On 4th February 2005 a storage tank containing 16,300 t of
96 % sulphuric acid ruptured. The entire contents of the tank were spilled out
into the bund and then overflowed out into the nearby dock. The environmental
consequences of the accident were quite significant,the sulphuric acid emission
had a serious effect on local flora in the inner and deepest parts of the
harbor and harbor entrance area. When the sulphuric acid came into contact with
the salt water an exothermic reaction occurred, producing a vapour cloud
consisting of hydrogen chloride that drifted northwards along the coastline in the
direction of the wind. Fortunately, the wind was blowing towards the sea and
away from populated land areas and the cloud diluted very quickly. After the
spill approximately 2,000 t of contaminated sulphuric acid remained in the
bund. The acid also soaked into about 100,000 square metres of the ground
surrounding the spill.
Causes
The cause of this incident was a leak in an underground
coolant supply pipe of reinforced concrete installed over forty years before that
resulted in a weakening of the ground under the tank farm. Apparently, water
forced its way out of the pipe, eroding the ground near and around the
sulphuric acid tank. This erosion damaged the ground under the tank which
ultimately failed due to the lack of support of the tank floor. A study of the
appearance of the involved part of the coolant supply pipe suggests that the
corrosion was a result of an acidic attack on the concrete.
Important findings
• The damage indicates that the acid exposure occurred over
a long period of time. However, it was not possible to determine the exact duration
of the exposure.
• The pipe had been in use over many years and the operator
had mno suspicion that the pipe was suffering severe degradation. The inspection
of the failed pipe after the incident detected little or no internal corrosion,
but heavy external corrosion to the concrete. In certain places the concrete
had corroded so severely that the reinforcing steel was exposed.
• According to the German standard, DIN 4030 (equivalent to
the European standard, EN 260) a strong attack on concrete occurs if the pH
level in surrounding water is < 5.5 and a very strong attack can occur if
the pH level is < 4.5. Fifteen years before the accident a ground pollution
study was carried out in the area, during which one of the sample taking points
was close to the failed coolant supply pipe. At this point the pH level was
measured at 4 in the shallow groundwater. With this knowledge the company drew
the conclusion that this pH level entailed risks for strong acidic attackson
the concrete.
Lessons learned
• The uneven corrosion on the outside of the pipe can
possibly be explained by the fact that it lay partly in groundwater flow. In
this environment, the acid can pass through the barrier more easily, and the
reaction products (gypsum) formed can be more easily dissolved. As such, the
concrete barrier was not as effective as on the part of the pipe that remained
in drier surroundings. Therefore, concrete piping exposed to ground water
should be should be subject to protective measures, monitoring and inspection
to take into consideration the increased risk from groundwater exposure.
• Similarly, underground piping that entail risks to
foundations should be inspected and measured..
• There are a number of strategies that can be applied to
piping where there is accelerated potential for degradation or where there are
high consequences should significant degradation occur.
Pipes may, for example, be tested for stability (remains in
place) and hydraulically checked on a regular basis. Alternatively,
consideration should be given to positioning the pipe above ground. The pipe
could also be placed in casings, especially where a leak may cause damage to
the surroundings or where pressure and ground deformation may cause damage to
the pipe.
Source: European commission
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