Storage tanks for sulfuric acid in strengths of 93% through 99% are usually fabricated from carbon steel. At normal temperatures there is some attack on the steel resulting in the evolution of hydrogen gas and the formation of an iron sulfate precipitate, most of which settles to the bottom of the tank. Wall thickness should be carefully calculated to provide for corrosion allowances and for the high specific gravity of the acid. Many storage tanks use anodic protection systems to minimize corrosion. Storage tank cleaning and inspection procedures are available on request. Other materials such as stainless steel and polyethylene are sometimes used for smaller tanks.
Hydrogen gas is potentially explosive and special care must be taken when working near tanks or performing maintenance on tanks. A hydrogen meter and spark-proof tools and lights should be used, and open flames and welding should be avoided.
Corrosion, inspection and cleaning
A program of routine internal inspection should be established to ensure early detection of excessive corrosion. It is necessary to clean out the tanks at regular intervals to allow for inspection. A suggested minimum frequency is every three years for tanks containing hot acid and every five years for tanks containing cold acid. Inspection frequency should be modified to suit the rates of corrosion evident in a particular tank, based on previous inspection results over time.
Foundation and secondary containment
It is recommended that storage tanks be enclosed by a secondary containment wall having a capacity no less than 120% of the largest tank volume (local regulations may vary). The containment area should be kept dry and clean. In the event of a leak, the acid should be neutralized and pumped out before it reaches a sewer or watercourse.
Heating and insulation
Heating and insulation are generally required for 96%, 98% and 99% acid, while 93% acid rarely requires either.
A good guide for the capacity of the storage tank is 1.5 times the size of the normal delivery or the normal delivery plus two weeks consumption, whichever is greater, although specific production parameters may dictate otherwise.
Sulfuric acid storage tanks must always have an open vent for normal breathing and to prevent dangerous pressure build-up due to hydrogen. Hydrogen gas may be produced from the action of acid on the steel tank and cause increased pressure and danger of explosion from potential sparks or flames. The vent line must be constructed of acid-resistant material such as polyvinyl chloride. Iron sulfate can build up in the line over time when carbon steel is used, requiring periodic checking to prevent blockage and possible tank collapse during acid transfer. The vent line should be flush with the inner surface at the highest point of the tank to ensure all hydrogen is purged, in order to minimize the danger of explosion.
Catwalks and overhead pipes
Metal catwalks should be provided for working on top of tanks as hidden weak spots may have developed from internal corrosion. Never walk on the tank skin and always use the catwalk. If dripping is noticed from a line suspended above, don't look up. Step well away before investigating.
Proximity to other materials
Although sulfuric acid is not flammable, it should not be stored near organic materials, nitrates, carbides, chlorates or metal powders. Contact between high concentrations of sulfuric acid and these materials may cause ignition.
All lines should be installed so that they drain toward the storage tank, or toward the point of consumption. This will prevent the accumulation of acid in low points, thereby eliminating possible safety hazards when repairs are necessary. The line from the loading point to storage should be equipped with a valve so that acid flow can be stopped at any time. The unloading line should also include a drain point that can be checked prior to disconnecting transfer hoses or pipes. Piping should be designed to ensure flow velocities are maintained to appropriate maximums, depending on the piping material used. All piping should receive regular ultrasonic inspection to confirm remaining wall thicknesses, especially at locations such as elbows and joints where flow characteristics can accelerate deterioration.