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Submitted Chris Haslego, Nov 21 2011 11:21 AM | Last updated Nov 21 2011 01:29 PM




Category: Refining
Question: Can molten sulfur pose a safety hazard in a sealed tank?
Keywords: sulfur,storage,tank,mawp,nitrogen,purge,design
Answer: It depends on the type of tank in which the molten sulfur is stored. Molten sulfur is probably not stored in pressure vessels. It is more likely stored in cone roof, flat bottom tanks and such tanks can only withstand a vapor space pressure of 0.5 psig or less whether they are storing molten sulfur, or water, or fuel oil, or anything else.Sulfur per se does not generate explosive gases at normal conditions, I would like to make this clear. But the system (in reality) could emit this type of gases as molten sulfur (from refining industries) usually contains dissolved quantities of such a lot of gases (such as H2S).Your liquid is subject to stabilization (gas evacuation) while in the tank, so you must provide for removing the generated free gases (H2S ?) as they are formed and pose a potential over-pressurization hazard. To design for this problem you can:a. Use a convenient and available vacuum system to vent the accumulated gases as they are released and route them through the vacuum system to a flare header or through your scrubber. If you are in a refinery area, you should have access to this type of convenient answer.b. If you don't have a vacuum flare header available, you can supply your own blower system and evacuate the generated gases on demand as the pressure increases in the storage tank. For this, you need a dead band for the instruments to be able to function reasonably. In other words, you should have a design tank pressure ample enough to activate the blower operation and be able to shut the blower off at a lower pressure. This lower pressure should be safe-guarded from a vacuum condition which you cannot tolerate. Normally, you must be prepared to break a potential vacuum with atmospheric air. If your gases cannot be brought into contact with atm. air, you are forced to use an inert gas (Nitrogen) to break the vacuum or maintain as an inert blanket. Either way, you are going to have to confront this need to identify the compatability of your gases (whatever they may be) with air. Otherwise, you cannot design any further. This will be the first and number one issue when you meet for your Process Hazard Analysis. If you apply Nitrogen, and evacuate at a rate to keep the vapor space purged continuously, the pressure could be a variety of values only subject to the tank's MAWP (Max. Allowable Working Pressure) which is set by you.


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