General

This standard provides recommended design, fabrication, and inspection practices for maintaining the mechanical integrity of and minimizing the potential for the occurrence of undetected corrosion in concentrated fresh sulfuric acid tanks, process sulfuric acid tanks, or oleum storage tanks at atmospheric and low pressure. Inspection guidelines that aid in detecting and monitoring corrosion are presented, with the overall aim being to avert catastrophic failures. This standard is intended for use by sulfuric acid manufacturers and end users that have stationary sulfuric acid storage tank systems.

This standard covers the storage of fresh and process sulfuric acids and oleum that may be handled in carbon steel equipment. Typical fresh acid concentrations are 93% and 98% sulfuric acid. Process acid is sulfuric acid between 65% and 99.5% concentration that is recycled, purified, and/or concentrated in process units as part of a manufacturing process. Fresh acid and process acid may be referred to collectively in this standard as concentrated acid. Typical oleum concentrations range up to 65%.

This standard is intended for bulk acid at ambient temperatures. Situations in which the inlet stream is hotter than 40°C (104°F), the tank has heaters, or the geographical location results in a metal temperature greater than 40°C (104°F) require special consideration. In these cases, a materials engineer should be consulted to determine the materials of construction and corrosion allowance (see Paragraphs 2.6 and 2.7 for materials of construction and corrosion allowance considerations, respectively).

This standard covers vertical tanks for atmospheric pressure and low pressure built in accordance with API Standard 650 and API Standard 620,³ respectively. API 650 covers atmospheric pressure tanks and API 620 covers tanks up to a gauge pressure of 100 kPa (15 psig). This standard covers horizontal tanks built in accordance with ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Division 1. Vessels pressurized to transfer acid to other vessels without a pump are not in the scope of this standard.

Spent sulfuric acids from alkylation units and chemical processes can have significantly different corrosion rates from fresh and process sulfuric acid. In addition, there is a potential for deflagration in the vapor space of alkylation and chemical spent sulfuric acid tanks. Therefore, alkylation and chemical spent sulfuric acids are not within the scope this standard. See NACE Standard RP02054 for information on storing alkylation unit spent sulfuric acid.

Fresh acid typically has very low contaminant levels that do not impact corrosion performance. Sometimes, low levels of hydrogen sulfide (H₂S), hydrogen cyanide (HCN), or arsenic (As) may be present in fresh acid, depending on the origin of the raw materials and the manufacturing method. For example, fresh sulfuric acid manufactured with smelter sulfur may contain As. Process sulfuric acid may contain H₂S, HCN, or As because of the way it is used in a manufacturing process. Therefore, the potential effects of H₂S, HCN, and As on the corrosion performance of fresh acid and process acid are addressed in this standard.

This standard is based on good engineering practice. The underlying philosophy is that major failures can be avoided and minor incidents reduced to a minimum by ensuring a high degree of storage tank system integrity through good design and construction, followed by adequate and periodic inspection and maintenance. The end user should refer to this standard for guidance, but in all cases, the end user should:

• Prepare a tank specification that includes design, materials of construction, fabrication, inspection, and testing. Major acid suppliers can assist by providing examples of their specifications.

• Select an experienced and qualified designer and tank fabrication and erection contractor.

• Arrange for inspection during all stages of fabrication and construction to ensure the specification is being followed.

• Operate the tank within design limits.

• Monitor and maintain the condition of the tank in service through periodic inspection and maintenance.

This standard applies to all stationary sulfuric acid storage tank systems that contain fresh or process sulfuric acid or oleum at conditions within the scope of this standard. The sulfuric acid storage tank system is defined as the tank and piping internal to the tank and immediately adjacent to the tank, including acid recirculation pump piping loops associated with the tank. Piping up to the first safety device, double block and bleed valves, and appurtenances (e.g., vent goosenecks) are part of the sulfuric acid storage tank system. The sulfuric acid storage tank system also includes the berms, diking, and some tank siting issues. Rail tank cars, tank trailers, barges, and portable tote containers for sulfuric acid are not within the scope of this standard.

The symbol % is used throughout this standard to indicate the mass ratio of sulfuric acid to the total mass for sulfuric acid-water mixtures only.

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• AWS D1.6/D1.6M : Structural Welding Code— Stainless Steel

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