Hazardous Area Classification For Mild Steel Sulphuric Acid Tanks. 1

[bigbang]

We have a project where we are considering installing high level instrumentation into mild steel acid tanks (approx 6000 Tonne capacity)

Does anyone have any guidance or thoughts on the hazardous area classification for mild steel sulphuric acid tanks.
In normal operation sulphuric acid will corrode the mild steel liberating hydrogen gas . This gas can then form a flammable atmosphere inside the tank (under the roof sheets).

We have two styles of tanks . One style has the roof beams above the roof plates and the other style the roof beams are under the roof plates. Both have top vents at the apex of the roof. Ventilation inside the tanks relies on the density difference between air and hydrogen (hydrogen naturally rising to the vent) . However the tanks with the internal roof beams can trap pockets of gas ( 200 mm deep).

According to the Australian Standards the hydrogen gas generation should be considered as a continuous grade release-"release which is continuous or is expected to occur frequently or for long periods".
The Lower Explosive Level for hydrogen in air is 4%

As I read it the Australian standards call for the roof space and the area around the tank vent should zoned as a Hazardous area ( Most likely Zone 0) and yet traditionally in my industry sulphuric acid tanks are not treated as hazardous areas. It is argued that the vents provide sufficient natural ventilation to keep down the hydrogen concentration inside the tanks. I tend to differ.

This leads to another point- Should the tank vents be fitted with flame arrestors to prevent any flames migrating back into the tanks say from a lightning strike.?

So,
Should the tanks be zoned as a hazardous area ?
Are the tank vents providing sufficient natural ventilation ?
Technically should we install flame arrestors which will be prone to sulphating over and blocking ?

 

[ash9144]

The tank does not need to be classified hazardous as long the vent is adequate size (6-10")and vents to atmosphere. The hydrogen rate is very slow so after tank is pickled the rate drops off to very little. I am assuming this 95% or greater sulfuric.

The biggest issue is with routine maintenance were as you mention internal roof beams and supports may trap acid which becomes dilute as corrosion progresses and builds hydrogen gas. Then as you cut into the gas traps you may get a flash.

No flame arrestor it would definitely plug/sulphate over.

 

[JStephen]

You may also want to look into frangible roof requirements. I think API and WRC have some documents on frangibility. The idea being that if you do get ignition in the tanks, you want it to blow the roof off rather than pulling the corner weld loose.

 

[kenvlach]

Some info on Sulfuric Acid Storage tanks:

http://www.sulphuric-acid.com/TechManual/Storage/storagetanks.htm

See also Corrosion on LHS menu. Gives charts for various materials (including carbon steel) vs. sulfuric acid concentration at different T's.

About your question -- would it be practical to purge the space above the acid with dry nitrogen? Might both prevent explosions & reduce corrosion. Although concentrated sulfuric behaves like a desiccant and removes moisture from air, don't know how fast it happens.

More info:

http://www.dupont.com/sulfurproducts/techdata/equipment.html

 

[bewdley]

In my experience, you can use mild steel for H2SO4 as long as this is anhydrous. This means that you cannot freely vent or, more precisely, you cannot introduce water vapour into the tank. There are special vents in which ,I believe, air goes through a silica gel bed. A common practice calls for a heavy corrosion allowance, considering 1-1.5 mm/year corrosion if the vent is controlled.
If you make a simple calculation, you will see that the amount of hydrogen you can release in one year is really minor.

 

[JLSeagull]

Use your own judgement and consider instruments rated for the hazardous area even if over-specifying the hazardous area rating.

Most instrumentation that I buy is inherently rated at least Class I Division 2 (zone 1). Much if not most is rated Division 1 (zone 0). Rosemount and many other manufacturers offer instruments rated groups A-D but some are only rated groups C & D. If you have any concern select instruments rated zone 0 group B for hydrogen service. Depending upon the manufacturers and technologies that you use, you may not have and difference in model numbers or pricing to specify the instrument rated zone 0. Look more closely if buying open or guided wave radar, magnetostrictive or some other technologies from manufacturers who are not strong in the oil, gas and petro-chemical business.

 

[BenThayer]

i agree with bewdley. you want to have some means to keep air from going in/out due to breathing of the tank.

either a nitrogen pad/depad or a constant purge.

high quality (low dewpoint) instrument air might be considered as well.

 

[ash9144]

My experience we used a 180 degree goose neck of SS316 of the vent nozzle to atmosphere. Any corrosion issues would be localized around that area, which you inspect during routine maintenance. The cost for N2 or dry air would be fairly high and not necessary.