LEL-HEL range, O2 can also be responsible for side reactions and corrosion in refinery equipment already at the ppm level under process conditions. That is one reason I would want to get the O2 level as far down as practically possible before I start the process running.

Some degradation mechanisms due to O2 are difficult to predict before hand and the designers assumed an O2 free environment. That could also be a reason why the industry standard is to get the O2 level down as far as possible before introducing process fluids.

Some examples of damage due to "higher" levels of O2, I can think of: degredation of MDEA - corrosive products will manifest themselves after months; pitting if there are Cl ions.

 

[dcasto]

gl431 is correct. There are some processes where 5 ppm O2 will cause violent reactions (polyethylene for example). So you you need to know whats going on in the equipment you are inerting too.