Seek help regarding the welding and repair of sand holes in sulfuric acid tanks
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I’m seeking help from all of you. My company has a sulfuric acid tank made of carbon steel, containing 93% concentrated sulfuric acid. There is a small weld defect at the bottom of the tank that needs to be repaired by welding. Currently, the liquid level is below the location of this defect. If welding is carried out directly on the tank body, the steel plate added after welding will definitely end up below the liquid surface; as a result, the remaining sulfuric acid in the tank cannot be properly drained, which could lead to an excessively high temperature in the tank and indirect heating of some of the sulfuric acid. Is there any danger in doing this? If it’s not allowed, what safety precautions should I take? I earnestly ask all the experts here for some guidance
Sulfuric acid is hygroscopic and readily absorbs moisture; concentrated sulfuric acid hanging on the walls of a tank will absorb water and turn into dilute sulfuric acid. You mentioned that steel tanks are being used, and reactions that produce hydrogen gas could occur, so it’s best not to carry out welding operations rashly to avoid explosions. If possible, fill the tank with nitrogen as a protective measure. The amount of heat generated by repairing small defects through welding is minimal, and it won’t cause much increase in the tank’s temperature; completing the welding in a short time will not have any impact on the medium contained within the tank.
What was said on the third floor is correct; otherwise, the sulfuric acid inside the tank should be completely replaced
If the storage tank is equipped with a top venting facility or if the manhole at the top of the tank is opened for a period of time, the hydrogen gas that may be generated can be released. The steel plates welded later are below the liquid level, so there’s not much of a problem; sulfuric acid is a non-volatile liquid.
The issue of hydrogen mentioned on the third floor is very important; it is recommended not to weld with materials present. Was there originally any welding strip or spots in the area where you have the sand eye?
It’s just my personal opinion – welding is possible with no problems. As for why such welding operations are carried out, our factory has done this at least 10 times. Let me explain how we proceed in our factory: first, the manhole on top of the tank must be opened, and a hydrogen detector is used to measure the hydrogen level. If the hydrogen concentration is high, there’s no need for gas displacement; simply venting the gas is sufficient. Second, during the welding process, the hydrogen level needs to be measured every 5 minutes. As for the necessary personal protective equipment, that goes without saying – it’s essential to wear it before starting the welding work, and the acid supply pipes must also be disconnected
This kind of situation has also occurred in our sulfuric acid tanks; we first introduce nitrogen for displacement, and then test the hydrogen level before carrying out any work that involves fire. As long as the requirements are met, the work can proceed. I’m not sure what size your storage tanks are – ours are small, 30 tons each.
Nitrogen displacement should continue without interruption during hot work. Furthermore, even short-term heating below the sulfuric acid level is not a problem, as the amount of sulfuric acid vaporization is very small; and even if a small amount of hydrogen is generated, it is quickly displaced by nitrogen. Before performing hot work, the inlet and outlet of the storage tank as well as the related valves must be closed and blind plates installed. Being bold and careful – no problem.
After reading your reply, there’s one thing I’m not quite sure about: is the purpose of using nitrogen to displace gas to push the hydrogen contained inside out of the equipment? But there is a problem: each sulfuric acid storage tank has manholes on it. The relative molecular weight of hydrogen is 2, so even when hydrogen accumulates, it stays at the top. Using nitrogen to displace it doesn’t seem meaningful in my opinion! ! !
There will definitely be valve flanges and similar components on the upper and top parts of the storage tank. If it is not convenient to open the valves, a gap can be left in the flanges, with the purpose of allowing hydrogen to be pushed out.