dipak2
By "cold flaring" I think you mean "venting" (not using a flame to burn the hydrogen).
I believe that you need the flammability limits of Hydrogen when diluted by steam.
These limits are not, commonly, available but can be estimated from the normal, published limits of flammability.
Because you are interested to STOP flammability we should use the lowest published limits, which are generally for upward propagation.
In air, the usual lean limit is 4% H2. This means that the dilution is 96 Air:4 H2 = 24:1 to stop combustion propagating in the mixture.
If you dilute with steam, the steam is roughly 1.35x more effective than the air so the limit in steam would be 18:1 or 5.3% vol H2 in a hydrogen/steam mixture.
It is best to always use a factor of safety. You must select your own FOS according to how you assess the danger of the flammable condition.
These numbers assume normal ambient temperature and pressure, AND that the hydrogen and steam are pre-mixed. If you have a steam assisted flare tip, and hope to achieve this result with external steam assist, the characteristics could be significantly different according to the design of the flare tip and would need a more complete and detailed assessment.
Remember too, that steam condenses in the line so you lose some of the steam to water, which then doesn't necessarily contribute to the quench effect
AND
when you stop the flow, the residual steam in the system condenses and contracts and will pull air back into the vent pipe, not what you need if you have hydrogen in there too.
You need to look very carefully at the operating conditions and not just assume that the mixture will always help. If, for some reason, you do get a flammable mixture of air and hydrogen back in the pipe, and you think that there is a possibility of an ignition source, you could be at risk for a flash back leading to detonation in the line. In that case, the forward velocity of any mixture from the stack has to be faster than the flame could possibly travel back into the top of the stack. (about 10 fps - again use an added FOS)
There are many considerations and I trust that you will apply good engineering judgement throughout to the entire problem, (of which I have insufficient knowledge) and not just run with my numbers without complete thought.
