Concentrated sulfuric acid in closed pipelines
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A carbon steel pipe closed with a valve at one end and blocked with a blind plate at the other end contains some concentrated sulfuric acid (98%). What changes can happen in the pipeline? ? (During disassembly, sulfuric acid spurted out from the pipes) Thank you for your help! ! !
Although carbon steel pipelines can be used for concentrated sulfuric acid, the main reason is that passivation occurs on the surface, and the blind plate you added is probably also made of carbon steel, so a small amount of gas may be generated when passivation occurs. Or because you stored it for some time, some gas was produced due to expansion due to rising temperature or some chemical reactions with the pipeline due to rising temperature.
I have personally experienced this problem. I wonder what kind of pipe you are using? Is it a carbon steel pipe? After my accident, the sulfuric acid content became dilute and the sulfuric acid solution turned green. The reason is that the analysis shows that the corrosion reaction between sulfuric acid and the iron in the pipeline produces hydrogen gas.
The materials here are all made of carbon steel~ The generated hydrogen should be at the highest point of the pipe, because our place is an approximately U-shaped connection (there is a flat pipe near the blind end). I don't understand why, but the sulfuric acid inside will spray out, but not much. ………………
my idea is : Your pipes (including blind plates and pipelines) contain iron. Although it can be passivated, due to concentrated sulfuric acid absorbing water and slow corrosion, it reacts and releases hydrogen, which increases the pressure inside the pipe.
It may be that the temperature of the sulfuric acid has changed, causing the pressure to rise slightly; or the sulfuric acid itself has a certain pressure before closing the valve, so a little sulfuric acid will spurt out when it is opened, and it will be OK when the pressure is relieved;
The main reason is that passivation occurs on the surface, and the blind plate you added is probably made of carbon steel, so a small amount of gas may be generated when passivation occurs.
Don't forget to check whether the valve is leaking. Also, check the material of the valve. If the material is wrong, the valve core may be corroded, causing the valve to leak.
1. Corrosion reaction generates hydrogen and generates pressure.； 2. There is already pressure in the pipeline when the valve is closed. ； 3. After the temperature rises, the sulfuric acid liquid expands in volume and generates pressure. Be especially careful when dismantling sulfuric acid pipelines. Don't think that if the valve is closed, there will be no pressure or material in the pipeline. Sometimes the valve will leak internally.
There is a rough standard for the concept of concentration and dilution of sulfuric acid.: The highest fuming sulfuric acid density is 1.96, containing 40% free SO3, and the total SO3 content reaches 89%. Generally speaking, sulfuric acid with a density of 1.84 is called concentrated sulfuric acid (note: I think generally those with a concentration of more than 75% are called concentrated sulfuric acid) which contains a total SO3 content of 82% and free SO3 4%, with a concentration of 18 mol/L. Medium concentration refers to a density of about 1.5 to 1.8, and their concentrations are 9.2 mol/L to 16 mol/L respectively. Then dilute sulfuric acid means that the density is below 1.5 and the concentration is below 9.2mol/L. Can 4% free SO3 turn into gaseous form and come out? ?