Sulphuric Acid Tank Vents

[jonelv]

Hello

I'm doing a review of some old (30 years plus) acid tanks, containing 96% sulphuric acid. Some are atmospheric tanks, some are designed for 2barg internal pressure. Although I can find no specific references in standards (BS or API) it is my understanding that atmospheric tanks should be fitted with separate vent and overflow pipes whereas pressure vessels may have a combined vent/overflow system - any thoughts?
Also when considering the vent size, as well as allowing for normal venting (during fill, drawdown, thermal expansion etc) the worst case that I can envisage is an accidental introduction of water or incompatible chemical leading to vigorous exothermic reaction in tank. Anyone any ideas on how one might go about calculating required vent diameter?

 

[EGT01]

Jonelv,

What you mean by "vent" could have a bearing on the answer and whether the destination could tolerate either vapor or liquid could also have a bearing on the answer. I have seen atmospheric tanks connected to a common vent collection system that also doubled as a common overflow collection system but even then, the designated vent line was connected to the tank roof while the overflow line was connected to the tank wall. Obviously, the overflow connection acted as a vent connection as long as the tank level was properly controlled. In this particular arrangement, the tanks were equipped with pressure/vacuum vent devices that were completely separate from the common vent/overflow collection system.

Without more details about what you are trying to accomplish, you should first determine the standard or codes to which your tanks were constructed from there, you should be able to determine if there are any specific requirements for venting and overflow of your tanks.

I can only speculate that your atmospheric tanks were constructed in accordance with API Standard 650. If that is the case, you should review section 3.10.8 Tank Venting in API 650. You will find that it will also refer you to API Standard 2000. In API 2000, you should review section 4.5.2 Installation of Pressure and Vacuum Relief Devices. Item "a" states:
"Provide direct communication with the vapor space and not be sealed off by the liquid contents of the tank."

I would think this precludes the use of the overflow connection for the vent connection when the vent connection is used as pressure and vacuum protection.

As far as determining your venting requirements, for your atmospheric storage tanks, I would also recommend further review of API 2000, Sections 4.2 and 5.2 - Causes of Overpressure and Vacuum.

For venting requirements of your pressure vessels (assuming you mean for overpressure protection), you should review API Recommended Practice 521, Section 2 - Causes of Overpressure and Section 3 - Determination of Individual Relieving Rates. See also API RP-520 Parts I and II sizing and installation of the relief devices.

You should also review any plant documentation such as PHAs or HAZOPs that may exist for your facilities that would have discussions about overpressure events.

 

[cub3bead]

Many concentrated acid tanks arre fitted with a desiccant dryer on the vent to absorb atmospheric moisture.

I have always included a very health corrosion allowance also.

 

[MJCronin]

jonelv

With regard to your question on vent size, My experience has been with concentrated sulfurica acid bulk tanks that were used only to supply a process.

The vent size for bulk storage tanks was typcally controlled by the maximum fill or discharge rate. A 3 or 4 inch NPS vent is the typical size used

Concentrated sulfuric acid (~93%)is usually stored in horizontal axis carbon steel tanks sized with a generous corrosion allowance (~.125 inches). Particular attention must be paid to the design and selection of materials for the tank inlet and outlet to prevent the dreaded "hydrogen grooving".

The vent of this type of atmospheric tank must be configured and selected to prevent moisture ingress during normal operations. If water is allowed ingress, a layer of dilute sulfuric acid will be slowly formed atop the concentratred acid. This dilute acid will attack a carbon steel tank interior causing a groove and weakspot on the tank shell. API-579 discusses this effect. Dessicant dryers are sold for this purpose, but they require periodic maintenance and/or regeneration.

There is more to discuss, but API-2000 is an excellent guide for vent sizing. What is the tank size, MOC and purpose ?

There is lots of information on sulfuric acid system design on the internet...

regards......


_MJC