Opinion: Fueling Station Safety and Hydrogen Detection
Andrei V. Tchouvelev, A.V. Tchouvelev & Associates Inc.
 Early hydrogen detection is one of the key risk mitigation measures associated with potential hydrogen releases from hydrogen containing equipment and pipework. But how early should it be implemented to effectively improve safety and not become an operational nuissance at the same time?
Hydrogen Detection at Fueling Stations: The 100 ppm requirement
Some people have a perception that if the hydrogen detection limit is as low as possible then safety will be improved (or risks reduced) at hydrogen fueling stations. This belief is particularly strong in Japan within the circle of experts that design, build and operate hydrogen stations. An informal survey of related Japanese industries on the minimum required detection limit of hydrogen concentrations revealed that the average weighted response was around a few hundred ppm. Thus, to ensure that a few hundred ppm are reliably measured, the requirement for a 100 ppm detection limit is justified.
Others (including myself) think that such a low detection limit has nothing to do with "safety" or "risk," is not scientifically based (on hydrogen properties and risk perspectives) and is purely emotional (perception-based). If we presume that a concentration of 4% hydrogen by volume (40,000 ppm) is flammable outdoors (which, we know, it is not) there is a huge disparity between the 40,000 ppm flammability level and the minimum detection limit – 100 ppm. If safety is primary concern, a factor of 10 rather that 400 would be more than adequate. From risk prospective, there is absolutely no reason to believe that hydrogen detection at 100 ppm will reduce risk of incidents during fueling instead of detecting hydrogen at, say, 4,000 ppm (which incorporates a safety factor of 10). Moreover, premature detection and initiated inadequate response may have adverse effects (see below), while timely detection of reasonably significant concentrations will allow the user to concentrate on mitigating real hazards. As Confucius advised: "Don’t use cannon against mosquitoes."
There is nothing wrong with a 100 ppm detection limit in principle; certainly there might be a need for this type of measurement. The point here is: it has nothing to do with improving fueling safety and it will likely be very impractical under the conditions of a fuelling station. It is ironic that in Japan, within the circle of experts discussing maximum allowable hydrogen concentrations that could be emitted from fuel cell vehicles' tailpipes, emissions containing up to 4% hydrogen by volume are considered to be potentially permissible. In the US during the development of a recommended practice for general fuel cell vehicle safety, the same tailpipe emissions with 4% hydrogen have been discussed. This means that a regular hydrogen fuel cell vehicle may routinely generate short term emissions of up to 4% hydrogen by volume (or 40,000 ppm) from its tail pipe during shut down or start up. This could happen immediately before or after fueling. One could predict that every normal (i.e. safe) fueling operation will likely set off an alarm of a highly sensitive detection apparatus. This might in turn have very “interesting” effects ranging from solidifying a sense of fear or unease in the public to strong annoyance from station operators against hydrogen detection followed by permanent disabling of detection system. This would really have an effect on safety--a negative effect.
Testing of detectors in enriched hydrogen gas
As pointed out by detection experts, it is very important that the sensors, when exposed to high concentrations of hydrogen, would continue to display over the range concentrations ("stay high") during the whole period of exposure. This is a very important safety feature. Suppose there is a large leak that quickly exceeds the measurement range of a sensor. If the sensor does not stay high and drops its reading to zero, an operator may think that there is no hydrogen around while in fact the hydrogen concentration is very high.
Experts advise that thermal conductivity sensors have no problem staying high while other technologies may. When exposed to high hydrogen concentrations, the accepted solution for hydrogen detectors is either to display an alert over the range of concentrations or send a latching alarm (the one that would require a deliberate action to be deactivated).
|
