h2so4 Storage Tank

Are there any other fluids connected to this box?

Of what material are the box and the tank made?

What concentration is the acid?

acid conc. is 98 wt% & inlet is from acid plant . moc is ms rubber lined

Concentrated sulphuric acid (if that is what it is) has a very strong affinity for water and will extract water vapour from the atmosphere, diluting itself in the process. It is also highly corrosive of course. Could your box and pipework be a system to prevent water vapour from entering the storage tank? Or could it be a system to prevent the escape of corrosive vapour from the tank? Or both?

Then there should not be overflow line to storage tank as it will absorb water vapor from atmosphere that means tank overflow will go to top vent line to acid box. Does providing overflow causes acid dilution?

sir, does providing acid box reduces fumes of acid that is splashing of acid?

i have read through acid handbook that dilute acid is more corrosive than conc. so acid box to prevent water absorbtion from atm. is seems convincing.

It is true that concentrated sulphuric acid is less corrosive than diluted, because it is essentially a covalent compound. Only in the presence of water will it ionize to H+ ions and attack metals and organic matter. Normal concentrated sulphuric acid is 98% pure, but "oleum" is c. H2SO4 in which sulphur trioxide has been dissolved. The SO3 reacts with H2O to form more H2SO4, so keeps the acid at 100% strength. I don't know if this is relevant to your situation.