Material selection, design and corrosion protection of sulfuric acid storage tanks

Safe storage is a very important part of the production, transportation, storage and use of sulphuric acid. In order to ensure the safe operation of the sulfuric acid storage tank, measures such as correct material selection, reasonable design, and appropriate anti-corrosion should be taken.

Material selection

Sulfuric acid corrosion has two major characteristics. First, the corrosion rate of carbon steel at a certain temperature increases with the increase of concentration, but after reaching a certain concentration, the corrosion rate decreases sharply with the increase of concentration. Second, the same concentration of sulfuric acid will increase the corrosion rate with increasing temperature. Therefore, for concentrated sulfuric acid storage at room temperature, carbon steel materials are usually used in order to ensure a certain corrosion margin and storage. In addition, suitable materials for sulfuric acid Storage Tanks include high-nickel-chromium alloys, molybdenum-containing stainless steels, titanium, rubber, ceramics, and glass reinforced plastics.

Dilute sulfuric acid is very corrosive even at lower temperatures, so corrosion resistant materials are required. 316L stainless steel and glass reinforced plastic (FRP) can be used at a certain concentration and temperature. The corrosion resistance of FRP depends mainly on the type of resin and its properties, so the key is to choose the right FRP resin. In addition to the application experience, it is also necessary to follow the instructions of the resin manufacturer. In addition to carbon steel or stainless steel cans, a large number of carbon steel materials with inner lining, such as polytetrafluoroethylene (PTFE), polyisobutylene (PIB), fluorocarbon, polyvinyl chloride, polypropylene, rubber, and glass reinforced plastic, can be used. These are all lining materials with better performance.

It is important to note that the feed nozzles, discharge nozzles, and venting nozzles (wires) of carbon steel storage tanks must be made of materials resistant to sulfuric acid corrosion. If the medium is ordinary sulfuric acid, it can be made of polyvinyl chloride-lined carbon steel, high nickel-chromium alloy (HASTELLOY, ALLOY20) and other materials. For fuming sulfuric acid, 316L stainless steel or a carbon steel tube lined with fluorocarbon can be used.

design

The design criteria for sulfuric acid storage tanks include pressure vessel type and atmospheric pressure large storage tanks. Because the vapor partial pressure of sulfuric acid medium is generally not high, close to normal pressure, generally speaking, when the nominal volume is >200m3 and the design pressure is not large, the atmospheric pressure Large Storage Tank can be designed according to GB50341/API650 standard, and the pressure can generally be taken as positive pressure ≥ 500Pa, negative pressure = -490Pa. The type of sulfuric acid storage tank does not require an internal floating disc because there is no large vapor pressure.

After selecting the material, it is necessary to reasonably determine the thickness of the tank of the sulfuric acid storage tank, and the steel storage tank should adopt a large corrosion allowance. In order to ensure good corrosion resistance of the tank, all welds under the liquid level should be fully welded and docked and subjected to local X-ray inspection. For large tanks where the product of height and diameter exceeds a certain value, the longitudinal weld can be 100% X-ray tested. The top of the tank can be lap welded. All tank bottom welds shall be full penetration butt welds with backing plates, and the bottom of the tank shall be vacuum chamber tested as required. Since the vapor pressure of sulfuric acid is not large, the pressure of the PN2.0 should be adopted for the general nozzle flange. Since the internal structure of the flange such as socket weld and threaded flange determines that corrosion is more likely to occur, it should be avoided. Class flange. When elbows are required, in order to reduce wear corrosion stress and corrosion cracking, seamless elbows with internal coating protection and large radius of curvature should be used. In the case of take-over welding, the joints of different thicknesses shall not be welded directly together to avoid abrupt changes in the thickness or shape of the weld, and the welds and structural discontinuities shall be separated during structural design. When using the gasket, the inner diameter of the gasket and the diameter of the nozzle should be the same to avoid wear and crevice corrosion.

Antiseptic

For the corrosion protection of sulfuric acid storage tanks, other measures can be taken, such as changing the flow state of sulfuric acid, electrochemical protection, passivation, using cover protection and strengthening and improving management.

Because of the corrosion resistance of carbon steel in sulfuric acid, it depends on a layer of ferrous sulfate protective film formed on the surface. When this protective film is destroyed, its corrosion resistance is greatly reduced. Therefore, the flow rate of sulfuric acid should be strictly limited. DuPont The company recommends a flow rate of sulfuric acid of 25 to 75 mm/s.

At present, the electrochemical protection technology mainly adopts the anode protection method, that is, an anode current is introduced into the protected metal (anode), and the anode potential is changed in the positive direction to reach the anode. This method can not only effectively reduce the corrosion rate, greatly reduce the corrosion rate of carbon steel, but also avoid intergranular corrosion.

The corrosivity of sulfuric acid increases with increasing temperature. To avoid the temperature rise of the sulfuric acid in the tank due to solar illumination, the outer wall of the tank should be sprayed with a light-colored (usually white) topcoat. In addition, in order to ensure the safe operation of the tank, the thickness of the tank should be tested at least once every 2 years, and at least once every 5 years.