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Safety and environmental standards for fuel storage sites
Final report
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107 On sites where hydrocarbon gas or liquid detection is used as a means of overflow detection,
the detector type, operation, maintenance and detector location are critical factors. Historically,
hydrocarbon detection systems have been found not to be highly reliable because their ability
to detect gas or liquid depends not only on the reliability of the instrument but also on their
positioning in a suitable location and their robust maintenance. Therefore, claims made for the
performance of an overflow detection system should include sufficient supporting evidence.
108 Care also needs to be taken to be realistic in specifying the required performance of an
overflow detection system because it is only a partial protection layer if it simply detects that the
storage tank is overflowing. For the protection layer to be complete and effective, it must also be
possible to take action which will stop the overflow before any vapour cloud formed can reach a
source of ignition. There are several important elements to this:
It must be possible for the overflow to be detected and stopped safely (ie without expecting
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an individual to approach close to the vapour cloud).
The means of stopping the overflow must be independent of other layers of protection – ie
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reliance cannot be put on closing valves or stopping pumps which form part of another
protection layer.
The time to stop the overflow requires careful consideration given the assumption of a very
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low wind speed. Under low wind speed conditions, any large vapour cloud may be persistent
and may be capable of being ignited and exploding for some time after the overflow has
stopped. Different considerations for response time would apply for an environmental
consequence where, for example, the consequence requires that the gasoline penetrates the
floor of the bund.
For any detection system relying on direct observation, careful consideration needs to
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be given to the human factors of the process, including the time taken for diagnosis,
communication, determination of the condition of any other failed protection layers and for
the correct action to be taken.
The human–machine interface, in particular the means of alerting the operator that an
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overflow has occurred and the human factors affecting the response of the operator.
Where relevant, the reliability and quality of the communications arrangements, including the
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presence of any radio ‘blind spots’ and areas of high background noise or distraction.
Where direct observation is assumed, consideration needs to be given to the means of
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observation. While the sense of smell may alert a knowledgeable person to the presence of
gasoline vapour and to the fact that the situation is abnormal, it is unlikely to allow the source
to be localised without further investigation. Even visual observation may not be sufficient if
the vapour cloud is large.
Where the operating procedures for the facility require operators to investigate potential leaks,
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a failure of the overflow detection protection layer may result in increased numbers of people
being vulnerable should the vapour cloud ignite. This may result in worse consequences than
would be expected from simple time-averaged observation of where people are and when.
Where the response to an indication of a tank overflow requires operator intervention,
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consideration needs to be given to:
the expected role of an operator on receipt of a signal from the gas or liquid detection
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system. (How will the operator be alerted? Will it be obvious which tank is overflowing?
Which operator is expected to respond? Where will the operator be when the alert is
received? How long will it take to diagnose the situation? Are there clear instructions on
what to do? Has the situation been rehearsed?);
their ability to take action (which valve needs to be closed? How is the valve identified? Is it
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accessible safely? How long will it take to close? How is the valve closed?);
the effectiveness of the action (will closing the valve in the required response time make
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much of a difference? Will the gas cloud already have reached a large size?).