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Sulfur Pit Explosion Protection -- NFPA 68 Interpretation

Sulfur Pit Explosion Protection -- NFPA 68 Interpretation

(OP)
I am working on a concrete sulfur pit, using an in-pit degassing technology.  Liquid sulfur enters the pit with approximately 350 ppmw H2S and is degassed to 10 ppmw H2S.  Because of this, the possibility of explosion, due to H2S concentration in the vapor space, exists (it is protected by numerous safety instrumented items, but since instruments may fail, mechanical protection is required as well).  We are currently working to design explosion protection, per NFPA 68, but have had some issues with various interpretations.  

The pit is separated, by weirs, into three separate compartments.  The weirs do not extend all the way to the roof, but do extend to ~75% of the pit height.  Per NFPA, an enclosure is defined as "a confined or partially confined volume"; based on this, I am led to believe that explosion protection is required for each of the three separate compartments.  I've made some initial vent area calculations so far, and have found that the third compartment (degassed liquid sulfur is being pumped out from our third compartment) requires more vent area than is available.  Space is limited in this third compartment, as there are many connections through the pit roof.

However, another statement in NFPA 68, states "If an enclosure is subdivided into compartments ... each compartment that contains a deflagration hazard shall be provided with its own vent."  As mentioned previously, this third compartment is degassed liquid sulfur; also, sweep air is brought in through two intakes in this compartment, and exits (with higher H2S concentration) from the first compartment.  While I realize that to reach the LEL will mean that the sweep air is not venting (adequately) and that the entire vapor space is the same, would it be possible to justify a slightly lower explosion vent area in this third compartment (and make up for it in the other two compartments, such that the total vent area required is met)?  


Another issue we are having is that NFPA 68 states, "Vent closure operation shall not be hindered by ... the buildup of deposits on their inside surfaces."  Inside the sulfur pit, it is highly likely that there will be some amount of solid sulfur buildup.  I've asked our Client to let me know how much buildup they normally see inside their pit between turn-arounds; I've been advised to assume the buildup will be up to 1" thick.  

Other than heating the vents to a temperature above sulfur melting point, I cannot come up with another way to prevent solids from building up.  Unfortunately, putting steam coils on the vents does not seem like a feasible (or even safe) option.  Also, opening up the panels to physically remove buildup is really not an option either.  So I need a way to take the additional weight of solid sulfur buildup into account.  Can I use the "effects of panel inertia" equations, or do I need to somehow adjust the Pstat?


One final question that has come up a number of times is whether we can have our Structural department provide some type of panel that can be used for both maintenance and explosion protection.  It sounds like this is what has been provided on some of our previous projects, but my best guess is that they didn't exactly comply with NFPA 68.  NFPA 68 states, "The vent closure manufacturer or designer shall be responsible for documenting the value and tolerance of Pstat of a vent closure", so unless our Structural team can test and guarantee the burst pressure and given tolerance of the panel(s), we would need to purchase from a manufacturer to be in compliance with the standard.


It seems that every time I read through the standard, I come across yet another thing that cannot be easily implemented.  I've talked with a number of people about providing explosion protection, but very few have been able to provide much assistance.  I'm hoping that someone on here can.  I'd really appreciate any help in this matter!
 

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