Storage/Loading/Unloading - Storage Tanks
April
2, 2008
Introduction
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.
Corrosion
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.
Uniform Corrosion
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.
Temperature
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.
Tank Utilization
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.
Acid Purity
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
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 Grooving
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
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
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. |
 |
Hydrogen
Blistering
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
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.
Containment
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
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.
Design Details
Corrosion
Allowance
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.
Acid Inlet Nozzles
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.
Tank Sizing
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.
Vents
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.
Oleum Vents
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.
