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Storage/Loading/Unloading - Storage Tanks
April 2, 2008
Acid inlet Nozzles
Existing codes are inadequate for the design of sulphuric acid storage tanks. The corrosion allowances and the design for corrosion control are left to the individual designer/owner/operator of the tank. Large storage tanks are usually built to the following codes:
API Standard 650 Welded Steel Tanks for Oil Storage
API Standard 620 Recommended Rules for Construction of Large, Welded, Low Pressure Storage Tanks
ASME Boiler and Pressure Vessel Code, Section VIII, Division I
These codes are sufficient for the mechanical design of the storage tank but are not adequate to address the peculiarities of the corrosion by sulphuric acid.
The corrosion resistance of carbon steel in the presence of sulphuric acid is due to iron sulphate which is formed in the initial contact period. Any condition causing sufficient turbulence to remove the sulphate film is likely to lead to corrosion.
Corrosion rates are usually stated in 'mpy' mils per year. One mil per year (1 mpy) is equivalent to 0.001 inches/year or 0.025 mm/year.
Metal loss increases as the acid strength decreases below 98% H2SO4. 77% H2SO4 is usually regarded as the practical lower limit at which sulphuric acid can be stored at ambient temperatures in unprotected carbon steel tanks.
The figure below illustrates the corrosion rate of steel by sulphuric acid as a function of concentration and temperature.
To compensate for uniform corrosion an adequate corrosion allowance has to be included in the design of the tank. It should be based on design life, with consideration given to factors such as temperature, tank utilization and acid purity.
The steel isocorrosion curves clearly illustrates that the corrosion rates increase with increasing acid temperature. Product acid from a plant should be cooled to a maximum of 40°C (104°F). Within the storage tank the bulk acid temperature should be kept as low as possible without freezing the acid.
Climatic conditions contribute to the temperature variation within a storage tank. Season temperature changes are generally not a problem because rarely will the ambient temperature be higher than 40°C (104°F). Of greater importance is the effect of the sun heating the tank contents. Higher corrosion rates have been reported on the side of tanks heated by the sun. To minimize this problems the tank should be painted with a heat reflecting colour.
The rate of corrosion generally decreases progressively up the shell of the tank because the upper sections are not exposed to acid as often as the lower parts.
The purity of acid can be relatively high leaving the acid plant with iron contents can be as low as 30 ppm. Acid will become more corrosive as the iron content is decreased. Corrosion rates will be higher than normal unless precaution are taken such as lining the tank or the use of anodic protection.
Non-uniform corrosion can severely reduce the life of a storage tank. Most occurrences can be minimized in the design of the tank and by performing regular inspections of the tank internals.
Hydrogen is generated when sulphuric acid corrodes carbon steel. The hydrogen gas formed rises towards the surface and the passage of these bubbles over the steel surface can result in the formation of grooves.
The grooves can be of varying width and depth. The loss of metal in this area occurs at a far greater rate than uniform corrosion at the same acid concentration and temperature.
The effects of hydrogen grooving can be minimized by a properly located acid inlet.
Hydrogen grooving can also occur at the top of the nozzle of a side manway.
Horizontal grooving occurs when a layer of dilute acid sits on top of a layer of stronger acid. Dilute acid is formed when water infiltrates the tank and contacts the acid surface resulting in an acid of lower strength. The lower specific gravity of the weaker acid means it will remain on the surface. The lower the acid concentration the more rapid the corrosion rate. Several horizontal grooves can be formed as the level in the tank fluctuates. In one incident, water was accidentally pumped into the storage tank with the result that the shell was cut in half.
Localized temperature increases can also result in horizontal grooving. Hot acid pumped into a tank will not easily mix with the cooler contents of a storage tank. The result will be a layer of hotter acid which will corrode the shell of the tank faster than the colder acid.
External corrosion of a storage can be as much a problem as internal corrosion. External corrosion can occur in three areas:
- Improper foundation which allows water and debris to come in contact with the bottom of the tank
- Crevices between the tank and supports or attachments that allow water or spilled acid to collect
- Wet insulation
When a storage tank is insulated to prevent freezing of its contents, corrosion under the insulation can occur if the insulation gets wet. This type of corrosion is very common and will occur where water can accumulate such as near nozzle necks, roof or shell attachments (i.e. handrails, stairs, etc.), insulation support rings, etc. Any cladding or insulation that has become damaged or is missing should be repaired immediately.
