Vapor Space Flammability concentration 1

[cntrlee]

Hi,
I have an atmospheric o-xylene storage tank with existing PVRV valves. I would like to determine with the existing vacuum breaker valves inbreathing capacity; if the stored o-xylene is within a flammable range in the storage tank vapor space during pump-out of o-xylene and inbreathing of atmospheric air?

 

[Latexman]

The flash point of o-xylene is 32 C. The flash point is the minimum temperature at which a flame will be sustained under air. So, if your tank is air blanketed and the contents are at 32 C or above, yes it is flammable *somewhere* in the tank between the liquid surface and the vent.

Good luck,
Latexman

 

[Montemayor]

cntrlee:

I don't have the data for Ortho-xylene at hand, but I remember the Flash Point is low and around ambient temperature. I also believe Latexman is right and is alerting you to the possible danger in having an air-blanketed tank.

Are you starting out with a N2 or inert gas blanket and actuating the vacuum breaker when the pump-out rate is high? I caution you about instrumenting or operating the tank that way with a potentially dangerous, low FP chemical like O-Xylene. Any static buildup or spark could ignite the very dangerous vapors in the tank, resulting in a major catastrophe. I sincerely hope this is not the situation.

 

[Assumptions]

Here is an MSDS for your substituted benzine derivative.

http://physchem.ox.ac.uk/MSDS/XY/o-xylene.html

Having been involved in a BTX plant years ago, I am surprised that these things are still connected to the atmosphere.

 

[cntrlee]

The existing atmospheric storage tank does not presently have an N2 Blanket. Presently, investigating whether or not to apply N2 Blanketing to existing vessel to mitigate possible flammability issues associated with storing o-xylene with the use of PVRV valving for inbreathing of atmospheric air into vapor space during pump out of process fluid.

 

[Montemayor]

cntrlee:

Granted, you're going to draw-down your vapor space pressure (N2) when you draw-down your O-Xylene inventory. However, when you get to the set point, the N2 supply control valve should activate to maintain the positive N2 pressure in the vapor space. Of course, at this point, you will "compress" the N2 vapor space when you subsequently pump in more O-Xylene and your pressure relief valve will open to relieve the excess N2 to the atmosphere. You'll waste the N2 used to keep the tank pressure positive. Sorry, but that's the price of N2 venting unless you select another system.

I would not incorporate a vacuum-breaking system that allows air to come into contact with O-Xylene vapors on a routine basis. You are differentially combining a potential explosive mixture and you've no idea when it gets to the dangerous level. If you have a need for an emergency vacuum break, that's a different story. But even then, I'd set off an alarm and shut the system down until you investigated and purged the vapor space to an inert level. This way, you are recognizing the fact of a potential hazardous scenario being formed and doing something about it in an emergency mode - not a routine affair. If I was managing the plant, I might not even accept emergency vacuum-breaking with air until it was proved to be totally safe.

 

[cntrlee]

Montemayor,
Thank you for your insight and comments they are much appreciated. I would ask that you would clarify your meaning of not allowing atmospheric air to come into contact with the o-xylene vapors on a routine basis. The fixed roof tank was originally used as a storage means for another process liquid and is now being considered for storage of o-xylene. I realize a floating roof tank would be more appropriate for the storage of o-xylene but that is not an option I have now.

The vacuum valve capacity was selected based on API2000 requirements for inbreathing requirements for the pump out rate from the storage tank. The vacuum break setpoint from tank design requirements for maximum vacuum. I would like to query regarding other options than N2 blanketing for protection of this storage vessel.

Thanks
cntrlee

 

[Montemayor]

cntrlee:

My concern with the BTX’s is not to create alarm over the handling of a very combustible compound. I want to direct your attention to the fact that you have a situation where you are differentially adding an oxidant into a combustible fuel tank – that’s the pure and simple fact of the situation. What’s of more concern is that you are unable to pin-point the moment you arrive at the envelope within your lower and higher limits. The moment you admit to introducing air (with inherent Oxygen content), you are admitting to progressively allowing a potential flammable mixture to form – and should you arrive at that point, a spontaneous combustion inside the tank could form a detonation that would be catastrophic. I’m not predicting this to happen; but how do you propose to arrest or control such a possible scenario if you continuously knowingly (or unknowingly) permit it? A floating roof tank does not resolve your situation. There is the inherent problem of ensuring 100% seal effectiveness – which I seriously doubt is possible for a large vessel. The basic concern still lingers.

I believe your best solution is the fixed roof, API tank, type of design – but with the caveats that you should have a reasonable MAWP and MAWV built into the tank’s construction. This is to furnish you with a workable instrumentation pressure range that will allow set points, alarm points and ensure a positive N2 pressure inside the tank at all times. I’ve done this numerous times in the past with old tanks in multiple services, so I know precisely what your predicament must be like. I empathize, but you must persevere and insist on having a tank with mechanical integrity suitable for the application you are intending to impose on it. It is common for plants to have storage tanks in service that have previously been specified and dealt with in a cavalier manner – simply pointing to “API” standards and letting it go at that. API tanks for chemical service are fine; however, they must be rigorously specified as to MAWP and MAWV – and applied to the corresponding chemical service with the related chemical properties and characteristics always in mind and mitigated.

There are systems to recover any N2 that has to be vented due to subsequent tank fill. However, once again, there is a trade off in costs, equipment, instrumentation, and complexity. There is no easy answer and that’s why an engineer is required to “engineer” the safe, operable, and economic solution. I have organized an Excel workbook on this subject and if you go to

http://www.cheresources.com/indexzz.shtml

and get to the Forums, you’ll find threads where you can find my email address or solicit the workbook. This should orient you as to what you have to look at with respect to N2 blanketing and safe tank relief.

I hope this helps.

 

[cntrlee]

Montemayer
I have read through the threads on the specified site and have not been able to find the Excel workbook regarding vapor flammability in storage tanks

"I have organized an Excel workbook on this subject and if you go to

http://www.cheresources.com/indexzz.shtml

and get to the Forums, you’ll find threads where you can find my email address or solicit the workbook."


Thanks,
cntrlee

 

[Guidoo]

Cntrlee,

You probably missed this one

http://www.cheresources.com/invision/index.php?showtopic=573

 

[JimCasey]

Undoubtedly the most prudent way to deal with this is to inert the tank with a blanketing system.

If there are obstacles to doing that immediately, I would suggest that the breather should pass through a flame arrestor. A conservation vent inherently reduces the likelihood of inhaling an ignition source, but a conservation vent coupled with a flame arrestor would provide substantially effective protection. These are listed in the catalogs from Groth and Protectoseal, and I am sure there are others. The conservation vent is NOT incompatible with adding a Nitrogen blanketing system in the future, but it is something you can do NOW if there is a delay in being able to supply nitrogen to the tank.

 

[unclesyd]

Adding to JimCasey's concerning the use of a conservation vent on a flammable liquid storage tank. We use conservation vents connected to N2 lines on all flammable liquid, O2 or moisture sensitive storage tanks.
These vents work very well and require very little maintenance except a periodic cleaning on some tanks.

There is one caveat in that as you drain the tank it is filled with N2 and will continue to feed N2 as the tank is opened for any reason. The nitrogen system has to taken care of by the breaking and entering procedure along with the vessel entry procedure.