Flash back in absence of ignition initiation

[Sawsan311]

Dear All,

I have a general question.. if we have a fluid which is handled above its flash point.. i.e. the operating temperature is above the flash point.. can flame development occur (in combination with air) with subsequent ignition occur even in absence of an external ignition flame ?

I saw several design standards dictating nitrogen blanketing for vessels which process fluids handled above their flash point.. surely the flammability range of the stored liquid is important.. but the relativity of the flash point with respect to the operating temperature also matters for the vapor space combustion i.e. flash point is the temperature which represents the limit of the LEL

additionally, In case your vessel is open to atmosphere, would you still provide a flame arrestor on vessels padded with nitrogen to cater for the scenarios of absence of the padding system and preventing flashback from external atmospheric ignition i.e. strike. Or would you prefer not to have a flame arrestor considering its potential blockage with subsequent overpressurization of the vessel. API 2000 7th edition for example Annexure F, allows having nitrogen blanketing in absence of flame arrestors on the condition that blanketing requirement reaches a minimum value of 50% of the computed normal thermal inbreathing rate+the inbreathing due pump out with no pump in.

Appreciate your views.

 

[MortenA]

How can you know that you dont have an ignition source? Static electricity, lightning ??

If you have an atmospheric tank that is breathing you will have to assume that you have a concentration between LEL/UEL if you operate above the flash point (maybe not all the time but you cant know when).

The flame arrestor question is difficult and i would say application specific. Experience shows that blockage of flame arrestors is quite rare - but i would recommend PM on the flame arrestors to ensure that they dont block. If you hvae N2 blanketing (and then the vessel will not have a vent line open to atm) then i would say that you dont need a flame arrestor

 

[Sawsan311]

Thanks Mr.MortenA for your response..

I am wondering that if the tank is blanketing with nitrogen then this nitrogen must be disposed off to safe location or to flare which eventually ends up with the fact that we would be open to atmoshphere and unless nitrogen blanketing is not continuous .. we will surely need other protection measures against flashback deflagartion with possible unstable detonation i.e. flame arrestors. Flame arrestors may be weak for detonation arresting therefore continous nitrogen blanketing must helps in reducing the flammability range of the vapor space.

Another question.. if the tank is in batch operation.. we can consider making the tank top sealed.. with a PSV due to the presence of potential flashed vapors caused by heat up from ambient.. this PSV would sure need to be routed to safe location.. flame arrestor would be needed anyays.

 

[TiCl4]

Sawsan311

To answer your first question, yes, ignition can occur inside a tank without having flashback into the tank. There have been several documented tank rupture cases by the CSB where suspected static ignition sources came from either falling flammable liquids or by a floating level gauge during tank filling. Lightning is also a concern. Unless O2 is needed (like in the case of monomer storage), I believe pure N2 blankets are rather ubiquitous when handling the storage of flammable liquids.

As far as sealing your tank question...What is the MAWP of your tank? If non-atmospheric, you can afford to pressurize it slightly with a N2 blanket. That will eliminate the need to have constant N2 flow to tank - N2 will only discharge from the tank when filling the tank. Note that in this design the vent valve that discharges when filling should NOT be your PSV, but rather be a weighted vent valve sized to handle normal filling rates. When drawing down on a inerted pressurized tank, the volume makeup comes from the N2 feed, not an in-breather valve. This design eliminates the possibility of passing through your flammable zone when drawing the tank level down and also requires N2 flow only when the tank level is dropping.

As far as your discharge location question...Yes, an appropriate vent valve should discharge through to an appropriate location. I doubt that discharge to atmosphere would be considered an appropriate location by your company or regulatory body. I don't know your application, but typically the appropriate location is a vent header that goes to a flare, thermal oxidizer, or scrubbing system. Yes, a flare/TO has the possibility of flashback. However, the flare/TO system should be designed to intercept/dissipate any flashback before it reaches the main header/branches. This eliminates the need to try to control flashback at each individual tank/source via flame arresters. A flame arrester would provide additional protection, but would incur additional costs and PM resources.

I work with various monomers, most of which are stored above their flash point. Every storage tank has a vent valve and a subsequent flame arrester - this is standard and recommended design practice by the manufacturers of each of these monomers. One monomer in particular, styrene, has a strong tendency to polymerize in the vapor space surfaces of tanks (creating stalactites >3 feet in length, in some cases). This can (and has!) plug the flame arrester, so the flame arrester is placed on a PM to handle the plugging issue. Unless your liquid has a known tendency to plug, there is not much risk of flame arrester plugs forming. Having said that, all of our flame arresters, not just the styrene tank arrester, are on quarterly inspection PMs to prevent the possibility of arrester plugs even though we have no plant history of plugs except for styrene. We do have flashback prevention from our thermal oxidizer, but each tank also has a flame arrester as additional protection.

Lastly, I would NOT place a flame arrester on the outlet of the tank PSV. The PSVs are normally sized for worst-case flow rates (i.e. pool fire) and have venting velocities that are in excess of the flame propagation speed. A flame arrester is simply not needed when discharging during those events. Moreover, the flame arrester would exhibit a very large pressure drop at high flow, necessitating an excessively large arrester (which happens to be unnecessary).