When not to use Fire case for PSV design

[Scall]

Hello all,

I'm sizing a PSV for an atmospheric tank containing 35% HCl. The fire case is by far the largest relief requirement, and would require a new nozzle to be cut into the tank. If the contents are not flammable, and there are no other flammable sources nearby, do i have to size for the fire case? Does anyone know which codes govern this decision. API-2000 doesn't offer too much up in this area.

Thanks in advance

Scall

 

[TechnicalBard]

IF you cannot envision a scenario where the tank or vessel is being heated by an external fire (ie. there are no flammable materials nearby that could jet or pool near the tank), then I would say you don't need a fire case PSV. Of course, local codes may govern and override good judgement.

 

[pleckner]

There is no Code that dictates the criteria for sizing relief cases. They rely on the owner's judgement to take into account all possible scenarios and determine the best way to protect against over pressure. Even the insurance company can't force you into sizing a relief device for a scenario that does not exist. Nor can the local authorities. You may have to prove to them a particular scenario does not exist if they so request that information but otherwise, it's totally up to you.

 

[BenThayer]

just be careful that you do not create a scenario in the future. e.g., a dike that can hold multiple tanks and someone adds a flammable material to a new tank in the future.

they might not remember to check the sizing for other tanks in the dike when doing the sizing for the new tank.

but if you have no valid scenarios, that is likely why you have the nozzle size you do.

best regards, ben

 

[Montemayor]

Scall:

I want to add a little additional experience to this thread because the subject matter, albeit basic, is so very important.

Two key individuals have contributed the basic, true facts:

Phil Leckner - "There is no Code that dictates the criteria for sizing relief cases....it's totally up to you."

BenThayer - "..be careful that you do not create a scenario in the future."

These two pieces of advice are pearls of wisdom. Now let me add something else: What Ben has pointed out is precisely why Management Of Change (MOC) was instituted and mandated by OSHA and some smart and experienced companies. As Phil indicates, it's your responsibility; therefore, the way you plan, organize, and control any future change is in your hands and if you are smart, you too will find a way to make sure you don't create a potential or fatal hazard by introducing a possible Fire Case in the future. Document any and all changes and continuously control them.

Merely installing the correct PSV is not the total answer when you are operating the unit. That attitude is reserved for those of us who are now contracted designers or work for engineering companies - we don't operate the units; we just design them, then walk away. If you operate, you have to live with the results and any future changes.

 

[roker]

hello all,

does an atmospheric tank need at all a PSV? not only pressure vessels?

regards,
roker

 

[Montemayor]

Roker:

In my opinion all enclosed vessels receiving and supplying fluids should be scrutinized for needing a Pressure or Vacuum (lack of pressure) relief device. Any API storage tank described as "atmospheric" probably will be found to require a device of one sort of another - even an overflow nozzle. It all depends on what you're doing with it and how you operate it.

Do not be naive and simply apply generalities to any application - even one as "mundane" as an atmospheric tank. I can show you photos of such "benign" applications that would raise the hair on your head. It doesn't take much pressure (or lack there of) to destroy such a piece of equipment - and it can be fatal.

A Pressure Safety Valve (PSV) is normally used on Pressure Vessels (reference API 520 & 521). A conservation vent and emergency relief (we call them PVSV and PSE where I work at) are used on API storage tanks. You can use whatever acronym you like - but make sure you acrutinize the safe operation of the tank, just as you would a pressure vessel - especially for vacuum scenarios.

 

[pleckner]

The term "atmospheric" when discussing tanks tends to lead us into a false sense of security concerning over pressure (or vacuum) conditions. As Art said, you can't generalize.

IF your "atmospheric" tank is one that is totally opened to the atmosphere via a vent, then you only need to make sure the vent size and piping (if existing) are sized appropriately to account for pump-in, pump-out, breathing and any other credible over pressrue scenario that you can think of, including fire. If the open vent pipe is not capable of handling it all then you would install some type of emergency relief device on the tank as well.

If the tank is designed to be an "atmospheric" tank but is totally enclosed for process or environmental reasons, then as Art points out, some device is absolutely required to protect the vessel from the same conditions, I state above.

 

[stookeyfpe]

Roker:

An atmospheric aboveground storage tank does require a means of pressure relief based on a fire scenario if it contains flammable or combustible liquids. NFPA 30 contains extensive requirements. It should be noted that NFPA 30 does not define it as pressure relief but instead defines it as emergency venting.