INERT CONFINED SPACE ENTRY ---WHY API 2217 A SETS A LIMIT FOR FLAMMABLE VAPOUR CONCENTRATION?

[p1a9l7a5]

The general understanding regarding inert entry is there is no need to worry about flammable vapour /gas concentration (LEL) since Oxygen is maintained at 5% or less. (hence no sufficient Oxygen to support fire).

However, in API 2217A (INERT CONFINED SPACE ENTRY STANDARD) there is a limit for flammable vapour/gas concentration.

It is 10% of LEL for hot work inside inert confined space
and 20% of LEL for doing cold work inside inert confined space.

If 20% of LEL is exceeded entrants shall evacuate inert confined space.

Can someone explain the logic behind these limits for flammable vapour/gas concentration even when Oxygen is less than 5%?

 

[EmmanuelTop]

Definition of flammable concentration is, by default, "the concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source". So, even at reduced levels of Oxygen, the ratio of concentrations of flammable components and Oxygen can be in the band between LFL and UFL.

Secondly, from safety perspective, the concentration is never considered constant or resistant to change - for vapors and gases it is easy to imagine why. There is natural convection, for example, or external factors that may alter/increase concentration of air significantly, thus creating flammable atmosphere in an area where vapors were dispersed in concentrations above UFL. This is a situation you have when e.g. a gas pipe/vessel bursts - a sudden expansion of gas that was way above its UFL into atmosphere, creating a flammable mixture. It is just one step between non-flammable and flammable mixture.

"Instability" of concentration is why all safety standards prescribe certain percentage of LFL (and not 95% LFL) as the maximum concentration for vessel entry, maintenance works, safety radius, etc.


Dejan IVANOVIC
Process Engineer, MSChE

 

[p1a9l7a5]

Thanks for your quick reply.

The issue is LFL & UFL values are valid only when oxygen concentration is 20.8%. (in air only). In Nitrogen atmosphere LFL & UFL values available in standard literature are not valid. they may be different.

Also, in an inert confined space Oxygen is kept very low (to avoid pyrophoric iron fire) by continuously purging the vessel with nitrogen in such a way that the nitrogen goes out through manway leaving no room for atmospheric air to enter the confined space.

And in case of cold work inside inerted confined space there is no ignition source also.

Still the standard sets LEL limit.

Hope we will find out the exact reason.

 

[EmmanuelTop]

Ignition does not occur in air only - it can happen all the way down to the point when the Limiting Oxygen Concentration (or Minimum Oxygen Concentration) is reached, which is less than 21% for all gases. See

http://www.processoperations.com/FireExplode/FE_Tables/FE_Table02.htm

and the attached article.



Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Typically, UFL for hydrocarbon vapors is about 95% by vol, when the rest is air per the API.

So, if the remaining 5% were to be air, the ratio of HC to O2 to support combustion would be 95 : 0.208x5 = 95:1, which is approx 1% by volume O2 in the mix.

So if you detect 5% by volume O2 in the vessel, it can still support combustion, assuming there is no effect from the N2 on this upper ratio limit of 95:1 for HC to O2.