Foundations that allow water to accumulate will result in corrosion of the tank bottom. This type of corrosion is difficult to detect since the bottom of the tank is generally not accessible for external examination. Corrosion can be a general loss of metal or a localized attack in the form of pitting.
An example of external corrosion is shown in the photo to the right. Here the tank bottom was attacked from below due to what is believed to be moisture under the steel. This type of corrosion is very difficult to detect during a tank inspection because it is so localized. The entire tank bottom would need to be scanned in order to determine the extent of the corrosion.
When carbon steel corrodes in the presence of sulphuric acid, atomic hydrogen is formed. Some of this atomic hydrogen diffuses into the metal and migrates to the exterior. If there is avoid or lamination in the plate, the hydrogen will accumulate there as molecular hydrogen. Molecular hydrogen does not diffuse as easily as atomic hydrogen so it becomes trapped in the void. As more hydrogen accumulates the pressure in the void increases significantly to the point where the plate will begin to bulge. As the pressure in the void increases the size of the blister may increase as the plate delaminates or the blister will rupture.
Special precautions must be taken if work is done near a hydrogen blisters. The heat generated from drilling or welding may be sufficient to ignite the hydrogen gas.
Defects such as voids or laminations are common in milled plate but are very difficult to detect. Experience has shown that blistering occurs in a large percentage of storage tanks.
Tank farms are common where a facility produces or handles acid of varying concentration and quality or when the quantity of acid to be stored exceeds the capacity of a single tank. Multiple tanks adds the flexibility of the facility to handle the unexpected.
Due to environmental pollution control a containment area will be required around all new tank installations. The dike or bund should be designed to contain 110% of the largest tank within the diked area.
Safe access into and out of the diked area must be provided as well as a sufficient number of escape routes.
The diked area should also provide for the draining and collection of water. The area should be sloped to a centrally located sump where any water or spills can be treated and disposed.
Pumps should be located inside the diked area as close to the tank as possible to avoid long suction lines. Pumps located outside a dike area means there may be pipes penetrating the dike. Differential settlement of the tank and the pump may lead to unacceptable strains on the piping, tank and pump nozzles.
The corrosion allowance should be based on the required service life, acid concentration, temperature, geographic location and upon whether or not anodic protection or an internal protective coating is installed.
In general, the roof of a storage tank should be designed with a minimum 3 mm corrosion allowance. The shell and tank bottom should have a minimum of 6 mm corrosion allowance.
For vertical storage tanks the inlet nozzles should be positioned as close to the centre of the roof to minimize the potential for hydrogen grooving of the shell. One source recommends locating the inlet nozzle a minimum of 2.44 m (8 ft) away from the shell. Another source requires a minimum of 3.66 m (12 ft). A third reference requires a minimum of 3.05 m (10 ft). In all cases the inlet nozzle should project into the tank at least 150 mm (6 in.).
When the required distance from the wall can not be accommodated (i.e.. in small tanks) all acid inlets should be equipped with dip pipes which will bring the acid to within 0.914 m (3 ft) of the tank bottom. The dip tube should be located a minimum of 1.83 m (6 ft) from the side wall. The bottom of the tank should be equipped with a protective impingement plate. The dip tube should include a siphon break in its design.
Some of the factors that will determine the number and size of tanks are:
- Plant production rate
- Number of different grades of acid
- Acid usage
- Acid sales
Once the number and size of each tank has been determined the individual tanks can be sized.
A good guide to determine the capacity of a tank is 1.5 times the size of the normal delivery or the normal delivery plus two weeks consumption which ever is greater.
For tank truck delivery, the capacity should be at least 40 tons.
For tank car delivery (100 tons), the storage tank capacity should be at least 200 tons.
Storage tank capacities are usually given based on the weight of acid in the tank rather than the volume (i.e.. 5000, 10000 tonne).
The shell of the tank will be fabricated from flat plate rolled to the required curvature. To minimize fabrication costs full plates should be used for each course.
The vent should be at least the same diameter as the fill line, the discharge line or drain connection, which ever is greater. Preferably, the vent should be one pipe size larger. The presence of other vent lines into the tank from an oleum tank should be considered when sizing the vent. The cross-sectional area of all incoming vents should be added to the vent line size.
For oleum tanks the vent should be sized using the above criteria but should be at least 75 mm (3 in) diameter. The vent should be at heat traced and insulated to prevent freezing of SO3 fumes. To contain the SO3 fumes the vent may be directed to the vapour space of a storage tank containing 93% or 98% sulphuric acid. Otherwise, the vent should be directed to a scrubber.