Recent vetting inspections have raised the profile of the carriage of gas detection instruments relating to toxicity with particular regard to atmospheric levels of Benzene, Toluene, Xylene (BTX) and associated additives such as MTBE. Some or all of which may be carried individually, or as a constituent part of, or as an additive, within a gasoline cargo.

The vapours generated by these substances all come under the generic term of Volatile Organic Compounds or VOC's.  It is believed that the oil companies are focusing on the early detection of these VOC's is in order to maintain safety of personnel.  However, the problem with detecting these gases is that the Threshold Limit Value TLV is very low - 10 ppm for Benzene and 40 ppm for MTBE (based on the US and Australian TLV values).  However the standard measuring tube for MTBE commences to read from 50 ppm and thus higher than the maximum of 40 ppm.     

A  point which needs to be considered is when more than one type of VOC is carried in a cargo it is very difficult to isolate the readings for specific products.  A good example of this is gasoline that will normally have a combination, including benzene and currently MTBE.  This can be compensated for on board, by identifying the limits for the most hazardous compound from the marine safety data sheets and used as the limiting value. 

Useful web link: The Draeger web site http://www.draeger.com/  and the following link http://voice.draeger.com/voice/owa/vn_such?L=1&C=1 Draeger list a hazardous substance database. By accessing this database TLV’s for the various products can be found and a further search will list the applicable Draeger tube and its measuring range. 

In addition, details of other measuring instruments can be found here that can be used in circumstances where for example a Tube is not produced that will measure down to 40ppm for MTBE. (The measuring range for this tube starts at 50ppm). 

There are a number of instruments on the market that are specifically designed to detect these VOC,s. Such as:  

1. Chip measuring  instruments with a chip measuring system which is available from a variety of manufacturers. With this type of instrument individual chips have to be supplied for each type of VOC required, e.g. one for Benzene, one for MTBE etc. Chip life is normally of the order of 2 years. 

2. Single and multigas instrument (Lower Explosive Limits H2S, O2) that claim to accurately detect the presence of most VOC,s.  These instruments use PID (photo ionisation detector) type sensors to cover a very broad band of VOC's, they are claimed to be very accurate and to work in all atmospheres including inerted spaces. 

3. Other single and multigas instruments are available on the market that will detect MTBE and other products, however these use catalytic sensors and are only accurate as a factor of the Lower Explosive Limits, therefore they will not work to the required accuracy and the presence of other hydrocarbons will change the readings. Catalytic sensors will not work in an inerted atmosphere as they require the presence of oxygen to function. 

Contact:  Howard Snaith on e-mail howard.snaith@intertanko.com for further information