Emergency vents for caustic soda solution

[Jagmtr]

Hi friends,

I need a help. I have to design a storage tank to store 40% caustic soda solution. My roof to shell joint is coming non frangible. So i think for emergency vents. But i understand that my stored product is non flammable so emergency vents are not required.

There is also heating coil inside the tank.

Please help me i should consider emergency vent or not

Regards,

 

[MJCronin]

Some questions:

1) What is the volume of your tank and what is the diameter ? Materials of construction and shell thickness ?

2) "Heating Coil" is kind of vague .... Electrical or steam heating or something else ?

3) You should have a generously sized overflow pipe as well as an electrically traced vent

Superbly written Caustic soda tank design FAQ here .......

https://www.eng-tips.com/faqs.cfm?fid=1096

MJCronin
Sr. Process Engineer

 

[horacio Torres]

Emergency vent line is not required for Naoh storage tanks. Vent line for commissioning, and maintenance procedure should be set.

Horacio

 

[Jagmtr]

Thanks horacio.

Thanks MJcronin. Its small size tank with MOC sa516gr60 with steam heating coil.

From your answers i am sure for no need of emergency vents. Overflow & normal vent nozzles are already considered.

Regards,

 

[Geoff13]

Emergency vents have nothing to do with the flammability of the stored product.

The API 2000 rules for the required venting capacity are based on an external fire causing boiling of the stored product. Boiling will create large vapour volumes which are handled through the emergency venting. It is sometimes possible the standard vents can handle the emergency venting requirements. The formulas provide factors reflecting the properties of the stored product, and thus the volume of vapour created. API 2000 does provides venting requirements for fire or explosion inside a tank.

However this still requires an external fire source. In the kinds of plants API typically applies to (refineries, etc) there are many flammable products so external fire is always a possibility. Only you can determine the fire hazard where your caustic soda tank is located. If external fire is an operating hazard that must be addressed, and 50% caustic soda can boil, then emergency venting may be required.

 

[Jagmtr]

Thanks Geoff13 for your kind response.

I think if we think about boiling of stored liquid then we will have to look the boiling point, heat transfer rate, exposed surfcace area, heating time and other points.

I am not aware of the boiling point of 40% caustic solution but for pure caustic soda it is 1388 deg c. So we have to think can it achieve such temp for external firing.

Regards,

 

[JStephen]

The problem is that API-650 is intended primarily for "oil" tanks, so some adaptation may be required for other uses. So while API-650 doesn't give any exemptions for the frangible/emergency venting, that is not normally applied to water tanks, molasses tanks, fertilizer tanks, etc, even when those tanks are designed to API-650.
The point behind the frangible roof/ emergency vent is to keep from breaking the shell-to-bottom weld, rocketing the tank into the air, and releasing the contents. Like this:

https://www.youtube.com/watch?v=6qcrwNM74sg

With a water tank, it may actually be advantageous to release the contents in a fire. With any flammable product, definitely not. With other industrial chemicals, it may vary.
The amount of heat input required versus the volume of vapor generated could vary considerably also, which would make required emergency venting much smaller.
If the other tanks in the area are not storing flammable products, it's much less likely to be an issue as well.
Also, if product stored does not require conservation vents, then normal free vents can simply be oversized to fulfill the emergency venting requirement.